Institute of Astronomy

 

Ask an Astronomer - Stars

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:

http://www.nobelprize.org/nobel_prizes/physics/articles/fusion/

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:

http://www.nobelprize.org/nobel_prizes/physics/laureates/1967/bethe-lecture.pdf

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.

http://www.nobelprize.org/nobel_prizes/physics/laureates/1983/fowler-lecture.pdf

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.

Lifetime of red dwarfs

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.

Is Betelgeuse about to explode?

Published on 01/05/2011 
Question: 

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 
Question: 

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.