Life Expectancy of Stars
By Quamrul Haider
Although stars in the sky seem eternal and unchanging to us, their permanence is an illusion. They seem unchanging because the changes occur over a time span of millions and billions of years. Like us, stars are born. They grow old and at regular intervals, they die. Star birth has been going on continuously since our galaxy, the Milky Way, took shape over 10 billion years ago.
Most of the stars spend their life post-nursery stage through adulthood in a phase termed as the main sequence. Since they spend the longest period of their life in the main sequence, astronomers refer to their time in the main sequence as lifetime.
All main sequence stars are living off their savings, the hydrogen they formed in the stellar nursery. The enormous amount of energy required to keep them shining for millions and billions of years comes from thermonuclear reaction fusion of hydrogen into helium in their cores. The tremendous outward pressure from all the energy produced by nuclear fusion reactions keeps the star from collapsing under its own weight.
But a star has a limited supply of hydrogen. Hence, it remains a hydrogen-fusing main sequence star for only a finite time. When a star exhausts its hydrogen fuel, it enters the old age phase and begins to build nuclei heavier than helium for its sustenance.
A star signs its death warrant once it begins making nuclei as heavy as iron and nickel. The core no longer has enough pressure to hold up the overlying layers. Gravity, which has so far been a patient observer, wins over pressure causing the core to collapse. At this point, a star has reached the end of its life cycle and leaves the main sequence.
How long do stars live? The life expectancy of a star depends on its mass. A simple mass-luminosity relationship tells us that greater is a stars mass, shorter is its life expectancy. This may seem strange and counter intuitive, since a more massive star obviously starts out with more available fuel for thermonuclear reaction.
Stars do not use their fuel at the same rate. Massive stars are analogous to big, gas-guzzling cars. Consequently, they consume fuel at a prodigious rate and their gas tank becomes empty within a short period of time.
Our own Sun has been around for 4.55 billion years and will run out of fuel in another 5 billion years or so. Thus, the total lifespan of a one solar mass star is about 10 billion years. A 20 solar mass star will die in only about six million years. On a cosmic scale, a few million years is a very short time, which is one reason why at a given moment only a few massive stars are around. Their population, however, does not dwindle because in death, they reseed the cosmos for creating new generations. The ones that are still alive were probably formed relatively recently.
A heavyweight star in our galaxy that has been in the news recently is the stellar powder keg Eta Carinae. Being at least 100 times more massive than our Sun, its lifespan is about 100,000 years. In fact, it is quite likely that Eta Carinae may have already met its doom, but the light bearing the news of its cataclysmic death has not yet reached us.
Low-mass stars with mass less than the solar mass burn their fuel at a much lower rate and can, therefore, shine for billions of years. They are analogous to small, fuel economy cars. The lifetime of a 0.5 solar mass star, for example, is 57 billion years. Since the Universe is only 13.7 billion years old, no star with half the mass of the Sun has ever left the main sequence.
The smallest stars, the red dwarfs may be a bit dim, but like Aesops tortoise, they are destined to win the survival race by a long margin. The life expectancy of Proxima Centauri, a red dwarf that is nearest to the Sun with 12.3 percent of solar mass, is predicted to be close to two trillion years. The long lifetime of low-mass stars mean that they outnumber every other type of star in the Universe. Also, those that were formed shortly after the Big Bang are still in their infancy, waiting to move into the main sequence.
Low-mass stars die not with a bang, but with a whimper. They slowly shrink away as they consume their fuels, ejecting their outer layers creating planetary nebulae. Despite the name, these objects are not related to planets. They are beautiful shell of short-lived about 25,000 years, glowing diffuse gas around an aging star. It is estimated that there are about 10,000 nebulae in our galaxy. They finally become red giants after which they are buried in the stellar graveyard as white dwarfs.
The high-mass stars, the rarest but brightest members of the stellar community, die in a dramatic fashion. They end their lives in a blaze of glory by blowing themselves apart in a spectacular but violent supernova explosion. Their final fate is a rapidly spinning neutron star about 20 km in diameter or a black hole, which is a single massive point with no dimensions and infinite density.
The writer is a Professor of Physics at Fordham University, New York. n
Photos: Google Image
