Rebirth Of A Star
In the backdrop of the recent discovery of white dwarf - G-29-38, S Ananthanarayanan takes a closer
look at the death of a star and formation of white dwarfs.
A visit to the neighbourhood of a dead star could give scientists an idea of what may happen, in the end, to planets. Scientists observing a white dwarf in the constellation Pisces saw a cloud of dust, which could be the remains of a comet, or a planet, in orbit around the star. Was ‘dust thou art, to dust returnest’ said of planets too?
Fate of stars
The life story of stars is that they go through cycles of compression and
expansion, due respectively to gravitation and nuclear reactions. When a star forms with the coming together, (through mutual gravitational attraction) of gas, mainly
hydrogen, the star heats up. When it gets hot enough, the hydrogen nuclei coalesce into helium nuclei, which is what happens in a hydrogen bomb, and huge amount of energy is given off.
This energy overcomes the gravitational collapse and the star explodes and expands, for millions of years. When the star expands it cools and ends up as ‘red giant’ - red because it is cool and a giant because it has expanded.
When the expansion stops, gravity takes over again and the star compresses, for more hydrogen to become helium, and later, for other elements to form. When new elements form, the heat causes expansion, which is followed by compression, followed by expansion and so on.
The White dwarf
Finally, when all the nuclear fuel is used up, there cannot be another expansion phase and the star settles down as a compressed, ie small and intensely hot object - a white (because it is hot) dwarf (because it is small). Stars that have more than a certain critical mass get so dense that the force of gravity at their surface bends space itself in such a way that no light can escape from the object, which has become a black hole. But stars with less mass end up as white dwarves.
William Reach of the Spitzer Science Center and Marc Kuchner of NASA/Goddard Space Flight Center used the Spitzer infra-red space telescope to look for matter in orbit around the white dwarf, G-29-38, located in Pisces. What they found was a cloud of dust, rich in silicates, the material of ordinary sand, surrounding the dying star.
Planets and comets
When a star goes through the cycle of expansion and compression, many planets get scattered into interstellar space. These fall into new orbits, generally with no relation to the original ones, and the massive tidal forces of the star in its throes generally shreds the planets to smithereens. This happens because the part of the planet nearer the star is attracted much more strongly than the part further away, causing internal stresses.
Instances of comets, which are bundles of dirt and ice, crumbling when they get too close to the sun are regularly observed. A dramatic display in recent times was in 1994, when the comet Shoemaker Levy 9 came too close to Jupiter. The icy comet crumbled into more than a dozen fragments, all of which finally crashed into Jupiter.
The dust cloud around the white dwarf G-29-38 is believed to have come about in the same way, with the disintegration of a comet. The dust is observable in infra red because of the light of the white dwarf that the dust particles absorb and then radiates as heat. But planets and comets are too cold to be detected. As the detectable dust particles radiate energy, which is more than they absorb, the particles finally spiral into the mother star. But the cloud has lasted millions of years. So it looks like there is a large population of planets and comets continuously getting ground up to keep the dust cloud alive!