Thursday, April 11, 2013

The Sun Part 1: Evidence of A Violent Star

A suggestion before you begin: If you read the Atoms series in this blog (see to the right on this page) first, you will be especially well equipped to understand the Sun's mysteries.

There is a monster lurking in our solar system - the Sun. The image below was taken using NASA's Solar Dynamics Observatory (SDO). The surface isn't uniform. Bright spots, dark spots, filament-like structures and giant wisps and loops of material are visible.

Don't let the orange colour fool you. This is a false-colour image of light captured in the extreme ultraviolet (invisible) region of the spectrum, where much of the sun's light output is concentrated. If you were to look at the Sun from space with your naked eye, it would be pure white, not yellow as we see it through Earth's atmosphere.

The Sun is the energy source for almost all life on Earth. Plants use the energy in sunlight to drive reactions that make carbohydrates and sugars from water and carbon dioxide, a process called photosynthesis. Plants form the base of almost every food chain in the world. Everything that eats plants or eats the organisms that eat plants ultimately receives the Sun's energy.

On a warm sunny day it is easy to imagine the Sun only as the benign life-giver that it is.

Inside every green leaf, for example, millions of tiny round green structures called chloroplasts (below) convert the Sun's energy into chemical bonds in molecules such as sugars.

This complex and elegant energy conversion process is called the Calvin cycle (below right). When the leaf is eaten and digested, chemical bonds are broken, releasing the energy that is needed to grow and live.

Kristian Peters;Wikipedi
Daniel Mayer;Wikipedia

The Sun has two personalities. It is a life-giver and it is also a gigantic seething ongoing thermonuclear explosion constrained only by its own gravitational force. It continuously spews particles and radiation in every direction, a phenomenon understatedly called solar wind, shown in this two-minute video:

About every 11 years or so, our star grows even more violent. The image below is a montage of Yohkoh SXT (soft X-ray) images of the Sun taken over a ten year period (1991 to 2001).

The image above highlights how widely solar activity varies during the solar cycle. The images in the middle of the horseshoe were taken during solar minimum.

Intense magnetic fields torture the Sun's surface, accelerating the particles that fly out from its surface to  almost light speed. During the maximum level of activity in the cycle, huge magnetic field lines twist and strain and explosively detach themselves. Magnetic field lines, calculated from 2010 data from the Helioseismic and Magnetic Imager (part of the SDO mission) are drawn over the Sun's image, below.

NASA SDO/Lockhead Martin Space Systems Company
These magnetic field lines, like flows of magnetic force, erupt outward into space along with all the particles caught up in them. This is called a coronal mass ejection, or CME.

Solar flares, patches of sudden brightening over the Sun's surface, and the coronal mass ejections associated with them, are among the most violent explosions in the solar system, with forces up to 100,000,000 atomic bombs. These phenomena produce blasts of radiation across almost the entire electromagnetic spectrum, from radio waves to X-rays. Most of this radiation is invisible to the human eye, so special instruments are required to observe them. Flares tend to occur in active regions around sunspots (regions with intense magnetic fields), and coronal mass ejections tend to follow them.  The graph below tracks average sunspot activity from 1760 to 2000, showing a clear 11-year cyclic trend.
Leland McInnes;Wikipedia
Although the graph above is a little dated, a maximum period is expected in 2013. We've already seen evidence of its approach.

Signs of increased activity came to Earth as early as July 14, 2012. You might recall some amazing footage of aurorae visible in the night sky as far south as California and Arkansas. Vivid multihued displays occurred here in Alberta as well.

This is an especially beautiful collage of aurorae shot last year in Russia by TSO Photography and set to music:

The Aurora from TSO Photography on Vimeo.

Below is an image taken in 2010 from the International Space Station, showing what an aurora looks like from space.

Although the light shows may seem as benign to us as sunlight itself, they betray just how violent solar storms are. Like the Sun, Earth is enveloped in a magnetic field. This field captures and accelerates plasma particles streaming from the Sun. They strike the atoms in Earth's upper atmosphere with so much force they glow. You can learn more about aurorae and where their colours come from in my article The Northern Lights.

The strength of a magnetic field is measured in Gauss. The field on the surface of the Sun is about 1 Gauss, twice as strong as Earth's magnetic field, but the Sun's surface is 12,000 times larger than Earth's surface so the Sun's overall magnetic field is far greater than Earth's. The Sun's magnetosphere (region of magnetic influence) envelops all the planets in the solar system. Around sunspots, regions of intensified magnetism, the Sun's magnetic field may be as strong as 1500 Gauss.

Earth's magnetic field, though less intense than the Sun's, is still very powerful and it can protect life from radiation from the most powerful solar storms. Earth's magnetosphere, shown right, extends far to the right in a teardrop shape (not shown) because solar wind from the Sun (to the left, also not shown) "blows" Earth's magnetosphere leeward.

I would just like to give you a general idea of the shape of Earth's magnetosphere. For the legend to the numbers, check the wiki webpage, where this image comes from. Notice that the magnetosphere billowing out from the magnetic poles of the Earth follows the shape you might expect of magnetic field lines around a bar magnet (shown below), which is what Earth and the Sun are - gigantic bar magnets.

If it were not for Earth's magnetic field, not only solar flares and CME's but even continuous less violent solar wind would be deadly to most living organisms. Earth's atmosphere would eventually be stripped away by solar wind leaving the surface to be bombarded with radiation, much of which is ultraviolet radiation. This radiation disrupts the chemical bonds in most organic molecules (molecules of life), especially DNA.

Even with natural magnetospheric protection, Earth's technology has never been more vulnerable. Last July, a roiling tangled net of magnetism more than fifteen times larger than Earth hurled a billion tons of seething plasma (a CME) at our planet (we will explore this plasma in the next article). This 4-minute NASA video describes the storm (and includes some extraordinary auroras seen then):

This geomagnetic storm, triggered by a CME, hammered Earth's magnetic field, pressing it down so close to the surface that some satellites were temporarily naked to the damaging plasma. The magnetic field protecting Earth reverberated like a trampoline, sending electric currents through the soil and making compass needles swing. Although no damage was done, a solar storm like this one is capable of creating radio blackouts, disrupting jet travel, causing widespread damage to power grids and disrupting all satellite and GPS communication, described in this 3-minute video:

Anything with digital components is vulnerable to the kind of electrical disruption that a large solar storm can cause, making every technologically advanced country vulnerable to widespread disruption of almost every major system from transportation to security to healthcare, requiring weeks to months to repair and replace damaged components such as transformers. Meanwhile, entire continents could be hurled back to a time before electricity was available. NASA offers an excellent, brief and sobering description of what could be in store for us in the case of such an event. Although the current solar maximum, which should arrive in August 2013, is forecast to be one of the weakest on record, it's a good reminder to be prepared.

How does the Sun generate such intense life-giving and life-threatening power? To answer this question we'll need to look deep inside the Sun, next, In The Sun Part 2.

Wednesday, April 10, 2013

The Sun Part 2: From Gas Cloud To Nuclear Fusion

Our Sun is enormous. It is a sphere with a diameter one hundred times bigger than Earth. It's mass, about 330,000 times that of Earth, accounts for almost 99.9% of all the mass in the solar system. Below, the size of the Sun and planets are shown to scale.

