Institute of Astronomy


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Ever had a question about astronomy you've want answered? Have a look through the previous questions which we've been asked and if you can't find find your answer, ask us!

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Meteorite identification

Published on 24/06/2013 

Hello there I was wondering if it is possible to test rocks to see if they are from space? I ask because, this is true! Whilst I was gardening today a rock fell into my wheel barrow.  It sounds crazy, but it did happen.  The object is black, looks like coal, and is very light, I suppose like pumice.  If It didn't fall from the sky, I would say it was a piece of normal earth rock, but as it came from above I'm very curious.

It can be quite difficult to distinguish meteorites from Earth rocks.  If it is a couple of centimetres in size or larger and it were a meteorite then I would expect it to have left a dent in your wheelbarrow.  A black exterior is typical since the outside of the rock would be heated to very high temperatures during it's descent, the exterior is often also pitted and can look obviously like it has been melted.  It would not necessarily have been hot to the touch however since it's passage through the atmosphere is very rapid so the interior remains at the temperature of space (extremely cold), and the exterior will cool very quickly.  The lightness would tend to suggest that it is not a meteorite, since meteorites are more often rather dense and heavy, but that isn't an absolute rule.  If you want more information the Natural History Museum in London houses the national meteorite collection and have an identification service that may be able to help.  If you do have a meteorite you are very lucky!

Light years and Red giants

Published on 24/06/2013 

I was wondering how astronomers know that a star is for example  50 light years away, how can they know when light travels from there and reaches here and even if they did, they would not wait for 50 years. I have asked my physics teacher at school and a student in Glasgow university but they did not have a clear answer. I also checked online but it is too hard for me to understand.
One more question what determines if a main sequence star will be a red giant or a super red giant?.

We know what the speed of light is from experiments conducted here on Earth, and there are various methods of working out how far away a star is.  For example if you take two images six months apart, so Earth is on opposite sides of it's orbit, then relatively nearby stars will seem to have moved slightly compared to distant galaxies since we are looking at them from a slightly different angle, an effect called parallax.  From the amount the star seems to move we can then work out how far away it is.  Once we know how far away the star is can translate that into how long it would take light from that star to reach us, which is where the measurement of a distance in light-years comes from.  Light-years are used just because measuring things in trillions of kilometres even for small distances gets a bit tiresome.

Regarding red giants vs red supergiants the main determining factor is the mass of the star.  Stars that are around half the mass of our Sun, up to ones about 8 times the mass of the Sun will become red giants, stars more than about 10 times the mass of the Sun will become red supergiants.

Eclipses on other planets

Published on 24/06/2013 

Hi, I asked my Dad if any other planets in our solar system have eclipses?  He doesn't know but thinks it is a great question so we would be grateful for your help.

Other places in our solar system can have eclipses, though nowhere has eclipses quite like those on we get on Earth.  The moons of the gas giants, Jupiter and Saturn, regularly experience eclipses because Jupiter and Saturn are so large, their eclipses last a lot longer than ones on Earth do though and cover the whole surface, rather like when the Moon is eclipsed by Earth.  In fact NASA's Cassini space probe took an image of Earth while the Sun was being eclipsed by Saturn a few years ago (, and is planning to take another one on 19th July (

Black holes and companion stars

Published on 22/05/2013 

I was wondering what it's called when there is a star next to a black hole and you can watch the black hole pulling in particles of light? I know there is a word for it i just cant find it.

Your description could match a couple of things. I think you are looking for "accretion from a binary companion". Accretion is the term we use for an object growing by accumulating matter that falls onto it. Many stars a found in multiple systems like binaries (where there are two stars). The more massive star will evolve more quickly as it burns up its fuel quicker, and so may collapse down to a black hole while its companion is still a regular star. As the companion evolves it may become a giant, puffing up in size, so the outer layers can get stripped off by the black hole. The material swirls around the black hole forming an accretion disc before eventually spiralling in. I think this is what you had in mind. The accretion disc can get very hot, hot enough to emit X-rays, in which case the system is referred to as an X-ray binary. Studying X-ray binaries has given us our best understanding of stellar mass black holes.

The other effect you could be referring to is gravitational lensing. This is when the trajectory of light appears curved because of the gravity of a massive object (often a black hole, a galaxy or a cluster of galaxies). This might match "pulling in particles of light", but you can't normally see this (since if the light is being pulled in, it can't escape for us to see): I think you're actually thinking of the stream of hot plasma being accreted from a star.

Seeing black holes

Published on 21/02/2013 

Why can't scientists detect a black hole with a telescope?

You actually can detect black holes with a telescope, but we will come to that. First, you cannot see black holes directly because they do not emit light: light is sucked into them after all. So, unlike stars, they are not directly visible.

However, there are a couple of ways you can spot them. The way we usually study black holes is by looking at matter falling into them. This often forms a disc about the black hole before spiralling in. This disc gets very hot and so emits X-rays. We can detect these with an X-ray telescope (not a conventional telescope) and infer the presence of a black hole. Another way is to watch a large number of stars for a long time. Eventually, if you are lucky, a black hole will drift between you and a particular star and you will notice a charge in the image. As light passes by the black hole its path gets bent, so the image is distorted. We call these microlensing events. This can be done with more conventional telescopes, but you have to watch many, many stars in order to find one of these rare alignments.