Stellar Objects Part 1: Introduction To Stars

Stellar Objects Part 1: Introduction To Stars

How did the universe come to be filled with stars? What are they made of and why are there so many kinds of them? How are they born and how do they die?

What is a star?

A star is a ball of mostly hydrogen and helium with enough mass that it can sustain nuclear fusion in its core. Our Sun is a star but stars come in a wide range of colours and vary a great deal in mass. The least massive possible star is about 80 times the mass of Jupiter. This is the theoretical minimum mass a star can have and still support fusion in its core. The most massive star possible is not really known but most physicists now peg it at about 250 times the Sun's mass. An example is the star, R136a1, its discovery published in July 2010. Tremendously massive stars like this one shed enormous amounts of energy and mass through a continuous stellar wind. They last only a few million years before exiting brilliantly as powerful supernovas. These stars are known as Wolf-Rayet stars. They far surpass what is called the Eddington limit. This limit is likely reached at around 120 solar masses, as which point a star starts to eject its envelope through intense solar wind because its outward radiation pressure exceeds the force of gravitational attraction. Radiation pressure is negative (outward) pressure resulting from the radiation of electrons, protons and other high-energy particles from the star. Solar wind is an example. This is why these huge stars blow off energy and mass at a very fast rate.

The longest-lived stars are the smallest ones. These are red dwarfs. They consume very small amounts of matter, put out very little energy over time and can live for up to 10 trillion years. That's far older than the universe is itself, which is about 14 billion years old. These are the most common stars in the universe today. 85% of the stars in the Milky Way are red dwarfs. 

This is an artist's conception of what a red dwarf might look like close up.

Star Birth

These are star-forming pillars of hydrogen gas and dust within the Eagle Nebula, as seen by the Hubble Space Telescope.

This region is also called the Pillars of Creation. It is the beginning of an immense star nursery. Small dark areas imbedded in the pillars are believed to be baby stars called protostars. There are countless numbers of these nurseries scattered throughout the universe. It is bittersweet to know that we are observing something that is probably already gone. The nebula is 7000 light years away from us and scientists have recent evidence that a supernova exploded nearby about 6000 years ago, destroying the pillars. We will not see the new shape of this nebula for another millennium.

How to Make a Star

Step one: wait. 75% of the matter in the universe is hydrogen and 23% is helium. These elements came from the Big Bang.  A nebular cloud of elemental gas and dust must wait, perhaps for millions of years, until a gravitational disturbance passes through it to turn on the process of star formation. This disturbance can be, for example, a passing star or a shock wave from an exploding star. In a pre-star environment, the disturbance might even be the result of a phase transition or the ionizing influence of primordial black holes. Once the disturbance occurs, matter begins to swirl and ripple and this sets off the process of accretion. This means that a locally dense region of gas and dust gravitationally attracts more and more gas and dust to itself. This will only occur if the cloud has sufficient mass. The gas and dust molecules in it have energy of their own that resists collapse and this energy must be overcome by the gravitational pull of the cloud. In other words the cloud must have a critical mass, about 80 times Jupiter's mass, before it can overcome the energy of the dust and gas particles as they randomly collide and bounce off each other, creating a negative pressure that resists gravitational attraction. If the critical mass is not reached, the cloud continues to swirl and clump but the clumps won't be permanent. They'll dissolve and reform over and over again. When critical mass is reached, the infalling material brings with it the sum of the angular momentum* of each its particles and this force begins to organize it into a disc than begins to rotate faster and faster as it continues to feed on surrounding material. This accretion disc stage can last for up to 10 million years, accumulating on average about 10^-8 solar masses per year.

* Every subatomic particle, every atom, and in fact, every rotating object has angular momentum.

Step two: let gravity do its work. Eventually, clumps of gas and dust grow bigger and together into one clump. When this clump reaches a high enough density it will stop losing heat to the surrounding nebula. This occurs because as its density increases, particles become closer and closer together and eventually they can exchange heat with each other and conserve energy compared to the looser arrangement of particles that surrounds them, which are too far apart to exchange heat (these begin to act like a Thermos container). Before the cloud begins to collapse, it may be very cold. The heat it acquires comes from gravitational potential energy. The collapsing material also becomes optically thick. This occurs when atoms become so energetic they ionize and the disc becomes full of freely scattering electrons. These free electrons collide into photons and prevent them from radiating away. It also means that gravitational potential energy can no longer be radiated away. This prevents any further cooling of the condensing gasses and their temperature now begins to spike rapidly. The gas falling toward the opaque center creates its own shock waves that further heat the core.

The clump is now called a protostar, shown here in this artist's rendering.






It has reached a state of hydrostatic equilibrium, in which internal pressure counteracts the force of gravity. When the accretion process is complete, the star is called a pre-main sequence star. It is a true star, when, at about 10 million K*, hydrogen begins to fuse in its core, igniting a fusion chain reaction and the star begins to shine. Solar wind quickly disperses the remaining dust and gas in its vicinity. This scenario is well documented for low mass stars, including the Sun. For stars larger than 8 solar masses the mechanism of star formation is not well understood but it occurs on a much faster timeline.

*K or Kelvins, the SI base unit of temperature, is used by astrophysicists. For example, absolute zero (0 K), the point at which all motion, even that of electrons, stops, is equivalent to -273°C.

An example of a very massive star is Eta Carinae; it is visible as a bright white spot in the center of the nebula shown here. It is thought to be more than 100 times the mass of the Sun.

It produces more than 1 million times the light of the Sun and it is quite rare. Only a dozen or so exist in the Milky Way.

Very low mass protostars become brown dwarfs. These are tiny stars, which are not technically stars, between 1/18 and 1/00 of one solar mass. They are no longer surrounded by their parent nebula but they are not hot enough to initiate fusion. The physical characteristics of these oddball "failed stars" fall somewhere between stars and giant gas planets.

This is an artist's rendering of a brown dwarf.

What did the first stars look like?

The fist stars in the universe turned on about 200 million years after the Big Bang and ushered in billions of years of intense stellar formation.

This is a simulated NASA image of what the first stars may have looked like within what are called faint blue galaxies. These first galaxies underwent large bursts of star formation resulting in blue light from young extremely massive stars.

Even at about 3-5 billion years old, the universe was a different star formation environment than it is today. Typical galaxies had up to 10 times more molecular gas than they do today and the rate of star formation was, as a result, up to 10 times higher then. These early stars accreted from gases that contained very few elements heavier than lithium-7 (for our purposes we call all these heavier elements "metals" as the astronomers do, not to be confused with actual metals) so very old stars are expected to contain no metals and yet all stars observed, some of them being very old, do. These stars are observed when they were young in their lifecycle so they could not have created their own metals through nucleogenesis (this process will be discussed later in this article). One explanation for this puzzle is that while only metal-free stars formed first, these stars were likely several hundred solar masses, far larger than what could form today. Physicists think this is because they had practically no elements heavier than hydrogen and helium. Heavier elements catalyze hydrogen fusion reactions, increasing a star's heat and therefore outward radiant pressure. These stars, in contrast, were able to amass much more gas before exceeding their Eddington limit. With such high inner densities of hydrogen, they lived very short lives and blew up spectacularly, blowing the first 26 elements up to iron, created inside them in the process of nucleogenesis, far into the galaxies in which they formed, thus relatively quickly seeding the early universe with metals. None of these very first stars have been observed but there is an active hunt for them within the oldest most distant galaxies. Another possible source for the metals observed in stars is black holes created by even more massive earliest stars. Instead of exploding into supernovae these truly enormous stars collapsed into black holes, and all their matter, including metals, eventually sprayed from their relativistic jets throughout the universe. The oldest observed stars have very low metallicities and subsequent generations of stars became more metal-enriched as the gas clouds from which they formed contained more and more metal-rich dust.

