Explained: The fascinating universe of stars

To all of us, stars are just dots in the sky, some bright, some faint. Some move in a great circle and never set, while some rise and set with fixed periodicity. Then there are some that seem to be wandering around. Very few of them, if observed carefully, seem to change their brightness well above the twinkling of stars. So how do astronomers claim all that they do?

Here's what astronomers gather from stars: a) their brightness b) their colour (light in different wavelengths – from radio to gamma rays) and c) variation in their brightness. With better telescopes, they can see smaller and smaller regions of the sky. They see nothing else. Everything else is inferred by logic, analysis of data and by applying our laboratory physics. 

Take for example the distance to stars. Just put the Sun in the middle and imagine observing the same star in January and July (six months apart when the Earth will be on opposite sides of the Sun) and you will notice that the stars that are close to us will appear to straggle a little with respect to the distant stars. This is not obvious as the fluctuations will be less than 1/3600 of a degree (1 second of arc) but a good telescope with photographic records can easily notice this. Beyond this, astronomers look at standard candles, objects whose intrinsic brightness you can calculate by other means - by taking a ratio of how bright they are, compared to how they appear to be, one can deduce their distance. How do you calculate the intrinsic brightness of stars? Well, some stars (called Cepheid stars) are unstable and they fluctuate in their brightness. These changes are related to how bright they really are. So knowing the rate of change of their brightness will tell you how intrinsically bright they are. Then there are other indicators, dying stars also explode with a more or less standard brightness and large stars are generally similar. So if you can find any of these, a Cepheid, an exploding star or even a large star, you can estimate the distances to very distant objects. 

Once you know the intrinsic brightness of a star, you can say a lot about the star. For example, the bigger the star, the more fuel it will burn to keep alive, so it will be hotter and not live a very long life. You remember that when you heat a coil, it begins by feeling warm before it begins to glow, and as it heats up, it turns red and then orange. If you could heat it more, it would become yellow (like our sun) and then go on to turn white and eventually become blue. A blue star can be as hot as 25,000 C compared to the Sun, which is barely 6000 C. Also, since blue stars don’t live long, a region with blue stars is a region where stars are being born while a region with more yellow stars means that the blue ones there have died and hence it is an old region where nothing much has happened in recent times.

Then we have a way of measuring the speed of different objects. This comes from the fact that stars are made up of atoms and atoms emit light at definite frequency. Like the sound of a train moving into or out of a station, the frequency of this light from these atoms changes slightly. Its frequency increases if the object is coming towards you or is blue shifted and decreases if the object is moving away from you, or is red shifted.

Stars also don’t live forever. They shine by burning nuclear fuel. Hydrogen is burnt to helium in the core of the stars and when they run out of hydrogen fuel, they begin to burn helium and carbon and silicon, each time producing a heavier element and in the process, producing heat that keeps the star stable. Once the iron burning stage is reached, they do not produce more heat and at that stage, the star has exhausted all its fuel. If a star is large (more than three times as big as the sun), it dies a spectacular death where the outer regions go out in a bang and the inner matter contracts into small ultra-compact objects. We call this event as a supernova. Nova means a new star, and an ultra-bright new star is a supernova. A close by supernova would be visible in the sky for a few weeks before it exhausts the energy of explosion and cools down into a large cloud.

But the universe is not just about stars. There is a lot of diffuse gas in the universe. This gas can be a residue of the time when the stars were born or can be residue of a dead star. The former will have gases that are leftover gases while the latter will contain a lot of material that was produced when the original star was alive and burning and producing heavier elements. These clouds can, over a period time, gather gas into dust, dust into rocks and rocks into planets or even stars. Such regions typically have lots of stars all within close proximity of each other, star cities if you wish, affecting the way they live and die. 

But we have found that stars are not randomly distributed all over the universe but come in bundles that we call galaxies. These galaxies typically have trillions of stars of all kind. These galaxies are very far from each other and harbour a gigantic dead star, so large that not even light will escape from these objects. They tend to be millions of times the size of our sun and have enough mass to hold a whole galaxy together. 

Here's one of the curiosities of the universe. It turns out that while galaxies can be in clusters bound by common gravity, these groups and individual galaxies seem to be moving away from each other at great speeds and the farther you see, the faster they move! This means that about 14 billion years ago, they were all at the same place at the same time. This is when a Big Bang occurred and all objects that are not bound to each other by gravity are moving away from each other. Everything in the universe was born during this Big Bang and the only thing that is happening since, is that energy goes from one form to another. Free matter gets compressed and heated and emits light and when the fuel is exhausted, the objects explode, at times leaving black holes from which nothing will escape. In this scenario therefore, in the end (a few hundred billion years from now) all material will either be in black holes or in very dilute gas with which nothing can be done. But black holes are not perfectly black and they too will eventually evaporate, giving back material to the universe. 

The other possibility is that like a projectile motion, may be these objects running away from each other may fall back on themselves forming a big crunch! While looking to see if the big crunch will happen or not, scientists made a startling discovery. Unlike in a projectile, the galaxies moving away from each other are not only not slowing down, but are being accelerated away from each other. We call this energy that is accelerating the breakup of the universe as dark energy (not to be confused with dark matter, which is ordinary matter – like the one in clouds that is not emitting any light). As far as we can tell, this dark energy will eventually rip the universe apart into nothingness. 

It is not the best thought to go to bed with, but then, the universe was not designed for us to be comfortable, it was designed with a certain amount of matter and some fundamental forces that were allowed to run free. It produced light, stars, galaxies on the largest scale, and life on a small scale seems to be a casual by product of this great show called the universe. We should consider ourselves lucky to be alive to see this greatest show in the universe!

Dr. Mayank Vahia is a scientist at TATA Institute of Fundamental Research in the department of Astronomy and Astrophysics

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