Multiple Stars

   

Most stars have companions. Two, three or more stars, born from the same gas cloud may orbit each other. Some giant stars are in contact as they orbit each other. The members of other multiple systems are so far apart, they may not belong to one system.

The more massive star of a pair will go through its life cycle more rapidly, and its death may be affected by its companion. In some binary systems, one star has aged until it has become a white dwarf. When the second star at last ages and swells, gas from its out layers can be sucked onto the surface of the white dwarf, which flares up as a bright , seemingly “new” star,  a nova, over 100,000 times brighter than the Sun. the white dwarf blows away the excess matter and subsides.

But if the mass of the white dwarf is above a certain limit, there can be a much more massive explosion a super nova but caused in a different way from those discussed before.

Supernovae

A supernova flares up until it is briefly brighter than the hundred billion fellow stars of its galaxy. The heavy atomic nuclei that it has synthesized at the end of its life are flung across space, mixing with the interstellar nebulae and forming raw materials for new planetary systems around new stars, whose formation may be triggered by the shock waves emanating from the supernova.

Remnants of supernova explosions can be seen as enormous rings and globe of gas. At the centre there may remain a fragment of the original star, with a mass similar to the Sun’s enormously compressed into a sphere perhaps 20Km across. Here electrons and atomic nuclei are crushed together to form a ball of neutrons (the uncharged particles found in the heart of all atoms). One teaspoon full of this neutron star, or pulsar, has a mass of 100 million tones. Beams of radiation jet from its poles generated by gas falling on to it and focused by the neutron star’s stars intense magnetic field. The star spins a few tens or hundreds of times a second, and if one of the radiation beams sweeps across the direction of the Earth, we observe it as pulsating radio source or pulsar.

When a star of around ten solar masses a supergiant dies, the remnant it leaves may be even denser than a neutron star, and becomes a black hole. The gravitation of such a body is so intense that it begins a collapse that continues for ever towards a geometric point ; the more it shrinks the stronger its gravity becomes Neither radiation nor matter can escape from within a sphere only a few kilometers across, the boundary of which is called the even horizon. Such matter has effectively left the universe.

But paradoxically black holes can be at the heart of intensely bright objects. Gas falling into a black hole- perhaps from a companion star- releases enormous amount of energy, much of it at very short X ray wave length.

Types of Stars

The great majority of stars, including the Sun, are “dwarfs”. Only minority that is approaching death have swollen to become ‘giants’ or ‘super giants’. Stars vary in color according to their temperature, from the cooler red stars to the hottest blue or white stars. By comparing magnitude and color it is possible to classify stars.

If the core of the star forming gas cloud is less than a twelfth as massive as the Sun, nuclear reactions can never begin, and the object glows dimly as a reddish ball of gas, a so called “brown dwarf”. The search is on for brown dwarfs.

Star only slightly less massive than the Sun reddish, with surface temperatures of a few thousand degree C. they burn so slowly that they will live for hundreds of billions of years before fading away.

Stars of mass similar to the sun are called ‘yellow dwarfs’ because they give out most of their radiations as yellow light. The surface temperature of such star is similar to that of the Sun, about 6000 degree C, and its life time as a normal hydrogen burning star is about ten billion years. After this it will briefly extend its life by ‘burning’  helium into heavier nuclei in the core, swelling to hundreds of times its original diameter as red giant, with a cool surface,  but still intensely bright because of its huge size. As the helium runs low, a red giant grows unstable, and becomes a variable star, swelling and shrinking and becoming brighter and fainter as it does so. It puffs off shells of gas and finally a collapsed core is left behind as an extremely dense white dwarf, with the mass of a star squeezed into the volume of the Earth. Though hot, it is small and faint, and it continues to cool and fade.

 The most massive stars are about a hundred times as massive as the Sun. The most massive stars are about a hundred times as massive as the Sun. They burn fuel so quickly that they are blue hot, with surfaces over 25,000 degree C, and diameters ten times greater than the 1,390,000 Km of the Sun. they use up their fuel in a few millions years, rather than ten billion. Then they swell into giant or supergiant stars such as Antares. In their intensely hot, dense cores, they burn first helium and then heavier nuclei. They built a range of nuclei. In the last seconds of the star’s life it builds nuclei as heavy as those of iron. These cannot be burned, and the star, its internal power supply cut off, collapses and then explodes as a supernova.

Stars and Nebulae

Great clouds of gas and dust, called nebulae, extend across space. Bight clouds are limited regions stimulated to shine by the light of nearby hot stars. An example is the faint patch of light just discernible in the sword of the constellation Orion. In reality this is a nebula about 15 light years across, the birthplace of thousands of new stars.

More extensive are the dark nebulae. The bulk of these is transparent gas, but the dust in them blocks the light from stars beyond. The dark “lanes” that we seen in the Milky Way are actually dust clouds blocking our view of the piled up banks of stars lying beyond and preventing us from seeing to the center of the galaxy, 30,000 light years away.

 Gas in the galaxy is pretty much the same as the primordial matter that emerged from the Big Bang, and from which the galaxy formed perhaps 12 billion years ago. Three quarters of its mass consists of hydrogen, nearly all the rest of helium, but some of the gas and all of the dust consists of new elements formed since then in stars. The hydrogen, though dark, can be mapped by ultra high frequency radio emissions.