Main Sequence Stars & Stability

What is a main sequence star?

A star that is fusing hydrogen into helium in its core.

What stage of its life does a star spend most of its time in?

The main sequence stage.

Why is a main sequence star stable?

Inward gravitational attraction is balanced by outward pressure.

What force acts inwards on a star?

Gravity.

What acts outwards to balance gravity in a stable star?

Pressure produced by the hot gas and energy released by fusion.

What happens when the inward and outward effects are balanced?

The star remains in equilibrium.

What is the main energy-producing reaction in a main sequence star?

Hydrogen nuclei fuse to form helium.

Why does the Sun shine?

Nuclear fusion releases energy in its core.

What type of star is the Sun currently?

A main sequence star.

What happens to the Sun's hydrogen supply over time?

Hydrogen in its core is gradually converted into helium.

What eventually causes a main sequence star to leave this stage?

Its core begins to run out of hydrogen fuel.

Why does a star not immediately collapse while fusion is occurring?

The outward pressure balances the inward pull of gravity.

How does a star's mass affect its lifetime?

More massive stars generally have shorter lifetimes because they use their nuclear fuel faster.

Why do massive stars use their fuel faster?

They have higher core temperatures and much faster rates of fusion.

Do all stars have the same lifetime?

No, stellar lifetime depends strongly on mass.

Why can smaller stars survive longer than larger stars?

They generally consume hydrogen fuel more slowly.

What happens to the balance of forces when core hydrogen becomes depleted?

The balance changes, allowing gravity to compress the core and the star to evolve.

What determines whether a star eventually becomes a white dwarf or undergoes a supernova?

Its mass.