by Isaac Asimov
The spectroscope was also applied to the light of the sun and the stars and soon turned up an amazing quantity of new information chemical and other wise. In 1862, the Swedish astronomer Anders Jonas Ångström identified hydrogen in the sun by the presence of spectral lines characteristic of that element.
Hydrogen can also be detected in the stars, although, by and large, the spectra of the stars vary among themselves because of differences in their chemical constitution (and other properties, too). In fact, stars can be classified according to the general nature of their spectral line pattern. Such a classification was first worked out by the Italian astronomer Pietro Angelo Secchi in 1867, on the basis of 4,000 spectra. By the 1890s, the American astronomer Edward Charles Pickering was studying stellar spectra by the tens of thousands, and the spectral classification could be made finer with the painstaking assistance of Annie J. Cannon and Antonia C. Maury.
Originally, the classification was by capital letters in alphabetical order, but as more was learned about the stars, it became necessary to alter that order to put the spectral classes into a logical arrangement. If the letters are arranged in order of stars of decreasing temperature, we have O, B, A, F, G, K, M, R, N, and S. Each classification can be further subdivided by numbers from 1 to 10. The sun is a star of intermediate temperature with a spectral class of G-0, while Alpha Centauri is G-2. The somewhat hotter Procyon is F-5, while the considerably hotter Sirius is A-0.
Just as the spectroscope could locate new elements on earth, so it could locate them in the heavens. In 1868, the French astronomer Pierre Jules César Janssen was observing a total eclipse of the sun in India and reported sighting a spectral line he could not identify with any produced by any known element. The English astronomer Sir Norman Lockyer, sure that the line represented a new element, named it helium, from the Greek word for “sun.” Not until nearly thirty years later was helium found on the earth.
The spectroscope eventually became a tool for measuring the radial velocity of stars, as we saw earlier in this chapter, and for exploring many other matters—the magnetic characteristics of a star, its temperature, whether the star is single or double, and so on.
Moreover, the spectral lines were a veritable encyclopedia of information about atomic structure, which, however, could not properly be utilized until after the 1890s, when the subatomic particles within the atom were first discovered. For instance, in 1885, the German physicist Johann Jakob Balmer showed that hydrogen produces a whole series of lines that are regularly spaced according to a rather simple formula. This was used, a generation later, to deduce an important picture of the structure of the hydrogen atom (see chapter 8). Lockyer himself showed that the spectral lines produced by a given element alter at high temperatures. This indicated some change in the atoms. Again, this was not appreciated until it was later found that an atom consists of smaller particles, some of which are driven off at high temperatures, altering the atomic structure and the nature of the lines the atom produced. (Such altered lines were sometimes mistaken for indications of new elements, but—alas—helium remained the only new element ever discovered in the heavens.)
PHOTOGRAPHY
When, in 1830, the French artist, Louis Jacques Maude Daguerre produced the first daguerreotypes and thus introduced photography, this, too, soon became an invaluable instrument for astronomy. Through the 1840s, various American astronomers photographed the moon; and one picture, by the American astronomer George Phillips Bond, was a sensation at the Great Exhibition of 1851 in London. They also photographed the sun. In 1860, Secchi made the first photograph of a total eclipse of the sun. By 1870, photographs of such eclipses had proved that the corona and prominences arc part of the sun and not of the moon.
Meanwhile, beginning in the 1850s, astronomers were also making pictures of the distant stars. By 1887, the Scottish astronomer David Gill was making stellar photography routine. Photography was well on its way to becoming more important than the human eye in observing the universe.
The technique of photography with telescopes steadily improved. A major stumbling block was the fact that a large telescope can cover only a very small field. If an attempt is made to enlarge the field, distortion creeps in at the edges. In 1930, the Russian-German optician Bernard Schmidt designed a method for introducing a correcting lens that would prevent such distortion. With such a lens, a wide swatch of sky can be photographed at one swoop and studied for interesting objects that can then be studied intensely by an ordinary telescope. Since such telescopes are almost invariably used for photographic work, they are called Schmidt cameras.
The largest Schmidt cameras now in use are a 53-inch instrument, first put to use in 1960 in Tautenberg, East Germany, and a 48-inch instrument used in conjunction with the 200-inch Hale telescope on Mount Palomar. The third largest is a 39-inch instrument put into use at an observatory in Soviet Armenia in 1961.
About 1800, William Herschel (the astronomer who first guessed the shape of our galaxy) performed a very simple but interesting experiment. In a beam of sunlight transmitted through a prism, he held a thermometer beyond the red end of the spectrum. The mercury climbed! Plainly some form of invisible radiation existed at wavelengths below the visible spectrum. The radiation Herschel had discovered became known as infrared—below the red; and, as we now know, fully 60 percent of the sun’s radiation is in the infrared.
In 1801, the German physicist Johann Wilhelm Ritter was exploring the other end of the spectrum. He found that silver nitrate, which breaks down to metallic silver and darkens when it is exposed to blue or violet light, would break down even more rapidly if it were placed beyond the point in the spectrum where violet fades out. Thus, Ritter discovered the “light” now called ultraviolet (“beyond the violet”). Between them, Herschel and Ritter had widened the time-honored spectrum and crossed into new realms of radiation.
