When the protostar starts fusing hydrogen, it enters the “main sequence” phase of its life. Stars on the main sequence are those that are fusing hydrogen into helium in their cores. Eventually the core of the star runs out of hydrogen. When that happens, the star can no longer hold up against gravity.

## How do you know if a star is main sequence?

A Hertzsprung–Russell diagram plots the luminosity (or absolute magnitude) of a star against its color index (represented as B−V). The main sequence is visible as a prominent diagonal band that runs from the upper left to the lower right.

## Do stars move along the main sequence?

No, the Main Sequence is more like a starting line. Most stars spend a long time on one point of it (10 billion years for the sun) while they fuse hydrogen into helium. Then they wander off it.

## What happens when nuclear fusion in the core of a star stops?

A star remains on the main sequence as long as there is hydrogen in its core that it can fuse into helium. Eventually the hydrogen fuel in the core runs out and fusion stops, shutting off the outward radiation pressure.

## How many days are left in the life of a star when it begins fusing silicon?

The silicon-burning sequence lasts about one day before being struck by the shock wave that was launched by the core collapse. Burning then becomes much more rapid at the elevated temperature and stops only when the rearrangement chain has been converted to nickel-56 or is stopped by supernova ejection and cooling.

Black holes

## How long does it take for a star to collapse?

So, how long does a supernova take to explode? A few million years for the star to die, less than a quarter of a second for its core to collapse, a few hours for the shockwave to reach the surface of the star, a few months to brighten, and then just few years to fade away.

## How much iron does it take to kill a star?

1 Answer. Phillip E. When the mass of iron in a star’s core reaches about 1.4 times the mass of the Sun, the star will effectively die.

## What happens when a star builds up too much iron?

An inert iron core builds up at this time where successive layers above the core consume the remaining fuel of lighter nuclei in the core. The infalling layers collapse so fast that they `bounce’ off the iron core at close to the speed of light. The rebound causes the star to explode as a supernova.

## What are the two forces that a star must constantly balance in order to stay alive?

Stars live out their lives in an exquisitely detailed equilibrium, or balance, between two powerful forces — outward pressure and the inward pull of gravity. The gravity force is a property of the mass of the star, and in order to support itself against gravity the star generates energy in its core.

## What two forces act on a star?

Gravitational forces act to contract the star. Fusion reactions and heat convection act to expand the star. The two forces are balanced and the star remains stable in size and reactions.

## What is a stable star?

In a stable star, the gas pressure pushing out from the center is equal with the gravity pulling atoms inward to the center – when these forces are equal, the star is at equilibrium. Once a star reaches equilibrium for the first time, it will start burning (fusing) hydrogen into helium.

## What happens to stars after they die?

As the hydrogen runs out, a star with a similar mass to our sun will expand and become a red giant. When a high-mass star has no hydrogen left to burn, it expands and becomes a red supergiant. While most stars quietly fade away, the supergiants destroy themselves in a huge explosion, called a supernova.