Monday, February 7, 2011

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.

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