An Alien Glow Reveals a Neutron Star

Hubble View of the Crab Nebula
NASA/ESA/STScI

When massive stars die in supernova explosions, they leave behind a messy scene. Hubble Space Telescope has often been used to look at the scenes of these distant events and always finds interesting clues. The Crab Nebula is a favorite and typical supernova explosion because it has a secret hidden among the clouds of debris surrounding it: a neutron star.

The typical supernova explosion that creates a scene like the Crab Nebula is referred to by astronomers as a Type II event.

That means the massive star that blew up did so because it ran out of fuel in its core to keep the nuclear fusion process going. When that occurs, the core can no longer support the mass of the layers of material above it, and it falls in on itself. That process is called "core collapse". When the outer layers fall in, they eventually rebound out again, and all that material explodes into space. It forms the shroud of gas and dust that surrounds the former star.

Forming a Pulsar From an Explosion

Not everything is lost to space, however. The remnant of the star—the former core—is crushed into a small ball of neutrons perhaps only a few kilometers across. In the case of the Crab Nebula, the neutron star is spinning very rapidly and sending out pulses of electromagnetic radiation (the strongest in radio waves). That's called a "pulsar". It radiates the surrounding cloud material, causing it to glow.

It's the tiny, star-like object in the middle of the cloud shown in the image provided by Hubble Space Telescope.

The Crab is one of the most commonly studied neutron stars and supernova remnants in the sky. It was first seen in 1054 A.D., probably when the light from the supernova reached Earth. The Crab is about 6,500 light-years from Earth, so the explosion really happened 6,500 years earlier.

It took that long for the light to travel that distance. Sky gazers at the time watched it brighten up to be brighter than Venus. Then, it steadily dimmed down over the next few weeks until it was too faint to see with the naked eye.

There are many accounts of its sighting by cultures around the world, mostly by Chinese, Japanese, Arabic, and Native American observers. There are remarkably few mentions of it in European literature. It remains a mystery why no one wrote about it, and there are plenty of theories about lost manuscripts, a schism in the Church, and various wars that might have kept people from mentioning such a sight in writing.

It really wasn't mentioned much until the 1700s, when Charles Messier ran across it during his search for comets in the sky. He dutifully recorded comet-like fuzzy objects that he found. The Crab Nebula was listed as Messier 1 (M1) in his catalog.

Pulsars Are Strong and Common

A neutron star is a curious object. It is one of a handful of pulsars that have been observed optically, although it appears more strong in radio and x-rays. It spins 30 times a second and has a tremendously strong magnetic field that can generate up to a million volts of electricity.

The field releases huge amounts of energy that radiates through the surrounding cloud, which looks like expanding rings of material in the Hubble image. As it releases energy, the pulsar is slowing down by 38 nanoseconds per day. The Crab Nebula pulsar is quite hot and incredibly massive. If you could capture just a spoonful of neutron star material, it would weigh 13 million tons.

The Crab Nebula neutron star isn't the only one around the galaxy. Astronomers suspect there are around 100 million or so of them in the Milky Way, and they exist in other galaxies, too. This makes sense since massive stars that can (and do) die in supernova explosions are common in galaxies. Not all neutron stars are like the Crab, however. Some are quite old and have cooled quite a bit. Their spin has slowed as well.

Today, astronomers continue to study this nebula and its pulsar with all manner of instruments, working to understand more about pulsars and supernovae in general. What they learn about further uncovers the workings of the weird neutron stars that inhabit the hearts of many supernova remnants.