Last updated on November 10th, 2020 at 02:55 pm
When you look at the dark night sky, it seems beautiful and mysterious. Living beneath this open dark sky, the earliest humans were aware of nightly changes as planets and stars marched across the sky. The earliest humans made some pictures by connecting certain groups of stars. Each of those groups is called a constellation and every constellation has a mythical story tagged along.
Our observable universe is divided among 88 constellations. Can you imagine how far from us are the stars that they contain? They are at a distance of some billions of light-years from us!
Formation of Stars
Stars are the basic building blocks of galaxies like our own Milky Way. These stars are born within clouds of dust and gas scattered throughout these galaxies. An example of such a dust cloud is the Orion nebula in the constellation of Orion (the Hunter). The gas and dust (mostly hydrogen gas) in the densest part of Nebula can begin to collapse under its own gravity. As the cloud collapses, the material at the centre begins to heat up and is known as a “protostar“. It is this hot core at the heart of the collapsing cloud that will one day become a star. However, not all of the material ends up as part of the star. The remaining dust can form a planet, asteroid, comet, or remain as mere dust in space. Any such star requires millions of years to mature from start to adulthood and it stays in this mature phase for a few billions of years.
Colours to stars
Have you seen something special in the night sky? There are different colours of stars depending on their temperature.
| Colour of Star | Temperature | Spectral Class |
| Blue | >28,000 Kelvin | O |
| Blue-White | 10,000 – 28,000 Kelvin | B |
| White | 7500 – 10,000 Kelvin | A |
| Yellow-White | 6000 – 7500 Kelvin | F |
| Yellow | 5000 – 6000 Kelvin | G |
| Orange | 3700 – 5000 Kelvin | K |
| Red | 2000 – 3700 Kelvin | M |
The youngest stars like Spica are blue in colour while our own Sun belongs to the spectral class G and is yellow in colour. Isn’t this amazing?
Evolution to face the end
Stars are fuelled by the nuclear fusion of hydrogen to form helium deep in the interiors. The outflow of energy from the central region of the star not only provides the pressure necessary to keep the star from collapsing under its own weight, but also the energy which allows it to shine. Larger the star, shorter will be its lifetime.
The most massive star lives for billions of years. When a star has fused all of the hydrogen in its core, nuclear reactions cease. The core begins to collapse into its own gravity and becomes much hotter. But some hydrogen is still available outside the core, so the process of fusion continues in the shell surrounding the core. Simultaneously, the hot core pushes the outer layer of the star outward for it to expand and cool, finally transforming the star into a red giant.
If any star is sufficiently massive, the collapsing core may become hot enough to support more exotic nuclear reactions that consume helium and produce a variety of heavier elements up to iron. Gradually the star’s internal nuclear fire becomes increasingly unstable, sometimes burning furiously, while at other times dying down. These variations cause the star to pulsate and throw off its outer layer, enshrouding itself in a cocoon of gas and dust, which we usually call a supernova.
What happens next depends on the size and mass of the core.
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Shivam graduated from the Department of Astronomy, Osmania University, Hyderabad. His research interests lie in cosmological modelling, compact objects, gravitational waves and much more.