Since pulsars are something we study here in the Astrophysics Science Division at NASA, and we know it’s a topic people are always curious about, we interviewed astrophysicist Dr. Tod Strohmayer to ask him all about them.
Blueshift: What’s a pulsar? Would you say they look sort of like lighthouses in space?
Tod: A short operational definition is that a pulsar is a spinning neutron star. The pulses that give a pulsar its name are seen each rotation period of the star as a radiation beam generated by the neutron star sweeps across our line of sight to the pulsar.
In that sense they are very much like lighthouses, but unlike terrestrial lighthouses, pulsars can send out beams of X-rays, radio waves, gamma-rays, and maybe even gravitational waves (though those have not been directly detected yet). Some pulsars spin faster than a high-speed kitchen blender! The fastest one currently known spins around 716 times each second. That means that its surface is traveling at about 15% of the speed of light! The scientific term for that is darn fast!
Blueshift: We hear that pulsars have an interesting history – what did early observers think they were?
Tod: The first pulsars were discovered with radio telescopes (radio pulsars), and they currently make up the largest population of known pulsars. The initial discovery was made by a graduate student, Jocelyn Bell! She and her graduate advisor, Anthony Hewish, were so surprised by this very regular, clock-like, pulsing radio signal (the first pulsar had a period of 1.337 seconds) that they thought there was at least some chance that it could be indicative of an extraterrestrial civilization, and they dubbed it LGM-1, for Little Green Men-1.
After other pulsars were discovered it became clear that they were not really little green men, but rotating neutron stars! Only a neutron star could spin as fast as observed and not fling itself apart. When a pulsar was discovered at the heart of the Crab Nebula supernova remnant, the neutron star hypothesis was proved beyond a doubt. A neutron star can be created in the supernova explosion of a massive star.
A composite image of the Crab Nebula showing the X-ray (blue), and optical (red) images superimposed. The size of the X-ray image is smaller because the higher energy X-ray emitting electrons radiate away their energy more quickly than the lower energy optically emitting electrons as they move. X-ray Image: NASA/CXC/ASU/J. Hester et al.
Optical Image: NASA/HST/ASU/J. Hester et al.