This is an image of globular cluster M 80, about 28,000 light years away. The densely packed metal-poor stars in this cluster are believed to be almost as old as the universe itself.

Our Sun

Our Sun is a young star. It contains a high metal content and it is these metal-rich stars that are most likely to have planetary systems formed from the accretion of metals.

The Sun is a main sequence star. This means that it is part of a sequence of stars on a graph on which surface temperature and brightness are plotted against each other, called a Hertzsprung–Russell diagram. Main sequence stars like the Sun are undergoing fusion of hydrogen into helium and are in hydrostatic equilibrium (compression due to gravity is balanced by outward pressure). The Sun is about halfway through its main sequence evolution. Stars as small as the Sun are essentially a proton-proton chain reaction contained by gravity. Higher mass main sequence stars undergo a different fusion reaction called the CNO (carbon-nitrogen-oxygen) cycle because they reach a higher core temperature.

This is a portrait of the Sun taken by Nasa's Solar Dynamics Observatory.

Three quarters of the Sun's mass is hydrogen and most of the rest consists of helium. Just 2% of its mass consists of heavier elements such as oxygen, carbon and iron. It makes up 99.8% of the total mass of the solar system. Within its core about 500 million tons of hydrogen are fused into helium every second at a rate of about 9 x 10³⁷ fusion reactions per second. The fusion rate of the core maintains a state of equilibrium. If fusion increases, the core heats up and expands. If the core expands, the fusion rate slows. It's a density-dependent reaction. This proton-proton fusion reaction releases 0.7% of the fused mass as energy, resulting in a total energy output of about 4 x 10²⁶ watts per second, as neutrinos and solar radiation.

Researchers have attempted to control the fusion process for over 50 years. It would be the answer to the world's energy crisis. Nuclear fusion, not to be confused with nuclear fission, the process undertaken within nuclear reactors, comes with some very difficult practical problems. First it takes a tremendous amount of input energy to get (positively charged and therefore repulsive) nuclei to fuse, even small hydrogen nuclei, because of the enormous electromagnetic repulsive force between them. If they are accelerated to very high speeds and energies, the attractive nuclear strong force can be sufficient to achieve fusion. Very powerful accelerators are required to achieve this. The beauty of nuclear fusion is that is has an energy density (energy produced per unit mass) many times greater than even nuclear fission does. However, even if powerful accelerators become cost effective, there are other barriers to overcome. One problem is once a self-sustaining reaction is achieved how do you contain it? Gravitational, magnetic and inertial containment are three options currently being investigated.

The Sun has the same chemical composition as the interstellar medium from which it formed almost 5 billion years ago. Its metallic portion (2% of its mass) was created within stars, which had since exploded as supernovae through the process of nucleogenesis. The entire solar system began with the gravitational collapse within a cloud of mostly molecular hydrogen.

This is a molecular cloud, similar to the one from which the solar system formed. It's a star nursery broken off of the Carina Nebula about 8000 light years away from us. The bright pinkish spots are new stars, which will "boil away" the surrounding gas and destroy this cloud within a few million years.

Most of the collapsing cloud formed the Sun while the rest contributed to an orbiting protoplanetary disc. It is from this disc that the planets, moons, asteroids and comets formed.

This is an artist's rendering of what a protoplanetary disk might look like.

The Sun will continue to fuse hydrogen into helium until almost all of its hydrogen is used up, in another 5 billion years.

The Sun isn't massive enough to explode into a supernova at that time. Instead, it will expand into a red giant. Its core will be full of inactive helium. Because nuclear fusion will no longer exert outward pressure, the core will begin to collapse under its own gravity. This will heat a shell outside the core where some hydrogen remains. Nuclear fusion will continue for a while longer within this shell as it balloons and the Sun expands, pushing the planets outward. The expansion will spread the heat over a much larger surface area so the surface cools and that is why the Sun will look red. The collapsing core will continue to heat up because of increasing pressure as helium accumulates. Mercury, Venus and likely Earth will be incinerated. Eventually it will ignite helium fusion in what is called a triple alpha process. At around 100 million K, 3 helium nuclei fuse into one carbon-12 nucleus. Some oxygen will also be produced. Larger stars at this point would repeat the fusion cycle, fusing heavier and heavier elements in successive phases but the Sun will never fuse carbon. It will instead end its life in what is called a helium flash. Its core will continue to grow hotter until runaway fusion of helium occurs. At this point, in a matter of seconds, up to 80% of the helium in the core will fuse into carbon and release an incredible amount of energy. A planetary nebula will be blasted out into space leaving behind a carbon-oxygen white dwarf. The dwarf's reduced mass will weaken its gravitational pull on the remaining planets, now frozen, and eventually they may be lured away by other stellar bodies that pass nearby.

A white dwarf is mostly made up of electron-degenerate matter. It is so dense that instead of ordinary atoms with electrons occupying stable orbitals, it consists of a collection of carbon and oxygen nuclei floating in a sea of loose electrons. White dwarfs are luminous not because they are generating any energy but because they have trapped huge amounts of it. Normal matter exerts pressure when it is heated but electron-degenerate matter cannot. It becomes super-compressed with nuclei pushed right up against one another, resulting in an extremely hot dense "solid." For stars the mass of the Sun, what is known as degeneracy pressure resists further collapse of matter. When stars larger than about 4 to 8 times the mass of the Sun collapse, atoms can collapse into each other even further creating a neutron star, which is itself supported by neutron degeneracy pressure. When very massive stars collapse, even neutron degeneracy pressure may fail to support the atoms and the matter then collapses completely into a black hole. The white dwarf that is left over from the helium flash will be about half the mass of the Sun and only slightly larger than the Earth. It will have a very thin atmosphere of purely hydrogen or helium surrounding a crystalline lattice of carbon and oxygen, almost like an impossibly dense diamond. And it will initially be very hot, over 100,000 K. White dwarfs are extremely stable once they are formed. It will take many billions of years to cool and eventually transform into a black dwarf. The only way a white dwarf can lose energy to cool off is through the process of proton decay, in which energy is radiated away through gamma ray photons. Protons are very stable, having a half-life of 7 x 10³³ years so this process is extremely slow. In fact no black dwarfs have ever been detected because the calculated time required for a white dwarf to cool into a black dwarf is longer than the age of the universe. As a black dwarf it will stop shining all together.

