A MEETING WITH THE UNIVERSE

Chapter 4-4

Life-Styles of the Stars

Normal stars

Before the Space Age, most astronomy concerned stars and systems of stars. The reason for this is that stars emit much of their energy as visible light, and this light can penetrate our atmosphere and be detected easily from the ground. Even though scientists were limited to studying this kind of starlight, much was learned. Stars were counted, analyzed, measured, weighed, and sorted into groups. Their nuclear energy sources were deduced. Their life histories, from birth to death, gradually were deciphered. The so-called "normal" stars, such as our Sun, shine steadily. They have a variety of colors: red, orange, yellow, white, and blue. Most are smaller than the Sun, many resemble it, and a few are much larger. In addition, there are several types of "abnormal" stars: giants, dwarfs, and a variety of vari able stars.

The Sun is about 1.4 million kilometers (865,000 miles) in diameter about 109 times the diameter of the Earth - and has a surface temperature of about 6000° C. It is a natural hydrogen-fueled nuclear power plant. Deep inside, the hydrogen that makes up 90 percent of the Sun is fused into helium atoms, releasing an intense flood of energy that finds its way to the surface and so out into space. Today the Sun is in a state of balance between two forces: gravity, which pulls it inward, and the pressure of the hot gas and outward streaming radiation from the central nuclear furnace.

The diameters of most normal stars range from one tenth to ten times as much as the solar diameter. The larger, more massive ones are blue or white, and notably hotter than the Sun. Sirius, in the constellation Canis Major, and Vega, in Lyra, are examples of hot, massive normal stars that are fairly close to the Sun (8.6 light years and about 26 light years away, respectively). They are white, several times more massive than the Sun, and have surface temperatures about 1O.000° C. Other, more distant normal stars have temperatures up to about 40,000° C. There are many normal red stars near the Sun, with temperatures of a few thousand de grees and masses much less than that of the Sun. None, however, is bright enough to be seen without a telescope.

All of the bright red stars in the night sky are red giants and supergiants, counted among those we term abnormal stars. Examples are the super giants Beteigeuse in Orion and Antares in Scorpius (each about 520 light years from Earth) and giant Aldebaran (68 light years) in Taurus. The Sun is slightly unusual in one respect: It has no companion star. Most stars seem to have companions, with which they orbit in binary, triple, or larger systems, and some stars are members of clusters, with from a few dozen to a few million members.

In the first half of the Twentieth Century, astrophysicists worked out the life cycle of the stars. Stars are born out of giant clouds of gas and dust called nebulae. We can see the young stars in such clouds as the Great Nebula in Orion. (This nebula is visible to the eye, and even with small binoculars one can see that it is a diffuse object and not a star.)

diagram illustration of the life of a star

Evolution of a massive star.
Bluer colors and higher temperatures are to the left; redder colors and cooler temperatures are to the right in this schematic diagram, while stellar luminosities are plotted so that the brighter values are higher, the dimmer ones lower. Seen, according to theory, is the collapse of an interstellar dust and gas cloud to form a massive blue star, which spends most of its life at a position to the left of center on the thick colored band at the center of the diagram. The arrow shows the position of the Sun on this band, known as the main sequence. Although the massive star may shed some matter in a stellar wind, it will remain on the main sequence until its central store of nuclear fuel is nearly exhausted. Then, it begins to expand. The visible "surface" of the star gets larger but cooler; its radius may become as great as that of the Earth's orbit, hence the term "red giant". After further mass shedding and nuclear burning, the star begins to pulsate, rhythmically grow ing larger and smaller. Finally, when nuclear burning no longer releases enough radiant energy to support the giant star, it collapses, its dense central core becoming either a compact white dwarf or a tiny neutron star. The collapse also triggers an explosion of the star's outer layers, which manifests itself as a supernova. In exceptional, very massive cases, the core or perhaps even the entire star may shrink into a black hole (symbolized by warped grid lines).

