Main Sequence Stars

Main sequence stars are those which are fusing hydrogen into helium in their cores and are seen on a colour magnitude diagram as a band stretching roughly diagonally from the top left (hot, blue stars) to the bottom right (cool, red stars). Stars live the majority of their lives in this balanced state, which is known as hydrostatic equilibrium. Our Sun, for example is thought to be approximately 5 billion years into its 10 billion year main sequence lifetime.

Hydrostatic Equilibrium

One of the dominant forces in the Universe is gravity. It's the force that holds us down on Earth, keeps the planets in orbit around the Sun, and the Sun in orbit around the centre of our Galaxy.

It also tends to make things collapse inwards. For solid objects like the Earth, this collapse is halted by the pressures in the solids and liquids that form the internal components of the planet. However, the Sun is a giant ball of gas, and the pressure to stop collapse must come from within the Sun. In fact, it is thermal and radiation pressures which create an outward expansion force which balances the inward contraction due to the force of gravity. This balance is called hydrostatic equilibrium.

Figure 1: Hydrostatic equilibrium.
Credit: Brian Woodahl (http://woodahl.physics.iupui.edu/Astro105/)

At first glance, it may seem rather unlikely that the gravitational and pressure forces should balance so neatly, but they actually self-regulate very well. Imagine that we have a star like the Sun where the internal heat produces sufficient pressure to balance the inward force of gravity. Then imagine that something happens to reduce the internal temperature slightly. Immediately, gravity will start to dominate and the star will shrink. However, as it does so, the gas that makes up the star is falling into a gravitational potential. This causes it to lose gravitational potential energy, and this energy is turned into heat. This heat raises the temperature of the star again slightly and a new balance is found - hydrostatic equilibrium is maintained, even if the star now has a slightly different radius.

A star's lifetime is determined mainly by its mass. Perhaps counter-intuitively, more massive stars live shorter lives than less massive ones, since more massive stars use their fuel up more rapidly. This relationship between a star's mass and its lifetime is therefore inverse. Towards the end of a star's life, it ceases to be able to keep these forces in balance and starts to undergo expansion as it evolves away from the main sequence to become either a red giant or a supergiant star.

Find out more about how low-mass stars evolve.

Find out more about high-mass stars evolve.

Read about how astronomers use colour magnitude diagrams to understand the main sequence.

What does the term 'Main Sequence' indicate?

The star is Sun-like in mass
No, stars of many different masses can be on the main sequence
The star is very hot
No, stars on the main sequence have a range of temperatures
The star is very cool
No, stars on the main sequence have a range of temperatures
The star is converting hydrogen into helium
Yes, this is correct - the star is converting hydrogen to helium in its core

Which one of the following statements is correct?

Massive stars live longer lives as they have more fuel
No, mass and lifetime are inversely proportional (massive stars live shorter lives)
Massive stars live shorter lives as they use their fuel more rapidly
Yes, this is a correct statement
All stars evolve into red giants then supergiants
No, a star's mass determines whether it will become a red giant or a supergiant
White dwarfs are the end-point for all stars
No, white dwarfs are the end-point for stars of low-mass only