Milky Way

Background

The light year

Light travels at a speed of almost 300,000 kilometres per second. Consequently, it takes light only about 500 seconds to travel the 150 million kilometres between the Sun and the Earth. If a ray of light travelled for one year, it would cover a distance of about 9.5 million million kilometres. This distance is called a light year. Note that a light year is a measure of distance, and not of time.

The distances to the stars are so vast that measuring them in kilometres (or even millions of kilometres) is not practical. Even the nearest stars to our Sun, those of the Alpha Centauri system, are about 40 million million kilometres distant. But light would travel this distance in only four-and-a-quarter years. So astronomers say that the distance to the Alpha Centauri system is about four light years. Other well-known stars are very much further away. Betelgeuse, the red supergiant star in Orion, is about 590 light years from us. Deneb, the brightest star in Cygnus (the Swan or 'Northern Cross') is even more remote, at 1800 light years distant. The distances to other galaxies can also be measured in light years. One of the nearest is the Andromeda galaxy. Its distance is over two million light years. Most galaxies are very much further away than this. Some of the most distant objects known are called quasars. Many of these are over ten thousand million (ten billion) light years away.

The Milky Way galaxy

Our galaxy, the Milky Way, is the giant star system to which our Sun belongs. It contains about 150 thousand million (150 billion) stars, arranged in the shape of a flattened disc. The disc is thin at the edges, but has a bulge at the center (the nucleus). The galaxy has a spiral structure. Our Sun is nowhere near the galactic centre. It lies close to the edge of one of the spiral arms, about two-thirds of the way out from the centre towards the edge. Because of this, when we look up into the night sky, the stars of the Milky Way are not scattered evenly across the heavens. If we look out in the direction of the flattened disc (the galactic plane), we see a fairly narrow, luminous band stretching across the sky. This is where the stars appear to be very crowded together. The galactic centre lies in the direction of the constellation Sagittarius. For observers in Europe and the United States, the band of the Milky Way passes through Aquila, Cygnus, Cassiopeia, Perseus, Auriga and parts of Gemini and Orion.

The diameter of the Milky Way is about 100,000 light years, and it's just over 30,000 light years from our Sun to the galactic centre. The maximum thickness of the Milky Way (across the central bulge) is about 20,000 light years. The galactic halo contains fewer stars than the main disc of the galaxy, and its full extent is uncertain. All the stars in the halo are old. They are believed to have formed early in the life of the Milky Way, maybe 12,000 million years ago. Most of the stars in the disc, and probably also in the nucleus, are of intermediate age. They are probably mostly between 3000 and 5000 million years old. The very youngest stars are confined to a layer about 1500 light years thick along the disc's central plane.

The spiral arms of the Milky Way contain many of the brightest stars. The arms wind outwards from the nucleus, close to the central plane of the disc. The Sun is situated only a few light years north of the galactic plane, near the inner edge of one of the spiral arms. The youngest stars in the galaxy, and the short-lived, hot, bright blue-white stars, are mainly found in the spiral arms. Three sections of the spiral arms lying close to the Sun have been traced out. These are the Orion arm, in which the Sun is situated; the Sagittarius arm, which lies about 6500 light years nearer to the galactic center; and the Perseus arm, which lies about 6500 light years further out.

About one-tenth of the mass of the galaxy appears to be made up of gas and dust, in the space between the stars. Huge quantities have been detected, lying in or close to the galactic plane. Large clouds of gas and dust are also found in the spiral arms. Young stars are still being formed from these clouds. The gas and dust also obscure many objects from view. We cannot see the centre of the galaxy directly, because it lies beyond the star-clouds in Sagittarius, and there is too much obscuring material in the way. The number of stars in a given volume of space increases rapidly towards the centre of the galaxy, and a powerful source of radio waves called Sagittarius A appears to lie right at the galactic centre.

The entire Milky Way is rotating about an axis through its centre. The disc rotates fairly rapidly, but the halo more slowly. At the Sun's distance from the centre, the disc is rotating at about 250 kilometres per second. The Sun takes around 225 million years to complete one revolution: this period is known as one cosmic year. The halo system is rotating with an average speed of only about 50 kilometres per second.

The birth of stars

Stars are huge balls of gas, held together by the force of gravity. Stars differ from planets not only in size; they generate energy in their cores by nuclear fusion reactions, which begin in all stars with the conversion of hydrogen into helium. This process releases energy which makes its way slowly to the star's surface before pouring out into space. Planets have no light of their own, but shine only by reflected starlight.

Stars are formed when clouds of interstellar dust and gas collapse. As more material falls onto a developing protostar, it becomes hotter. Eventually, when the temperature in the core of the star exceeds a critical value of about 7 million degrees, nuclear fusion reactions begin. Stars are, in fact, a temporary halting point in the process of collapse: they introduce a balancing force, the outward flow of radiation (radiation pressure) generated within. The source of that radiation is nuclear fusion, the combination of hydrogen atoms to make helium. The temperature rises within a collapsing cloud because the energy of infalling atoms is liberated as heat as they collide, and it rises highest in the core of the star. The fusion of hydrogen to make helium occurs fastest in the hottest regions, so this process is concentrated in the core. The energy released has to force its way out through the gas, and at every point within the star the gravitational pull of the parts within exactly balances the outward thrust of the radiation pressure.