The large blue supergiant stars have up to 100 times the mass of the Sun, while small, red dwarf stars have less than one-tenth the mass of the Sun. (For comparison, the planet Jupiter has slightly less than one thousandth the mass of the Sun.) The biggest stars burn hotly and rapidly, consuming all their nuclear fuel quickly, sometimes in less than a million years. Stars like the Sun, on the other hand, burn slowly and steadily; their hydrogen fuel may last for 10 billion years or more. The red dwarf stars burn up so slowly that trillions of years would have to elapse before their hydrogen is exhausted. When a star has used up its hydrogen fuel, it leaves the "normal" state. This occurs when the core of the star has been converted from hydrogen to helium by the nuclear reactions. Now the burning process moves outward to higher and higher layers. The atmosphere of the star expands greatly and it becomes a red giant. "Giant" is an apt name; if a red giant were placed where the Sun is now, the innermost planet, Mercury, might fall inside it, and a larger red "supergiant" might extend out past the orbit of the planet Mars. As nuclear evolution continues, the star may become a variable, pulsating in size and brightness over periods of several months to a year. The visual brightness of such a star may vary by a factor of 100, while its total output of energy changes by only a factor of two or three.

Stars in a New Light

Space astronomy has allowed us to understand some of the really hot stars in the universe. When a star shines with a temperature of about 6000° C, like the Sun, most of the energy is emitted as visible light. A 1O.000° C star produces much ultraviolet radiation. Unusual, very small stars, with temperatures around one million degrees, generate X-rays. But we can never see these extremely hot stars from the ground. The X-rays are absorbed by our atmosphere. In fact, they were discovered with instruments flown in space.

Ultraviolet telescopes in orbit have observed hot blue supergiants, such as Rigel in Orion (about 900 light years away), that are much more massive than the Sun. To our surprise, these massive stars turned out to have intense stellar winds, streams of atoms that boil off the top of the star's atmosphere and race into space.

Although the winds from the hot supergiant stars are invisible to telescopes on the ground, they are hundreds of millions of times more power ful than the wind from our own Sun.

Underground observers of distant space.
In a basement room at the NASA-Goddard Space Flight Center, astronomers work with an orbiting telescope far above. The telescope is aboard the International Ultraviolet Explorer, a spacecraft suspended over the south Atlantic, in Earth-synchronous orbit. In continuous touch with the IUE telescope, the astronomers can view starfields through it or examine the ultraviolet spectrum of a star (as shown here on large screen at top of photograph) or nebula shortly after a time exposure observation is completed. (Photograph courtesy of Fred Espenak.)

NASA ground control room for the obiting telescope

These winds sweep away the interstellar gas and dust around their stars, sometimes producing an "interstellar bubble" over 10 light years in diameter. The wind "blows" at thousands of kilometers per second and carries away enough of the star's mass to make a whole Sun every million years. In the lifetime of a blue supergiant, which may be 10 million years, a substantial fraction of the original mass of the star may be expelled into space.

Studying the X-rays from stars has given us more surprises. With the X-ray telescope on the second High Energy Astronomy Observatory (HEAO-2), stars of all kinds have been observed through the X-rays they produce. Contrary to what scientists expected, massive stars were found to have coronae: thin, hot gaseous envelopes surrounding their lower atmospheres. These coronae, with temperatures up to several million degrees, generate the X-rays. Normal yellow stars like the Sun seem to make much fewer X-rays. Even some cool stars make more X-rays than the Sun. New theories are being developed to account for this discovery. The space observations indicate that the speed of a star's rotation may play a more important role than its temperature in determining its X-ray luminosity, and indeed the Sun is a slowly rotating star. Faster-turning stars seem to outshine slower ones of the same type in X-rays.

The interplay between space telescopes and ground-based astronomy has not only given us a new look at familiar objects, it has also turned up a number of very strange and un familiar ones. One example is the remarkable, still somewhat mysterious object known as SS 433. The light from SS 433 was observed to have spectral lines that did not correspond to the spectra of any known stars. More detailed observations revealed that these lines moved very quickly from one wavelength to another, indicating a surprising change in the velocity of the gas emitting the light. Over several months, the range in velocity amounted to nearly one-third of the speed of light. This was sufficient to shift some infrared and ultraviolet wavelengths alternately into the range of visible light. No wonder the spectral lines were hard to identify! The high-speed movement is characteristic of gas at a temperature of close to a billion degrees. The width of the lines, however, showed that the gas is cool, with a temperature of only about 10,000° C. How the gas in SS 433 can move so very fast and still remain cool is one of the outstanding mysteries of the 1980's in astrophysics. X-ray observations from satellites first called our attention to this star, stimulating the spectral studies that revealed the enormous velocities.