How Are Stars Formed? (Part2)

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20 Jan 2024

109

What is a Main Sequence Star?

Stars are actually like human beings: they are born, they live and they die. Human beings breathe throughout their lives, they need energy to survive in the stars, and just like we breathe, they also convert hydrogen into helium throughout their lives (90% of their lives). Well, stars that do this are said to be on the main sequence and are called dwarf stars, just like our star, the sun. So, does a star on the main sequence spend its entire life on the main sequence? Answer is no. When these stars run out of hydrogen fuel, they expand to become giant stars and first become red giants and then white dwarfs.

So, what is the factor that determines the duration of time a star will be on the main sequence? There are two factors that determine this; the initial mass and luminosity of the star. But the most important thing is the mass of the star. Because its mass is the most important element that determines how the star will evolve in its later life and even what it will turn into when it dies. For example; Large stars burn their fuel very quickly and therefore have short lifetimes. That is, the more mass a star has, the shorter its lifespan.

Main sequence is the name that defines the stars that are included in the group called main sequence in the HR diagram developed by Ejnar Hertzsprung and Henry Russel. In the image, we see an example HR diagram showing the classification of stars. (Source: Universe Today)

Many stars fall into the main sequence class and spend most of their evolution as main sequence stars. However, it should be noted that the most important factor that determines this process is the mass of the star.

A Star's Mass Determines Its Evolution and What It Will Become After Its Death

We said that in order for an object to be called a star, it must convert the hydrogen in its core into helium, but this is not the only condition for being a star, the object must also have a certain mass. According to research, scientists found that the minimum amount of mass that an object must have in order to become a star is equal to 0.084 solar masses. The maximum amount of mass is 1.8.1032, that is, 90 solar masses. So why does there have to be a minimum and maximum amount of mass?

We know that a certain temperature value is required for nuclear fusion reactions to begin, so a lower mass value must also exist for nuclear fusion to begin. Because the initial mass determines how much the star will collapse, how hot and bright it will be, and the nuclear reaction cannot start until the star reaches a certain mass.

In cases where the mass is too much, the object cannot carry this mass. The excess mass increases the temperature, as a result the pressure increases, and as we mentioned in star formation, this pressure begins to oppose gravity. The more the mass, the greater the pressure, and with the increasing force outward, the star faces disintegration.

A visual depicting the pressure in the core of the Sun opposing gravity. (Source: Socratic.org)

So, what happens to objects that do not have enough mass for nuclear fusion to occur? What do these objects turn into instead of stars? Reply; to a brown dwarf. What we call brown dwarfs are called failed stars by scientists and are the names given to objects that fall in between in terms of mass. Brown dwarfs are just like ordinary stars; They form from the collapse of gas and dust nebulae, but they do not have large enough mass to perform nuclear fusion. However, it should be noted that when an object becomes a failed star (i.e. cannot become a star), it does not always turn into a brown dwarf. These objects may also be among the gas giant planets. Just like Jupiter, the enormous planet in our Solar System.