Institute of Astronomy


Ask an Astronomer - Stars

Is Betelgeuse about to explode?

Published on 01/05/2011 

I have never seen Betelgeuse so red with naked eye - is it about to blow?

Betelgeuse is remarkably red - but maybe it's just looked so brilliant at the moment because of the lovely clear nights we've been having! You're right that as Betelgeuse is a 'red giant' star it's in the final stages of its life and is (as you so poetically put it...) about to blow, in the form of a dramatic supernova explosion. The catch here is that when astronomers mean 'about', it could be any time in the next few  thousands of years... no-one knows exactly when it might happen, and there aren't any observations suggesting it's particularly imminent... We'll certainly know when it does occur; at around 640 light-years distant Betelgeuse will become very much brighter in the sky, perhaps briefly becoming bright enough to be visible during the day.

Black Hole Formation

Published on 02/04/2011 

Are all black holes formed after the death of a massive star? If not, how are these non stellar black holes formed?

Black holes fall broadly into two categories. Firstly there are galactic, stellar mass black holes, which as the name suggests are found throughout galaxies and have masses similar to that of stars (of order 10 times the mass of the Sun). These form at the end of the lives of stars that are too massive to become white dwarfs or neutron stars. This occurs when they are still greater than two or three times the mass of the Sun by the time they have shed material through stellar winds or supernova explosions during their death. The gravitational force causing them to collapse is too strong for the pressure of electrons or neutrons to support the star as in the case of white dwarfs and neutron stars, so it keeps collapsing down to a black hole.

Secondly, there are so-called supermassive black holes, which tend to be around 100 million times as massive as the Sun. These are believed to be found at the centre of most galaxies about which the stars in that galaxy orbit (in fact our own galaxy, the Milky Way, has a black hole 4 million times the mass of the Sun at its centre). While the exact mechanism of their formation is still unknown, it is believed they form when a large gas cloud collapses to the centre of the large gravitational 'well' in the centre of a galaxy as it forms.

In addition there may be 'intermediate mass black holes' which could explain a number of observed phenomena such as 'ultraluminous X-ray sources.' When matter falls into a black hole, it emits radiation, part of which is seen as X-rays. These appear much brighter than stellar mass black holes, implying their mass is around 1000 times that of the Sun, but they are not at the centre of galaxies. Their formation is a mystery. They are too massive to form from a single star, but they could be formed when multiple stars or stellar mass black holes are pulled together by gravity and merge. Alternatively, they could be the central black hole from a smaller galaxy that has merged with the galaxy in which they are found and in the process, were thrown out from the centre.

Twinkling stars

Published on 28/02/2011 

Why do stars twinkle and planets do not?

In fact, both stars and planets twinkle! The twinkling is due to the turbulent air in the Earth's atmosphere, blurring and distorting the image of the star. The twinkling therefore has more of an effect nearer to the horizon, where the light must travel through more of the densest parts of the Earth's atmosphere. You can see this for yourself! Compare the twinkling of a star near the horizon (such as Sirius), and one close to zenith (straight up). Objects such as the Sun, Moon and the planets are called extended sources, because the light is emitted from a disc. Objects such as distant stars are called point sources, because they appear to be a point as they are very far away. In fact, the light from extended sources can be thought of as many point sources spread over an area. The turbulent air in the atmosphere causes a point source to appear to move around on the sky ever so slightly. However if we spread many point sources over the face of the planet, all point sources move around, but we do not notice a change in the total light from the object.

The brightest star

Published on 28/02/2011 

What is the brightest star in the night sky?

We believe our galaxy, the Milky Way, contains roughly 200 THOUSAND MILLION stars (that is, 200,000,000,000). However, if you were to count the number of stars in the night sky with a naked eye, you would only be able to count about 6,000. Besides our star, the Sun, the brightest star in the night sky is Sirius, in the constellation Canis Major (the “Big Dog”). Sirius is almost twice as bright as the next brightest star, Canopus, in the constellation Carina.


Published on 24/02/2011 

What is a star?

A star is a huge sphere of hot, glowing gas. The star produces its own heat and light through a process called nuclear fusion. This process forces lighter chemical elements to become heavier chemical elements. When this happens a large amount of energy is released and this energy is what causes the star to heat up and 'shine'.

Stars come in a range of sizes and colours. Our Sun is a fairly typical yellow or G type star. Stars which are hotter than our Sun tend to look bluer while those which are cooler look red. So the next time you look at the night sky, see if you can see stars of different colours!