Despite its size, the Sun is one of the smaller stars in the Milky Way. It is classified as a G-type main sequence star, shown below third from the left.

The stellar classification system using letters, shown above, groups stars together based on their surface temperature. The coolest stars are red (M class) and the hottest stars are bluish-white (O class). Stars also vary greatly in terms of mass and size. Stellar mass ranges from about half the Sun's mass up to 150 times the Sun's mass. Generally, stars with more mass tend to be larger, but star remnants break this trend. At the end of its lifespan, a star may blow up as a supernova or simply blow its outer layers away, depending on its mass. What remains is a core-like remnant that may vary in density from a white dwarf to a neutron star to a black hole. These remnants can be quite small (the Sun will someday be an Earth-sized white dwarf) but intensely massive (it will have about half the Sun's present mass).

About 8% of main sequence stars are Class G ones like the Sun. Most, 76%, are class M stars (far left) - red dwarfs (more common, shown) and red giants (less common, not shown). The behemoth to the right is a Class O star. An example of this type is Theta1 Orionis C of the Orion Nebula. This massive star (40X the Sun's mass) is one of the most luminous stars known, with the highest surface temperature (45,000°C, which means it is "blue-hot") of any known star. This monster generates so much ultraviolet light and solar wind that it is slowly ionizing and blowing the gases of the Orion Nebula away.

Two elements - hydrogen and helium - make up almost all the material in the universe, and the Sun, like all stars, is made of this material. In general, three quarters of the Sun's mass consists of hydrogen, while the rest is helium. Less than 2% of the Sun's mass comes from heavier elements such as oxygen, carbon, neon and iron. The relative abundance of hydrogen and helium, however, varies within the Sun. Scientists can analyze the Sun's surface by using spectroscopy. Every element emits a specific spectrum of light when its atoms are in an excited state. This tells scientists what the surface of the Sun is made of. Spectral analysis of the Sun reveals that its surface is 91% hydrogen and about 9% helium plus various trace elements. The core of the Sun is believed to have a much different ratio - 35% hydrogen to 62% helium. When the Sun formed it originally had a ratio of 70% hydrogen to 27% helium in its core (like all stars when they first form), but the Sun has been fusing hydrogen into helium for 4.6 billion years, so that ratio has changed, and it is continuing to change.

The Sun Was Formed From A Gas Cloud

The Sun formed from a giant rotating disk of gas and dust ( Cosmic dust consists of elements that were ejected from ancient stars and supernovae, some of which later condensed into molecules such as carbon compounds and mineral grains (

This gas and dust was part of a giant molecular cloud - composed mostly of molecular (H2) gas (70%), helium gas (about 27%) and a trace of heavier elements. (Helium, a noble gas, is always an atomic gas, He. My article Atoms and Chemistry shows you why this is the case.) All of the hydrogen and most of the helium in this gas was created when the universe formed, almost 14 billion years ago. Heavier elements were created and then ejected from giant stars when they exploded at the end of their lives as supernovae. Supernovae are still adding heavy elements to the universe. To learn more about where the elements came from, try my article How Atoms Are Made.

In the Milky Way galaxy, the bulk of molecular gas tends to be concentrated in a ring between 3.5 and 7.5 kiloparsecs (kpc) from the galactic center, which is the bright disk in the image below. Our Sun is located just outside of this ring, about 8.5 kpc from the center.

credit: Jon Lomberg;Kepler/NASA
When the Sun was forming, its solar neighbourhood of the Milky Way contained much more gas than it does now. These gas clouds seem to be transient and irregular structures that are constantly changing within the galaxy. The Milky Way is still making new stars from the gas it gravitationally draws in from various satellite galaxies.

Although the process of making new stars isn't entirely understood, most researchers accept the nebular hypothesis of star formation, which I'll describe here. A cloud of gas will remain a cloud of gas as long as two forces are balanced. Like any gas, these clouds exert outward pressure that comes from the kinetic energy of the molecules inside them. This outward-directed force is balanced by the inward-directed force caused by gravitational potential energy. Like an apple falling to Earth, molecules are drawn to the gravitational center of the cloud. The balance may be a delicate one. If the cloud has sufficient mass, any disturbance, such as a shockwave from a nearby supernova, can trigger it to collapse. As it collapses, the molecular cloud breaks into smaller and smaller pieces until each one reaches roughly stellar mass. Stellar mass obviously varies; this model works well for stars up to 20X the Sun's mass, but the formation of high mass stars is not as well understood. Our Sun was once one of these fragments.

A recent article in Scientific American Magazine (March 2013), called The Inner Life Of Star Clusters by astrophysicist Steven Stahler, describes his current research on how gas clouds contract to form clusters of stars, and why some clusters remain close groupings of stars for many millions of years while other star groups rapidly disperse. Also, try this 45-second Quicktime video of an Orion Nebula fly-through (click on the resolution you want). It starts with a journey through the Orion Nebula and ends with a close-up of proplyd HST-10, a forming star with a disk from which planets will form - all based on data from the Hubble telescope.

The Large Magellanic Cloud, a giant gas cloud filled with bright young bluish (hot) stars, is shown right in an image taken by the Hubble telescope. This cloud is an irregular satellite galaxy of the Milky Way, and one of the most active star-forming regions in the universe. You can see it with the naked eye if you are in the Southern hemisphere, where it looks like a separate piece of the Milky Way. Our Sun's "childhood neighbourhood" might have looked something like this.

Each gas cloud fragment has its own angular momentum which comes from the combined angular momentum of its atoms. It begins to rotate, flatten into a disk, and collapse inward. As the cloud collapses inward, it rotates faster because the total angular momentum of the system is conserved. Gas within the centre heats up as its density increases. The molecules collide with each other more often as they are forced closer together. This increased kinetic energy is measured as heat. When the molecules become energetic enough, they begin to radiate energy in the form of light outward, energy that is ultimately released from gravitational potential energy. The center becomes luminous or bright.

Planets may form as dust, ice and gas in the surrounding disk aggregate into larger and larger clumps. Most researchers believe both electrostatic attraction and gravitational attraction play roles in bringing these clumps together. As the density at the center of the disk increases further, the center eventually becomes opaque to radiation. This means that it can no longer radiate energy away as efficiently. The temperature here starts to go up very fast and the rotating hot sphere of gas forms a star embryo called a protostar. It becomes so hot (so energetic) that the bonds in hydrogen molecules are torn apart, releasing hydrogen atoms. The hydrogen and helium atoms themselves are so energetic they become ionized into plasma. We are going to explore this fourth physical state of matter in depth because almost all of the Sun's matter is in this state. In completely ionized plasma, all electrons are so energetic they overcome their electrostatic attraction to the nuclei. Plasma, therefore, is a "soup" of free nuclei and free electrons, rather than a collection of neutral atoms.

The temperature in the core of the protostar continues to increase until it is high enough to trigger nuclear fusion. Atomic nuclei are positively charged thanks to their protons. This means they repel each other strongly. They must have enough kinetic energy to overcome what is called the coulomb fusion barrier before they can fuse into new larger elements. Within the protostar core, nuclei are forced so close together that the attraction of the strong nuclear force overcomes the repulsive coulomb barrier, and they fuse.

