Stellar Evolution

The Life Cycle of a Star

When we observe the night sky, it can seem like the stars that can be seen are fixed and unchanging, returning year after year to the same point across the sky. This is the basis of navigating by the stars and has been used for thousands of years by sailors to cross all the oceans of our planet.

Navigating by the Stars

The image above shows how 'North' can be found every clear night of the year. Polaris (the 'North Star') is a star at the North Celestial Pole. This means that as the stars move across the sky, they appear to rotate around Polaris. As this star lies at the north celestial pole, it itself appears stationary night after night, and therefore, if you face Polaris, you face north.

The image below shows how over the course of a night, the stars appear to rotate around Polaris:-

Note - This apparent motion is caused not by the stars themselves moving, but by the Earth's rotation.

In order to find Polaris (a fairly unremarkable star) the constellation Ursa Major (the Big Dipper) is used. By identifying the stars Dubhe and Merak (the two stars at the 'bucket' of the Big Dipper) and drawing a line upwards, you will reach Polaris.

Stellar Evolution

This process of navigation has worked for thousands of years, and will for thousands more because the life cycle of a star operates on time-scales many orders of magnitude larger than this. Over our lifetimes, the sky appears fixed and unchanging but when we look at stars over millions of years, huge changes in size, temperature, colour and position can occur.

Life Cycle of a Sun-Type Star

Our Sun is an unremarkable G-type yellow star of moderate size. At present, the Sun is about half-way through its 10 billion year life span. This life span is positioned approximately in the middle of the range of lifespans of stars:-

1. High Mass Supergiant Stars - use up their fuel very quickly, lifespan ~ several million years.

2. Sun-like star - use up fuel more slowly, lifespan ~ 10 billion years.

3. Low Mass Dwarf Stars - use fuel up very slowly, lifespan possibly 100s of billions of years.

Star Formation

All stars regardless of size are formed inside huge clouds of dust and gas. Gravitational Forces between dust particles and gas cause the molecular cloud to 'clump' together. As more and more of these clump together, their combined gravity draws in more and more material. As this happens, the core of the forming star (the Protostar) starts to heat up due to the effects of Gravity and pressure.

As more and more mass collapses towards the core, the temperature and pressure increase until nuclear fusion (Hydrogen is fused together to create Helium) starts to take place. Once this occurs, it is called as a star.

Note - At this point, the thermal pressure outwards balances the Gravitational Force inwards and the star maintains a fixed size until the source of the thermal pressure (generated by fusion) stops, and the star starts to collapse under its own Gravity.

Stellar Death

The final fate of a star depends on the mass of the star.

A Sun-like star will run out of Hydrogen after ~10 billion years. At this point, the star will increase in size to become a Red Giant star and start to fuse Helium at its core. Once the Helium fusion stops, the Sun will collapse under its own gravity, throwing off its outer layers into space, leaving behind a Planetary Nebula with a white Dwarf Star at its centre.

If the star is more massive than the Sun it will run out of Hydrogen after ~several million years. Once the star runs out of Hydrogen at its core, the Star increases in size to become a Red Supergiant Star and starts to fuse Helium at its core. Unlike a Sun-like star, once the Helium fusion stops and the star starts to collapse, the Gravity and Pressure are strong enough to trigger further fusion processes of heavier Elements (up to Iron). Once this occurs, the core collapse continues and the Star explodes in a Supernova.

Mid-sized Massive Star - Core collapse strong enough to force Protons and Electrons to combine to form a Neutron Star.

Large-sized Massive Star - Core collapse cannot be stopped and the Core continues to collapse to a Singularity - a Black Hole.

The video below gives a brief overview into the life-cycle of a star:-