G. C. Sloan: The Life of Stars

The Life of Stars

What is a star?

This description may not be typical, but it explains things in a nutshell: A star is a continuous (and eventually doomed) battle with the inexorable force of gravity. From the moment an interstellar cloud of gas and dust begins to collapse under its own gravitational forces, it is doomed to one day become a cold, dense, degenerate object like a white dwarf, a neutron star, or possibly, if it's massive enough, even a black hole.

Every stage of a star's life represents a pause in this battle, as the star finds some energy source which can temporarily halt its inevitable collapse.

Young stellar objects

A star begins its life as part of the interstellar medium, usually as a dense core in a giant molecular cloud. If something (like a nearby supernova or a spiral density wave) can compress this cloud core some, it will begin to collapse under its own gravity. Most of the mass will quickly settle in the center of the cloud core while the envelope slowly falls inward. At this stage, astronomers refer to the source as a protostar. Because of its very cold temperature, astronomers must look in the far infrared or at millimeter wavelengths to see it. At visual wavelengths, protostars are completely dark.

Protostars continue to contract gravitationally while material continues to accrete onto their surface. As they contract, their temperature rises, making them visible first in the infrared and eventually as star-like objects in the visual. Usually, these young stars are embedded within the clouds from which they formed, and they are often obscured from view.

Very massive stars can actually ionize the dust cloud around them, producing spectacular nebulae like the Great Nebula in Orion or the Eagle Nebula in Serpens. For some images from the Hubble Space Telescope, go to the page on the birth of stars (There are lots of pretty pictures here).

The main sequence

Eventually the interior of the star gets so hot, thermo-nuclear fusion reactions begin. These reactions produce tremendous amounts of energy, halting the collapse process and allowing the star to settle onto what is called the main sequence. Main sequence stars provide their energy by fusing hydrogen atoms together to produce helium.

The more massive a star is, the more energy it requires to counteract its own gravity. As a consequence, very massive stars burn the available hydrogen in their cores much more quickly than low-mass stars. The more massive a star is, the shorter its life on the main sequence will be. Stars with the mass of the Sun will last on the main sequence about 9 billion years. Very massive stars, like those in the Trapezium in Orion, will only last a million years or less. Low-mass stars can survive for 100 billion years or more.

Red giants

When all of the hydrogen in the core of a star has been converted to helium, there is nothing left to prevent the core from collapsing under its own gravity. Hydrogen to helium fusion continues in a shell around this core, and as the core contracts, the temperature increases. As the temperature increases, the luminosity of the star increases, in some cases by a factor of 1000 or more. This extra energy production will cause the outer layers of the star to expand, even while the core is collapsing. In this bizarre geometry, even though the interior temperature is far hotter than it was on the main sequence, the outer layers expand and cool, producing a red giant.

The helium-burning main sequence

Eventually, the temperatures in the stellar core get so high, helium fusion can occur. The star has now found a new energy source to hold itself up, although it won't last anywhere near as long as the hydrogen-burning main sequence. The star settles down, decreasing in brightness and contracting.

The asymptotic giant branch

When the star exhausts its supply of helium in the core, it repeats the process that sent it up the red giant branch. The core begins to contract and heat up again, and the envelope expands and reddens, getting even larger than before. When the Sun reaches this phase, its radius will extend past Earth's orbit. Not only will Earth be cooked, but it will actually be swallowed up by the Sun! The interior of a star on the asymptotic giant branch has several layers. Around the inert (but toasty) core, there's a thin shell of helium fusion. Around this an inert shell of helium (the temperature isn't high enough to allow helium fusion), and around this a layer where hydrogen fuses into helium.

This is not a very stable configuration, and astrophysicists have been able to show that stars on the asymptotic giant branch are unstable to pulsations. Once they start, they're hard to stop. These pulsations push gas so far above the star, it cools enough for dust to condense. When the Sun dies, it will lose half its mass this way. More massive stars might shed 90% of themselves back into space! With enough mass loss, a star will eventually be surrounded by a dust shell so thick that optical radiation is trapped. The shell reprocesses the radiation and emits it in the infrared. The star disappears from the visual sky and becomes an infrared source.

