When very large stars die, they go out with a huge explosion called a supernova. The outer layers of the star are ejected outwards in a magnificent, extremely bright explosion that can outshine an entire galaxy; the inner core of the star collapses inwards, and if the core is between 1.4 and 3 times the mass of our sun, it forms a neutron star.
Neutron stars are often seen as pulsars. But what is a pulsar? Pulsars are rotating neutron stars that have highly focused jets that point towards us at regular intervals. To explain this type of object and what we see, astronomers like to use a lighthouse analogy:
When you are near a lighthouse at night, you see a sight just like the one in the above link. A beam of light shines out in a specific direction, which rotates at a fixed rate. If you’re close to the lighthouse, you see the light beam regardless of the direction, but what happens when you are far away from the lighthouse? When you are far enough away, you will only see the light from the lighthouse when it is pointing directly at you. And you’ll only see a dot of light when it is pointed at you. So over time, you will see a blinking dot coming from the lighthouse.
The same thing is true with a pulsar. Pulsars can only be seen when the focused beam coming from it is pointed directly at us. So what we observe is a blinking dot of light, or rather a repeating signal. These blips are called pulses (hence the name pulsar).
Because many pulsars won’t be pointing towards Earth, there are many more that will never be discovered (unless we travel to a different star system that the other pulsars point at).
In general, pulsars rotate really fast (compared to Earth, anyway), but some rotate much faster than others. They can rotate many times each second or faster! Over time, pulsars slow down, so comparing rotation rates can give us a good qualitative estimate of a pulsar’s age.
When pulsars were first discovered, it was thought that they could be alien signals. Don’t laugh–put yourself in the shoes of Jocelyn Bell (and her thesis advisor Antony Hewish) who discovered repeating signals coming in at 1.3 second intervals, much faster than any known stellar rotation rates (and in the radio, no less)! What would you think it was? Eventually it was realized that this was not the case, and the Little Green Men (LGM) theory was put to rest–but Hewish received the Nobel Prize for the discovery (yeah–in typical sexist fashion, Bell was pushed aside and didn’t get her just reward).
To end this entry, some fun facts about pulsars:
1) The Crab Nebula, which resulted from an explosion observed from Earth in 1054 AD, has a pulsar in the center that rotates at a rate of 30.2 times each second.
2) The fastest known pulsar rotates at a rate of 716 times each second!
3) Pulsars have extremely strong magnetic fields, ranging from roughly 1 trillion to multiple quadrillions of times larger than the Earth’s magnetic field.
4) Not all neutron stars are pulsars. All pulsars are neutron stars.