Institute of Astronomy


Ask an Astronomer at the IoA

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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Number of stars in a galaxy

Published on 24/06/2013 

As an astronomer when you look at distant galaxies do you sometimes wonder what it would be like in those galaxies? Perhaps ellipictal and dwarf galaxies not spiral galaxies because our milky way is one of them. Also how do astronomers approximate the number of stars in each galaxy?

It is indeed interesting to think about what it would be like in a different galaxy, or even in a different part of our own galaxy.  Nearer to the galactic centre for example there would be many more stars in the night sky.

To approximate the number of stars in a galaxy essentially what we do is look at how bright the whole galaxy is and then divide that by the brightness of an individual star.  There are additional things we do to make it more accurate however, not all stars are the same mass and brightness for example, and there may be dust in the galaxy obscuring some of the stars, but we can look at the spectrum of light from the galaxy (how bright it is at different wavelengths) to help us disentangle these problems.

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.