A Special Section on Nucleogenesis

I have mentioned that the Sun will never create any element larger than carbon, except for small amount of oxygen. It will create these elements when three helium atoms fuse into one carbon atom and four helium atoms fuse into one oxygen atom. This will begin to happen after the helium flash, mentioned earlier. Larger stars can compress their cores enough to fuse higher elements. At a core temperature of about 600 million K, carbon atoms can fuse into oxygen, neon, sodium and magnesium. At 1 billion K, oxygen atoms can fuse into silicon and the elements from magnesium to silver on the periodic table. These atoms can in turn fuse into elements as massive as iron. Stellar fusion (nucleogenesis) stops at iron because fusing atoms larger than iron requires more energy than it releases. Fusion can produce energy only as long as the sum of the masses of the new nuclei is less than that of the original nuclei. When iron nuclei (26 protons and 30 neutrons) begin to fuse with other nuclei the resulting nuclei have more mass, the process consumes more energy than it produces and the process stops. An iron core then begins to form inside the star.

How are elements heavier than iron created? There are two processes, one slow and one very fast. Over time, free neutrons occasionally slam into atomic nuclei. When they do so, they decay into protons and release an electron. Adding a proton creates the next element on the periodic table. Massive stars last long enough to allow this process to happen and they make atoms as large as bismuth (83 protons) as they do so. Atoms larger than bismuth can only be created in a Type II supernova. Successive fusion reactions within a large star leave an iron core. Eventually this core reaches what is called Chandrasekhar mass, which is about 1.4 solar masses. At this point, not even electron degeneracy pressure can hold it up. It collapses pushing protons and electrons together to form neutrons and neutrinos. Neutrinos do not normally interact with matter but at this density they exert a tremendous outward pressure. Even so, the core continues to collapse and approach neutron degeneracy. The outer layers crash inward and rebound, creating a neutrino outburst and a shock wave, and when they do so, they trigger an enormously powerful Type II supernova explosion.

Twenty years ago, astronomers witnessed one of the brightest stellar explosions in more than 400 years. The supernova SN 1987A exploded with the power of 100 million Suns. This NASA image shows its shock wave, visible as a glowing pink ring of gases.

This explosion, which occurs within a timescale of milliseconds and creates an initial temperature as high as 100 billion K, creates a tremendous number of neutrons, and these neutrons slam into atomic nuclei, resulting in all the elements heavier than bismuth, including all of the radioactive elements, and blasts them far into space.

All the metals within stars, planets and asteroids, all the metal-containing interstellar dust, originally came from the initial fusion of hydrogen and helium. You would expect a decreasing abundance of elements corresponding to the frequency of various fusion reactions within the universe and roughly equivalent to increasing mass, and this is generally the case. The heavier "metal" elements are far more rare than the lighter ones - only 2% of the Milky Way disc is composed of metallic matter by mass. The metallicity of a stellar object can be used to determine past stellar activity within its region of space.

You might be wondering why the nucleogenesis of metals didn't occur just after the Big Bang when nucleons formed. Temperatures were more than hot enough then, just under 2 trillion K. Actually, fusion would have continued then just as it does in large stars and in supernovae if the universe hadn't very quickly entered a period of extremely rapid expansion in which density and temperature dropped below what would be required to sustain fusion. That is why the first baryonic matter (to be distinguished from dark matter) in the universe consisted almost entirely of hydrogen and helium, and no elements heavier than beryllium existed before the first stars approached the ends of their lives.

I leave you now with NASA's star gallery. Click on an image to learn about the story behind it. Such fascinating and beautiful objects stars are!

Next up: Neutron Stars.

Stellar Objects Part 2: Neutron Stars

Stellar Objects Part 2: Neutron Stars

A neutron star is a supernova remnant of degenerate matter so dense that it represents matter at the limit of physical laws, atoms balanced at the cusp of total annihilation, a black hole.

A neutron star is a stellar remnant from a supernova explosion. The tiny white dot indicated by the arrow in the photograph shown below represents the first neutron star directly observed in visible light. Neutron stars comprise the densest form of matter known to exist. Matter denser than this collapses into a black hole.

About 2000 known neutron stars populate the Milky Way and the Magellanic Clouds. The closest one, shown here, is 424 light years away from us.

There are various classifications of neutron stars, generally according to what kinds of energy they emit. When these stars are very young, they rapidly pulse radio or X-rays. These pulses are believed to be caused by particles accelerating near the star's magnetic poles. The mechanism for these pulses is not well understood but the beams are coherent and synchronized to the rotation of the star, although the magnetic and rotational axes are not aligned, so the beams sweep around as the star rotates like the spotlight of a lighthouse. Neutron stars that emit these pulses are called pulsars. The rotation of pulsars very gradually slows down over time and the pulses eventually die out because magnetic torque acts against the spin. Pulsar radiation is not generally dangerous to life on Earth. Possible exceptions are neutron stars that may be soft gamma ray repeaters. However, magnetars, which are discussed in a different article, are more commonly associated with soft gamma ray emission. Gamma rays are very energetic photons and a nearby gamma ray burst directed at Earth from a supernova, for example, could cause a mass extinction. Soft gamma rays are slightly less energetic but harmful nonetheless. A gamma ray burst could alternatively come from the merging of two neutron stars or a neutron star and a black hole.

Sometimes a neutron star will experience a glitch in which its rotation momentarily speeds up. This may be caused by transitions in the vortices in its neutron superfluid core into a lower energy state. What results is a star quake.





This is an artist's concept of a 2004 neutron star quake that flared so brightly it momentarily blinded all X-ray satellites in orbit. Some neutron stars aren't isolated but instead are part of a binary system. A neutron star's accretion from its companion star or from gases near a black hole may also affect its rotation and its fate.

How Neutron Stars Form

Stars that are about 4 to 10 solar masses have cores that are hot and dense enough to fuse elements up to iron. Fusion (nucleogenesis) stops at iron because fusing atoms larger than iron requires more energy than it releases. Fusion can produce energy only as long as the sum of the masses of the new nuclei is less than that of the original nuclei. When iron nuclei (26 protons and 30 neutrons) begin to fuse with other nuclei the resulting nuclei have more mass, the process consumes more energy than it produces, and the process stops. An iron core then begins to form inside the star.

Eventually this core reaches what is called Chandrasekhar mass, which is about 1.4 solar masses (this explains why all neutron stars are about the same mass). At this point, not even electron degeneracy pressure can hold it up. It collapses, pushing protons and electrons together to form neutrons and neutrinos. Neutrinos do not normally interact with matter but at this density they exert a tremendous outward pressure. Even so, the core continues to collapse under its own gravity and approach neutron degeneracy (discussed in the next paragraph). The outer layers crash inward and rebound, creating a neutrino outburst and a shock wave, and when they do so, they trigger an enormously powerful supernova explosion.

What remains is a stellar remnant composed almost entirely of neutrons. This extremely hot neutron star is supported against further collapse by neutron degeneracy pressure. This pressure results when neutrons become so tightly packed together they occupy all the lowest possible energy states with some neutrons left over that must then occupy higher energy states. These high-energy neutrons create the outward directed degeneracy pressure. This is a quantum mechanical effect and as a result it is insensitive to temperature. That means that the neutrons stay packed together even as the neutron star cools off, as it eventually does, through neutrino radiation.