For more information on these dust shells, go to the circumstellar dust page.

Planetary nebulae, white dwarfs, and novae

As an infrared source, the star continues to lose mass until eventually, so much of the stellar envelope is gone, the hot stellar interior is exposed. The temperature of the core is so hot, it produces enough ultraviolet radiation to ionize the shell of gas and dust around it.

In a classic example of astronomers' poor choice of names, these ionized shells are called planetary nebulae. Why? In the smaller telescopes used in the 19th century, these nebulae resembled planets some. They didn't move around the sky like planets do, but they did look like discs of light instead of point sources.

For some nice pictures of some planetary nebulae taken by the Hubble Space Telescope, go to the page on the death of stars.

Some people mistakenly call the phase when a star evolves into a planetary nebulae a "nova." I can recall an episode of Star Trek where Scotty warns Captain Kirk to get off of a planet because its sun is "gonna go nova any minute now." Actually, while it must be a beautiful event to witness, it takes thousands of years for a dying star to produce a thick circumstellar dust shell and then ionize it. The details of this process remain poorly understood.

The hot stellar interior of a planetary nebula collapses into what is called a white dwarf. Imagine half the mass of the sun crammed into a volume the size of the Earth! At this point, every electron in every atom has been compressed down as close to the atomic nucleus as it can get (according to the Pauli Exclusion Principle). The force preventing any further collapse is called electron degeneracy pressure. For most stars, this is the final resting place, a dense, degenerate object slowly cooling into what one day could be described as a black dwarf.

Sometimes in binary systems, you'll get a white dwarf in orbit around a red giant. If the giant expands enough, it'll start to dump material onto the surface of the white dwarf. When this gets hot enough, it'll ignite in a runaway thermo-nuclear reaction, and that's what a nova really is.

Supernovae and neutron stars

In the 1930s, a young astrophysicist named Chandrasekhar showed that if a white dwarf was more massive than 1.44 times the mass of the Sun, something amazing happened. The force of gravity was enough to overcome the electron degeneracy pressure, and the star would collapse into an even denser state. Stars more massive than approximately 3-5 times the mass of the Sun don't become giants - they become supergiants. Their mass is so tremendous, the core can get hot enough to fuse elements even larger than helium into more massive elements. As the star evolves, it will get hot enough in its core to fuse heavier elements, one at a time. Its interior will be layered like an onion, with progressively hotter layers deeper and deeper, fusing heavier and heavier elements. Each layer moves outward from the core and represents a temporary source of energy to halt the collapse process. The dying star exhausts each more massive fuel source in the core more furiously and faster than the one before.

The problem with this strategy is that it is doomed to failure. As long as you're fusing elements lighter than iron, you get energy out of the reaction. But try a reaction where the end product is more massive than iron and you'll discover that the reaction absorbs energy instead of produces it. The same principle explains why both hydrogen fusion and uranium fission produce energy. As long as you're moving to atomic numbers closer to iron, you're generating energy.

So all of a sudden, the star, trying to hold itself up, runs out of fuel. The consequence is sudden and dramatic: a supernova. The star collapses in on itself, and material rebounds off of the central core on a tremendous explosion. When a nearby star goes supernova, it will shine so brightly that we can see it during daylight. If a supernova happens really close, we'd be in VERY BIG TROUBLE!

In the core, the force of gravity is sufficient to overcome the electron degeneracy pressure, and the electrons are driven into the atomic nuclei. Each electron combines with a proton, producing a massive sphere of neutrons. A typical neutron star weighs more than 1.44 times the mass of the sun, and yet it's so dense, it's only a few kilometers across! Most towns are larger than that.

Black holes

You might guess that with sufficient mass, even the neutron degeneracy pressure holding up a neutron star can be overcome. When this happens, the star collapses in on itself completely. It becomes so dense, only objects travelling faster than the speed of light could escape its gravity. And since nothing can travel faster than the speed of light (according to Einstein), nothing can escape, not even light itself.

It's a common misconception that all stars eventually become black holes. (The people at Trivial Pursuit seem to think so - I once lost a game because of that question). Actually, most stars will eventually become white dwarfs. Most of what's left become neutron stars. Only the most massive of stars become black holes.

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