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.

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