The Life of a Star

What is a Star? A star is a hot body of glowing gases that emits light and undergoes nuclear reaction. This is the only difference one can point out between a star and a planet. There are billions and billions of stars found in our galaxy, The Milky Way. There are thousands of galaxies in the Universe. It is said that there are more stars in the universe than the number of grains of sand found in all the beaches of the Earth. It is predicted that there are 100 billion stars in the Milky Way. Stars vary in their size, mass, temperature, density, etc. No two stars are alike just as no two humans are alike (left alone the identical twins). But yes, the stars do look alike as you watch them in the night but if you watch them carefully you can see that some stars are bright and some are comparatively less bright.

Stars, just like life, have birth and death of its own. Stars live for a period of time and then it dies. By saying stars I also include the Sun. At average stars live upto 10 billion years. Let’s see how a star is born.

Birth of a Star:

Usually a star is born in a region of high density Nebula. Nebula is nothing but a cloud of dust in the space in which stars and planets are born. Nebulae are visible to our naked eye as a tiny coloured patch of light. The Orion Nebula is one of the brightest nebula situated in the Orion Constellation.

The picture above shows the orion nebula at the bottom left and horse head nebula at the top right. This nebulae is found in the constellation of Orion.

When the nebula condenses and contract under its own gravity it creates a new star. The region of condensing matter will begin to heat up and it starts to glow. These glowing bodies are called as protostars.

The above picture shows a protostar in a nebula. The x-ray version does not show the protostar but you can clearly see them in an Infrared version of the image.

When a protostar contains enough matter the central temperature reaches 15 million degrees centigrade. At this temperature nuclear reactions starts where Hydrogen fuses to form Helium. The star then begins to release energy stopping it from contracting. Now it is called as Main Sequence Star. Sun is in Main Sequence Star level. A star is said to be in its Main Sequence Level for 10 billion years before it starts to die. Sun is said to be 5 billion years old and it is said to live for 5 billion more years.

Death of a Star:

A star is considered dead when all the hydrogen is burnt into helium. But what happens to a star after its death? There are two possibilities based on their mass.

Mass of the Star is under 1.5 times the mass of the Sun:

If the mass of the star is less than 1.5 times the mass of the Sun then as the hydrogen gets less the star begins to expand. The expanding star is called as a Red Giant.

The above picture shows the size comparison of a Red Giant with the Sun and the Earth.

The helium core runs out, and the outer layers drift of away from the core as a gaseous shell, this gas that surrounds the core is called a Planetary Nebula.

The picture above shows the Boomerang planetary nebula where a shell covers around a star.

The remaining core (that’s 80% of the original star) is now in its final stages. The core becomes a White Dwarf the star eventually cools and dims. Sirius, the brighest star in the sky is a White Dwarf. When it stops shining, the now dead star is called a Black Dwarf.

The picture below shows the white dwarfs which are circled.



Mass of the Star is greater than 1.5 times the mass of the Sun:

If the mass of the star is greater than 1.5 times the mass of the Sun then as the hydrogen gets less the star begins to expand just as the previous case. But here the star becomes massive in size and it is called as a Red SuperGiant.

The above picture is a size comparion of a Red SuperGiant Aldebaran with the Sun.

The SuperGiant then starts of with a helium core surrounded by a shell of cooling, expanding gas. In the next million years a series of nuclear reactions occur forming different elements in shells around the iron core. The core collapses in less than a second, causing an explosion called a Supernova, in which a shock wave blows of the outer layers of the star. The actual supernova shines brighter than the entire galaxy for a short time. The bright object at the top left corner (arrowed) is a supernova explosion.

Sometimes the core of the star survives the explosion. If the surviving core is between 1.5 to 3 times the mass of the sun, it contracts to become a tiny, very dense Neutron Star. If the core is much greater than 3 times the mass of the sun, the core contracts to become a Black Hole.