An absorbing question

IF you take a look to the northwest in the evening tonight, you will easily spot a quite bright star that has an obvious reddish-orange colour.

It’s called Betelgeuse, and it is one of the main stars in the constellation of Orion, The Hunter.

Astronomers have recently suggested that Betelgeuse, a red supergiant star in the late stages of its “life”, may be a cannibal: it may have absorbed a companion star a thousand centuries ago. They suspect this to be the case because Betelgeuse is spinning at a surprising rate.

Betelgeuse is really huge. With a diameter of about 1200 million kilometres, it is nearly equal to the size of Jupiter’s orbit around the sun in our solar system. Replacing our sun with Betelgeuse would mean that Mercury, Venus, Earth and Mars would all be inside the star!

In studying the physical characteristics of Betelgeuse, astronomers have found that it is spinning at a speed of about 15 kilometres per second at its equator. They know this because as one side of Betelgeuse approaches us and the other side recedes, the wavelengths of the light from each side are different.

Making these measurements is not unlike the physics behind a driver’s speed being measured with a radar gun, although the process is rather more complicated with starlight.

You may think that 15 kilometres per second is not very fast compared with the enormous size of Betelgeuse, but most stars of this type rotate much more slowly.

One explanation is that we are observing Betelgeuse at a particular well-defined point in its evolution that lasts only about a thousand years: hardly any time at all in the lifetime of a star.

However, a possible and even likely scenario, say the researchers, is that Betelgeuse has absorbed a companion star, and when the two stars merged, it resulted in the rotation of Betelgeuse speeding up.

There is some evidence for this. The Herschel Space Observatory has taken a picture in infra-red light showing a ring of material that may be the result of the dramatic event.

Betelgeuse hasn’t always been this large. When stars age, they expand and cool, resulting in the red colour. We think of red as a hot colour in our everyday lives, but in the physics of any object glowing as a result of its temperature, red is actually a cooler colour than blue.

So as Betelgeuse expanded, it not only cooled and became more red, but it grew so much that it became larger than the orbit of its companion star.

We know that this sort of thing can happen, although there is still some uncertainty as to whether this is actually the case with Betelgeuse.

In fact, there is a lot of uncertainty about this star — including its distance from us, which is very approximately 640 light years. The light from the star, if at that distance, left on its journey to us about the year 1380.

Betelgeuse is seen in the lower right part of the constellation of Orion, The Hunter, from our part of the world.

The name Betelgeuse is one of the most confusing of all star names. It is thought to have originated as Yad Al Jauzah, said to mean “hand of the giant”. However, it is unclear why Al Jauzah was used, because Al Jabbar would be the correct term for ‘the giant’. The Bet at the beginning of the name, which is more correctly Ibt, means “armpit”, and it is thought that the difference was due to a translation error.

The name has quite a history! Martin George is manager of the Launceston Planetarium (QVMAG).