What Exactly Is A Star?

Have you ever wondered what stars actually are and why they shine so much?

Well, they are cosmic energy engines that produce light, heat, x-rays, ultraviolet rays, and other forms of radiation. If we talk about their composition, they are mostly gas and plasma – a superheated state of matter consisting of several subatomic particles.

You must already know that the sun is also a star – and perhaps the one that most people are familiar with. While this star stands alone, three of every four stars are a part of a binary system that contains two mutually orbiting stars.

All of us have wondered at some point in time about the total number of stars that exist in our universe. Well, the exact number is unknown but it sure would be staggering! To give a fair idea, let us tell you that there are more than 100 billion galaxies in our universe, and each of those galaxies may have more than 100 billion stars.

However, even if the night is dark and clear, our naked eye is able to see about 3000 stars only on the Earth’s sky. Interestingly, people of many cultures have mapped out the heavens by these stars.


You must have noticed that some stars stand out on the sky and shine brighter than the others do. Their degree of brightness depends upon their luminosity – the level of energy they put out – and their distance from Earth. So, the most shining stars you often see in the sky are the ones with more energy and less distance from the Earth.

Since the temperatures of the stars in the heavens are not all the same, they appear to have different colors. The hot stars usually are white or blue colored whereas, the cooler stars appear to show orange or red hues.

When it comes to the size of stars, there seems to have a wide range – typically from dwarfs to supergiants. Surprisingly, supergiants may have thousand times larger radii than that of the sun!

Photo Via. Sun.org

Moving on, the primary building block of stars is hydrogen. Hydrogen forms circles throughout the space in the form of cosmic dust clouds called nebulae. These clouds condense due to gravity and they are soon forced to collapse in on themselves. As their size decreases, the clouds begin to spin faster given the conservation of angular momentum – the same principle that explains the speeding up of a spinning skater upon pulling in her arms.

Building pressures lead to an increase in the temperature of a nascent star. And when the temperature increases to about 27 million degrees Fahrenheit, that’s when nuclear fusion starts.


As young stars aka protostars develop, they gather mass from the clouds around them and grow into main sequence stars. These stars, in the state of nuclear fusion, convert hydrogen to helium and thus, emit energy for billions of years.

This is how stars develop through the years.

After going through the main sequence state, stars exist in many other states depending upon their size and characteristics.

The larger the mass of a star, the shorter its lifespan will be.

Towards the end of a star’s life, most of its hydrogen is converted into helium. Helium rises the star’s temperature and causes its outer shell to expand. Such large, swelling stars are called red giants.

A red giant star then transforms into a small, dense body called a white dwarf. White dwarfs cool for years until they go dark and out of energy – and are known as black dwarfs.

However, some stars directly explode as supernovae, leaving behind a small core that may become a neutron star or even a black hole.