This incredible density comes about because of how neutron stars form. A star is held together by a balance between gravity trying to contract it and an outward pressure created by nuclear fusion processes in its core. If the initial star is around 20 solar masses or more, the core collapses into a black hole instead.

A neutron star is about 20 km in diameter and has the mass of about 1.4 times that of our Sun. This means that a neutron star is so dense that on Earth, one teaspoonful would weigh a billion tons!

The usual definition of nuclear density gives for its density: ρnucleus = m / V = 238 x 1.66 x 10-27 / (1.73 x 10-42) = 2.3 x 1017 kg/m3.

Pulsars are quickly rotating neutron stars — under something like 10 miles in size, rotating with periods less than about 1 second, made up of neutrons (plus some other stuff). A neutron star is apparently the product of a supernova explosion. It’s the leftover core of the star that went supernova.

The charged particles exert a reaction force on the magnetic field slowing it and the pulsar down. Eventually, the pulsar dies away when the neutron star is rotating too slowly (periods over several seconds long) to produce the beams of radiation.

Geminga

about 200 million years old

The universe is full of weird objects, but pulsars take the prize as the strangest things scientists can study directly. Astronomers can see pulsars only because electromagnetic radiation, especially radio waves, streams from their magnetic poles. As the pulsars spin, these streams point, once per go-around, at Earth.

Except for a few pulsars in our neighbouring galaxies, the Magellanic Clouds, most pulsars are found to be well outside of our solar system but within our Galaxy. The youngest pulsars (we call them young, but these pulsars are many thousands of years old) are found to lie within the plane of our Milky Way Galaxy.

Like other neutron stars, magnetars are around 20 kilometres (12 mi) in diameter and have a mass about 1.4 solar masses. They are formed by the collapse of a star with a mass 10–25 times that of the Sun. A magnetar’s magnetic field gives rise to very strong and characteristic bursts of X-rays and gamma rays.

Pulsars were a serendipitous experimental discovery made in 1968 by Jocelyn Bell and Anthony Hewish. They were discovered using a radio telescope array just outside Cambridge. The telescope scanned across the sky with time due to the rotation of the Earth.

Pulsars orbiting within the curved space-time around Sgr A*, the supermassive black hole at the center of the Milky Way, could serve as probes of gravity in the strong-field regime.

From Earth, pulsars often look like flickering stars. On and off, on and off, they seem to blink with a regular rhythm. But the light from pulsars does not actually flicker or pulse, and these objects are not actually stars. Pulsars radiate two steady, narrow beams of light in opposite directions.

It has a diameter of about 78 billion miles. For perspective, that’s about 40% the size of our solar system, according to some estimates. And it’s estimated to be about 21 billion times the mass of our sun. So there you have it, black holes can be millions of times larger than suns and planets or as small as a city.

This is a pulsar. If the stellar core is more than about 4 solar masses gravity overcomes neutron degeneracy pressure. Once the core collapses below its Schwarzschild radius, spacetime is curved to the point where not even light can escape. This is a black hole.

Magnetars vs. “The gravity from the black hole will always be stronger, because the lowest mass black hole is always more massive than the most massive neutron star,” Plait says. “[But] the magnetism of the magnetar will be stronger, in general.”

Quasars are very compact objects – the word “quasar” and the acronym “QSO” are short for “quasi-stellar radio source” and “Quasi-stellar object” respectively, due to their ‘star-like’ appearance. Quasars were originally discovered with radio telescopes in the 1950s – hence the “r” in “quasar”.

Although quasars are known to drive strong winds and jets of relativistic particles that can be dangerous in their own right, Forbes and Loeb looked at the damage caused by their light alone.

Quasars inhabit the centers of active galaxies and are among the most luminous, powerful, and energetic objects known in the universe, emitting up to a thousand times the energy output of the Milky Way, which contains 200–400 billion stars.