By the time a protostar becomes optically visible, it has already begun to settle down within, and is called a pre-main-sequence star. Thereafter its temperature will not change, but it will become less luminous. The time taken for the birth process depends on a protostar's mass. A star 10 times the mass of our Sun will only take 300,000 years, while a star with a mass similar to that of our Sun would take 30 million years. A star's mass will influence the course of its life. For a star to shine at all, it requires a mass of at least one-fifteenth that of our Sun - about 80 times the mass of the largest planet, Jupiter. However, there are stars much larger and more massive than the Sun, the largest being around 100 solar masses. A star's mass will also determine its temperature and luminosity during its life.

The development and death of stars

Once the nuclear fusion process has begun, stars have temperatures and luminosities prescribed by their masses (with only a slight variation depending on their chemical composition), and these define the Main Sequence. Massive stars are very luminous and hot, and therefore blue. Stars in the middle range are less hot and are yellow (the Sun being a good example), while the least massive stars are the coolest and glow a dull red. A star lives most of its life on the Main Sequence, and again, the mass of a star is the main factor governing how long it will spend there. The Sun's Main Sequence lifespan is 10 billion years, of which roughly half has already elapsed. For a star like Sirius, which is twice as massive as the Sun and 20 times as luminous, the lifespan is one billion years. For a star of 30 solar masses with a luminosity 100,000 times that of the Sun, the lifespan drops dramatically to a mere one million years. Thus massive stars have much shorter lifetimes than the Sun, despite having more fuel to start with, because they consume their fuel so much faster.

As stars grow old, they begin to change dramatically. As the available hydrogen fuel is consumed, a core of hot but inert helium grows within, and the hydrogen fusion proceeds on the outside of this core. The core becomes hotter, reaching a point where helium too can participate in nuclear fusion, producing carbon and oxygen. The change in the central regions is sudden, and is called the 'helium flash'. The core shrinks to an enormous density, and the outer parts expand. The luminosity rises somewhat (more for smaller stars than for larger ones) and the temperature falls rapidly. The star becomes a giant, perhaps 100 times its original diameter, and its outer layers cool so that it becomes orange-red or red in colour.

The giant star changes slowly, generally growing brighter and cooler; further fusion reactions within cause additional fluctuations. At length it becomes a variable star, prone to oscillate in size and brightness for some time. At this stage the internal chemistry can change, or gas of a different chemical composition deep within can rise to the surface by convection. The star may then appear as a carbon star, which has strong absorption bands of titanium oxide and zirconium oxide and large numbers of metallic lines. Eventually the giant sheds its outer layers, forming a 'bubble' around the dying star, called a planetary nebula, and revealing the hot white dwarf core of extreme density. It is believed that white dwarfs go on cooling indefinitely, becoming black dwarfs - mere cinders of former stars.

The above description of the later life of a star applies only to those less than a few solar masses. If the mass of the core exceeds 1.4 solar masses (the Chandrasekhar limit) the star's fate is different. Massive stars exhaust their hydrogen quickly, and in progressing towards the red giant stage they may be caught up in a more catastrophic internal convulsion. If they do make it to the red giant stage, they will be even larger and more luminous, and are therefore called supergiants. Red supergiants are so luminous that we can easily see them right across the Milky Way and even in other galaxies.

In a very massive star, the core is compressed and heated far enough to ignite carbon, which allows it to keep burning when its helium is used up. A succession of nuclear reactions takes place that creates heavier and heavier elements, and at the same time supplies energy for the star. Oxygen, neon, magnesium and eventually silicon are formed, but because higher and higher temperatures are needed for each successive stage, each new shell is confined to a smaller and hotter region around the star's core. Eventually, the star begins to burn silicon into iron. The formation of an iron core signals the end of the massive star's life. Nuclear fusion stops with iron, and a star with an iron core is out of fuel. The star is blown apart in a spectacular and catastrophic supernova explosion.

Stable stars maintain a delicate balance between their internal radiation pressure and their own force of gravity. The disruption of this balance is the underlying cause of a supernova. As the fusion reactions cease, the internal radiation pressure disappears, and the star's interior begins to collapse. The effect of this implosion is to crush the core, forcing the protons and electrons together to produce a ball of neutrons. The plunging outer layers of the star strike the neutron core, crushing it still more, and the infalling gas is heated to billions of degrees. The pressure surges and blasts the outer layers away from the star in a mighty supernova explosion. The explosive events do not take place exactly at the centre of the star, but on the exterior of the dense core. The explosion throws off several solar masses of gas at speeds of well over 10,000 kilometres per second, and crushes the core with unimaginable force. At the very least, a superdense neutron star about 10 kilometres in diameter is formed, and the star may sometimes be compressed still further to become a black hole.