Question: Why do stars turn into red giants

Isabel Pires answered on 9 Mar 2015:

Although I am not an astronomer, I do love this topic and it has fascinated me since I was very young, so lets give this a go 🙂

A red giant is how a yellow dwarf star, such as our sun, will look like when it is reaching the end if its life. In actual fact, it is more orange-yellow than red.

An active yellow dwarf star (such as the sun is now) uses hydrogen it is core (its centre), fusing it into molecules of helium. This stage in its life can last a long time, around 10 billion years for a star like the sun.

At the end of this period of activity, the star has spent most of its supply of hydrogen in its core and can no longer do hydrogen fusion into helium. It changes the nature of chemical reactions around its core and its outer layers (which surround the core) start expanding, making it larger, glow brighter and finally becoming a red giant, which marks the start of the end of its life. At this stage, the outer layers of the star actually cool down, which means that the visible light that it emits changes, making it look red-orange in colour rather than yellow.

In the case of our sun, this will happen in 5-6 billion years. As it expands, the sun will grow so massive it will occupy an area as far as the Earth. When I was younger (and even to this day), the thought of the sun potentially engulfing the Earth made me quite sad!