What is a Star?

Look up on a clear night and you will see thousands of twinkling lights shining down on you. Each one of these is a star, many millions of kilometers away.

Multiple stars in the double cluster in Perseus. Image Credit: Paul Smith

Although we don’t see it very often from Ireland, the Sun is our closest star. The Sun is a star not dissimilar to others in the sky, but stars come in many sizes, types and colors. Most of the stars we see in the sky are actually a lot bigger than the Sun but the vast distance to them makes them look very small. Take the star Antares which is in the constellation of Scorpius. It can be seen low in the south in the summer months. This is a huge star, much bigger than the Sun. It is about 680 times the diameter and 11,000 times brighter. The reason you don’t get sunburn from is that it is a huge 554 light years from Earth. That means that it is so far away it take the light from it 554 years to get here. In comparison, light from the Sun reaches us in 8 minutes.

Stars are really just big balls of gas, mostly hydrogen and helium. They are formed in clouds of gas and dust called nebulae. Over thousands of years, the gas in these nebulae start to clump to together. These clumps of gas become dense enough that they collapse under their own weight forming what is called a protostar. As the pressure in star rises so does the temperature. Once the temperature is high enough (around 15 millions degrees) a process called nuclear fusion begins. Nuclear fusion is where two or more atomic nuclei combine to form a different type atomic nuclei. During the process energy is released and part of this energy is released as light and heat. In the case of the Sun and most other stars this reaction is hydrogen atoms combining to form helium.

The Carina nebula is an example of a star forming region. Image Credit: NASA

At this point the star enters a phase known as the main sequence. Most stars in the galaxy, including the Sun are main sequence stars. The star can remain in this phase for billions of years. Scientists estimate that the Suns expected life is around 8 billion years and it is currently about 4.6 billion years old. Stars are categorised into different groups using different variables including color and luminosity. For example, the Sun is called a yellow dwarf. You can tell what temperature a star is by it’s colour. A white or blue star is hotter and a red or orange star is cooler.

When the star starts to run out of fuel (hydrogen), the star starts to swell. This type of star is called a red giant. Betelgeuse, the star on the top left corner of Orion is an example of a red giant. Once the star runs out of fuel the next stage depends on its size. Average sized stars like the Sun will shed its outside layers to form a planetary nebula. These can be really beautiful. What is left at the centre is the core of the star which is then called a white dwarf.

Planetary nebula NGC 6565, Image credit : NASA

If the star is big enough it will die in a huge explosion. The outside of the star appears to expand but beneath this the core will collapse and explode in a huge explosion known as a supernova. A supernova can be so bright that if it happens in another galaxy it can briefly outshine the rest of the galaxy or if one were to happen in the Milky Way it would be visible in daylight! What is left after this explosion is either a black hole or a neutron star.

The next night it is clear go out and take a look up. These little twinkling lights you see may seem small and gentle but that is far from the truth.

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