(The following narratives are taken from the NASA publication "A Meeting with the Universe" edited by Bevan M. French and Stephen P. Maran. It has many beautiful color photos and is sold by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. The document is identified as NASA EP-177.)
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 variable 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 10,000 degrees C. Other, more distant normal stars have temperatures up to about 40,000 degrees C. There are many normal red stars near the Sun, with temperatures of a few thousand degrees 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 supergiants Betelgeuse 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.)
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.
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 6 000 degrees C, like the Sun, most of the energy is emitted as visible light. A 10 000 degrees 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 powerful than the wind from our own Sun.
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 (HEA0-2), 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 unfamiliar 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 degrees 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.
Perhaps the greatest surprise of the Space Age has been the realization that "dead" stars that have used all their nuclear fuel can sometimes produce more energy than they did when "alive." We have discovered that there are three possible ends for a burnt-out star. If the star has about the mass of the Sun, it will collapse under its own gravity until the collective resistance of the electrons within it finally halts the process. The star has become a white dwarf and may be comparable in size to the Earth. A star with a mass of about 1.5 to 2 or 3 times that of our Sun will collapse even further, ending up as a neutron star, perhaps 20 kilometers in diameter. In neutron stars, the force of gravity has overwhelmed the resistance of electrons to compression and has forced them to combine with protons to form neutrons. Even the nuclei of atoms are obliterated in this process, and finally the collective resistance of neutrons to compression halts the collapse. At this point, the star's matter is so dense that each cubic centimeter weighs several billion tons. For stars that end their life weighing more than a few times the mass of the Sun, even the resistance of neutrons is not enough to stop the inexorable gravitational collapse. The star ultimately becomes a black hole, a region in space so massive that no light or matter can ever escape from it.
The existence of white dwarfs has been known for some time, and many have been detected with ground-based telescopes. However, neutron stars and black holes existed only in much-disputed theory until the Space Age.
The discovery and understanding of neutron stars involve studies of two poorly understood types of space objects, supernovae and pulsars. Supernovae are extremely violent explosions, in which a star suddenly detonates, pouring out so much energy that for a few days it may outshine all the other stars in its galaxy put together. Pulsars, first detected by radio astronomers in 1967, are sources of very accurately spaced bursts of radio waves. These bursts were so regular, in fact, that the scientists who detected them wondered briefly if they had found artificially generated signals from an interstellar civilization.
The discovery of a pulsar in the Crab Nebula supernova remnant led to a great synthesis in our understanding of pulsars and supernovae. Supernovae occur at the end of a massive star's life, when it is a red supergiant, with its nuclear fuel almost spent. When the central core becomes so dense that electrons and protons begin to form neutrons, it collapses catastrophically to form a neutron star. In the process, more energy is released than the star ever generated from its nuclear fuel, producing an explosion in which every atom in the outer parts of the star is heated to well over a million degrees. The star is literally destroyed in an instant, but the debris from the explosion shines briefly with the energy of a billion suns.
Besides splattering stellar debris into space, supernova explosions leave behind a "cinder" (the dense, collapsed core, made of neutrons) where there once was a star. The weak magnetic field of the original star is greatly enhanced in the collapse, and the remnant core,the neutron star, may have a magnetic field trillions of times stronger than the magnetic field of the Earth. The rotation of the star also increases dramatically during collapse, and the resulting neutron star spins many times a second. Beams of radio waves, X-rays, and other radiation, perhaps focused by the powerful magnetic field, sweep through space like the revolving beam of a lighthouse. The neutron star has become a pulsar.
Pulsars were discovered accidently during a study of "twinkling" radio sources in the sky. This twinkling is not due to our atmosphere, as is the twinkling of stars. Instead it is caused by the highly rarefied interstellar gas, which affects the passage of radio waves. As the study went on, the scientists at Cambridge University noticed that in some sources the twinkling was periodic, the signals came at regular intervals of 1 or 2 seconds or less.
Gradually, more pulsars were discovered. The fastest one known so far, which rotates at 30 times a second, is in the Crab Nebula, the remnant of a supernova explosion that was observed in 1054 A.D. When this rapid pulsar was found, it was quickly realized that it must be a neutron star. Only a neutron star could remain intact under such rapid rotation without breaking up. (A rotating black hole would remain intact, but it would not produce a regular signal.)
Now that we can see the universe by the light of X-rays and gamma rays, further unexpected properties of pulsars have been found. The theories that were rather successful in explaining the Crab Nebula pulsar failed to predict or account for phenomena found in the brightest gamma ray pulsar, located in the constellation Vela. New theories are needed to explain how pulsars can create intense radio waves, visible light, X-rays, and gamma rays, all at the same time.
Many neutron stars of another kind have been found with orbiting X-ray telescopes. We usually cannot detect the heat left over from their collapse, but instead we detect X-rays from matter that is heated intensely as it falls rapidly towards the surface of the star. The realization that neutron stars suck up surrounding matter came from the discovery in 1971 of an X-ray pulsar, Hercules X-l. Detailed study of this X-ray source revealed very small variations in the 1.2 second period of pulsation. More study proved that these small variations were caused by motion of the neutron star in orbit around another star.
We have now learned that most X-ray emitting neutron stars are in orbit around other, otherwise normal stars. In some cases the stars are so close that the intense gravity of the neutron star actually pulls gas away from the atmosphere of its companion. Even when the stars are farther apart, the neutron stars may collect material from the stellar winds of the companions. As the gas is pulled from the normal star down to the surface of the neutron star, the gravitational energy of the neutron star heats the gas to millions of degrees. The hot gas gives off X-rays that mark for us the location of the otherwise invisible neutron star. X-ray pulsars derive their energy from the accretion of matter; the pulsars discovered by the radio astronomers are mostly single stars that are using up their energy of rotation and thus are gradually slowing down.
When the gravity of a collapsing star is too strong for even neutrons to resist, a black hole may be formed. A black hole is a point mass in space, surrounded by a literally black region in which the gravity is so strong that no matter, nor even light, can escape it. But, just as in the case of a neutron star, matter that falls toward the black hole is intensely heated, producing copious X-rays that can be detected with telescopes flown above the atmosphere.
A few of the brightest X-ray sources in our galaxy are probably black holes orbiting closely with relatively ordinary stars. The X-ray source called Cygnus X-l is a famous example. In 1971, astronomers learned that Cygnus X-l was associated with a visible star that also is a radio source. This discovery is an important example of how ground-based optical and radio telescopes work in consort with orbiting X-ray telescopes to solve the problems of Space Age astronomy. The identity of the stellar companion was confirmed when both the radio source and the X-ray source were observed to change dramatically and simultaneously in intensity. Observations of the spectrum of the visible star and its changes in velocity as it and its X-ray source companion followed their orbits led to an estimate of the mass of the X-ray source. This unseen star that does produce X-rays appears to have at least six times the mass of our Sun, much more than can possibly be supported by the resistance of neutrons. Comparing the deduced mass with the theoretical limits on the masses of neutron stars, we conclude that the unseen X-ray source in the Cygnus X-l binary star system must be a black hole. However, the proof necessarily is limited‹you can't see a black hole‹ and further studies of this and other cosmic X-ray sources are needed.