A neutron stars packs a mass of between 1.4 and 2 solar masses into a sphere about 20 km in radius. A 1 cm cube of neutron star would weigh as much as a mountain. These stars have been observed in supernova remnants and in binary systems. Four of them are thought to have planets. As a general rule, stars that are between 0.5 and 4 solar masses eventually mature into black dwarfs. More massive stars, between 4 and 10 solar masses, mature into neutron stars and very massive stars, more than 10 solar masses mature into black holes. Neutron stars retain most of their angular momentum. This means that because they are only a small fraction of their parent star's radius, they have extremely high rotational speeds, somewhere between ¼ millisecond to 30 seconds per revolution. These stars, due to their density, also have extreme surface gravity, up to 7 x 10¹² m/s², compared to Earth's 9.98 m/s². If an object fell from a height of 1 meter onto the surface of a neutron star it would take 1 microsecond to land and it would be landing at a velocity of 7 million km/h! The gravity on a neutron star's surface results in an escape velocity of 100,000 km/s, about 1/3 the speed of light. The extreme gravitational field around a neutron star acts as a gravitational lens, bending radiation that passes through it. This means that distant objects behind neutron stars become visible.

Mysterious Innards

Neutron stars are very hot when they form, about 10¹² K, but they cool rapidly through neutrino radiation, to about 10⁶ K in a few years. As they cool they are thought to form layers and as you explore deeper within these layers, the kinds of matter you might encounter are, well, are just plain bizarre. Remember, neutron stars are made of the densest matter possible. In fact, physicists are beginning to wonder if the name "neutron" is itself a misnomer because they think that in the center of neutron stars the neutrons themselves may be squished down into more exotic types of matter. Some recent computer models have suggested that these stellar corpses may be filled with free quarks, the constituents of neutrons, or even hyperons or kaon condensates.* The October 2010 discovery of a neutron star called J1614-2230 located 4000 light years away not only has broken the record for mass, 2 times solar mass, but it deals a death blow to many proposed models for the kind of matter that makes up a neutron star. Free quarks, kaons and hyperons are out. A neutron star composed of any of these materials would collapse to form a black hole before it could reach 2 times solar mass. What a neutron star is actually made of remains a mystery.

*Under normal conditions all quarks are bound up in atoms as neutrons or protons. Free quarks do not exist except under extraordinary heat and pressure in the form of quark-gluon plasma. Hyperons, like neutrons and protons, are made of three quarks, but the quarks that make them up are of a different kind.  Kaons are particles called mesons that consist of two quarks. Both Kaons and hyperons contain strange quarks. These particles constitute what is called strange matter. When neutrons are compressed beyond a certain limit they dissociate into strange matter quarks. These strange matter quarks in turn transform into a bound state called a strangelet, composed of roughly equal numbers of up, down and strange quarks. Such a state could be as small as the mass of a hydrogen nucleus or as large as meters across and these strangelets are believed to be what make up something very exotic called quark stars or strange stars as they are sometimes called (these stars are discussed in more detail in another article). Normally, strange quarks are unstable and don't exist for long, but in large numbers in neutrons, for example, they may represent the lowest possible energy state. Having three kinds of quarks allows them to be packed in together more efficiently. If this strange matter hypothesis is correct, it could have some potentially dire consequences. For example, if a strangelet came into contact with an ordinary atomic nucleus in a clump of matter on Earth, it would convert that matter into strange matter. By doing so it would release energy, producing a larger more stable strangelet, and this process would continue until all nuclei in all the matter in Earth were converted. Earth would as a result be converted into a hot large clump of strange matter. Not to worry though, about the only way this matter would come into contact with Earth would be if a quark star slammed into it, very unlikely, and we would have bigger worries at that point. If a strangelet hit a neutron star it would theoretically convert it into a quark star or strange star. The strange matter hypothesis is unproven. One nagging question is why aren't all neutron stars strange stars, since strange matter seems to represent a lower energy state. There is an ongoing effort to determine whether the surfaces of known neutron stars consist of strange matter or nuclear matter. The phenomenon of X-ray bursts is well explained in terms of nuclear matter and seismic vibrations of magnetars (see the article on magnetars) also support nuclear matter.

I recommend Dr. Coleman Miller's Neutron Star page as a fun read.  Some of his information is based on his own personal speculations but he backs everything up with scientific argument and some very helpful diagrams.

An Exciting Update

University of Alberta astronomer Craig Heinke and his colleagues have just increased our understanding of the physical nature of the matter inside neutron stars, as described in a February 2011 Edmonton Journal article, specifically how matter behaves inside the core of the neutron star, Cassiopeia A, a remnant from a supernova that exploded 11,000 light years away in the Milky Way.

This is a false colour image of Cassiopeia A, using the Hubble and Spritzer telescopes as well as the Chandra X-ray observatory.






They found direct evidence that the core contains a frictionless superfluid (a fluid that flows with absolutely no friction) that seems to defy gravity, as well as a superconductor (a material through which electrons can flow without losing any energy along the way). They have been observing the neutron star's surface temperature (it's about 2 million °C) for 10 years and have found that only a superfluid core could explain its rapid rate of cooling, of about 4% per year.  The only superfluids observed on Earth are extremely cold, just above absolute zero. And superconductors are observed here only when temepratures drop below - 100 °C. Evidence of superfluid and superconducting hot matter inside neutron stars will hopefully spark a new wave of theoretical research supported by computer modelling as we try to probe further into the workings of matter that is squeezed to its absolute limit, and perhaps, beyond.

Sept 19, 2014 addition: A new 5-minute video just released by Kurzgesagt (which means "in a nutshell" in German) explains neutron stars in a fun and very clear (and accurate!) way. It's an excellent watch:



Next up: Quark Stars.

Stellar Objects Part 3: Quark Stars

Stellar Objects Part 3: Quark Stars

Quark Stars are what happens when matter is squeezed as tightly together as it can be before it implodes into a black hole. Matter in this state has some very unusual properties. For example, its pressure is largely independent of temperature, and because its atoms are so tightly squeezed, only the strong force has any significant influence on its behaviour.

Quark stars, or strange stars as they are sometimes called, are a theoretical type of neutron star. Some researchers suspect that about 0.1% of all neutron stars are quark stars. And, like neutron stars, these stars pack between 1.5 and 2.0 solar masses into a volume about the size of a small city. They, in theory, consist of matter composed of up, down and strange quarks. The idea here is that under extreme pressure, ordinary nuclear matter (organized into protons and neutrons, both consisting of arrangements of up and down quarks) dissociates into quarks and some of these up and down quarks then transform into strange quarks. This arrangement of three kinds of quarks may allow the matter to be packed together more efficiently. The critical pressure required for the transformation from nuclear matter to strange matter is currently unknown. Not enough is known yet about the strong force that governs the behaviour of quarks. What is known, however, is that under ordinary densities and temperatures, the strong force confines quarks into hadrons (protons and neutrons for example). The scale across which this force can act is very short, about 10^-15 m and that's about the size of a hadron. However, when density increases to the point where quarks are squeezed together closer than 10^-15 m, the hadrons "melt" into quarks and the strong force becomes the dominant force of the entire star. This defines the theoretical quark matter state. This is thought to be the same physical state of matter as the quark-gluon plasma state that dominated the several-microsecond-old universe.

