I couldn't pursue my hobby much during those days, as having my own telescope was not feasible and not much information was available at hand. So some time later this hobby took a back seat. But now a days with the advent of the Internet, the entire universe is at your fingertips. Since past few months, the once dormant astronomy bug, is back in action. I have been taking time-off for reading scores of material available on the net, especially the web sites of the observatories, my favorite being APOD[Astronomy picture of the day] which is being hosted by the Goddard Space Flight Center, NASA.
For those who don't know what a Nebula is, after millions to billions of years, depending on their initial masses, stars run out of their main fuel - hydrogen. Once the ready supply of hydrogen in the core is gone, nuclear processes occurring there cease. Without the outward pressure generated from these reactions to counteract the force of gravity, the outer layers of the star begin to collapse inward toward the core. When the material contracts, the temperature and pressure increase. This newly generated heat temporarily counteracts the force of gravity, and the outer layers of the star are now pushed outward. The star expands to larger than it ever was during its lifetime, a few to about a hundred times bigger. The star has now become a red giant. What happens next in the life of a star depends on its initial mass. Whether it was a “massive” star (some 5 or more times the mass of our Sun) or whether it was a “low or
medium mass” star (about 0.4 to 3.4 times the mass of our Sun), the next steps after the red
giant phase are very, very different.
Once a medium size star(such as our Sun) has reached the red giant phase, its outer layers continue to expand, the core contracts inward, and helium atoms in the core fuse together to form carbon. This fusion releases energy and the star gets a temporary reprieve. The atomic structure of carbon being too strong to be further compressed by the mass of the surrounding material. The core is thus stabilized.
The star will now begin to shed its outer layers as a diffuse cloud called a planetary nebula.
Eventually, only about 20% of the star’s initial mass remains and the star spends the rest of its days cooling and shrinking until it is only a few thousand miles in diameter. It has now become a white dwarf. White dwarfs are stable because the inward pull of gravity is balanced by the electrons in the core of the star repulsing each other. With no fuel left to burn, the hot star radiates its remaining heat into the coldness of space for many billions of years. In the end, it will just sit in space as a cold dark mass sometimes referred to as a black dwarf.
Featured above is a picture of the Helix Nebula[one of my favorites] from a galaxy about 650 light years away. This probably our sun would undergo a few million years from now. Spectacular isn't it. The irony of a nebula is, though it being the death of a star, its very fascinating.
