What is a Red Giant?

Red_Giant_Chi_Cygni

Here we explain just exactly what is a Red Giant……..

What happens to a star when it runs out of Hydrogen?

As a star exhausts its hydrogen you might expect it to slowly flicker away, but for stars like our sun, the opposite happens. Having spent millions or billions of years with the core as its beating heart, a star that is running out of hydrogen in fact swells up to potentially hundreds of times its original size. Such stars are known as red giants.

Our closest Red Giant

One of the closest red giants to earth is the star Alpha Orionis, better known as Betelgeuse, the ninth-brightest star in our night sky and one of our nearest neighbours in cosmic terms, a mere 500 light years away. Betelgeuse, the ninth-brightest star in our night sky and one of our nearest neighbours in cosmic terms, a mere 500 light years away. Betelgeuse has long been familiar to stargazers, notable for its brightness and reddish tinge that is clearly visible to the naked eye. Sir John Herschel studied the star intensely in the nineteenth century, recording the dramatic variations in its brightness.

How it was discovered

However, it was only when three astronomers from the Mount Wilson Observatory in California tried to measure its diameter that we realised this was no ordinary star. Albert Michelson, Francis Pease and John Anderson used a specially designed telescope to measure the scale of this red star using a technique known as interferometry. By measuring the angular diameter, they came up with a number that, although it’s been refined since, revealed something profound: Betelgeuse is a true giant in every sense. This star is about twenty times the mass of our sun but its size rather more impressive. If you put Betelgeuse at the centre of our solar system it would dwarf our sun. In fact, Betelgeuse would extend past the Earth’s orbit, encompassing everything out to jupiter. Current estimates suggest it is around 800 million kilometres in diameter; a vast, ethereal wonder that would fill our solar system with a single wispy star.

What its allowed us to do

Due to it immense size and relative proximity, we can study Betelgeuse in incredible detail. In 1996, the Hubble Space Telescopic took a picture of Betelgeuse that was the first direct image of another star to reveal its disc and surface features. We’ve even imaged sunspots on its surface and been able to study its atmosphere in ever-increasing detail. However, it’s not the surface of the red giant that holds the clue to where the heavy elements are made; to understand that, we need to journey deep into its dying heart.