Our closest star is of course, the Sun. It is incredibly important to us here on Earth. Without the heat and light from the Sun, there would not be any life on Earth.
However, it is rather humbling to realise that there are billions of stars in the universe. And even worse, our Sun is an unremarkable star.
There are many stars that are bigger than our Sun and many are smaller. Some are older, others younger.
We also now know, through modern detection methods, that other stars have their own planets in orbit around them.
So what is a star and how does it come into being?
What Is A Star?
A star is just a large ball of gas, nothing much more. For all its apparent fury and violent energy generation, it is just gas.
The gas is mainly hydrogen and helium. If you have ever learned about the Periodic Table, you will know that these are the two simplest elements, with an atomic number of one and two, respectively.
The heat and light generated comes from nuclear fusion reaction, in which hydrogen atoms are combined to form helium, releasing enormous amounts of energy.
The star has so much mass and therefore so much gravitational force acting inwards towards its centre, that fusion processes keep going for millions of years, generating the heat and light that we are familiar with.
How Do Stars Form?
With telescopes, we can observe regions in our galaxy where there are vast clouds of gas.
It is believed that stars form in these clouds of gas, as it begins to coalesce. So these gas clouds are “stellar birthplaces”.
As the gas coalesces, denser clouds form and gravity increases. This leads in turn, to still greater density. The forming star begins to rotate.
Eventually, if there is enough material, temperature and pressure increase to the point where nuclear fusion begins and energy generation starts in earnest.
A “star is born”.
The star then settles into a long period of equilibrium, during which the energy generation that increases pressure and forces material outwards, is just balanced by the forces of gravity holding the material into a sphere.
The more massive the star at this point of equilibrium, the brighter it will be.
This is what we observe in the sky. Stars come in a range of sizes and brightnesses.
How Do Stars End?
Eventually, the hydrogen fuel for nuclear fusion begins to run out and the star generates less energy and cools.
What happens next, depends on how massive the star is.
With stars of low and medium mass like our Sun, the outer layers will expand in size and cool, forming a “Red Giant” star. This will be many times larger in diameter than before.
Eventually the outer layers will dissipate and the central core collapse into a “white dwarf” star.
Alternatively, for more massive stars, the outer layers may blow off to shroud the star in a nebula of gas, called a “planetary nebula”. A good example of a planetary nebula is M57 (the Ring Nebula), in the constellation of Lyra.
Much more massive stars may end catastrophically , with the star exploding as a “supernova“.
Part of this process involves the core of the star collapsing into a “neutron star” or a spinning, flashing “pulsar”.
An example is M1, the Crab Nebula in Taurus. This is the remainder of a supernova seen in the year 1054 and it has a pulsar at its centre.
In the case of very massive supernovae the core may collapse to form an incredibly dense “black hole”, from which no light or other radiation can escape.
