Stellar Evolution

The material between stars occurs in clouds of varying mass. By processes that are still not completely clear, but involve cooling of the cloud-center with the formation of molecules, and the squeezing of the cloud by outside star light or perhaps a stellar explosion, the cloud begins to collapse under its own self-gravity. The collapse of the cloud results in the material becoming hotter simply from the squeezing of the collapse. At this point, the interior of the star churns. This churning process is called convection. Its rate of collapse is determined by the rate at which it can lose energy from its surface. Atomic processes keep the surface near a constant temperature so that a rapid collapse is slowed by the radiating surface area shrinking during the collapse. The star simply gets fainter while the interior gets progressively hotter.

Finally, the internal temperature rises to the point where atoms located at the center of the star, where the temperature is the hottest, are moving so fast from the heat that they begin to stick together. This process is called nuclear fusion, and it results in an additional production of energy. Thus the star has a new source of heat. The subsequent evolution of the star will be largely determined by its mass.

If the mass of the star is about like that of the Sun or less, the nuclear "fires" which now provide the energy for the star to shine will determine its internal structure. A central radiative core is surrounded by a convective envelope. In the radiative core the material remains quiescent, while energy generated by nuclear fusion of hydrogen to helium simply diffuses through it like the light from auto headlights shines through a fog. It is at the very center of this radiative core that the helium "ash" of the nuclear "fires" accumulates as the star ages. Beyond the radiative core lies the churning convective envelope through which the energy is carried by blobs of hot matter rising past returning cooler blobs. At the atmospheric surface, the energy again flows as it did in the core until it physically leaves the star as starlight.

The structure of stars more than twice the mass of the sun is essentially the reverse of the low-mass stars. The cores of these stars are fully convective so that the energy produced by nuclear fusion is carried outward by the churning motion of the material in the core. The surrounding radiative envelope behaves much like the cores of lower-mass stars except no new energy is produced there. The churning motion of the material in the convective core causes the nuclear ash of helium to be well-mixed with the surrounding hydrogen fuel. This motion ensures that virtually all the hydrogen will be available to the nuclear fires which heat the star.

Both high- and low-mass stars respond to the depletion of hydrogen fuel in a similar manner. In order to supply the heat to oppose its own self-gravity, the star's core again responds by shrinking. In a sort of reflex reaction, the outer regions of the star expand, causing a great increase of its radiating surface area. Although the total energy output of the star increases during this phase, the greatly enhanced surface area results in a cooling of the surface and the star takes on a redder appearance. The size and color change lead to the name of red giant for these stars. If the star is very massive, it may become what is called a red supergiant.

For the low-mass stars, the expansion to the red giant phase will begin when about 90% of its hydrogen has been converted to helium. During the contraction of its core, a complicated sequence of events occurs. The shrinkage required to produce the energy radiated by the large giant causes the core to shrink to the dimensions of a white dwarf, while hydrogen continues to burn by nuclear fusion in a thin shell surrounding the core. It is this shell that provides most of the energy that is radiated away by the star. However, the core material, having attained the dimensions of a white dwarf, behaves very differently than the high-density gas that it was earlier in its life. No longer must it be heated to generate the pressure required to oppose the weight of the overlying material. When matter reaches this state it is called degenerate matter. The degenerate core just sits there, becoming hotter from the energy released by the surrounding hydrogen-burning shell and growing slowly from the helium ash generated by the shell. The hydrogen-burning shell is required to produce increasing amounts of energy from decreasing amounts of hydrogen fuel to sustain the brightening red giant. This continuing increase in the energy output from the shell heats the core, which finally reaches a temperature where the helium begins to undergo nuclear reactions, producing carbon. In this fusion process three helium nuclei collide yielding one carbon nucleus and additional energy for the support of the star.

The star now has a new energy source. However, the degenerate nature of the core doesn't allow it to expand and cool as would a core made of ordinary gas. Thus the onset of helium burning leads to a rapid rise in core temperature which is not balanced by a cooling expansion. The increased core temperature leads to a dramatic increase in helium burning. This sequence, known as the helium flash, continues until the degeneracy of the material making up the core is removed by the intense heat. The return of the material to its ordinary gaseous state leads to a rapid expansion which cools the core and reduces the helium burning. An equilibrium is established with the star generating progressively more energy from helium fusion, while the energy from the hydrogen burning shell is reduced, and it ultimately goes out from lack of fuel. The star continues to shine through the red giant phase by converting helium into carbon through nuclear fusion.

As the helium becomes depleted, the outer layers of the star become unstable and rather gently lift off the star to be slowly blown away by the light from the star. (Such an image was captured in 1998 by the Hubble Space Telescope, of a dying star known as NGC7027, located about 3,000 light-years from the sun in the direction of Cygnus the Swan.) Such shells of expanding gas are observed as greenish disks that eventually become greenish rings and the material becomes less dense. These greenish clouds are called planetary nebulae, even though they have nothing whatever to do with planets. Astronomers of two centuries ago gave them that name, for their telescopic appearance from Earth was like that of the outer planets Uranus and Neptune.

The remaining core of the red giant, now exposed, cools rapidly, again becomes degenerate, and is known as a white dwarf. A white dwarf has reached a stalemate between its own self-gravity and the nature of the degenerate stuff of which it is now composed. It may now simply cool off to become a dark stellar cinder about the size of the earth.

The evolution of a massive stars follow a somewhat different course. The churning of the convective core makes most of the hydrogen fuel available for consumption in the nuclear fires. Thus, these stars will not suffer the effects of core contraction until more than 99.9% of their hydrogen has been consumed. Even though they can consume more of their hydrogen (on a percentage basis), and they have more fuel to burn, they also shine much brighter than the low-mass stars. Thus their overall lifetimes will be far less than the low-mass stars. While the lifetime of a star like the sun may approach ten billion years, a star with ten times the mass of the sun may last less than ten million years.

