What is a "Red Giant"?

Briefly, it is a stage in the lifespan of a star. I was soon to find there was a lot more to this subject than I'd realised however...

A star will spend the majority of its existance on the main sequence (an examination of the Hertzsprung-Russell diagram is highly recommended), during which it exists in a stable, very slowly changing condition, usually without any significant deviations in its temperature or brightness.. During this time, it converts hydrogen atoms into helium atoms by means of nuclear fusion. In the course of this transformation, there is an amount of excess mass, which is in turn changed into energy (determined by means of e=mc2). This emitted as visible light and other forms of radiated energy, commonly refered to as "sunshine".

Although this period of stability is measured in billions of years, nothing lasts forever...

All very reasonable, but why does the star increase in size?

The hydrogen, which forms the major part of a star is also its fuel source, and like all fuel sources it is eventually going to run out. When hydrogen-based fusion eventually ceases in the core, the lack of outward force (known as radiation pressure) produced by those reactions is insufficient to counter the influence of gravity. The core becomes compressed and temperatures intensify dramatically. The heat caused by the contraction of the core causes surrounding "shells" of hydrogen to start their own fusion reactions, and the process begins anew.

Because of these increased temperatures, now much higher than the star exhibited previously, the speed with which hydrogen is depleted within the shells surrounding the core accelerates also. The expansion in size expends a lot of energy, causing the star's radiated light emissions to drop off sharply, along with a reduction in temperature to around 3500 - 5000 Kelvin, giving the stellar object its distinctive red colour. A star of this temperature range is known as Type K Star

To get an idea of how much bigger a star can get let's consider our own Sun - if transformed into a red giant we'd be looking at an increase in radius of around 200 times. That wouldn't quite reach Mars, but the inner three planets would have been swallowed... Naturally, no one still on Earth would survive this. The reduced gravity of the red giant means they also tend to lose large amounts of mass from their surface.

Eventually in the case of red giants, all that helium which has gathered at the core is under tremendous amounts of pressure and subjected to such intense temperatures that it in turn is begun to be consumed as a new level of fusion is triggered. This helium is transformed into even denser elements, such as beryllium and carbon. This rapid transition is known as a helium flash, and when that happens anyone in the vicinity will definitely notice it happening as it is quite a explosive (although brief) occurence.

Will our Sun ever turn into a Red Giant?

At present our own star, the Sun (composed of roughly 75% hydrogen and 25% helium, with a smattering of other elements present in relatively small proportions), is about halfway through its time on the main sequence. Given its mass and other characteristics, it seems certain that we will have a red giant star in our solar system in about 5 billion years. This red giant will continue to evolve into a new type of stellar object, known as a white dwarf.

Well, that's all I have to say about red giants, so I thought I'd finish with an old joke I heard some years ago...

A professional gambler is playing cards with a local university professor he's just met. As they play, the professor has been chipping in with bits and pieces about his work, and he casually mentions that our Sun will expire in about 5 billion years. The gambler who has been concentrating on his cards hears something of this, turns pale-faced with terror and says "What did you say?!"

The professor repeats himself, and the gambler looks relieved - "Thank goodness for that, I thought you said 5 million years!"

(drum roll and hi-hat optional)

"Hyperspace" by Michio Kaku

Many Thanks to strawberryfrog and especially tmaq for their feedback. (Work is still in progress, but anyone please feel free to message me in the meantime, as it's my first dabbling with astrophysics. Why choose this as a subject? Well, for this reason...)

Last Updated : 6th March, 2003

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