Miss

Name: Rubina Visuvanathan

Imagine a clear empty sky, not a cloud in sight. Your telescope aimed towards a nondescript spot in the sky. A slight glimmer catches your attention. Did that star just wink at you? Well, maybe it's just a trick of the eye. Perhaps, just perhaps... you've just witnessed the birth of a star. Well, highly unlikely.

Why? Read on and you shall understand.

For the layman, the very idea of star formation is difficult to grasp. The magnitude needs to be clarified. How does one understand collapsing clouds of gas and dust? When you think gas, the layman thinks 'lighter than air'. Of course, the scientist realizes that there are various types, both lighter & heavier and in the topic of star formation, we are talking about massive masses/volumes of gases many hundreds & thousands of times larger than our Earth.

Now let us look back in time. Stars have been important to civilizations throughout the world. Stars have been used in religious practices, celestial navigation and orientation. It was also used in the creation of the Gregorian calendar and not to mention astrology.

These days, cosmologists are suggesting that the universe was quite dark in the first billion years. During these so called “dark ages”, the universe contained clouds of hydrogen gas. [1] So how did it churn into a huge ball of burning gas light years away? Now, let us look further into:

1. How and where are these stars formed?

2. Evolution and stellar luminosities.

3. Clustered star formation and why young stars are rarely found in isolation

4. How astronomers search for and observe young stars

5. What are the youngest stages of star formation that we can detect

6. Conclusion.

1. So how and where are these stars formed?

At a view, we see that the space between the stars seems to be empty, but if we look closer, it is filled with a thin gas laced with microscopic particles. The combination of both particles and gas is called an interstellar medium.

The clouds which are within the interstellar space are called a Nebula. It is in these nebulae that stars are formed.

How?

The regions inside a molecular cloud are extremely cold (~10 to 20K). At these state, gases such as CO and H2 group together and causes it to clump to high density. Once the density has reached a certain point, clusteration starts taking place and a star is formed.

Since some of the regions are dense, there is no source of light coming from it, therefore particles and no visible light can be seen. This is known as the dark nebula.

Star formation begins when the denser part of the cloud collapses under it’s own gravity. The cores have masses around 104 solar masses in the form of gas and dust. The cores of these clouds are denser than the outer cloud, so it collapses. As the core collapses, it forms into clumps around 0.1 parsecs in size and 10 to 50 solar masses in mass. This process takes about 10 million years and this is the beginning stage of the formation of a star which is called a protostar.

Certain celestial events can trigger star formation as well. Events like a nearby supernova explosion, or a collision between young clouds or a nearby massive young star can cause these clouds to begin to contract.. [6]

1.1. What is a protostar?

A clump which has broken free from the cloud core has its own unique gravity and identity.

Once a clump has broken free from the cloud core, it has its own unique gravity and identity. This clump is called a protostar.

So how does a protostar evolve into a star?

As the protostar is being formed, loose gas falls into its centre. Kinetic energy is then released in the form of heat. Once things have started to progress inside the protostar, the temperature and pressure in the centre of the protostar increases. Once it reaches thousands of degrees, the protostar then becomes an infra-red source.

It is very difficult to see what is going at this stage because the star is hidden inside the accreting dust and is still in its embryonic stage. However, the energy inside the protostar heats this and it begins to shine as a blackbody. No fusion takes place at this point of time and the protostar radiates in the infra-red.

To begin with, a protostar has only about 1% of its final mass. The star however, continues to grow as the in-falling material is accreted.

After a few million years, thermonuclear fusion begins in its core which converts Hydrogen into Helium. A strong stellar wind is produced which stops the in-fall of new mass and blows of the excess stellar clouds.

Our protostar is now a young star since its mass is now fixed and visible.

2. Evolution of stars:

There are many types of stars in the galaxy. Astronomers are still discovering many types of stars. The types of stars that are known are:

[10] Main sequence stars: These stars are stars like our Sun. These stars burn hydrogen to helium in their core. It is the longest stage in a star’s life, approximately around 90%.

Red giants: These stars are enormous cool stars which have high luminosity. This star is formed after it has used up the fuel at its core.

White dwarfs: These stars mark the end of a low to medium star’s evolution. These stars are at this stage because they have utilized all of their fuel, collapsing itself due to gravity. This leaves an extremely compact star. This star will continue emitting light for a several billion years because there is still heat energy trapped in its core.

2.1 Stellar properties

Stars however are classified via the surface temperatures or stellar luminosity as stated in Wien’s Law. As stated in Stellar Spectral types