The new star glows much more brightly and settles into a new state of equilibrium where the internal outward pressure of the nuclear reaction balances inward gravitational force. This reaction blows dust and gas outward, perhaps leaving behind a solar system of planets, asteroids and comets, like ours. This 3-minute narrated video models how a solar system like ours formed:

The Sun Fuses Hydrogen Into Helium

For 4.6 billion years the Sun has been fusing about 620 million tons of hydrogen into helium every second. It is now middle aged and it has maintained equilibrium even as its composition has evolved. The rate of nuclear fusion is temperature-dependent, so as hydrogen gradually becomes less available for fusion, the fusion rate decreases, and gravitational force overcomes nuclear fusion pressure. The plasma collapses inward, heating up further and triggering an increase in fusion rate. Even at this astounding rate of consumption, the Sun's fusion reaction will continue for another 5.4 billion years, until the supply of hydrogen in the core is exhausted.

A schematic of the Sun's fusion reactions is shown below.

In plasma, atoms are so energetic that electrons move around free from their atomic nuclei. If it is hot enough and under enough pressure, hydrogen nuclei (at the top) will fuse into helium nuclei (bottom), releasing radiation such as gamma rays and neutrinos. Positrons (the white spheres) are antimatter twins of electrons, and are also emitted. Each positron emitted will annihilate with a free electron in the plasma, releasing even more gamma radiation.

Only in the core is the temperature high enough to sustain nuclear fusion. The core makes up about a quarter of the Sun's radius. The entire Sun is composed of plasma but its energy, composition and behaviour vary greatly, from extremely energetic thermonuclear plasma in the core to plasma at the Sun's surface and within its atmosphere, which may be only partly ionized, which means it contains some neutral atoms. All plasma is defined as having at least some free electrons and these free electrons can give plasma fascinating electric and magnetic properties. We will examine solar plasma closely as we continue to explore the Sun, in The Sun Part 3.

Tuesday, April 9, 2013

The Sun Part 3: The Sun Is Full Of Plasma

The Sun is incredibly hot. The core, shown below, where all fusion takes place, is about 15,000,000°C.

In the core, hydrogen, helium and a trace of heavier elements exist in the physical state called plasma. Plasma is shown below right as part of a phase diagram.

Enthalpy is the total energy of a system. Plasma has more energy than a gas state does. Unlike solids, liquids and gases, plasma is a mixture of electrons and ions rather than atoms. You might be surprised to learn that plasma is by far the most common physical state of matter in the universe, by mass and by volume. All the stars and both intergalactic and interplanetary space exist as plasma, with the exception of molecular clouds.

The diagram below compares solids, liquids, gases and plasma.

To ionize into plasma, one or more electrons in an atom must have enough energy to overcome the electrostatic force binding it to the nucleus. The negative charge of the electron is attracted to the positive charge of the protons in the nucleus. If the energy of the system falls, ions and electrons will recombine into neutrally charged atoms. An atom may be partially (lose some electrons) or completely ionized (lose all electrons). The diagram below shows the difference between partially and completely ionized atoms using hydrogen and helium as examples.

Hydrogen can only be completely ionized but helium can be partly or completely ionized because it has more than one electron. Larger atoms have even more partial ionization possibilities, by losing one, two, three or more electrons.

What Is Plasma?

Plasmas vary widely. The ionized gases in a plasma TV or a neon light, for example, are many magnitudes less energetic than the plasma in the core of the Sun. The TV kind of plasma is called cold plasma. Plasmas such as this can be created by applying an electrical current to a gas that is at very low density. When electrical energy is applied to the gas, electrons, which have much less mass than the nuclear ions, will have much higher kinetic energy. Particles are far enough apart that infrequent collisions between electrons and ions do not transfer significant energy between them. The electrons are "hot," they have a lot of kinetic energy, but the ions, containing the bulk of the mass of the atoms, are not. Their kinetic energy is much lower. This means that the average kinetic energy, or temperature, of the system is relatively low. In cold plasma, there is no appreciable pressure exerted by the thermal (average kinetic) motion of the particles, and magnetic forces can also be ignored. Earth's ionosphere is an example of cold plasma. It is sparsely populated by ionized gas as well as excited gas atoms if there is a solar storm. Excited atoms are atoms with one or more electrons that have enough energy to climb to a higher-energy orbital but not enough to leave the atom altogether. In the ionosphere, excited atoms glow, creating an aurora. Many cold plasmas may contain excited neutral atoms as well as atoms at various degrees of ionization. The diagram below compares hydrogen atoms, excited hydrogen atoms and hydrogen plasma.

Hydrogen actually has many excited states, each one associated with a photon of a particular wavelength. I explore excited hydrogen in detail in my article Atoms and Light.

You can even create ultracold plasma, which may exist at less than 1°C above absolute zero, by laser-ionizing super-cooled atoms, giving only the outermost electrons just enough energy to escape the nucleus.

It doesn't take much ionization for a gas to begin to exhibit plasma behaviours such as electrical conductivity. Plasmas are electrically conductive because they contain freely mobile electrons that can flow in the direction of an electrical potential, creating an electrical current. Many cold plasmas become electrically conductive when as few as one in every thousand atoms loses one outermost electron.

In order to be called plasma, the collection of electrons and ions must be quasi-neutral. This means it must have more or less the same number of electrons as protons in it. Any collection of (neutral) atoms will ionize into neutral plasma. A beam of electrons or other charged particles, however, is not plasma. Plasma must also exhibit collective behaviour, but this definition can sometimes be fuzzy. It means it must have electrical and magnetic properties that describe it as a whole. Most (but not all) researchers, for example, don't consider a candle flame to be plasma. Though it contains very weakly ionized gases, it doesn't exhibit significant collective behaviour.

Hot, or thermal, plasma is created from gas under high pressure. Ions and electrons are forced close together so collisions are frequent, and energy is continuously transferred between them. Unlike cold plasma, electrons and ions exist in thermal equilibrium with each other. As you increase the temperature of thermal plasma you increase the degree of ionization. It takes a great deal of energy to completely ionize all the atoms in most gases. Under extremely high pressure, you can have very hot plasma with 100% ionization. The helium/hydrogen plasma in the core of the Sun is completely ionized.

The energy required to ionize gas into plasma comes in different forms. Heat, electromagnetic radiation and electrical charge can all supply the energy to ionize atoms into plasma. In the TV, electricity does the job. In ultracold plasma, electromagnetic radiation (a laser) does the job. In Earth's ionosphere, collisions with fast electrons ionize atmospheric gases. The gas in the Sun is fully ionized by heat and pressure.

Extreme Plasmas

The Sun's plasma is not the only kind of star plasma. If we explore plasma inside different kinds of stars, we can glimpse what happens when we continue to add energy to plasma, and we can end up at the extreme edge of matter itself.

