Space - The Final Frontier

Photographs are from Astronomy Picture of the Day - http://antwrp.gsfc.nasa.gov/apod/archivepix.html

Text is in part from History of the Universe, a fabulous site - http://cgi.www.historyoftheuniverse.com/

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Universe - The young universe was mostly hydrogen gas and some helium gas.  It grew bigger and cooler.  Atoms began to feel force pulling them towards each other due to gravity.  

Clusters - About 2 billion years after the universe formed the force of gravity made the gases
break up into clouds, billions of them.   Each cloud of gas shrank under gravity and large empty spaces opened up between them.  These clouds have become clusters of galaxies today. They measure tens of millions of light years across. 

Galaxy - A galaxy is an island of billions of stars, separated from other galaxies by a vast ocean of almost empty space.  We are in the Milky Way galaxy.  But we should not forget that, scattered far and wide across the Universe, there are billions of other galaxies, probably very similar to ours. 

Galaxies are either spiral (about 70% of galaxies - similar to the Milky Way) or elliptical (about 30%). A few are other shapes. It is not clear how the different shapes arose. Spirals are probably more interesting than ellipticals, since stars are
formed continuously in them. It is probably this which has allowed life to form in the spiral galaxy where we live. 

The squashing of the gas and dust as it enters a spiral arm of the Galaxy makes new stars form all the time. The gas collects in cold dark clouds and stars form inside it, often in groups of several tens of stars. It is unfortunate that star birth happens in these cloudy cocoons, because they make it difficult for us to watch what is happening.  We think it is gravity which created the spiral pattern, although the details are not clear. The spiral arms are places where the gas is squashed together. 

Globular star clusters - Galaxies shrink under the force of gravity. The gas breaks up into hundreds of smaller clouds. Gravity made each of these little clouds shrink even more. Stars began to form inside them. Each cloud formed millions of stars.  With a good telescope we can still see these large ancient globular star clusters today, dotted around the outside of the Galaxy where we live. 

Nebula - Stars surrounded by cocoons of gas blown off in the late stages of stellar evolution; a dying star throwing off shells of glowing gas.

Sun - A star (such as the Sun) is a ball of gas which is a nuclear fusion reactor.  Hydrogen is being burned to Helium.  The Sun is the source of almost all the energy used by living things, including humans. We could not survive without it.  If we could copy the Sun in a small and controlled way, we believe we could obtain a great deal of energy on Earth without creating a lot of pollution.  Stars are the places where large atoms are built. Past generations of stars formed the gas and dust from which the planets and life were made.  Our Sun is a second generation star.  Note that most stars have companion stars and are called binary stars.  Here are two close-ups of our sun: Sunburst and Sunflare

Red giant - After a few billion years the center of a star runs out of hydrogen nuclei . What is left is a core of helium nuclei. The outer layers of the star still contain hydrogen, but they are not hot enough to fuse.  Because it has run out of fuel, the star begins to cool, and contract. The outer layers of the star fall inwards under gravity, and as they fall in, they heat up. A shell surrounding the central core becomes hot enough to fuse protons into helium. So the star gains a new source of energy. The core of the star is now hotter than it was during its normal life and this heat causes the outer parts of the star to swell. The star becomes a giant. The radiation from the fusing shell has grown weak by the time it reaches the surface of the star. Weak radiation is red, so the star becomes a red giant. 

Meanwhile inside the shell, the core of the star shrinks and heats up enough to fuse the helium nuclei together into even heavier ones. Among the commonest nuclei are carbon, nitrogen and oxygen. Heavier and heavier nuclei are created inside a red giant, the heaviest nearest the middle. At its center are iron nuclei. 

These fusions release only a little more energy, so they keep the red giant burning for a little longer. But they do not produce as much energy as the fusion of protons (hydrogen). Iron nuclei cannot be used as fuel because they need to be given energy to make them fuse. So iron nuclei collect in the heart of a red giant star.  The Sun will run out of fuel and become a red giant in about 4 billion years. 

White dwarf - Some red giants die quietly. A white dwarf is created when a small red giant (with about as much mass as the Sun) runs out of fuel, cools and begin to contract under gravity. The contraction of the inner parts of the star releases heat which causes the outer parts to expand and gently get blown away. In this way the gas of old stars is released and can be formed into new stars.  The inner part of the star continues to contract until it reaches the size of the Earth. Then its atoms are crushed together so tightly that their electrons begin to overlap. Because two electrons cannot occupy the same space, they repel each other. This is called the exclusion principle. The electrons cannot be compressed further and so create a force called degeneracy pressure. This small squashed cinder of matter is called a white dwarf star, although it does not shine like a normal star. Note that if the red giant is more than 1.5 times the mass of the Sun it does not become a white dwarf. Instead it becomes a neutron star or a black hole. 

