# Ask an Astronomer - Galaxies

## Star cluster densities

Published on 09/04/2014
Question:

In star clusters like M55 what is the average distance between them? They look all bunched together but I am curious to know just how close together they are.

Globular clusters like M55 are indeed very dense.  As you may know the nearest other star system to our own is Alpha Centauri, about 4.2 light years away.  In a globular cluster however the typical distance between stars is only 1 light year.  In the centre the density is higher still, the typical distance between stars can be as little as a few hundred AU, comparable to the size of the solar system.

## Number of stars in a galaxy

Published on 24/06/2013
Question:

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.

## Looking back in time

Published on 22/01/2013
Question:

When a picture is taken of deep space and it is said that it is from when the universe was 500,000,000 years old.  Mainly saying that you're looking into the past.  That doesn't make sense to me for the fact that you're able to capture a picture.  Distance and time can coincide but in this case i dont get how this theory works with space?  I understand at such a distance it takes time for light to reach us, the point I'm trying to make is that how can it be said that what we view from deep space is the past not the present?

The effects of large distances and time in astronomy can be a little confusing.  Take as an example Proxima Centauri, the nearest star to our Solar System.  This is 4.2 light years away, which means that it takes light 4.2 years to get from Proxima Centauri to us.  Now since the only way we can see something that has happened at Proxima Centauri is through light, this means anything we see at Proxima Centauri actually happened 4.2 years ago.  If there were a person on Proxima Centauri and they had an exceptionally powerful torch, which the flashed at Earth, it would take 4.2 years for the torch flash to reach us, so by the time we saw it the person would actually have flashed the torch 4.2 years ago.  Now as I said Proxima Centauri is very nearby, when we look at objects in the distant universe they are much farther away, billions of light years, so when we see them we are seeing light that left them billions of years ago, when the universe was much younger.  As a result we can in a way think of looking at objects that are very far away in the distant universe as looking back in time, because the light has taken so long to reach us that the universe has changed a lot in the time it has taken the light to get here.

## Motion of the Galaxy

Published on 03/01/2013
Question:

I'm a teacher in Belgium and get a lot of questions about the universe. I did my research but never found out one thing which is also my question. Is there proof that the whole Galaxy (the earth, sun, moon everything) also travels with a certain amount of speed?

There is indeed evidence that the whole galaxy, and indeed the whole of the Local Group of galaxies (the small cluster of galaxies that the Milky Way is part of) is moving relative to the rest of the universe.  What we use is the Cosmic Microwave Background.  This is radiation left over from the very early universe and is very smooth (the variations are less than 1 part in 100,000).  Since it comes to us from everywhere on the sky equally it makes a good reference to measure our speed against, and since it is very smooth you can see even quite small velocities.  If you look at this image the big gradient that you can see is due to our motion through the Universe, the radiation seems slightly hotter in the direction we are moving towards and slightly cooler in the opposite direction due to the Doppler effect.  Now of course Earth is orbiting the Sun, and the whole Solar system orbits the centre of the galaxy, but we know what these motions are and they don't account for all of the motion in that image, so the Milky Way must also be moving.

## What is the bulgy centre of spiral galaxies like Andromeda?

Published on 09/05/2011
Question:

What is the yoke like bulgy centre of galaxy that we see in pictures like Andromeda, what is it made up of?

All spiral galaxies like Andromeda have a round bulge at the centre of the flat disc that contains the spiral arms. This 'bulge' is just a large ball of stars all tightly packed together; they are yellowy-red in colour as they are a lot older than the blue stars being formed in the spiral arms.