With the exception of the Sun, most of what I have described so far can be called common plasma. The electron orbitals are partly deteriorated but the atoms hold onto some of their (inner orbital) electrons. The Sun's core, however, contains thermonuclear plasma. Here, the electron orbitals are gone. The atoms are completely ionized into ions and free electrons. The plasma in an exploding supernova is far hotter and under much more pressure than the core of the Sun. Some researchers call this nucleon plasma. The nuclei themselves are shattered, so it is a mixture of free electrons, protons and neutrons. This plasma is an intensely creative zone where all the heavy elements in the universe are created. If this plasma is pressurized further, for example inside the star remnant leftover from the supernova explosion, the free protons will absorb the free electrons and what remains is an ultra-dense star filled with tightly squeezed neutrons, a neutron star. Under even higher energy conditions, such as when the universe was under a millisecond old or perhaps inside quark stars, which are especially massive neutron stars, researchers expect even the neutrons themselves to be shattered. This plasma, called quark-gluon plasma, contains only free quarks and gluons. At energies beyond this, researchers speculate that quarks may break down into even smaller units called preons, so a kind of preon plasma may exist, perhaps in the core of a maximum mass neutron star that is not quite massive enough to form a black hole, where matter has collapsed all together.

When you increase the energy of an already extremely energetic system, you may begin to see a trend where building blocks of matter degenerate into more and more fundamental particles, a process called matter degeneracy. Reversing this trend gives you an idea of how matter originally formed in the universe. We'll revisit degenerate matter in the next article.

Stellar Plasma Dynamics Is a Complex Field

Heat and pressure are not the only factors that make the Sun behave the way it does. Stellar plasma is a very complex physical state. Radiation, pressure, gravity, electrical charge and magnetism all play critical roles in the Sun's activity. Even when all the atoms in a gas mixture are completely ionized, as they are in the Sun's core, plasma is complex and difficult to model. How does it transfer heat? How do electrical currents and magnetic flux that originate from drifting or rotating plasma further influence its motion? How do changes in pressure and temperature change it? Later on in the Sun series, we'll explore some of the models of Sun plasma behaviour, which are especially important for understanding and predicting solar weather.

A Closer Look at the Hydrogen and Helium in Solar Plasma

The Sun's core is basically a mixture of two completely ionized gases - helium and hydrogen. Let's look at what it takes to ionize these atoms. Hydrogen and helium have very different ionization energies. In fact, every atom has its own unique ionization energies. Hydrogen, for example has only one ionization energy, 1312 kJ/mol (although it has many excitation energies as we saw earlier). 1312 kJ/mol is the energy required to remove its single electron. Helium has two ionization energies, 2372 kJ/mol and 5250 kJ/mol. Compared to hydrogen, it takes almost twice as much energy to remove helium's first electron and almost twice as much again to remove both electrons and completely ionize the atom. In fact, the second number, 5250 kJ/mol, is the highest ionization energy of any element. Why is helium so different?

A high number generally means a strong attraction to the nucleus, but as we'll see, the picture can be a bit more complex. Element first ionization energies follow a general wavelike pattern, shown below.

The pattern hints at how electrons bind to atoms. First ionization energy is the energy required to remove one of the most loosely bound outermost electrons from an atom. The energy depends on the charge (how many protons) of the nucleus, the distance of the electron's orbital from the nucleus, the number of electrons between the outermost electron and the nucleus (inner electrons "screen" the positive charge from the outermost electron), and whether the electron is paired or not. When electrons are paired in the outermost orbital, they experience some repulsion from each other. This offsets some of the attraction to the nucleus so they are removed more easily.

Helium is an especially interesting atom. It exhibits unique symmetry. The charge cloud of its nucleus, an alpha particle with two neutrons and two protons, and the charge cloud of its two electrons both occupy spherical 1s orbitals. To review what a 1s orbital is, try my article Atomic Orbitals and Bonding. Both quantum mechanical clouds are perfect spheres centered on the center of the atom, as shown below right.

A femtometre (fm) is 10-15 m. The blue and red spheres represent neutrons and protons in the nucleus, shown as a purple dot in the center. The grey sphere is the charge cloud of the atom (the shape of the 1s orbital). (The nucleus would be more than 100,000 times smaller than the orbital size, invisibly small in this image, if it were drawn to scale.)

Neither of these orbitals possesses any orbital angular momentum, and each pair of particles cancels out each other's intrinsic spin. This puts the atom in a uniquely low potential energy state that makes it very stable. Adding one more electron, neutron or proton would drastically increase potential energy by introducing angular momentum. This is why there is no stable atom with five nucleons. That arrangement would have too much potential energy to be stable. The stable arrangement of helium means that it takes tremendous energy to remove electrons from, or ionize, it (and it also explains why helium exists only as an atomic gas and why it won't react with any other atoms to form any compounds).

The Sun's core has enough energy to completely ionize helium and hydrogen. On Earth, these elements exist in very small concentrations as gases in the atmosphere (although while helium is always found as a gas, hydrogen is also, very abundantly, bound up in various compounds). Inside the Sun's core, they exist as plasma under intense pressure. To compare, hydrogen gas on Earth has a density of 0.00009 g/cm3 at 0°C at sea level. In the core of the Sun, it is compressed to 150 g/cm3, a density almost two million times higher.

The Sun's Core Is Evolving

Helium nuclei are heavier than hydrogen nuclei so helium sinks into the center of the Sun's core. A ball of dense helium plasma is currently building up there. For now, it sits inert as the Sun maintains a state of equilibrium where fusion pressure balances gravitational pressure. The Sun is in the main sequence stage of its life, where equilibrium is maintained, and it will remain in this phase for another 4.5 billion years, contracting slowly all the while to compensate for energy lost through nuclear fusion and radiation. The temperature and pressure in the core, meanwhile, slowly increase as well.

The rate of nuclear fusion will eventually slow down as hydrogen is consumed. The Sun will evolve into a red giant star and then into a white dwarf, as shown below.

The Sun's core gradually shrinks while helium builds up inside it. See the diagram below. When the Sun is about 9.5 billion years old, nuclear fusion will take place only in a thin shell (light blue) around a sphere of helium (dark blue) while heat from core contraction causes the Sun's outer (almost all hydrogen) layers to expand. The Sun will no longer be a main sequence star as it enters a new phase called the subgiant phase. It will be about twice its current size and orange in colour because expansion will cause the surface to cool. This phase is relatively short, lasting only about 1 billion years. Hydrogen in the core is now completely exhausted as the temperature of the core continues to increase, igniting nuclear fusion in a shell of hydrogen surrounding the core (not shown in the diagram below). This causes the rate of expansion of the outer layers of the Sun to accelerate, ballooning the Sun up to 200 times larger than it is now, into a red giant star.

The Sun will last as a red giant for another billion years. The core hydrogen is used up, so the helium inner core of the Sun becomes unstable as gravity starts to overcome waning fusion pressure. The core will shrink even more and the inner helium plasma sphere will be compressed further, into an even more bizarre state, one that will set off a series of events that ultimately seal the Sun's fate, next, in The Sun Part 4.

Monday, April 8, 2013

The Sun Part 4: Helium Eventually Seals The Sun's Fate

The Sun Approaches The End of The Main Sequence Phase

The Sun's core consists of protons (hydrogen nuclei) and alpha particles swarming with very fast moving electrons. Alpha particles (helium nuclei) are the heaviest particles in the plasma. They tend to condense toward the center of the core, forming a helium-rich inner core zone where no fusion takes place (yet). This inner core of helium gradually forces the nuclear fusion reaction outward. This means fusion is forced into a slightly cooler less dense zone, where it is not as easily sustained. As fusion slows, outward pressure decreases and gravity causes the core to condense. As long as the Sun remains in the main sequence phase, with enough fusable hydrogen available, the core condenses and heats up and the fusion rate increases once again. This process maintains a finely tuned equilibrium that operates over most of the Sun's lifespan, but eventually overall fusion pressure falls as the pool of available hydrogen, under sufficient pressure and heat to fuse, grows smaller, and what's left is continually forced outward. Inward gravitational force begins to win out over outward fusion pressure and the density of the core begins to increase rapidly, adding pressure to the already intensely pressurized helium-rich zone in the center.