These new realms bear promise of yielding much information. The ultraviolet portion of the solar spectrum, invisible to the eye, shows up in nice detail by way of photography. In fact, if a quartz prism is used (quartz transmits ultraviolet light, whereas ordinary glass absorbs most of it), quite a complicated ultraviolet spectrum can be recorded, as was first demonstrated in 1852 by the British physicist George Gabriel Stokes. Unforturiately, the atmosphere trans mits only the near ultraviolet—that part with wavelength almost as long as violet light. The far ultraviolet, with its particularly short wavelengths, is absorbed in the upper atmosphere.
RADIO ASTRONOMY
In 1860, the Scottish physicist James Clerk Maxwell worked out a theory that predicted a whole family of radiation associated with electric and magnetic phenomena (electromagnetic radiation)—a family of which ordinary light was only one small portion. The first definite evidence bearing out his prediction came a quarter of a century later, seven years after Maxwell’s premature death through cancer. In 1887, the German physicist Heinrich Rudolf Hertz, generating an oscillating current from the spark of an induction coil, produced and detected radiation of extremely long wavelengths—much longer than those of ordinary infrared. These came to be called radio waves.
The wavelengths of visible light can be measured in micrometers (millionths of a meter). They range from 0.39 micrometers (extreme violet) to 0.78 micrometers (extreme red). Next come the near infrared (0.78 to 3 micrometers, the middle infrared (3 to 30 micrometers), and then the far infrared (30 to 1,000 micrometers). It is here that radio waves begin: the so-called microwaves run from 1,000 to 160,000 micrometers and long-wave radio goes as as many billions of micrometers.
Radiation can be characterized not only by wavelength but also by frequency, the number of waves of radiation produced in each second. This value is so high for visible light and the infrared that it is not commonly used in cases. For the radio waves, however, frequency reaches down into lower figures and comes into its own. One thousand waves per second is a kilocycle, while 1 million waves per second is a megacycle. The microwave region runs from 300,000 megacyc
les down to 1,000 megacycles. The much longer radio waves used in ordinary radio stations are down in the kilocycle range.
Within a decade after Hertz’s discovery, the other end of the spectrum opened up similarly. In 1895, the German physicist Wilhelm Konrad Roentgen accidentally discovered a mysterious radiation which he called X rays. Their wavelengths turned out to be shorter than ultraviolet. Later, gamma rays, associated with radioactivity, were shown by Rutherford to have wave lengths even smaller than those of X rays.
The short-wave half of the spectrum is now divided roughly as follows: the wavelengths from 0.39 down to 0.17 micrometers belong to the near ultraviolet; from 0.17 down to 0.01 micrometers, to the far ultraviolet; from 0.01 to 0.00001 micrometers, to X rays; and gamma rays range from this down to less than one billionth of a micrometer.
Newton’s original spectrum was thus expanded enormously. If we consider each doubling of wavelength as equivalent to 1 octave (as is the case in sound), the electromagnetic spectrum over the full range studied amounts to almost 60 octaves. Visible light occupies just 1 octave near the center of the spectrum.
With a wider spectrum, of course, we can get a fuller view of the stars. We know, for instance, that sunshine is rich in ultraviolet and in infrared. Our atmosphere cuts off most of these radiations; but in 1931, quite by accident, a radio window to the universe was discovered.
Karl Jansky, a young radio engineer at the Bell Telephone Laboratories, was studying the static that always accompanies radio reception. He came across a very faint, very steady noise which could not be coming from any of the usual sources. He finally decided that the static was caused by radio waves from outer space.
At first, the radio signals from space seemed strongest in the direction of the sun; but day by day, the direction of strongest reception slowly drifted away from the sun and made a circuit of the sky. By 1933, Jansky decided the radio waves were coming from the Milky Way and, in particular, from the direction of Sagittarius, toward the center of the galaxy.
Thus was born radio astronomy. Astronomers did not take to it immediately, for it had serious drawbacks. It gave no neat pictures—only wiggles on a chart which were not easy to interpret. More important, radio waves are much too long to resolve a source as small as a star. The radio signals from space had wavelengths hundreds of thousands, and even millions, of times the wavelength of light, and no ordinary radio receiver could give anything more than a general idea of the direction they were coming from. A radio telescope would have to have a receiving “dish” a million times as wide as the mirror of an optical telescope to produce as sharp a picture of the sky. For a radio dish to be the equivalent of the 200-inch telescope it would have to 1” 3,150 miles across and have twice the area of the United States—manifestly impossible.
These difficulties obscured the importance of the new discovery, but a young radio ham named Grote Reber carried on, for no reason other than personal curiosity. Through 1937 he spent time and money building in his backyard a small radio telescope with a parabolic dish about 30 feet in diameter to receive and concentrate the radio waves. Beginning in 1938, he found a number of sources of radio waves other than the one in Sagittarius—one in the constellation Cygnus, for instance, and another in Cassiopeia. (Such sources of radiation were at first called radio stars, whether the sources were actually stars or not, but are now usually called radio sources.)
