What Is #10: A Neutron Star?

The 'What Is' Series

A couple of months ago I started a new series on this blog titled "What Is". Thus far the focus is on the field of astronomy where we hear all sorts of cool science taking place in space. The goal of this series is to explain the details of different objects we hear about relatively frequently.

Stars

I have recently covered Brown Dwarfs and White Dwarfs. Brown Dwarfs are objects that were not quite large enough to fuse hydrogen, so they never reached full star status. White Dwarfs are the final stage of a low mass star such as the Sun. After these low mass stars shed away their outer layers, the stellar core leftover is called a White Dwarf. The end stage process is different for stars that start out with more mass. Stars starting with a higher mass of 5-8 or more solar masses, will end their lives much differently. They will explode as a supernova (to be covered in a later post) and leave behind a small stellar core called a Neutron Star.

Neutron Stars

If a star begins its life with more than 5-8 solar masses, it will take a different path in the latter stages of life. It still fuses hydrogen to helium in the core, just as all stars do for most of their lives. Smaller mass stars like the Sun can fuse up to carbon in their cores. At this point the core can no longer contract. But a higher mass star has greater gravity and the core continues to collapse, allowing it to fuse all the way up to iron. At this point the star explodes as a supernova.

The first direct observation of a neutron star in visible light. The neutron star being en:RX J185635-3754. Credit: Fred Walter (State University of New York at Stony Brook) and NASA. Source: ST Scl. neutron stars are really small and can't be seen by the eye

In these higher mass stars, the core contracts until it is only 10-15 km in size, so very small. Yet it has the mass of several suns. Therefore it has a very large gravitational force on its surface. In fact, if you could land on a neutron star and survive, you'd have to launch with a velocity of about half the speed of light to leave and not be pulled back. Wow!

Most high mass stars end their lives with a supernova explosion and leftover Neutron Star. The very highest mass stars, however, take it one step further. In these stars, the mass is so great and the gravitational force is so great, the stellar core doesn't stop collapsing. It continues to collapse into an infinitesimally small space called black hole. That's a topic for another day.