Stellar Evolution: Answers

What colour are the hottest stars?

The hottest stars emit high frequency light which corresponds to the blue part of the spectrum. This means that the bluest stars are the hottest.

Describe the formation of a star.

Stars form when dust and gas particles come together to form a nebula. Gravity pulls the dust and gas together which results in the formation of a protostar.

What two forces act on a main sequence star during the stable part of its life cycle?

Nuclear fusion of hydrogen nuclei to helium nuclei generates an outward force on a star which counteracts the inward acting force of gravity. These two forces balance out to keep the star stable.

What happens to an average sized star when nuclear fusion ends?

Nuclear fusion ends when all hydrogen is used up, at which point the outward force of gravity causes the star to collapse inwards. The heat generated during this collapse causes the temperature to increase and the star expands. If the star is an average-sized star, it will become a red giant. If it is a high mass star, it will become a red supergiant.

What is the relationship between the brightness of a star and its temperature?

The brightest stars are the coldest ones. This is because the coldest stars (red giants & red supergiants) are the largest so they give off more light. The relationship between the brightness of a star and its surface temperature is shown on a Hertzsprung-Russell diagram.