Stellar evolution
The births, lives, and deaths of the stars.

Stellar evolution.
Our galaxy's spiral arms are literally filled with dust and gas. Occasionally, a cloud of this gas begins collapsing (for reasons not completely understood) under its own gravity, and a star begins to form. Due to conservation of angular momentum, this cloud forms into the shape of a rotating disk, with the inner regions rotating faster than the outer ones. After a certain time, the center of this "accretion disk" begins to heat up from the pressure of the gas particles, and, one day, thermonuclear reactions start in its core. Two hydrogen atoms are squeezed together with such pressure and heat that they fuse into one atom of helium, thus liberating considerable energy. [footnote 1] This energy is what drives the stars. When this happens, the object is now a protostar -- the first stage of a star's life.

The thermonuclear reactions in the core of a protostar cause pressure -- the star wants to fly apart. However, gravity keeps it from flinging its guts into space. This constant interplay between radiation pressure and gravitation dictates the course of a star's life -- and, eventually, the star's end.

The protostar's accretion disk sometimes buds off into small vortices of condensing matter, away from the tumult of the stellar furnace. These protoplanets eventually become the other bodies orbiting in the system. [footnote 2]

The protostar, when it finally "turns on," pushes the gas and dust in its vicinity away. This clears the system out, and leaves the protoplanets orbiting around their young sun. [footnote 3]

After a few million years the system begins to mature. The protostar, which might have been wildly variable -- changing in size and luminosity -- begins to settle down. The protoplanets begin forming their more recognizable cousins, the planets, asteroids, comets, and other interplanetary bodies. The star has now become a main-sequence star, the most populous of all stars in the galaxy. [footnote 4]

The course of a star's evolution is dictated almost entirely by its initial mass. The lower the mass of a star, the longer it will live, and the less spectacularly it will die. The higher the mass of the star, the shorter its life, but the more violent the death. Red dwarfs, the coolest stars, are very small, but can live for hundreds of billions of years -- while blue main-sequence stars, some of the hottest, can burn themselves out in only a hundred million years or so. Our Sun, a yellow main-sequence dwarf, somewhere in between, will live ten billion years, from birth to death.

When a star begins to age, it wanders off the main-sequence. It begins to run out of hydrogen in its core, and the star starts to quiver. It is now a subgiant. It begins to swell, and starts becoming unstable. The reactions in the core are beginning to change, and so the balance between expansion and gravitation becomes upset. At this point in its life, the star can again become wildly unstable and begin to pulsate, change brightness, or even change color.

After a time, the hydrogen in the core runs out almost completely, and it collapses. New reactions, such as helium-to-carbon, begin to take place in the core. These reactions liberate more energy than do the hydrogen-to-helium reactions, and so the star expands. When a star roughly the mass of our Sun or below comes to this stage, it is a red giant. Its radius has increased many times -- so much so that our Sun, when it eventually reaches this stage in five billion years or so, will engulf the inner planets, out to Mars.

As red giants remain in this stage, their cores, as they run out of their new fuel sources, switch to others. Helium-to-carbon, carbon-to-oxygen, oxygen-to-silicon, and so on. [footnote 5] New elements are formed in the cores of stars all the way up to iron. But iron is the limit to nuclear reactions in stars -- iron is too heavy to be fused into higher elements with a gain of energy. The giant has reached its evolutionary dead-end. When a giant reaches the end of its life, and the reactions proceeding in its core are delivering a very large amount of energy indeed, it begins to shed its outer layers, since gravity is not strong enough to hold onto them anymore. These layers are flung into space, leaving a fiery core. Such an object is called a planetary nebula. [footnote 6]

Eventually, when the core runs out of fuel and has converted the majority of its mass into heavy elements, such as iron, it collapses -- there is insufficient outward pressure from thermonuclear reactions with which to keep gravity at bay. It collapses until electron pressure -- the electrical repulsive force between electrons, halts the collapse. Such an object is very dense, and is called a white dwarf (or degenerate star). It is the mass of the Sun compressed into a sphere the size of the earth. [footnote 7] Eventually, the white dwarf cools, venting its energy into space, until it no longer shines. This dead star is called a brown dwarf, or an exhausted star. Such a fate eventually awaits our Sun.

More massive stars, however, do not have such a kindly death. When a star is quite a bit more massive than the Sun, then the powerful reactions in its core turn it into a supergiant -- some supergiants have radii corresponding to Jupiter's, Saturn's, or even out to Neptune's orbit. Supergiant stars are extremely luminous, and have tremendous surface areas. When a supergiant runs out of its fuel (again, iron is the limit), then it begins to quake. Supergiant stars, occasionally, undergo tremendous change. Their cores, with no fuel left to sustain reactions, collapse violently. This collapse triggers a tremendous explosion, called a supernova. Supernovae are so brilliant that they can outshine an entire galaxy. [footnote 8] The outer layers of the star are thrown off violently, and the remaining core has two possible fates.

If the star was only moderately more massive than our Sun, then electron pressure in its core is not strong enough to halt the collapse. The core continues to collapse, plunging in on itself, until the protons and electrons in its core are forced together so violently that they are transformed into neutrons. Thus, when almost all of the core of the star is transformed into neutrons, the collapse is halted by this new pressure. Such an object is called a neutron star. It is the mass of several Suns compressed into the size of the center of a city -- only 10 kilometers in diameter or so. Neutron stars are made up of stuff called neutronium, which is the densest matter known. [footnote 9] The neutron star, still ringing from its formation, slowly cools, and dies. [footnote 10]

However, if a star is quite a bit more massive than our Sun, even the pressure from the neutrons squeezed together in its core is not enough to stop the contraction. Such an object falls in on itself, until the escape velocity of its surface exceeds that of light itself. The object disappears from view in our universe, and is now a black hole, or collapsar. Black holes are so dense that not even light can escape -- hence the reason they are called black.