The exhaustion of the hydrogen convective core leads to its contraction and the expansion of the outer layers, as was the case with the low-mass stars. However, the fate of the core is rather different from that of the low-mass stars. The core is far too massive to reach equilibrium as a degenerate structure like a white dwarf, so that contraction continues heating the core until the ignition of helium fusion is achieved. Unlike the lower-mass stars where the onset of helium burning occurs with a flash, the helium fusion in massive stars begins slowly and systematically takes over from the hydrogen-burning shell surrounding the core. Throughout the red giant or supergiant phase the role of energy production steadily shifts from hydrogen burning to helium burning. Eventually, helium becomes exhausted around a growing carbon core. While helium continues to undergo fusion in a shell surrounding the core, carbon fusion is ignited. Just as a degenerate helium core gives rise to the unstable ignition of helium, called the helium flash in low-mass stars, so the degenerate carbon core of moderate mass stars can result in an unstable ignition of carbon. However, whereas the helium flash is quickly quelled in low-mass stars, the carbon ignites explosively in the cores of these moderate-mass stars. This process is called carbon deflagration and may result in the destruction of the star.

In even more massive stars, the onset of carbon burning is a controlled process and the star develops multiple shells of energy sources involving carbon fusion, helium fusion, and even some hydrogen fusion in the outer regions of the star. The ignition by nuclear fusion of each new element yields less energy than the one before it. In addition, the increased temperature required for the nuclear fusion of these additional sources leads to an increase in the stellar luminosity. The result is an ever-increasing rate of the formation of less-efficient energy sources. When nuclear fusion in the core of the star yields iron, further nuclear fusion will no longer yield energy. Instead, nuclear fusion of iron will use up energy-robbing thermal energy from the surrounding material. This sudden cooling of the core will bring about its collapse.

As the density increases in the collapsing core, there is less and less room for the free electrons that have been stripped from the atomic nuclei by the extreme temperature. These electrons must go somewhere, so they will begin to be "absorbed" in the protons of the atomic nuclei, turning them into neutrons. The process is called neutronization. This reaction generates particles called neutrinos, which interact very weakly with ordinary matter, and so normally escape directly from the star. The energy robbed from the core by the neutrinos also adds to the energy crises in the core and contributes to the core collapse.

The production of elements with masses greater than iron also produces large quantities of neutrinos, so that whichever process dominates, a great deal of energy is lost directly from the star, resulting in a catastrophic gravitational collapse of the core. This is followed promptly by the collapse of the entire star. The rapid increase in the density of the collapsing core finally reaches the point where the material becomes opaque to the energy-robbing neutrinos, and their continued escape is stopped. The sudden deposition of the neutrino energy in the collapsing core reverses the collapse, bringing about an explosion of unprecedented magnitude. The infalling matter and trapped photons are hurled into space, liberating as much energy in a few minutes as the star has radiated in its lifetime of millions of years.

The remains of this titanic explosion depend on the initial mass of the collapsing star. Very-massive stars may leave a black hole of completely collapsed matter behind. Should the collapse involve a star of less mass, the remainder may be something called a neutron star, similar to that formed by the collapse of a white dwarf. In some instances, the entire star may be involved in the explosion and there will be no remains at all. While there have been recent attempts to refine the classification of these explosions, astronomers still refer to the explosion of a massive star as a supernova of type II. Supernovae of type I are thought to result from the collapse of a white dwarf which has exceeded its critical mass. Unlike the evolution of low-mass stars, in which an accommodation between the forces of gravity and degenerate structure of the star is achieved through the formation of a white dwarf, the evolution of a massive star must end in a violent stellar explosion. Gravity appears to win its struggle with nuclear physics, but at the last moment, the energy of collapse is turned to an explosion leaving either a collapsed corpse, or perhaps nothing at all.

The accession of the Hubble Space Telescope had given astronomers a valuable tool to study the evolution of stars in the universe, at the same time challenging their understanding. In 1997, Hubble detected rogue stars that belong to no galaxy, displaced long ago and now hanging in empty intergalactic space among star clusters like the Virgo Cluster, about 60 million light-years from Earth.

In 1996, astronomers found evidence of many isolated, dim brown dwarfs, lacking sufficient mass to start nuclear fusion. They detected light spectra from the element lithium, which quickly burns in true stars. These brown dwarfs, called "L dwarfs," are typically smaller then our sun but much larger than even Jupiter, and some may resemble Saturn's moon Titan.

On the opposite scale, in 1997 Hubble detected the then brightest star ever seen. Discovered at the core of our own galaxy and named the Pistol Star, it has the energy of ten million Suns and would fill the distance of the Earth's orbit around the Sun. The Pistol Star is about 25,000 light-years from Earth; it is so turbulent that its eruptions create a gas cloud four light-years across. It had been thought that a star so big could not have formed without blowing itself apart, and so the Pistol Star will require astronomers to reexamine their ideas about stellar formation, especially of supermassive stars near the centers of galaxies.

By 2003, other observations, including x-ray observations from the ROSAT Observatory and NASA's Chandra x-ray Observatory, allowed the identification of high intensity ultra-bright x-ray sources that many astronomers argued were evidence of black holes in star-forming galaxies. Although there are other explanations for these phenomena, the fact that they provide additional confirmation of black holes is enhanced by Hubble observations of stars rotating around stellar cores of these galaxies.

In early 2003, the Chandra x ray Observatory, provided extended observations of Sagittarius A (or Sgr A), the supermassive black hole at the center of Earth's own Milky Way galaxy.