During the Second World War, while British scientists were developing radar, they discovered that the sun was interfering by sending out signals in the microwave region. This aroused their interest in radio astronomy, and after the war the British pursued their tuning-in on the sun. In 1950, they found that many of the sun’s radio signals were associated with sunspots. (Jansky had conducted his experiments during a period of minimal sunspot activity, which is why he detected the galactic radiation rather than that of the sun.)
What is more, since radar technology made use of the same wavelengths as radio astronomy did, by the end of the Second World War, astronomers had available a large array of instruments adapted to the manipulation of microwaves which did not exist before the war. These were rapidly improved, and interest in radio astronomy leap-frogged.
The British pioneered in building large antennae to sharpen reception and pinpoint radio stars. Their 250-foot dish at Jodrell Bank in England, built under the supervision of Sir Bernard Lovell, was the first really large radio telescope.
Ways to sharpen reception were found. It was not necessary to build impossibly huge radio telescopes to get high resolution. Instead, one might build a sizable radio telescope in one place and another one a long distance away. If both dishes are timed by superaccurate atomic clocks and are made to move in unison by clever computerization, the two together can give results similar to those produced by a single large dish of the combined width, over the distance of separation. Such combinations of dishes are said to be long baseline and even very long baseline radio telescopes. Australian astronomers, with a large, relatively empty land at their disposal, pioneered this advance; . and, by now, cooperating dishes in California and Australia have produced a baseline of 6,600 miles.
Hence, radio telescopes are not fuzz producers far behind the sharp-eyed optical telescopes. Radio telescopes can actually make out more detail than optical telescopes can. To be sure, such very long baseline radio telescopes have gone about as far as they can on the earth’s surface, but astronomers are dreaming of radio telescopes in space cooperating with one another and with dishes on the earth to make still longer baselines.
Nevertheless, long before radio telescopes advanced to present levels, they Were making important discoveries. In 1947 the Australian astronomer John Bolton narrowed down the third strongest radio source in the sky, which proved to be none other than the Crab Nebula. Of the radio sources detected here and there in the sky, this was the first to be pinned down to an actual visible object. It seemed unlikely that a star was giving rise to such intense radiation, since other stars did not. The source was much more likely to be the cloud of expanding gas in the nebula.
This discovery strengthened other evidence that cosmic radio signals arise primarily from turbulent gas. The turbulent gas of the outer atmosphere of the sun gives rise to radio waves, so that what is called the radio sun is much larger than the visible sun. Then, too, Jupiter, Saturn, and Venus, each with a turbulent atmosphere, have been found to be radio emitters.
Jansky, who started it all, was largely unappreciated in his lifetime and died in 1950 at the age of 44, just as radio astronomy was hitting its stride. He received posthumous recognition in that the strength of radio emission is now measured in janskies.
LOOKING BEYOND OUR GALAXY
Radio astronomy probed far out into space. Within our galaxy, there is a strong radio source (the strongest outside the solar system), which is called Cass because it is located in Cassiopeia. Walter Baade and Rudolph Minkowski at Palomar trained the 200-inch telescope on the spot where this source was pinpointed by British radio telescopes, and found streaks of turbulent gas. It is possible that these may be remnants of the supernova of 1572, which Tycho observed in Cassiopeia.
A still more distant discovery was made in 1951. The second strongest radio source lies in the constellation Cygnus. Reber first reported it in 1944. As radio telescopes later narrowed down its location, it began to appear that this radio source was outside our galaxy—the first to be pinpointed beyond the Milky Way. Then, in 1951, Baade, studying the indicated portion of the sky with the 200-inch telescope, found an odd galaxy in the center of the field. It had a double center and seemed to be distorted. Baade at once suspected that this odd, distorted, double-centered galaxy was not one galaxy but two, joined broadside—to like a pair of clashing cymbals. Baade thought they were two colliding galaxies—a possibility he had already discussed with other astronomers. The evidence seemed to support the view; and for a while, colliding galaxies were accepted as fact. Since most galaxies exist in rather compact clusters
in which they move like bees in a swarm, there seemed nothing unlikely about such collisions.
The radio source in Cygnus was adjusted to be about 260 million light-years away, yet the radio signals were stronger than those of the Crab Nebula in our own stellar neighborhood. This was the first indication that radio telescopes would be able to penetrate greater distances than optical telescopes could. Even the 250-foot Jodrell Bank radio telescope, tiny by present standards, could outrange the 200-inch optical telescope.
And yet as the number of radio sources found among the distant galaxies increased and passed the hundred mark, astronomers grew uneasy. Surely they could not all be brought about by colliding galaxies. That would be overdoing a good thing.
In fact, the whole notion of galactic collisions in the sky grew shaky. The Soviet astrophysicist Victor Amazaspovich Ambartsumian advanced theoretical reasons in 1955 for supposing that radio galaxies were exploding rather than colliding.
This possibility has been greatly strengthened by the discovery, in 1963, that the galaxy M-82, in the constellation of Ursa Major (a strong radio source about 10 million light-years away), is such an exploding galaxy.