These stars might be almost entirely composed of strange matter surrounded by a thin envelope of nuclear matter, or perhaps all neutron stars, especially those near the mass limit of about 2 solar masses, have within them a strange matter core. Physicists are currently looking for a possible strange matter signature as they observe neutron stars.

If two neutron stars with strange matter cores or two quark stars collide, one would expect pieces of strange matter to fly off into space. What might happen when these bits of strange matter collide with the ordinary nuclear matter of another star or a planet such as Earth? This very remote possibility is explored in the last section of my Neutron Star article.

In 2008, researchers at the University of Calgary in Canada proposed that three recently observed powerful supernova, each about 100 times brighter than a typical supernova, might have been neutron stars exploding into quark stars. They call this the quark-nova hypothesis.

Even though quark stars are only hypothetical at this point, I include them because of the possibilities they offer for getting us closer to a fuller understanding of the mysterious workings of the universe. As researchers continue to explore the possibility of quark stars, we will understand more about the physics of strange matter and quark plasma as well. You can even think of going back in time to the beginnings of the universe as you dig deeper into a quark star. Observations of these stars and the intense supernovas that may produce them might help us understand the incredibly intense first microseconds of the universe's existence.

Next up: Magnetars.

Stellar Objects Part 4: Magnetars

Stellar Objects Part 4: Magnetars

Magnetars, like all neutron stars, are composed of matter that cannot be compressed any further without  collapsing into a black hole. These stars emit the most powerful magnetic fields in the universe, so powerful in fact that no one is sure how matter and energy behave inside them.

Like quark stars, magnetars are a special type of neutron star. I'd like to point out that these subtypes of neutron stars should best be thought of arbitrary points along a neutron star spectrum. Astronomers are discovering many stars that fit somewhere in between categories and they are in the process right now of coming up with new subcategories in which to catalogue the various radiation patterns they are observing from these stars. Unlike quark stars, magnetars are known to exist. In fact, several magnetars have been discovered, including this one by the not-so-pretty name, SGR 1900+14.

This image, courtesy of NASA, shows a ghostly ring seven light years across surrounding the magnetar (bright white). It is the remnant of a collapsed star, as all neutron stars are.

Magnetars, like all neutron stars, are composed of the densest matter known. They rotate rapidly as pulsars do (pulsars are discussed in the Neutron Star article) and their active life is short as well, in this case no more than about 10,000 years. The reason their lives are so short is because their strong magnetic fields rapidly slow their rotation, by about 1 second every 300 years. Eventually, a magnetar will no longer rotate fast enough to support any magnetic field. When this happens, the star cools and eventually it becomes dark dead rock of incredible mass. There may be 30 million or more of these massive dead lumps in the Milky Way. 

The Whole Star Is a Magnet

Magnetars are so named because of their very powerful magnetic fields.  For example, SGR 1900+14 exhibits an enormous field strength of 10¹¹ teslas. A magnet this strong located halfway between you and the Moon would erase all of your credit cards and suck your pens out of your pocket! A distance within 1000 km of this magnetar would be lethal because the water molecules in your body are actually magnetic dipoles and they would be attracted so strongly they would tear your tissues apart. Eeewww! Magnetars are the strongest known magnets in the universe.

This is an artist's conception of a typical magnetar with its magnetic field lines shown in red. Its actual magnetic field is far greater than these lines imply.

The Earth, the Sun, and in fact most stars and most of the Sun's planets have magnetic fields. The Sun's magnetic field, for example, extends beyond the furthest planets and protects them from extrasolar radiation. Earth's magnetic field protects all living things, as well as all of our electronic hardware, from solar radiation. We could not exist without it.  As important to us as Earth's magnetic field is, it is extremely insignificant compared to SGR 1900+14, about 10^-7 teslas compared to 10¹¹ teslas, respectively. A magnetic field this strong radically alters not only the star's matter but also the quantum vacuum around it (Scientific American "Magnetars" February 2003). Stars such as these ones can emit a million times more radiation than the Eddington limit allows (the concept of the Eddington limit is introduced in the article "Introduction to Stars"). This is the point at which a star's outward radiation pressure exceeds the force of gravitational attraction, and its mass simply blows away. Why doesn't this happen with magnetars? This is currently the focus of intense study. Researchers are trying to identify a magnetic version of the Eddington limit that might constrain a magnetar's magnetic field into a confined emission spike, as well as place a limit on the distance reached by its X-ray bursts. This could enable a more accurate measurement of the gravitational redshift of magnetars. It might also locate the emitting region at the magnetar's surface. This kind of study might tell us how the radiative bursts of magnetars work. A more technical but possible answer to this question is "magnetic fields greater than 10¹⁴ Gauss [this is roughly equivalent to 10¹¹ teslas] reduce the Thompson cross section [remember Thompson scattering?] allowing bursts to exceed the Eddington limit by suppressing electron cross section and decreasing the scattering opacity. This allows higher fluxes to escape."

How Does a Magnetar Work?

Let's start with pulsars. These neutron stars spin rapidly and emit radio waves. Strong electric currents flowing deep within the star create magnetic poles from which radio waves are emitted. The magnetism of pulsars is very strong and this magnetism is for the most part explained by its compression. The original star's core collapsed by a factor of about 10⁵ as the pulsar formed. For every time its radius is halved, its magnetic field strength quadruples. That means that pulsars should have a magnetic field strength about 10¹⁰ times stronger than that of the original star's core (all pulsars tend to have very similar radii as explained in the Neutron Star article). That works out to about 10⁸ teslas for an average pulsar, an enormous magnetic field, but still many magnitudes smaller than that of a magnetar.

Most researchers believe that the initial rotational rate of the core determines whether it will become a pulsar or a magnetar. Within the collapsing core, superheated plasma circulates by convection. This plasma is very electrically conductive and it drags any magnetic field lines threading through it as it moves. This phenomenon, called dynamo action, can greatly magnify the magnetic field if the newly born neutron star is rotating fast enough. This is how it works:  Computer simulations demonstrate that the convective period of new neutron stars is about 10 milliseconds. All stars tend to give up about a tenth of their kinetic energy to the magnetic field as the furiously convective plasma drags along magnetic field lines.  If the newborn neutron star's rotational period is less than the convective period, it creates a dynamo effect that greatly magnifies its magnetic field. In this sense, newborn neutron stars that become pulsars rather than magnetars can be thought of as failed dynamos.

As the magnetic field blinks into life, it drives enormous electric currents outside the star. These currents generate powerful X-rays. The magnetic field also moves through the outer crust of the star and, as it does so, it stretches and distorts it. The crust eventually snaps back in what is called a starquake. This action releases a burst of soft gamma radiation. This action gives magnetars their name, soft gamma repeaters or SGR's.

