A Trip Through The Universe (Episode 9)

by Carson

Talking about exotic objects, how about white dwarfs, neutron stars or black holes? They are all stellar remnants, or dead stars.

White Dwarfs

When a low-mass star under 8 solar masses dies, it collapses and ends up Earth-sized because of electron degeneracy pressure. Imagine Sun being shrunk to a millionth of its volume… That’s it, a white dwarf! These are extraordinarily dense objects so its gravitational pull is so strong that it would flatten you.

White dwarfs are hot but dim because their volume is so small. Even though it can be twice hotter than Sun, it radiates at only a tiny fraction of Sun’s luminosity. That’s why white dwarfs can live for billions of years to become a black dwarf, which doesn’t emit any light.

White dwarfs in binaries can also trigger a nova or a supernova. When matter flows into a white dwarf, it could trigger hydrogen fusion in a narrow zone if enough matter is transferred. If it reaches a critical mass, the white dwarf explodes and formed a type Ia supernova.

The closest white dwarf to Earth, Sirius B, is just 8.6 light years away. Other examples include Procyon B, IK Pegasi b, Van Maanen’s Star, BPM 37093, etc.

Neutron Stars

When a high-mass star dies, it forms a neutron star. Neutron stars are so dense that a teaspoon of it weighs more than a billion tons. That’s because neutron stars are almost purely neutrons. The protons and electrons squeezed together and collided into neutrons because of the extreme pressure inside it, created by gravity. The mass of a neutron star is no less than 1.5 times that of Sun but squeezed into a sphere about 20 kilometres wide.

Their temperature is about a million degrees so even though they are small, they have three quarters the luminosity of Sun. Also, their magnetic fields are billions of times stronger than Earth’s. Some neutron stars have stronger magnetic fields, making them pulsars, which emits electromagnetic radiation from their poles. If the beam is pointed to us, we can detect them.

Some of pulsars can even be magnetars, which has a very strong magnetic field. They are the strongest magnets in the Universe. Magnetars don’t last long. They will either become a pulsar or a normal neutron star in a short time.

Neutron stars have a very hard crust. If the crust moves or breaks, a starquake happens. A starquake from the magnetar SGR 1806-20 50,000 light years away, compressed Earth’s magnetic field and partially ionized Earth’s upper atmosphere. If a starquake occurred closer to Earth, the result would be catastrophic and devastating.

Black Holes

When a star with even higher mass — exceeding 20 solar masses dies, it becomes a stellar mass black hole. There is a kind of black hole called supermassive black holes, I’ll talk about it later. Black holes are infinitely dense because its centre — the singularity is a spot (like a pixel) in spacetime, and its mass isn’t limited. When its mass increases, its gravitational field strengthens so its event horizon, which is where the escape velocity equals the speed of light, expands.

When you approach a black hole (especially a stellar mass one), you will be spaghettified by the extreme tidal forces and die before going into the event horizon. Since its gravitational effects are infinitely strong, at the singularity, spacetime is infinitely bent.

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