MILKY WAY AND THE UNIVERSE Our Sun and all the stars visible in the night sky are members of a vast aggregation of stars known as the Galaxy (given a capital G to distinguish it from any other galaxy). Our Galaxy is spiral in shape, with arms composed of stars and nebulae winding outwards from a central bulge of stars. It is about 100,000 light years in diameter; the Sun lies in a spiral arm 30,000 light years from the Galaxy's centre. Astronomers estimate that the Galaxy contains some 250,000 million stars. Most of the stars in the Galaxy lie in a disk about 2000 light years thick. Seen from our position within the Galaxy, this disk of stars appears as a faint, hazy band crossing the sky on clear, dark nights. We call this band the Milky Way, and the name Milky Way is often used for our entire Galaxy. The starfields of the Milky Way are particularly dense in the region of Sagittarius, which is the direction of the Galaxy's centre. Note that the plane of the Milky Way is tilted at 63° with respect to the celestial equator. This results from a combination of the tilt of the Earth's axis, and the fact that the plane of the Earth's orbit is tilted with respect to the plane of the Galaxy. Our Galaxy has two small companion galaxies called the Magellanic Clouds. To the naked eye they appear like detached portions of the Milky Way, in the southern constellations Dorado and Tucana. The Large Magellanic Cloud contains about a tenth the number of stars in our Galaxy and lies about 170,000 light years from us. The Small Magellanic Cloud has only about a fifth as many stars as the Large Magellanic Cloud, and lies somewhat farther off, about 200,000 light years. Both clouds contain numerous star clusters and bright nebulae, and are rich territories for sweeping with instruments of all sizes. One can but imagine the magnificent view of our Galaxy that any astronomers living in the Magellanic Clouds would have. Countless other galaxies are dotted like islands in the Universe as far as the largest telescopes can see. Most galaxies are members of clusters containing up to thousands of galaxies. Our own Milky Way is the second-largest member of a small cluster of some three dozen galaxies known as the Local Group. The largest galaxy in the Local Group is visible to the naked eye as a fuzzy, elongated patch in the constellation Andromeda. The Andromeda Galaxy is estimated to contain about twice as many stars as our own Galaxy, and to be about 25 per cent greater in diameter. It lies about 2.5 million light years away. Long-exposure photographs reveal that the Andromeda Galaxy is a spiral, but tilted so that we see it almost edge-on. If our own Galaxy were viewed from outside, it would look much like this. Amateur telescopes show that the Andromeda Galaxy has two small companion galaxies, its equivalent of the Magellanic Clouds. The only other member of the Local Group within easy reach of amateur instruments is M33 in Triangulum, another spiral galaxy, somewhat farther from us than the Andromeda Galaxy and containing considerably fewer stars. It can be picked up in binoculars under clear, dark skies. The nearest rich cluster of galaxies to the Local Group lies in the constellation of Virgo, part of it spilling over into neighbouring Coma Berenices. Of its 3000 or so known members, dozens are within the reach of amateur telescopes. GALAXY CLASSIFICATIONS Astronomers classify galaxies into three main types: elliptical, spiral, and barred spiral. Elliptical galaxies range in shape from virtually spherical, designated E0, to flattened lens shapes, designated E7. They include both the largest and the smallest galaxies in the Universe. Supergiant ellipticals composed of up to 10 million million stars are the most luminous galaxies known. An example is M87 in the Virgo Cluster. At the other end of the scale, dwarf ellipticals resemble large globular clusters. Dwarf ellipticals may be the most abundant type of galaxy in the Universe, but their faintness makes them difficult to see. Spiral galaxies (type S), such as the Andromeda Galaxy, have arms winding out from a central bulge. There are usually two arms, but sometimes more. In barred spirals (type SB) the arms emerge from the ends of a bar of stars that runs across the galaxy's centre. Spiral and barred spiral galaxies are subdivided according to how tightly their arms are wound: types Sa and SBa have the tightest-wound arms, while types Sc and SBc have the loosest-wound arms. M31 in Andromeda is type Sb. Until recently, our own Galaxy was thought to fall midway between Sb and Sc, but there is growing evidence that it might instead be a barred spiral. Most of the galaxies we see in the Universe are large, bright spirals but, as mentioned above, they may actually be outnumbered by dwarf galaxies that are too faint to detect over great distances. In addition to these three main types, there are certain galaxies classified as irregular; the Magellanic Clouds are usually reckoned to fall into this class, although there is some semblance of spiral structure in the Large Magellanic Cloud. Peculiar things are going on within some galaxies. For example, certain galaxies give off vast amounts of energy as radio waves; these radio galaxies include the supergiant ellipticals M87 in Virgo and NGC 5128 in Centaurus. Some spiral galaxies have unusually bright nuclei. These are termed Seyfert galaxies, after the American astronomer Carl Seyfert, who was the first to draw attention to them, in 1943. M77 in Cetus is the brightest Seyfert galaxy. Most peculiar of all are the objects known as quasars, which emit as much energy as hundreds of normal galaxies from an area less than a light year in diameter. Despite their exceptional nature, quasars are not at all exciting visually, which is why they were overlooked until 1963.The brightest quasar, 3C 273, appears as an unimposing 13-th magnitude star in Virgo. Observations with the Hubble Space Telescope have shown that quasars are actually the highly luminous centres of galaxies far off in the Universe. Radio galaxies, Seyferts, and quasars are probably all related; perhaps they are all young galaxies seen at different stages of their evolution. Their central powerhouse is thought to be a massive black hole that gobbles up stars and gas from the surrounding galaxy, and its activity can be boosted by infalling gas when two galaxies collide. THE BIG BANG In 1929 the American astronomer Edwin Hubble made the most significant of all discoveries in cosmology: the galaxies are moving apart from one another as though the Universe is expanding, like a balloon being inflated. (But clusters of galaxies such as the Local Group do not themselves expand - they are held together by their mutual gravitational attraction.) Hubble's discovery that the Universe is expanding came from a study of the spectrum of each galaxy's light. This revealed that the light from the galaxies was being lengthened in wavelength as a result of high-speed recession (this is called the Doppler effect). Such a lengthening of wavelength is called a redshift, because the light from the galaxy is moved towards the red (longer-wavelength) end of the spectrum. Incidentally, the redshift does not make galaxies actually look redder, because the blue end of the spectrum is filled in by light that was formerly at ultraviolet wavelengths. Hubble found that the amount of redshift in a galaxy's light is directly related to its distance, the most distant galaxies having the greatest redshifts. Therefore, by measuring the redshift of a galaxy astronomers can tell how far away it is. Quasars, for instance, exhibit such enormous redshifts that they must be the most distant objects visible in the Universe, over 10,000 million light years away. Since the Universe is expanding, it is logical to conclude that it was once smaller and more densely packed than it is now. According to the most widely accepted theory, the entire Universe was originally a compressed, superdense blob which, for some unknown reason, exploded in a cataclysm known as the Big Bang. The galaxies are the fragments from that explosion, still flying outwards. As far as anyone can tell at present, the Universe will continue to expand for ever. According to the best estimates the Big Bang took place around 13,000 million years ago; that is the age of the Universe as we know it. It is impossible to tell what, if anything, happened before the Big Bang.