Nova - We have seen that most stars have a companion tied to it by gravity, forming a binary or multiple star. In most binaries one star is larger than the other. The large star evolves quicker than the small one, and becomes first a red giant and then a white dwarf.  When the other star eventually becomes a red giant, its outer parts reach out towards the white dwarf. Gas is pulled by gravity towards the white dwarf, forming an accretion disc around it. Here it gathers in a layer on the surface of the white dwarf until the temperature and pressure at the base of the layer become high enough for nuclear fusion to start. The energy builds up and the temperature rises until a nuclear explosion is triggered, producing the nova and an expanding shell of gas.  In any one galaxy, typically a few tens of novae occur every year. 

We have seen that nova and supernova are major ways in which the new, heavy nuclei made in red giant stars are sent out into the galaxy, ready to be incorporated into new stars and planets. If this re-cycling did not happen, planets and life could never have begun. 

Supernova - We have seen that a small red giant, up to 1.5 times the size of the Sun, turns into a white dwarf when it dies. Larger red giants, however, die in a more spectacular way. Once the nuclear fuel is exhausted in a red giant, the core starts to cool and the internal pressure falls, leading to contraction. In large red giants this is a sudden and catastrophic event so that the star collapses. As the outer layers of the star fall they gain heat. This triggers nuclear fusion in these outer layers and they explode in a spectacular explosion called a supernova, becoming for a few days brighter than a whole galaxy. 

With so much energy it is possible to fuse iron nuclei into even heavier ones such as uranium nuclei. As the star explodes it throws out the nuclei which it has made. On their way out they pick up electrons and become atoms. The helium, oxygen, carbon, nitrogen, iron, uranium and other heavy atoms made by the star are scattered back to dust in the disc of the galaxy. In this way the atoms made in one generation of stars are passed on to be used by the next.  So all the atoms in your body (except hydrogen) were made in a supernova 5 billion years or more ago.  What happens next depends on the size of the original star. 

Neutron stars and Pulsars - A supernova with a mass between about 1.5 and 3 times the mass of the Sun collapses under gravity until it consists almost entirely of neutrons. Nuclear forces prevent these squashing any closer. Neutron stars are only about 10 kilometers across and have a density of 1 hundred thousand billion (1014) times as dense as water.  Pulsars are rotating neutron stars. The rapid pulses of radiation they emit are generated by their very strong magnetic fields, which are characteristic of all neutron stars.  Note that a star smaller than this will collapse into a white dwarf, while a larger star will collapse into a black hole. 

Black Holes - A small region of space which contains so much matter and has such a strong gravitational field that nothing, not even light, can escape. The region therefore looks dark, hence the name. There are thought to be three kinds of black hole: 

Super massive black holes 
These lie at the center of quasars and certain active galactic nuclei that appear to be exploding. Millions or billions of stars together with gas, dust and perhaps planets can fall into the black hole and give off enormous amounts of energy. In 1994 the Hubble Space Telescope provided conclusive evidence for the existence of a super massive black hole at the center of the M87 galaxy. It has a mass equal to two to three billion stars but is only as big as the solar system. 

Stellar black holes 
These form at the end of the life of a red giant star more than three times as large as the Sun. Stars with a less mass evolve into white dwarfs or neutron stars. When the star has no more atoms to fuse it collapses into a singularity, perhaps the size of a full stop. This is the black hole. Any object or light photon passing within a few kilometers gets pulled in. This is called the event horizon and its radius is called the Schwarzschild radius. Inside the event horizon the velocity required to escape from exceeds the speed of light, so that not even rays of light can escape into space. 

Mini black holes 
These might have formed in the big bang. They were proposed by Stephen Hawking. Many tiny primordial black holes, each with a mass of an asteroid, might have been created. Protons and antiprotons may be created very near a mini black hole. One of them may fall into the black hole while the other escapes so appearing to come out of the black hole. This effectively removes energy from the mini black hole and it evaporates over time. 

Observation of black holes 
Being black, black holes can never be seen directly. Their existence can only be deduced from their gravitational effects and the radiation emitted by material falling into them.  A Super massive black hole's existence near the center of a galaxy can be deduced from its effects of its enormous gravitational fields on the stars and gas. The stars here are moving so fast that if there was not an enormously heavy small object there they would fly off.  Stellar black holes can be detected in binary stars such as Cygnus X-1. Here a blue super giant and a stellar black hole orbit each other. Gas is pulled off the super giant and falls into the black hole, heating and radiating X rays before entering the event horizon. Many galactic centers are also the source of X rays and matter, probably because of other matter falling into a super massive black hole there.  Mini black holes have long since evaporated, so cannot now be seen.