Stellar Evolution

Just like humans, stars go through a life-cycle from young protostars to elderly white dwarfs. We classify stars according to their colour, which is related to their temperatures. There is also a relationship between the colour of stars and their brightness, which is represented on Hertzsprung-Russell diagrams.

Classification of stars

Stars can be classified according to their colour. The hottest stars will emit more high frequency light whereas the cooler stars emit more light with lower frequencies. Low frequency light corresponds to the red end of the visible light spectrum whereas high frequency light corresponds to the blue end. This means that stars which emit blue light are hotter than white stars, white stars are hotter than yellow stars, yellow stars are hotter than orange stars and orange stars are hotter than red stars, which are the coldest type of star there is.

Life cycle of a star

Star formation is initiated when a cloud of dust and gas aggregates to form a nebula. Nebulae are mainly composed of hydrogen. As the dust and gas particles are pulled closer together by the force of gravity, a protostar is formed. Its temperature rises, providing the right conditions for hydrogen nuclei to fuse and make helium in the process of nuclear fusion. Nuclear fusion releases energy, keeping the core of the star nice and hot.

The star goes through a very stable phase where the outward pressure generated by nuclear fusion is equal to the force of gravity which pulls the star inwards. Stars can spend several billion years in this stable phase and this is the stage that our Sun is currently in. Eventually all the hydrogen will be used up and the force of gravity exceeds the pressure generated by nuclear fusion. The star is compressed due to gravity which generates heat, which results in the star expanding. The expanded star is much cooler and the surface becomes a red colour. If the star is a small to medium size (like the Sun), the star will be called a red giant. Larger stars are referred to as red supergiants.

Red giants transform into white dwarfs by ejecting its outer layer of gas and dust. What remains is a dense, hot core which now emits white light as its surface temperature has increased.

Red supergiants will continue fusing heavier and heavier elements, getting hotter and expanding until they explode in a supernova. The supernova expels dust and gas into space, leaving behind a neutron star, or in the case of very big stars, a black hole.

Absolute Magnitude

The brightness of a star also provides information about the size and temperature of the star. The larger and hotter the star is, the brighter it appears. The problem is that the closer the star is to Earth, the brighter it appears to us. To account for this, astronomers use absolute magnitude where they determine how bright a star would look if it was a fixed distance from Earth. This makes sure that distance to Earth doesn’t affect the data collected about brightness, and therefore temperature, of different stars. The lower the absolute magnitude, the brighter the star is. Stars that have an absolute magnitude greater than 6 cannot be seen with the human eye and can only be visualised using a telescope.

Hertzsprung-Russell Diagram

The Hertzsprung-Russell diagram shows the relationship between the temperature of a star and its brightness. Absolute magnitude, or brightness, is shown on the y-axis (the vertical axis) and decreasing temperature is shown in the x-axis (the horizontal axis). This means that red giants and red supergiants are found in the top right corner because they are cold and very big - the large size makes these stars very bright. White dwarfs are found in the bottom right corner as they are very hot and small. Main sequence stars are found in the middle of the graph in a sloped line from the top left corner to the bottom right.