Magnetic fields around magnetars exceed the quantum dynamic threshold. When this happens, "X-ray photons readily split into two or merge together. The vacuum itself is polarized, becoming strongly birefringent, like a calcite crystal. Atoms are deformed into long cylinders thinner than the quantum-relativistic wavelength of an electron" (Scientific American "Magnetars" February 2003).  Such extreme gravitational fields as well as electromagnetic fields created by magnetars wreck havoc with matter and energy. (I think this gives a hint as to how magnetars exceed the Eddington limit.)

Although we have some ideas about how magnetars come into being and how they work, they remain enveloped in a shroud of mystery. Like all neutron stars, the composition of the innermost core is a complete mystery. And, especially in the case of magnetars, the laws of physics themselves are pushed right to their limits, resulting in energies so extreme they are almost impossible to imagine.

Now that we've cut our teeth on magnetars, lets move on now to a phenomenon even more extreme, black holes.

Stellar Objects Part 5: Black Holes

Stellar Objects Part 5: Black Holes

Black Holes are the puzzles of the universe. The physics of matter and energy break down at their mysterious event horizons, presenting theorists with fertile ground from which many of our current cosmological theories are growing.

This is a NASA image of the youngest known black hole, only 30 years old. This image is a compilation of data from several sources including the Chandra X-ray observatory. The black hole is the yellowish-white spot labeled SN 1979C within the galaxy M100, which is about 50 million light years away.

Researchers think this black hole is the result of the recent collapse of a star of about 20 solar masses.

A black hole is a concentration of mass so great that the force of gravity it creates is so intense that nothing can escape it, not even light. A common misconception is that black holes will eventually suck everything in the universe up. However, only when something, including light, gets close enough will it be unable to escape a black hole. Stars have been observed in stable orbits around black holes, just as if they were orbiting a star of the same mass.  You might be wondering why light, which consists of massless photons of electromagnetic radiation, is affected by gravity at all. The answer is not that photons are interacting with gravity but that gravitational fields, especially strong ones, bend the fabric of spacetime itself. The traveling photons respond to and follow the curvature of spacetime. Spacetime is so strongly bent inward toward a black hole that photons cannot escape if they come very close. This idea will be fleshed out more as we go.

This is a NASA concept drawing of what a black hole might look like closer up. It is essentially an invisible black sphere made visible by gases swirling around it that are so hot they emit radiation. The blue curved lines are possible magnetic lines along which hot gases may be dragged.

Composition of A Typical Black Hole

There are theoretically four basic types of black holes. These types are based on four known black hole solutions to Einstein's theory of general relativity. Two kinds rotate. An electrically charged rotating black hole is a Kerr-Newman black hole. A rotating black hole of zero charge is a Kerr black hole. A nonrotating charged black hole is a Reissner-Nordstrom black hole. And a nonrotating black hole of zero charge is a Schwarzchild black hole. Most black holes rotate because the stars that formed them were spinning. In other words, angular momentum is conserved. The charged black holes can be thought of simply as theoretical solutions because physicists do not expect real black holes to have charge. No astronomical objects with any appreciable electrical charge have ever been observed and no such thing as a black hole electron has ever theoretically stood up. We will simply compare nonrotating and rotating black holes from here on in.  All black holes consist of an event horizon, which is an envelope surrounding a central singularity. The singularity is a geometrical point of zero radius. Many theorists are uncomfortable with this notion and perhaps a solution, combining quantum mechanics, which describes particles not as physical points but as regions of maximum probability, combined with relativity might reveal a better description of the singularity. The event horizon is not a physical shell but is better thought of as "the point of no return" where nothing, not even light, once it ventures this close, can escape. In the image above, the black shell is the event horizon. It is impossible to observe or directly test the singularity within it.

A photon sphere exists just outside the event horizon. It operates like this:  A photon can escape from just outside the event horizon if it is traveling straight outward away from the black hole. The directions close enough to straight out that allow a photon to escape form a cone called an exit cone. Photons traveling outside the cone will fall back into the black hole. Photons pointed outward that are right on the boundary of the cone neither escape nor fall back in but instead orbit the black hole. This is the photon sphere.

A rotating black hole has an additional feature, an ergosphere, which is ellipsoidal in shape and touches the event horizon at the two poles of the black hole. This is not a physical boundary but a theoretical region outside the event horizon in which spacetime is dragged along in the direction of the black hole's spin with respect to the rest of the universe. Within this region objects are dragged along as well. Any object near the black hole will tend to start moving in the direction of rotation. As we explore how the ergosphere works, it is important to remember that objects do not break the light speed barrier, but the fabric of spacetime itself can and does. An object close to the event horizon, within the ergosphere, would have to move faster than the speed of light in the opposite direction to just stand still. In other words, within the ergosphere, spacetime is moving faster than the speed of light. The outer edge of the ergosphere is called the stationary limit. Here objects moving at the speed of light are stationary with respect to the rest of the universe because spacetime itself is dragged at exactly the speed of light. Spacetime outside the stationary limit is also dragged but slower than the speed of light. Objects within the ergosphere, because they have not reached the event horizon, can still escape the black hole, and in fact an object traveling on a tangent to the black hole and entering its ergosphere should be able to gain energy from the black hole - this is called the Penrose process. The object being boosted can theoretically reduce the black hole's energy by as much as a third and rob it of its angular momentum, dissolving the ergosphere with it. The result is a nonrotating black hole. The Penrose process may be a possible source for gamma ray bursts, the most energetic phenomena observed in the universe, and it is also really the only possible source of a nonrotating black hole.

Approaching the Event Horizon

Imagine that we are several light years away from a black hole, a safe distance, and from this vantage point we notice a derelict spacecraft that is about to be sucked in. We are bewildered by what we see. It seems to be slowing down as it gets closer! It should be speeding up because the increasingly intense gravitational field is drawing it in. The reality is that the spaceship is approaching the speed of light away from us as it falls inward. Yet it seems to us that it is taking forever to fall in. How can things speed up and slow down at the same time? We are witnessing Einstein's theory of general relativity in action. As the force of gravity approaches infinity, time itself slows to a complete stop from the perspective of an independent observer. For a rat, say, marooned on the derelict spacecraft, time would tick along as usual and it would not sense a thing (that is, until it becomes spaghettified - this is Stephen Hawking's word - as it falls closer into the black hole it will simply be crushed and its body will experience such an intense gravitational gradient that its tissues, then its cells, and then even its atoms will be torn apart).

To us watching it, the spacecraft slows to a stop as it approaches the boundary of the black hole called the event horizon.  Even though the event horizon is not a physical barrier, physicists can describe the event horizon as a mathematical membrane with physical properties. For example it behaves as though it contains heat, as if it were a hot material of some kind. And its temperature is inversely proportional to the mass of the black hole. Stephen Hawking came to an interesting conclusion about this heat. Like any hot body, the black hole should radiate energy and particles into space. This radiation comes from just outside the event horizon and not from within the black hole. This means that the radiation, called Hawking radiation, doesn't violate the theory that nothing, not even light, can escape. Over time an isolated black hole should radiate away all its mass and disappear. This is a teaser; we will build on this idea shortly.

