Question: how do you find out which stars are neutron stars ?

Evan Keane answered on 13 Jun 2011:

Great question! I will tell you how we know a star is a neutron star, but first I’ll tell you what a neutron star is …

A neutron star is one of the three types of “dead stars”. When a star has shone for its entire lifetime and it is out of fuel it will eventually die. The lighter ones (like our Sun) will end up as white dwarfs (these are white, and small!). Heavier ones than this will have a big explosion (called a supernova) and the bit that’s leftover is a neutron star. The heaviest stars also die in big explosions but there is nothing left over accept a black hole (if you want to know what a neutron star or black hole is I can tell you more if you ask!). One thing to know is that all of these stars are “small” compared to living stars (like the Sun), and together astronomers refer to them as “compact objects”. What is small? White dwarfs can be about 5000 kilometres in radius (this sounds big but it is actually smaller than the Earth, and so very small for a star!). Neutron stars are about 10 kilometres in radius. Black holes (of same mass as the Sun) are only 3 kilometres in size (very small but VERY heavy!). Ok, so now we know the sizes of things …

Remember when I said that a pulsar is a star which spins very fast and emits a ray of light towards us? Then when it spins it is like a lighthouse. Well we know that pulsars rotate very fast – about every second but some are VERY fast and the fastest one we know about spins 716 times every second (that’s fast!). There is an equation in physics which says how fast something can rotate before it blows apart and for something as heavy as a pulsar (and we can measure its mass from observing how strong its gravity is) we realise that it can only be 40 kilometres across (that is about 25 miles) AT MOST! And actually our most precise measurements tell us they are about 20 kilometres across. That is very small for a star! And we know it is too small to be a white dwarf so it must be a neutron star or black hole!

But I said it was a neutron star, how do we know it is not a black hole? Well, something you should know is that everything radiates light. Yes – even you! What type of light depends on your temperature. The Sun emits visible light (this is why our eyes have evolved to work for this kind of light). The Sun is 6000 degrees – very hot! Humans are much cooler than this so they emit a different type of light – infrared. Have you ever seen “night vision” cameras? These are just infrared cameras and you can see people very well with them as they emit so much infrared light! Now what if you are hotter than the Sun – well if we look at pulsars we find that they emit X-rays which means they are very hot – a million degrees in fact! You don’t want to go to a pulsar to get a tan or you would be vapourised! From the temperature and distance we can work out the size of the star also (just like if I measured how much light there is from the Sun I can tell what size the Sun is, and if I measure how much infrared light there is from you I could work out what size you were). It turns out pulsars are too big to be black holes. But they are too small to be white dwarfs. Aha! They must be neutron stars!

Just so you know there are many others ways to know this but I just picked these as examples of the kind of detective work we can do in astronomy 🙂

@bouncyrabbit it seems that rabbits are a popular thing for usernames as I also had a question from @mallfunbunny ! 😀