New study has shown that even black holes can die. A black hole emits radiation like any hot body. As it does, it loses mass, and eventually it dies in a puff of photons and exotic particles. [footnote 11]

It is interesting to note that the early universe, according to cosmologists, was composed of entirely hydrogen and helium. All of the other elements -- the carbon in our bodies, the oxygen in our atmosphere, the silicon in the rocks -- were formed in the process of evolution of the stars. The elements up to iron were formed in the cores of massive stars, and all of the other elements -- those higher than iron -- were formed in supernovae. [footnote 12] The supernova explosion is so powerful that it forces atoms together to fuse into very high elements indeed -- making silver, gold, uranium, and all of the other heavy elements.

We are all undeniably tied to the history of the universe.

Actually, the energy created by nuclear fusion in the Sun's core is in the form of high-frequency gamma rays. Why then do we see visible light from the surface? Because the Sun is so dense, light cannot make the journey directly from the core to the surface, only about 700 000 kilometers. It follows what physicists call a "random walk," where a photon of light moves only a few centimeters and gets absorbed by an atom; then it is reradiated by the atom in a random direction and absorbed shortly thereafter again. In the process it loses a little energy (sometimes it is reradiated as two or more photons). Thus it takes an individual photon of light a long time to get to the surface after its formation. How long? It has been estimated that it takes -- on the average -- one million years.

Interestingly enough, another subatomic particle is created in the nuclear fires inside the Sun's core -- the neutrino. The neutrino travels at or near the speed of light, but, unlike photons, it does not interact strongly with matter. It makes the trip from the Sun's core in three seconds. In fact, a neutrino has a roughly 50-50 chance of being stopped by several light-years of solid lead!

footnote 1
Scientists have found evidence of planetary systems in the process of forming. A star quite close to us, Beta Hydri, was noticed to have a fuzziness to it -- a disk of gas and dust surrounding the star. This is probably the accretion disk of matter of a forming solar system.

Another "disk star," as they are sometimes called, is designated MWC 349 and is in the constellation Cygnus. Unlike Beta Hydri, however, this one is very far away -- around 8000 light-years. It is a huge supergiant star with a large disk of dust and gas around it. Unfortunately, since MWC 349 is burning itself up so fast, it will probably go supernova before a life has a chance to form within its newly-forming solar system.

footnote 2
Some astrophysicists also speculate that this is what causes other systems to be formed. The wavefront of gas and dust, caused by the photon pressure of the new star, crashes into other, more uniform fields of gas, causing another collapse and another star to be born.

footnote 3
There is a division of population within main-sequence stars as well. Cool, low-mass main-sequence stars are much more populous than hot, high-mass ones. The most common star in the universe appears to be a red main-sequence dwarf -- there are large number of them in our immediate vicinity. Our Sun, a yellow main- sequence dwarf, once thought to be a common, ordinary star, has gotten some of its prestige back: yellow dwarfs aren't as common as we thought them to be.

footnote 4
The reactions are more complicated than simple fusion. For instance, nitrogen fuses into silicon, and some of the silicon decomposes into magnesium. Then silicon and magnesium fuse to form iron.

footnote 5
Actually, the term planetary nebula is a misnomer. In 1785, when William Herschel looked through his eyepiece at the first discovered planetary nebula, he named it such because of its similarity in appearance to the blue-green disk of the distant gas planets, such as Uranus or Neptune. From the text, however, we can see that planetary nebulae really have little to do with planetary systems.

footnote 6
Some white dwarfs, at certain ages, temperatures and densities, could be composed of crystalline carbon (the chief element left over from the nuclear furnace inside its parent star). Without a doubt they would be the largest diamonds in the Universe -- about the size of the Earth.

footnote 7
Supernovae, because of their great ferocity, are relatively rare events. There have only been seven supernovae in our Galaxy in all of recorded human history. Since we can see supernovae in other galaxies (the supernovae occasionally outshine the galaxy that they are contained in), however, we have more of an opportunity to study them.

footnote 8
Neutronium is, literally, the density of the nucleus of an atom: the neutrons are so close in the neutron star that they are touching, so you can think of the entire body as one gigantic atom.

footnote 9
Neutronium is, literally, the density of the nucleus of an atom: the neutrons are so close in the neutron star that they are touching, so you can think of the entire body as one gigantic atom.

footnote 10
This black hole radiation, or Hawking radiation (named after the physicist who suggested it, Stephen W. Hawking), is a direct consequence of quantum physics. According to quantum physics, particle-antiparticle pairs (or virtual particle pairs) are being created all the time. They appear and then annihilate each other, returning to equilibrium. Hawking radiation involves one of these virtual particle pairs being created near the black hole's event horizon. The particle with negative energy is swallowed by the black hole, and the other escapes. Negative energy means a reduction in energy, so the black hole loses mass. The particle that escaped appeared to have been emitted, and so we call it "radiation," even though it is somewhat misleading.

footnote 11
The vast amounts of gold, uranium, and other elements with high atomic numbers on the Earth give you some idea of the size of titanic explosions that we call supernovae.

footnote 12
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