For us, the event horizon is simply a black disk; we see the spacecraft's approach slow to a stop and then it simply hangs there suspended, an effect called gravitational time dilation.  At the same time, the spacecraft has been getting redder and dimmer as it approaches the horizon because all processes acting on this spacecraft (light in this case) slow down from our perspective. Eventually it will become less and less visible to us and disappear all together, an effect called gravitational redshift.  We are now left alone wondering what happened to it. We can get some clues by examining how black holes work.

The Nuts and Bolts of Black Holes

Lets start by refreshing our concept of spacetime. We experience a four-dimensional universe consisting of three spatial dimensions and one dimension of time. These dimensions operate together, weaving a fabric that influences the behaviour of all objects. The fabric of spacetime can be visualized as the elastic surface of a trampoline (keep in mind that we are making a two-dimensional model of a four-dimensional system). A massive object, a large star for example, indents the fabric, or bends spacetime. The curvature of spacetime is directly related to the mass of the object. Planets might orbit around it and they are held in place by the curvature created by the star (and they too contribute their own curvatures).  At the event horizon of a black hole the curvature of spacetime is infinite. If we use the trampoline analogy, the indent in the trampoline fabric would be infinitely long, extending right through Earth across to the farthest reach of the universe, and theoretically, beyond. Mathematically, and thanks to four dimensions operating rather than two, the distortion of spacetime created by a black hole is a much more complicated geometry than an infinite spike.

General relativity not only tells us that black holes are possible, it actually predicts that a black hole will form whenever mass gets packed in tightly enough to create a gravitational collapse. One way to make a black hole is to let a massive star reach its natural conclusion. Aging stars collapse in on themselves when they exhaust their nuclear fuel. The outward pressure created by nuclear fusion is no longer there to balance the star's tremendous gravitational pull inward. If the star is not too massive, it will collapse into a core of atomic nuclei floating in a sea of electrons. It remains supported from further collapse by electron degeneracy pressure. These are white dwarf stars, described in the article, "Introduction to Stars." If the star is more massive, nuclei themselves collapse into neutron plasma or even quark plasma, and the remaining core is prevented from complete collapse only by neutron degeneracy pressure (or quark degeneracy pressure). These are neutron stars, discussed in the article, "Neutron Stars." Now, when a star is very massive, its gravitational pull inward is so intense when it collapses that no intra-atomic force can hold up against it. All particles of matter become infinitely squeezed together, into what is called a singularity. At this point, any information about the matter that goes into a black hole is lost forever. And this is where we once again pick up on Stephen Hawking radiation.

Hawking Radiation

Hawking radiation is related to thermal radiation and it is predicted to be emitted by black holes due to quantum effects, which will be described shortly. This radiation should, over time, reduce the mass of a black hole until it eventually evaporates away entirely, if it does not consume as much matter as it loses. Micro black holes should emit more net radiation than larger black holes. In fact the smallest black hole possible, a Planck-size black hole, should evaporate as instantly as it forms. One way of explaining Hawking radiation is as follows:  The radiation doesn't come from the black hole itself but is the result of virtual particles being sufficiently energized by the black hole's gravitation into becoming real particles of radiation. This is how it works in more detail:  Virtual particles, according to quantum dynamics, arise spontaneously near the black hole due to quantum fluctuations of spacetime, as particle-antiparticle pairs. Normally these pairs instantly annihilate each other when they form and disappear as a result. But one of the pair may fall into a black hole while the other one escapes. In order to preserve the total energy of the system, the particle that falls into the black hole must have negative energy (this idea will be expanded upon soon), with respect to an outside observer. As a result the black hole loses energy and therefore mass and again, with respect to an outside observer, it will appear to have just emitted a particle (this is the leftover one of the pair and as a result it becomes real). Hawking radiation differs from thermal radiation in that thermal radiation contains information about the body that emitted it and Hawking radiation depends only on the mass, momentum and charge of the black hole. In September 2010, researchers at the University of Milan claimed to have observed Hawking radiation experimentally for the first time.

Black Hole Information Paradox

A black hole's information loss can be is summed up by the no-hair theorem. This theorem holds that every black hole can be completely characterized by only three externally observable parameters: mass, electric charge and angular momentum. Any other information (and this is where the "hair" metaphor comes in) about the matter that goes into one is permanently inaccessible to external observers. This is called the black hole information paradox. The problem is with all the other information about the matter that goes into a black hole and is irreversibly lost, such as baryon number and lepton number. Quantum dynamics is built on the principle that information like this can't be lost from a system. In other words, quantum dynamics is based on the principle of time reversibility. For example, if a particle interacts with another particle, it may be absorbed or reflected or even broken apart into its constituent particles. But you can always run the sequence of events backward and reconstruct the process that occurred. Events that occur at the event horizon of a black hole are, in contrast, irreversible. They are lost and cannot be reconstructed. This illustrates an epic battle between quantum dynamics and general relativity. Both theories must be satisfied or one or the other or both must be modified to explain black holes (and still explain all other phenomena in the universe). And the finger is pointed at gravity. I explored this idea in more depth in the article, "Our Universe: A Baby Picture." When enormous mass is concentrated in an extremely tiny volume, the normally weak gravitational force (so weak in fact that this force is ignored all together when working with particle physics) becomes as significant as the other fundamental forces. In this sense, black hole theories might become a platform from which to build the as yet elusive concept of quantum gravity, which would marry the two theoretical systems of quantum dynamics and general relativity.

Enter, Once Again, String Theory

Let's re-imagine that we are watching the derelict spacecraft falling into a black hole. Remember that to us it appears to slow down to a stop because all physical processes working on it stop. Lets now focus on one of the ship's atoms as it slows. Think of this mental game as analogous to watching one of those nature films where the film is slowed down to the point that a hummingbird's wings become separate and visible. First the atom looks like we expect it to look, like a whirling cloud of negative charge.  Soon the electrons slow down enough to become visible. As they freeze, the protons and neutrons in the nucleus become visible, and a moment later, we can see the individual quarks that make up these nucleons. As the quarks themselves now slow down we begin to make out the strings from which they are composed. They are minute, Planck-size to be exact, and as their vibrations slow down, more and more of them become visible. When higher modes of vibration freeze out, the strings become identical to each other. An electron string looks the same as a quark string and so on. According to the mathematical calculations done for this scenario, the strings and all the information they once carried simply become smeared across the event horizon. And here is where our loss of information could be explained. When time from our perspective is slowed to a stop, even the fundamental strings of matter stop vibrating and it is within these vibrations that information about the various particles exists. From the perspective of the atom we are studying, all of its functions are carrying on as usual, just like the derelict spacecraft rat as it approaches the event horizon. However, like the rat, the atom is also experiencing an ever-increasing gravitational field and its velocity is approaching the speed of light. As it does so, its mass is increasing as well. According to general relativity, its mass will continue to increase to infinity and its volume will decrease to zero. According to quantum mechanics, its mass will approach a maximum possible mass called Planck mass and its volume will decrease to a minimum possible Planck volume. We can see here how the two theories approach each other but do not offer a cohesive picture. There are other competing theories about black hole operation and indeed about the nature of matter itself, but I especially like the simplicity of string theory here.

