Neutron stars – and their very closely related cousins, pulsars – are in the running for the strangest objects in the Universe. Their tiny size and their unusual properties put them in a class of their own.

 

Consider the following:

  1. Neutron stars are the husks of large old exploded stars

You are likely to be familiar with supernovae, those spectacular events in which, at a certain age, the cores of stars explode and violently scatter their mass in all directions over a relatively short period of time.

But what comes after the supernova? Well, it all depends on the size of the star. If the star is relatively small, like our Sun, the remnant is a much cooler orb called a white dwarf. This dwarf star will continue to eject its mass over the next billion or so years as it cools and its gas slowly expands outward as a planetary nebula.

A neutron star is what’s left after a medium-sized supernova explodes. These stars are the smallest and most dense known to exist.

And in the case of a truly massive megastar, what happens is considerably stranger. The huge blast leaves in its wake a black hole, which emits no light and consumes all matter in its proximity.

But when a medium-size star explodes – say, one that has at least two or three times the mass of our Sun but is not enormously larger – its supernova will create a rapidly spinning ball of densely packed heat, matter, gas, and energy. It becomes so dense, in fact, that the protons and electrons are compressed into neutrons and neutrinos. As the neutrinos are expelled outward, nothing but neutrons are left in the core; hence, they are called neutron stars.

This neutron star–containing nebula is about 17,000 light-years away from Earth. Remarkably, it resembles a hand, with the light source, a rapidly spinning pulsar, in the bright central region. The magnetized, 20 kilometer-diameter neutron star spins seven times per second. Image credit: P. Slane (Harvard-Smithsonian CfA) et al., CXC, NASA

Such stars are so small as to be generally invisible from Earth; their existence must be inferred from their behavior. For example, in binary systems, neutron stars can be found continuously drawing matter from their partner, emitting electromagnetic radiation powered by gravitational energy. That is, gas from the normal star in a binary system will be continuously pulled toward the neutron star.

  1. Neutron stars are the smallest known stars in the Universe

Stars come in many sizes, but no other type that is known is nearly as small as a neutron star. In fact, a typical neutron star has a diameter roughly equivalent to the size of Washington, D.C. But that tiny size belies its strength. It pulls gas to itself and sends out electromagnetic waves that are as strong as any in the Universe.

  1. Neutron stars have the densest cores of any known stars

As small as they are, it’s hard to imagine just how dense neutron stars are. They have more mass than our Sun, but they are 60,000 times smaller. Their cores are so dense that a thimbleful of core material would weigh more than the combined weight of every person on Earth.

The Crab Pulsar Wind Nebula, a magnetized neutron star spinning 30 times a second, lies at the center of this nebula. A magnetized neutron star, or a pulsar, spins out energy visible in the X-ray spectrum. This star has more mass than the Sun and possesses the density of an atomic nucleus. Image credit: NASA / CXC / SAO / F. D. Seward, W. H. Tucker, R. A. Fesen

  1. Neutron stars with strong electromagnetic pulses are called pulsars

Certain highly magnetized neutron stars do something very strange: they give off a constant light from both of their magnetic poles. And since they also revolve at stunningly fast, stable speeds, their light sweeps across the heavens in a highly predictable rotation.

The fastest known neutron star pulsates at nearly a quarter the speed of light.

From Earth, we can see these sweeps of light only in seemingly instantaneous flashes – they don’t appear to sweep, they just appear to blink. Think of a lighthouse along the coast: its light sweeps across the sky, but sailors out at sea can see the light only when viewed straight on. To the sailors, the light appears to flash, providing warning and orientation.

 

That’s how neutron stars of this type operate, projecting a brilliant light visible on Earth as intermittent, blinking pulses. Such stars are called pulsars.

  1. Pulsars play a game of cosmic peekaboo

Because of the spinning, intermittent light, appearing and disappearing from view, they create what might be called a cosmic game of peekaboo: Now you see one, then you don’t, in an endlessly repeating cycle.

This image of the Crab Nebula shows the aftermath of the detonation of a supernova that was recorded in China in 1054 C.E. The magnetized neutron star in the middle of this nebula is only 10 km. wide but is more massive than the Sun. Image credit: J. Hester and P. Scowen (ASU), NASA

  1. Neutron stars revolve at a mind-boggling speed

Neutron stars and pulsars are some of the fastest-spinning orbs in the Universe. While the slowest may gyrate at just a few rotations per second, the fastest one measured to date revolves 1,122 times every second – more than a quarter the speed of light!

Strange Behavior, But Far From Rare

With behavior so strange, a size so small, and virtual invisibility (except for pulsars), you might think that very few neutron stars exist in the Universe. But they are surprisingly common. In fact, there are estimates that just in our own galaxy there may be as many as 100,000 neutron stars. They may be invisible to the unaided eye, but there’s no denying their outsize power and performance.