What Is It? Neutron Star
The astronomy world was rocked recently by an announcement that an international collaboration of physicists and astronomers detected gravitational waves from two colliding neutron stars. Not only were gravitational waves detected, but scientists collected data on the event from across the electromagnetic spectrum: ultraviolet, x-rays, infrared, radio, and the optical. It was a truly impressive feat that opens entirely new avenues in observational astronomy.
But wait…what even is a neutron star?
The short answer is that nobody knows for sure. We know they exist – thanks, Jocelyn Bell Burnell! – but their internal structure is a bit of a mystery. They are one of those strange and fascinating results of general relativity. When a star that is about 1.4 times the mass of the sun runs out of fuel, its core collapses and results in a massive explosion, leaving behind a dense remnant.
OK, that doesn’t sound so weird. Actually, wrong. It is weird. Listen, think about the sun. The sun is a fairly typical yellow dwarf on the main sequence. Stars, even middle-sized stars like the sun, are incredibly massive. They start their lives under the influence of gravity, but are prevented from collapsing all the way down by the thermal pressure generated by the nuclear fusion in the core.
The sun will ultimately blow off its outer layers and transform into a white dwarf. If you get much bigger than the sun, however, we get fireworks.
The resulting supernova, however, doesn’t completely obliterate the star. If the star is between the Chandrasekhar limit of 1.4 solar masses and three solar masses, the explosion leaves behind a small, fantastically dense remnant. We’re talking a diameter of about 20 miles and a density of about one billion tons per cubic inch.
It seems impossible, right? How does it work? After a massive star uses up all of its fuel, there’s nothing left to keep the star from succumbing to its own gravity. So the star squishes down further and further until the protons and electrons literally fuse to form neutrons. These neutrons, as tiny as they are, are what keep some massive stars from collapsing straight into black holes. One neutron can’t occupy the same state as another neutron and the pressure exerted by the neutrons is enough to stave off gravitational collapse.
All of this means that the inside of neutron stars are under intense pressure. So much pressure, in fact, that it’s hard to determine what the inside of such an object would look like. That’s actually what makes the gravitational wave finding so exciting. Neutron stars are so dim that they are hard to detect with normal telescopes. Gravitational waves give us another avenue to explore these truly strange objects.