by Prasad Ganti
About three decades ago, when I came out of college I heard that all Stars are born and eventually die, it was a shocking revelation for me. Like it is for a child who learns that a human being or an animal can die. Innocence, based on the permanence of the Sun, got shattered. Recently I heard that even galaxies die. So what is next ? Will the Universe die sometime ? Let us look at what death means in each of these cases.
A star is formed when a huge mass of gas, mostly hydrogen, comes under the influence of gravity and gets squeezed into the shape of a ball. This raises the temperature and stokes the nuclear furnace whereby the hydrogen atoms fuse to form helium and release enormous amounts of energy. Once all the hydrogen turns into Helium, the star goes into death throes. There could be other nuclear reactions fusing helium to higher elements like Carbon, all the way up to Iron, before the star ultimately dies. Depending on the mass of the star, the death and the ultimate state takes different forms. Medium stars like Sun will blow out and end up as a compact white dwarf. While heavier stars will blow up as Supernovas and end up as a compact neutron star. There are many other subtle variations of the ultimate state possible, but this is the gist.
Stars and their planets were one of the earlier astronomical structures to be formed in the Universe. Later the solar systems accumulated to form higher structures like a galaxy. Then galaxies accumulated to form a local cluster of galaxies, and then a super cluster. Galaxies typically contain millions or billions of stars each. Star formation happens all the time in galaxies. And stars die too. The cycle of birth and death continues. Stars use hydrogen as a fuel and cook higher elements and at the end of their lives spew them back, which forms the raw material for the next generation of stars.
Gravity is the master sculptor in the Universe. The same force discovered by Sir Isaac Newton as responsible for the fall of an apple from a tree to the ground, is responsible for creation of stars and their planets, the galaxies, clusters of galaxies etc.
While stars are formed from gas and dust, they breathe the gas until the end of their lives. And star formation makes Galaxies active and lively. Gas feeds the star formation in any galaxy. If the gas is stripped away by either the external or internal forces, the star formation will stop. Once the star formation stops, the galaxy will be alive as long as the existing stars are alive. As these stars die, the galaxy reaches its death throes. One of the likely culprits is overcrowding. if a galaxy is in a busy group or cluster, its collection of gas from the surrounding environment might face disruption, commencing the strangulation process. The same gravity which leads to clustering and crowding in the first place, can also lead to overcrowding and strangulation.
Maybe the same concept extends to higher structures like local clusters and super clusters. Will the Universe be the next to die ? We will not be around to see our Sun die, or our Milky Way die. Certainly not for the Universe to die. We can only speculate as to what may happen. As per our current understanding, the Universe is expanding at an accelerating rate due to the dark energy. What does the death of the Universe mean ? Even if all the galaxies die, the vacuum of the space is supposed to possess some energy. Will this energy die down ? Will space as such cease to exist ? Or, will the dimensions, including time, simply vanish ?
Seems like human learning is more of a relentless journey towards a direction but without a destination.