Question

A main sequence star is:
A) a star that shines steadily due to nuclear fusion
B) several stars close together
C) a star with planets orbiting it
D) a nebula that skips a protostar and is instantly turns into a star

Answer 1

Hint:- A main sequence star fuses the hydrogen atoms to form the helium atoms. About $90\%$ of the stars in the universe which including the sun, are the main sequence stars. These stars can range from about a tenth of the mass of the sun to up to $200$ times as massive.

Complete step by step solution:
Stars start their lives as clouds of dust and gas. Gravitational force helps to draw these clouds together. A small protostar forms, powered by the collapsing material. Protostars often form in densely packed clouds of gas and can be challenging to detect.

Smaller bodies with less than $0.08$ the sun's mass cannot reach the stage of nuclear fusion at their core. Instead, they become brown dwarfs, stars that never ignite. But if the body has sufficient mass, the collapsing gas and dust burns hotter, eventually reaching temperatures sufficient to fuse hydrogen into helium. The star turns on and becomes a main sequence star, powered by hydrogen fusion. Fusion produces an outward pressure that balances with the inward pressure caused by gravity, stabilizing the star.

How long a main sequence star lives depends on how massive it is. A higher-mass star may have more material, but it burns through it faster due to higher core temperatures caused by greater gravitational forces. While the sun will spend about $10$ billion years on the main sequence, a star $10$ times as massive will stick around for only $20$ million years. A red dwarf, which is half as massive as the sun, can last $80$ to $100$ billion years, which is far longer than the universe's age of $13.8$ billion years. This long lifetime is one reason red dwarfs are considered to be good sources for planets hosting life, because they are stable for such a long time.

Note: The total amount of energy that a star can generate through the nuclear fusion of the hydrogen is limited by the amount of the hydrogen fuel that can be consumed at the core. For a star in equilibrium, the energy generated at the core must be at least equal to the energy radiated at the surface.