Huge and Tiny Black Holes

Physicists speculate that the very early universe contained many extremely massive stars, which have since collapsed into black holes as massive as hundreds of solar masses. These heavy black holes may have been the seeds of immense galactic black holes consisting of millions to billions of solar masses and spewing out massive amounts of radiation. These monsters will be explored in the article, "Quasars." The recent (2011) discovery of a truly gigantic black hole with a mass equivalent to 6.6 billion Suns places a new upper limit on black hole size. It is inside the galaxy M87 about 50 million light years away.

Physicists are also fairly confident that micro or Planck-size black holes also exist. In fact, they pop into existence and just as instantly evaporate every time two Planck-energy particles collide. This occurs in the case of cosmic radiation. These extreme-energy collisions may also be a clue to understanding the ultimate structure of particles.

How To Observe a Black Hole

Black holes do not emit light, with the possible exception of Hawking radiation from the event horizon, so phenomena such as gravitational lensing (light is bent in the direction of extreme gravitational fields) and stars that appear to orbit an invisible body are used to locate massive black holes in space. This latter technique recently revealed that the Milky Way harbours a supermassive black hole, called Sagittarius A*.  This is a CHANDRA image of Sgr A* (white dot) based on data from a series of observations.

This is not an optical image but a radio image. The bright dot near the center is a very concentrated radio source. Physicists think that the radio emissions are not centered on the black hole itself but rather from a region around it close to the event horizon.  The radio emissions most likely come from gas and dust heated to millions of Kelvins as it accelerates into the black hole.  The mass of this black hole is just over 4 million solar masses confined within a 44 million km diameter sphere (the size of the sphere reflects the measurement of the event horizon, not the singularity within it; as well, a larger horizon means a singularity of greater mass and vice versa). It is believed to emit a small amount of Hawking radiation as well. Many astrophysicists now suspect that a supermassive black hole lurks within the center of most if not all large galaxies.

Matter falling into a black hole sometimes results in spectacular effects.  Moving like water spiraling down a drain, matter collects in an extremely hot and fast-spinning disc called an accretion disc before it is swallowed by the black hole in its center. Friction within the disc causes the angular momentum of the particles to be transported outward allowing the matter to fall inward. This causes a release of potential energy that increases the temperature of the matter. Many accretion discs are accompanied by relativistic jets that are emitted from the poles of the black hole, and which carry away tremendous amounts of energy.

This NASA image shows how extragalactic relativistic jets form from a galactic black hole like Sgr A*.

Particles are spewed out from the vicinity of the black hole at speeds up to 99.98% the speed of light and they traverse distances longer than the diameters of galaxies. What accelerates these particles to near light speed and what kinds of particles make up the jets is still largely unknown. But some physicists believe that magnetic fields in the accretion disk may be responsible for expelling charged subatomic particles outward at near the speed of light and as the charged particles interact with the magnetic field, they emit radiation, in the case of Sgr A*, powerful radio waves.

In this NASA image, matter from an orbiting companion star is drawn toward a black hole, forming an accretion disc (blue).

An example of this system may be Cygnus X-1, one of the strongest X-ray sources observed from Earth. This black hole probably has a mass of about 9 times that of the Sun and the radius of its event horizon is thought to be about 26 km. The other star, the one being "eaten," is a supergiant called HDE 226868.  Cygnus X-1 might be the black hole remnant of a star that was once more than 40 solar masses that has since blown up in a supernova.

How Dangerous Are Black Holes?

The closest black hole is V 4641, just 1600 light years from Earth on the way to the center of the Milky Way in the direction of the constellation Sagittarius.  This relatively small black hole is part of an X-ray binary system and it is believed to be between 3 and 10 solar masses. Despite its size, it is unusually energetic, emitting occasional but very powerful radiation bursts, much like a quasar does. As a result some astronomers call it microquasar (these are briefly discussed in the Quasar article). Its jet is oriented at an angle toward Earth but, fortunately for us, not directly at Earth. There are currently no black holes close enough to Earth to worry about.

On the other hand, there has been some public concern that the Large Hadron Collider (LHC) might create a microscopic black hole that might in turn swallow the Earth. The Standard Model of particle physics holds that the LHC energies are too low to create a black hole but string theory suggests that extra spatial dimensions exist and it might be possible to create a micro black hole within one or more of these tightly folded up dimensions. Yet, even if a micro black hole were to pop into existence, it would be expected to almost instantly decay as it released its energy via Hawking radiation. Even if such miniature black holes were created and they were stable, the safety assessment group at the LHC remind us that these micro black holes would also have been produced by cosmic ray collisions with the special matter in neutron stars and white dwarfs where the creation conditions are very similar to the those in the LHC, and the stability of these stars gives us good evidence that they aren't dangerous.

Can We Use Black Holes?

There has been much speculation about using a black hole to space-jump. Perhaps we could enter the ergosphere of one where spacetime is dragged around at ultra-relativistic speeds and use it as a slingshot to another part of the universe. There are two problems with this idea. First, your spaceship must be traveling at very close to the speed of light to enter the ergosphere on a tangent and avoid the event horizon and we have no technology yet to achieve anything close to this velocity. Second, objects even in the near vicinity of black holes experience enormous gravitational forces that can rip apart stars let alone space ships. I have read about the possibility of a black hole connected to a white hole via a wormhole (a white hole is a black hole antithesis where matter is shot outward. There is some speculation that when a black hole forms a big bang occurs at the core and a new universe is created. This expanding universe is in effect a white hole and the cosmological horizon is its event horizon). This idea too has all the problems associated with trying to enter or come near a black hole. But, before we toss out the baby with the bath water, there actually are theoretically possible wormholes based on general relativity. None have yet been observed. You can explore them in the article, "Wormholes."

Primordial Black Holes

A sea of tiny Planck-size black holes may have been created during the birth of the universe. Total gravitational collapse of matter is required to make a black hole, and this in turn requires very high densities of matter. Shortly after the Big Bang, when baryonic matter first appeared, the density of matter would have been very high. Uniform density alone is not enough to make a black hole, however. For a non-stellar black hole birth, a perturbation is required, much like star formation itself requires. Different computer models of the early universe vary widely in the size of these initial perturbations of matter but they, to varying extents, were expected to exist. Once Planck-size black holes formed they would be expected to grow very rapidly feeding on the molecular gases around them, and many would be able to grow into the behemoth matter-gobbling black holes called quasars, of which the early universe had many more than the universe does today. These black holes might predate the first stellar black holes created as the first massive stars went supernova. NASA's Fermi Gamma-ray Space Telescope, launched in 2008, is searching for the Hawking radiation signature of evaporating primordial black holes (with the caveat that if Hawking radiation doesn't exist, these black holes would be virtually impossible to detect if they existed).

Black holes express the mystery that lies beyond the absolute limits of nature. They extend beyond our current understanding and it is up to us to build theoretical frameworks that can scaffold us to a more complete understanding of them (and find ways to test those theories!). To astrophysicists and physicists this presents an opportunity for the most fun kind of play. And as our understanding of how black holes work advances, so will our understanding of how physics itself works.

Next up: Quasars.