# Ask an Astronomer - Stars

## 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.

## Light years and Red giants

Published on 24/06/2013
Question:

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.

## What to read on stellar theory?

Published on 26/01/2013
Question:

I am a current A2 Physics student and part of my course is a research project on a topic of our choice. Stars have always been interesting to me. To me, understanding how stars evolve and produce the elements that make up the world we see before us is fascinating. Could you recommend any scientific papers, journals or books?

Stars are indeed fascinating. There are many interesting areas of physics involved in understanding stellar evolution, from fluid mechanics to atomic physics. Understanding nucleosynthesis is not only important for understanding where the elements come from, but also how stars generate their energy.

You can find a lot of information online on this subject. Wikipedia is a good place to start (although you should know to be careful, as it's not a perfect source). I would also recommend this article by John Bahcall:

Bahcall worked a lot on stellar models, in particular looking at the solar neutrino problem (which may be an interesting aside for you).

In terms of journal papers, it is difficult to make recommendations as (i) they are likely mostly too advanced and (ii) you will usually require a subscription to read them. Many more recent articles are available for free via arXiv, however most of the key research on nucleosynthesis was done in the early 20th century before the arXiv existed. However, there are two Nobel lectures (that would later be published as scientific reviews) that you should be able to read:

This is by Hans Bethe, who was very smart. He invented quantum electrodynamics on a the train home from a conference. He was actually a theoretical particle physicist, and only did a little work on stars.

This is by Willy Fowler, who did spend much of his career on nuclear reactions.

Textbooks are similarly difficult to recommend, as they are expensive. Really you need a nice library to purchase things for you. It might be best to see if you can find any books on stellar evolution locally and work with what you have. If you are looking for concrete recommendations, then I would say Stellar Structure and Evolution by Kippenhahn & Weigert is a good choice. Principles of Stellar Evolution and Nucleosynthesis by Clayton would be more detailed, but is also a little more old fashioned, and perhaps not as readable.

Published on 13/11/2012
Question:

I want to know how long is the longest lifespan a red dwarf star can ever have? Also, I want to know the approximate percentage of red dwarf stars that can live for trillions of years.

Models of stellar evolution suggest that the lowest mass red dwarfs (about 8% the mass of the Sun, these are the smallest objects that can fuse hydrogen) can last for something like 10 trillion years.  Red dwarfs are usually defined to be stars with less than about half the mass of the Sun.  At about a quarter the mass of the Sun a star becomes completely convective, so the gas in the star circulates all the way from the core to the outer envelope, whereas stars like our Sun only have a region near the surface that is convective.  Being completely convective means that the star can access, and burn, all of its hydrogen reserves whereas stars like our Sun will still have significant amounts of hydrogen when they leave the main sequence and die.  This, combined with the decrease in the speed at which a star fuses hydrogen as it decreases in mass, means there is thought to be a jump in lifetime at about 25-30% the mass of the Sun to over a trillion years.  More than half of all red dwarfs probably fall into this mass range and so will likely live for over a trillion years.  Incidentally these completely convective stars will also never become red giants, but will simply gradually run out of nuclear fuel and transition directly into white dwarfs.

Bear in mind however that there is considerable uncertainty in these estimates since the present age of the Universe is about 13.5 billion years, which means there has simply not yet been time for any star with a mass less than about 80% that of the Sun to complete its evolution.  As such we cannot observe any red dwarfs in these advanced stages of their lives to check whether our models are correct.

## What are the drifting stars that we see at night?

Published on 09/05/2011
Question:

What are the drifting stars that we see at night?

If by 'drifting stars' you mean stars that appear to cross the sky in a matter of only around 4 minutes or so, then they're not stars, but artificial satellites. They reflect the sunlight down to the Earth's surface as they orbit around it. Some of these can appear very bright indeed - such as the 'iridium' network of satellites which have large, very reflective antennae. The International Space Station can also sometimes be seen at morning or evening twilight; and many other satellites can be seen at much fainter brightnesses throughout the night.