Hydrogen now fuses only in a thin shell around the core. The Sun, at this point is becoming a red giant. The density of the core has been increasing sharply, heating it so intensely that it rapidly heats a layer surrounding it. Despite its distance from the core, the hydrogen in this layer is heated enough to begin to fuse at a rapid pace. This new phase of intense fusion activity increases the Sun's luminosity by a factor of thousands. This rapid rate of fusion, however, can only last so long, perhaps as briefly as a few hundred million years. On the Sun's scale that's a very short period of time.

The Sun Evolves Into A Red Giant

Meanwhile, the outer layers of hydrogen plasma expand greatly as they are heated. The Sun's radius balloons outward hundreds of times larger than it is now. The outer envelope of gases cools at it expands because its thermal energy is spread out over an increasing area. Its lower surface temperature now gives it a red appearance as the Sun's blackbody radiation shifts from white (around 5500°C) toward the red end of the visible spectrum (around 3500°C).


The diagram left shows how drastically the Sun will change as it evolves into a red giant.

Meanwhile, the helium center has been compressed into a new state of matter called degenerate matter.

This kind of matter operates under a new set of behavioural rules, and it brings the Sun, along with most other stars, to various kinds of violent endings.

Helium In The Core Becomes Degenerate

At this point, the helium no longer follows ordinary gas laws. Before this point, the pressure of the completely ionized helium plasma followed the ideal gas law where pressure = density x temperature x the Boltzmann constant. Now, the plasma no longer exerts an outward pressure dependent on temperature. Its pressure instead comes only from degenerate pressure. The intense pressure in the helium core tries to force all the electrons into lowest possible quantum energy states. It's a bit like forcing them back into an atom-like state but going much further. In this case, it tries to force countless electrons into the same 1s orbital, regardless of the spin-determined two-electron orbital maximum that stems from the quantum mechanical nature of electrons. The electrons, being fermions, resist because they follow the Pauli exclusion principle. In atoms, electrons tend to fill the lowest energy levels first, but in normal gases, there are actually many empty energy levels and the electrons in each atom are free to move up and down them, depending on their energy. In degenerate plasma, electrons (with tremendous kinetic energy) are forced to fill up the lowest energy levels and they are locked in as a group there. An electron in this state of matter, being stuck in a minimum energy quantum state, can't give up any extra energy by moving into a lower energy level and emitting a photon. It can't absorb an electron and move into a higher energy state either. However, it is still "laterally" mobile because the matter is still in a plasma state. The difference between an ordinary gas atom and a degenerate state atom is shown below right.

On the right hand side above, electrons resist being forced into the same quantum state because they are following the Pauli exclusion principle. This resistance manifests as outward pressure on the system, called degenerate pressure. Unlike an ideal gas that experiences outward pressure as the kinetic motion of its atoms increases, degenerate plasma experiences only degenerate pressure, which does not depend on temperature at all. This means there is no longer any stabilizing and cooling expansion possible in the core. Even though the outer layers of the Sun as a red giant expand greatly, an inner layer of plasma around the helium center continues to contract under gravitational pressure, and the temperature continues to rise. Suddenly, the temperature spikes at over 100,000,000°C and at this point helium nuclei are hot enough to fuse, using the triple alpha process, shown below left.


This is a runaway reaction called helium flash. In a matter of seconds, three quarters of the Sun's helium fuses and the temperature soars, creating so much energy that the luminosity would equal that of the entire Milky Way, if it could be observed. This energy burst, however, takes place deep inside the core, where the energy goes into a soaring helium fusion rate rather than blowing up the star.

The Sun Enters The Horizontal Branch

After the helium flash, the Sun shrinks back down to about ten times its current size. It is now on what is called the horizontal branch of star evolution. Over the following 100 million years it will be about 50 times more luminous than it is today and it will expand once again, but this time very gradually. However, as helium runs out, the Sun will once again go through an intensified expansion phase where it will once again become a red giant. The sudden expansion into a red giant happens much faster this time and the Sun will become even larger and more luminous than it did before. The Sun will last only about 20 million years in this second red giant phase, called the asymptotic branch, because it is growing increasingly unstable all the while.

The Sun's core now has a new composition. Most of the helium has fused into carbon as well as oxygen. If you look at the diagram above of the triple alpha process, you'll notice that three helium nuclei (4He) don't fuse directly into one carbon nucleus (12C). When the core is hot enough it begins to fuse helium into beryllium (8Be). However, no new stable nuclei form yet because beryllium-8 is very unstable. As the core continues to grow even hotter, helium nuclei begin to fuse faster than the beryllium can decay. Now that both beryllium and helium are available, it is energetically favourable for them to fuse together to create a stable carbon nucleus, and carbon builds up. Some carbon nuclei will then fuse with additional helium nuclei to create stable oxygen (12O) nuclei as well. The oxygen-creation reaction is not shown in the diagram above. Each of the three fusions releases gamma radiation and a tremendous amount of energy.

The Sun Expands Into An Unstable  Red Giant

Meanwhile, in a thin shell around the core, helium is soon hot enough to fuse. Outside this fusion layer, the Sun's plasma rapidly expands even further outward than it did during its first red giant phase, this time probably far enough to engulf Earth. In a thin shell around the helium, hydrogen is also hot enough to fuse. This reaction restricts the very thin layer of helium fusing underneath it, preventing it from fusing stably. Helium in the hydrogen shell builds up until the helium shell beneath it ignites in a massive nuclear explosion. The Sun expands and cools, shutting off hydrogen shell fusion. Then, the Sun contracts gravitationally and the whole cycle begins again. Each of these thermal pulses, as they are called, lasts a mere 100,000 years or so, becoming more intense each time, and each time the Sun loses a significant amount of mass as material blasts outward in the explosion. Most researchers expect the Sun to experience four pulses before it loses its outer envelope of plasma altogether, leaving the Sun with about half of its current mass.

The graph below follows changes in luminosity and surface temperature as the Sun leaves the main sequence phase (the black line), expands into a red giant, enters the horizontal phase, and then expands once again into a red giant, before it moves on to a much more quiescent phase as a white dwarf star (top line). The second, and final, red giant phase is simplified in the graph. It will likely include four "mini-phases" as described above, where luminosity and surface temperature swing up and down following the Sun's expansion and contraction. This eventful solar "last stand" will begin about 4.5 billion years from now, when the Sun is about 10 billion years old.
When the Sun reaches its final red giant phase, Earth will probably be incinerated but distant stars and moons, such as Titan, may be offered a brief window of opportunity for life to evolve from the energy boost to the rich compliment of organic compounds there.

Star cores generally experience three kinds of outward pressure: 1) radiation pressure, 2) kinetic pressure according to the ideal gas law, and 3) degenerate pressure. Degenerate pressure is technically always present in matter but its effects are overshadowed when matter is not in a degenerate state.

The helium burst creates tremendous radiation pressure. The sudden flash of intense gamma radiation exerts outward pressure because each photon created carries momentum. A solar sail could someday utilize the (much lower) radiation pressure from the current Sun for interplanetary travel, although the pressure in this case more accurately comes from fast-moving particles as well as from electromagnetic radiation.

The Sun Enters Its Final Phase As A White Dwarf

Once the Sun ejects its outer plasma layers, forming a planetary nebula, which will surround it at a distance, much of what's left is carbon-oxygen core material. Ultimately the Sun becomes a white dwarf, a dense stellar remnant in a degenerate matter state. It is very hot, emitting blue-white light. We can see from a distance what our Sun as a white dwarf might look like in the photograph below. The star itself is a very tiny white dot slightly off center to the left. It is surrounded by a planetary nebula called NGC 2440 (photograph taken by the Hubble space telescope).

Ultraviolet radiation from the white dwarf makes the gases glow. This is one of the hottest white dwarfs known, with a surface temperature of 200,000°C.

All fusion has been exhausted so it will eventually cool, but the rate of cooling is astoundingly slow. Our current Sun would take about 20 million years to cool off if nuclear fusion suddenly stopped in the core. The entire current Sun is composed of ordinary matter so all of it can lose heat through thermal radiation. The degenerate matter in a white dwarf has no mechanism to release energy. Only the thin outer shell of ordinary matter surrounding it can release energy as thermal radiation. When a white dwarf starts out, it is very luminous. The one above is emitting as much light as 500 current Suns. This radiation comes from the thin outer shell of ordinary matter. The degenerate matter underneath is in a locked state, where no photons can be absorbed or emitted because the electrons are locked in low energy orbitals.

This means that the inside of a white dwarf stays very hot for a very long time. Even the oldest white dwarfs in the universe maintain an almost constant internal temperature of about 10,000,000°C, and a surface temperature of just under 2800°C.

The outer shell of non-degenerate matter in a white dwarf releases some energy, so it will eventually cool from 10,000,000°C to about 9000°C, and then maintain this temperature, radiating blackbody radiation that will make it appear bluish white for tens of billions of years. It should very gradually dim to red and then, theoretically, eventually, to black, but our universe may not exist long enough to house such objects. It is difficult to say with certainty how long it would take the Sun as a white dwarf to cool completely. Researchers believe a white dwarf, like all objects, should contain dark matter, which might decay very slowly, releasing heat as it does so. Protons in the degenerate matter may also eventually decay and they too could contribute some heat to the star.

White Dwarf Degenerate Matter is a Fermi Gas

Thermal radiation isn't possible from degenerate matter, but this matter is highly electrically conductive because the electrons are "laterally" mobile, much like they are inside metals. Although neither degenerate matter nor metals are what we'd think of as gas states, physicists can describe this kind of group electron behaviour as a Fermi gas.

As a white dwarf, about half the Sun's current mass will be compressed into a sphere the size of Earth. That represents a density similar to a car crushed down to a sugar cube. It will stay this way forever, even as it cools. The degenerate matter in a white dwarf will never spring back into a normal atomic state, because there is no mechanism available to unlock the electrons within it. The pressure in degenerate matter comes only from the kinetic energy of the degenerate electrons in it. Adding heat doesn't increase this energy and losing heat doesn't decrease it, so the pressure squeezing the matter into its degenerate state remains in place even as the star cools. Nothing will decrease the degeneracy pressure but additional mass will increase it, and instead of making it larger as you would expect with ordinary matter, it would make it smaller still.

White Dwarfs, Neutron Stars and Black Holes

So far I have been describing electron degenerate matter. Neutron degenerate matter is also possible inside stars, but it all depends on mass. A white dwarf remnant is always left with fairly low mass. About half the Sun's mass, for example, will be blown off in the planetary nebula, so the Sun as a white dwarf will be about 0.5 solar mass. No white dwarf can exceed a maximum of 1.4 solar masses. Only stars of low to medium mass, like the Sun, end as white dwarfs (electron degenerate matter). Almost all stars in the Milky Way, most of which are red dwarfs, fall into this category. They will all eventually end as white dwarfs. More massive stars end much more spectacularly. Stars with between 8 and 20 solar mass explode as supernovae, leaving behind remnant cores called neutron stars. These stars are made of neutron degenerate matter.

A white dwarf can pull off a supernova, but only if it attracts enough mass. If a white dwarf were to receive mass from a companion star or if it gradually accreted enough nearby gas and dust, it could eventually exceed the mass limit of 1.4 solar masses. It would explode as a Type 1a supernova, thanks to a process called carbon detonation. This is a runaway carbon fusion reaction that, unlike the runaway helium fusion reaction described earlier, releases enough energy to unbind the star altogether. Type 1a supernovae are fairly common and they always explode with identical brightness, making them perfect standard candles for astronomers.

Very massive stars, more than about 20 times the Sun's mass, explode and leave behind black holes, in which matter entirely collapses, shown to the right in the diagram below.

Comparing White Dwarf Electron Degenerate Plasma With Metals

Does the white dwarf cartoon, above left, remind you a bit of a metallic lattice? For a refresher, scroll down to Metallic Bonding in this Scientific Explorer article. In a metal, the outermost electrons are so loosely bound to their atoms that they move about freely. Some researchers call this an "electron sea." All the electrons exist in one shared molecular orbital, and this shared (same energy) quantum state, while obeying the Pauli exclusion principle, allows electrons to flow, making the metal electrically conductive and allowing some metals to have magnetic properties as well. Both metals and white dwarf plasma are considered by some researchers to be examples of electron degenerate matter. They both behave like Fermi gases, as mentioned earlier. Both examples also exhibit a crystalline structure. The electron degenerate plasma in a white dwarf is thought to resemble a diamond-like crystalline solid. There are important differences though. Degenerate pressure may be one reason why metals resist compression but it is not the most important source of internal outward pressure, whereas in white dwarf plasma it is. Only the outermost conducting electrons in metals are degenerate. The inner electrons remain in ordinary electron orbitals. An exception here is metallic hydrogen because it has only one electron per atom to free up. Metallic hydrogen, like white dwarf matter, is degenerate matter. It relies only on degeneracy pressure. Like white dwarf matter, it is expected to have a crystalline structure composed of positive nuclei (protons here) surrounded by an electron sea. I should note too that Wikipedia limits electron degenerate matter to matter that is compressed into degeneracy, technically excluding metals then, but other sources do not.

Electron degeneracy is a fascinating concept to ponder on. It reveals some of the oddness of the quantum mechanical nature of electrons, and it ties in with a puzzle I explored recently in the article Electrons, Strings and Spooky Action, if you are curious.

To sum up this article, try this Naked Science 45-minute video, describing the Sun and its ultimate fate:

Our Sun has another 5 billion years to go before it ends its life as a white dwarf. Before then, it will continue to experience a mysterious regular cyclic pattern of activity that peaks every 11 years. During these peaks, we enjoy more colourful auroras around the poles and, sometimes, we endure intense and destructive geomagnetic solar storms as well.

This 11-year cycle of activity is the focus of intense current solar research in an effort to learn how to predict damaging and dangerous solar storms. Scientists know that each cycle ends with a dramatic magnetic flip-flop, like a bar magnet suddenly reversing its poles. We will start by peering deep into the Sun to see how its magnetism works, next, in The Sun Part 5.

Sunday, April 7, 2013

The Sun Part 5: How The Inner Layers Work


The energy from fusion in the Sun's core travels outward and eventually leaves the Sun altogether in the form of electromagnetic (EM) radiation.  Protons, electrons and neutrinos also leave the Sun's surface, with a great deal of kinetic energy. These particles plus the EM radiation is what solar wind is made of and it eventually reaches far past the orbit of Pluto .

Gamma Ray's Long Journey

How does electromagnetic energy, originating in the core, make its way outward through the thick dense hot plasma of the Sun? In the Sun's core, energy from fusion dissipates outward into the surrounding radiative zone, the thick orangey yellow layer in the diagram below.

The plasma surrounding the core is so incredibly dense that gamma rays emitted during the fusion reaction can hardly travel anywhere before they are absorbed by electrons. These electrons then re-emit the photons in all directions, with each re-emission taking a having ever so slightly lower energy because each collision absorbs a tiny bit of it. Because the radiative zone cools in an outward gradient, net energy eventually moves outward, but these countless absorption-reemission events and the indirect path taken should slow the net movement of energy greatly. Exactly how much these processes slow the movement of photons outward is still up for debate and estimates vary widely.

Some online courses mention another factor that may slow down outward photon movement.  In the core environment, the energy is so intense that matter and light exist close to thermodynamic equilibrium. This could slow down the outward dissipation of energy even more. Nuclear fusion creates positrons as well as gamma rays. The positrons react with electrons in the plasma, annihilating each other and creating even more gamma rays, and possibly interfering with the slow outward absorption-emission progress outward. The reaction, an electron (e-) annihilating with a positron (e+), creating two gamma photons (γ), and vice versa, is shown by a double arrow, below.

γ + γ ↔ e+ + e-

However, even in the center of the Sun, temperatures are not likely to be high enough to make the reaction above truly reversible. A temperature of 1010°C is required, and the core is about 16 x 106°C, so the reaction is favoured to the right. It seems likely, then, that no new electrons are created in the Sun's core, affecting net outward photon movement. However, because new positrons are created, additional gamma rays flood the core plasma and these photons may impact net photon movement.

The above reaction is quite interesting. At everyday energies, you will never see an electron annihilating with a positron and emitting gamma radiation because at these low energies, the reaction is very strongly favoured to the right, but at intense energies, such as inside a collider, many positrons are often emitted and these quickly annihilate with electrons and release gamma radiation. What is interesting is that even though at everyday energies the reaction favours both electrons and positrons (everything to the right of the arrow), you will not see any positrons. This is part of a big unanswered question in physics: Why isn't there antimatter in our current universe? I invite you to explore this question in the Scientific Explorer article, The Hadron Epoch. To conclude this reaction discussion, inside the intense heat and pressure in the solar core, the reaction only weakly favours the production of photons, if it does at all, so the reason it takes a very long time for energy to actually move outward through the core is mostly due to countless absorption-emission events and very convoluted zigzag paths photons would have to take because of absorption and re-emission in random directions.

All these factors slow the movement of radiation outward so much that most physicists estimate it takes about 20 million years for a photon created in the core to make it the surface (although there is debate as I mentioned). From there it takes a mere 8 minutes or so to reach Earth, as sunlight, because almost no collisions interfere with the photon trajectories in the near-empty space in between. 20 million years is also how long physicists estimate it would take the Sun to cool off to a stable state if nuclear fusion suddenly stopped.

Each gamma photon is ultimately converted into several million much lower energy photons that eventually escape the surface of the Sun, and strike Earth.  Almost all sunlight photons range in wavelength between 100 nm (nanometers) and 1 mm. Sunlight is mostly made of visible photons, but significant ultraviolet and infrared photons contribute as well, resulting in a fairly short band (between the red lines) in the EM spectrum shown below.

The graph below shows the radiation spectrum of sunlight at the top of the atmosphere as well as at sea level. Although the graph cuts off at both ends, small amounts of higher energy photons such as X-rays and very low energy photons such as radio waves also come from the Sun. We will look at the origin of X-rays in a moment.
Robert A. Rohde;wikipedia
The Sun Glows White-Hot

This distribution is similar to the distribution of photons created by a black body at about 5500°C, the Sun's surface temperature, and that spectrum is basically what you see above. Both radiation and heat come from nuclear fusion. This means that all the gamma rays produced in the Sun's core and transformed into lower energy photons at the surface are mixed up in the Sun's "heat glow." This means the photons in the graph above come from two sources - some come from the core fusion reaction and others come directly from the kinetic motion of white hot plasma and gases at the surface, like the glow from white-hot steel. The majority of photons from both sources fall into the same energy range.

If you look at the radiation spectrum above you'll notice several dips. These are absorption bands of common gases - oxygen, water vapour and carbon dioxide - in the atmosphere. Electrons in gas molecules absorb radiation just like free electrons in plasma do, preventing some of it from striking Earth's surface.

Neutrinos, X-rays and Red Light

Along with gamma rays, neutrinos are also produced in the fusion reaction. Neutrinos aren't a kind of photon. They are fundamental fermion particles with no charge and possibly no mass as well. They are almost entirely invisible to matter. They experience almost no interactions in the plasma so they escape immediately out through the layers of the Sun and then go right through Earth as well, with almost no interaction with matter here either.

The surface and atmosphere of the Sun are sources of yet more photons. Just above the surface, hydrogen is cool enough to exist as excited atomic hydrogen. These excited atoms emit a series of photons depending on the energy level of the excited electron. The majority of emissions are of a specific wavelength in the visible red spectrum. This is why this layer of the Sun's atmosphere glows red, and it is called the chromosphere (it is overwhelmed by other photons so the Sun doesn't look red; you need a special filter to see it). We will explore this layer in more detail in The Sun Part 6. There is a layer of atmosphere above this one, within the corona, that is mysteriously extremely hot. Electrons in the hot plasma, accelerated by powerful magnetic fields, emit X-rays and extreme ultraviolet photons. An X-ray imager can pick out bright loops of this plasma arcing within the corona. We will look at these loops as well as why this upper layer is so unexpectedly hot in The Sun Part 7.

The immensely hot compressed plasma of the Sun is a very complex fluid system. In the previous Sun articles, we've seen how fusion works and how it ties into the Sun's life cycle. This plasma is also loaded with powerful electrical and magnetic properties. We'll explore how these properties create amazing solar surface structures such as sunspots, coronal loops and coronal mass ejections - structures that are visible from Earth and make up solar weather.


What is magnetism?

Magnetism comes from two sources - electrical currents (induction) and the intrinsic magnetic moment of every electron, proton and neutron. Both electric currents and magnetic moments create fields of force called magnetic fields. You feel this force as a push or pull when you bring two magnets together. They will attract each other or push against each other, because magnetism is a dipole - it exhibits two opposite poles, which attract each other while like poles repel. Explore the origin of magnetism in my series of articles called Magnetism Explained.

Magnetism, like electricity, comes from atoms. Every electron is a tiny magnet. It is a magnetic dipole because it has two qualities - a special kind of intrinsic spin and an electric charge. Any rotating charged body creates a changing electric field and a changing electric field generates a magnetic field. Electricity and magnetism are intimately intertwined. A changing magnetic field also generates an electric field. In fact, these two fields are different aspects of one single force, called electromagnetism in physics.

An atom has a magnetic moment that is the sum of all the magnetic moments of its particles. Each atomic nucleus has a magnetic moment that comes from its protons and neutrons. However, if the number of protons and neutrons is equal (as in an alpha particle in solar plasma) then the total magnetic moment is zero because their spins cancel out. Electrons also contribute magnetism. Each magnetic moment has a direction, just as a magnet does. In most materials, the magnetic moments are directed in all different directions. They cancel each other out, and these materials display no noticeable magnetism. But in some materials, the atoms line up in such a way that the dipoles tend to be pointed in the same direction. These materials display magnetism, a macroscopic phenomenon composed of countless microscopic phenomena all added together. Metal atoms are arranged in neat crystal lattices in a sea of mobile conducting electrons. Certain iron compounds have atomic metal lattices that are especially well suited to lining up electron spins so these materials display powerful magnetism.

The magnetism in a star is a bit different. Here, magnetism is not so much the sum of all the magnetic moments of the free electrons in the plasma (although they contribute magnetism), but rather electromagnetic induction. The Sun's core and radiative zone plasma rotates and the plasma in the convective zone surrounding them rotates and experiences convection as well. To get an idea of what convection looks like, see the image below.
This is a snapshot model of thermal convection in Earth's mantle. It is made of solid crystalline pressurized rock that undergoes slow creeping convective movement. It's not plasma so it lacks the turbulence caused by powerful magnetic forces and electrical currents, making this a much simpler system that is far easier to model than the convective zone plasma in the Sun.

In order to have convection you need a sufficient temperature gradient to drive the motion. The temperature gradient is not sufficient to drive convection in the core or in the surrounding radiative zone, so despite enormous temperatures and intense subatomic kinetic energy, these regions are very calm. Plasma in the core and in radiative zone rotates smoothly, following the rotation of the Sun itself. It's the convective zone where matters get very complicated.

Electrons flow inside the convective zone of the Sun as they follow the plasma's convective currents and rotational currents. These electron flows are changing electric currents that induce corresponding magnetic fields. Likewise, changing magnetic fields inside the Sun induce electric currents, so there is always feedback going on, which greatly complicates the system. The complex interplay of electric and magnetic fields generates tremendous forces that can compress the plasma fluid and accelerate plasma particles. The gigantic plasma system of the Sun is highly turbulent and chaotic, generating tremendous forces that make its motion very difficult to predict.

The Sun is a complicated collection of magnetic fields of varying strengths pointing in different directions, some of which line up to create ultra-powerful localized magnetic fields such as sunspots and other, even more powerful surface phenomena that we will look at in following articles.

The Sun Is A Magnetic Dynamo

There is a relatively thin layer of plasma between the convection zone and the calm radiative zone underneath where a tremendously powerful magnetic generator is set up, generating a field so powerful that it pervades the entire solar system. This is the transition layer. There is a sharp transition between the uniform rotation of plasma in the radiative zone and the much more complex movement of plasma in the convective zone. The graph below shows how plasma rotates uniformly (single red line left) in the radiative zone and then changes into several different rotation rates in the convective zone. The transition layer is where the red lines branch off.

Puzl bustre;en.wikipedia
The transition region experiences tremendous shear, as several horizontal layers slide past one another, and it is here where physicists believe a magnetic dynamo generates an extraordinarily intense magnetic field in addition to, and far more powerful than, the induced internal magnetic fields that complicate the motion of plasma in the convective zone.

The shear produced in the transition layer means that electric currents within the plasma are dragged through the pre-existing magnetic fields set up by convection, distorting them in the process. The drawing of an armature in a motor, shown below, might help you visualize a dynamo. An armature is made up of copper wire wound around an iron core (the circular structure) that is made to rotate inside a stationary magnetic field created by two magnets (see the three N-S poles).

The rotation drags and distorts the magnetic field lines created by the two stationary magnets. In the same way, magnetic field lines are dragged along with the plasma fluid in the Sun, amplifying the pre-existing magnetic field (see how close the field lines become; that means it's more powerful there).

Every 11 Years, The Sun's Magnetic Field Reverses Polarity

The dragging of magnetic lines is also behind the mysterious 11-year cycle in solar activity. Magnetic fields are drawn as magnetic field lines so we can visualize them. They show us the direction of the field and how relatively strong it is. Around a magnetic dipole (the Sun), lines of force, called magnetic field lines, run from the North pole to the South pole. These lines have elasticity and tension like a rubber band does. As the Sun rotates, the equator rotates faster than the poles do and magnetic field lines stretch out and wind around it, like stretchy thread wound around a spool. The effect is shown below (imagine the Sun is the yellow sphere).

The magnetic field lines experience increasing tension. Their potential energy increases, an effect called the Omega effect. The lines themselves also twist, like twisting a length of twine, because of the motion of convection - columns of hot plasma rise and sink. This too increases the potential energy of the magnetic field, and this is called is the Alpha effect.

Like a rubber band that is increasingly stretched and twisted, eventually something has to give. Around the time of solar maximum, once every 11 years or so, the magnetic field reverses itself, so North becomes South and vice versa, and the stored up potential energy is released, a bit like a tightly wound spool unwinding itself all at once. As reversal approaches, sunspots (areas where intense magnetic loops poke up through the photosphere) appear, peppering the Sun's surface. The magnetic fields of sunspots can be extremely powerful, as much as a thousand times more powerful than the background dipole magnetic field of the Sun. Solar flares, sudden areas of brightening on the Sun's surface, are more common and severe as well.

The differential rotation of the Sun builds up magnetic potential energy, but what finally triggers a reversal? The answer appears to be yet another kind of plasma flow in the Sun. The Sun also experiences a kind of plasma flow called meridional flow, and it is this flow that might be the actual trigger for magnetic reversal. Plasma flows from the equator toward the poles at the surface and in the opposite direction just beneath the surface. The surface flow is slow but the subsurface flow is even slower, taking about 11 years to travel from the poles back to the equator. The strength and the structure of the meridional flow varies greatly over each 11-year cycle. Both the Omega and Alpha effects, on the other hand, are constant, they vary very little. This suggests that the meridional flow has something to do with the 11-year solar cycle. The surface meridional flow carries the powerful sunspot magnetic fields toward the poles. This means that south-directed magnetic flux goes to the north magnetic pole and vice versa, weakening the dipole magnetic field, perhaps until it eventually breaks down and changes polarity altogether. The 11-year solar cycle isn't completely understood by physicists. The SOHO spacecraft, a joint project between NASA and ESA, is currently investigating the solar cycle. It continuously takes colourful and fascinating images of the Sun using a variety of imagers. To view the very latest collection, click here.

For a more technical discussion, try this review of current dynamo models of the solar cycle, available in PDF and HTML.

Next we will explore the outer layers of the Sun - the photosphere and corona, where violent storms take place, in The Sun Part 6.