D-29: The Unquiet Dead

The following is new content.


Good morning and welcome back. Today we’re going to talk about ghosts. Meet PSR B1257+12.

Catchy name, right? This might be why, when the IAU invited suggestions for a name in 2011, it was renamed Lich. This is one of those names which shows you just how geeky scientists are. “Lich” is an old English word meaning “person”, but which gradually came to mean “corpse.” If you go walking in small English villages you will occasionally discover cemeteries that are referred to as “lichyards.”

Lich is indeed a corpse. She’s a neutron star: the shrunken core of a star at least 8 times the size of the Sun. She’s not dense enough to form a black hole but a neutron star is still a fairly scary place.

She spins phenomenally quickly, doing one orbit every 6.22 milliseconds. We know this because stars that spin this quickly have an unusual effect where they narrow their light into a beam, which sweeps around as they spin, like a lighthouse. This beam goes past us once every 6.22 milliseconds, making her appear to “pulse.” This is why stars like this are also called “pulsars.”

You may remember that the core of the Crab Nebula is a pulsar too. This is where they come from: from whatever’s left over after a supernova, which collapses in and maintains a weird sort of life-after-death. Neutron stars are no longer burning. They’ve exhausted all their fuel and only glow because of leftover heat from when they were alive, and will gradually fade as the heat is radiated into space. (Contrary to what the Covenant song may tell you, dead stars do not still burn.) Calling them ghosts is pretty accurate. But this isn’t the only ghost we’re going to mention today.

A supernova is a violent thing, coming only after a massive star has exhausted its fuel, swollen up into a giant, and then burned out the ashes of that fuel. If any planets orbited the star – and stars this big are unlikely to have planets – then they would be destroyed either by the swelling-into-a-giant or by the supernova itself. This means that by the time the star’s remnants form a pulsar, the one thing they absolutely cannot have is planets.

Meet Lich’s planets.


(Image courtesy Yuri Milachay at cienciaescolar.wordpress.com. Earth is included for size reference.)

This was a shock to the scientific community in 1991 when we first heard of them, because neutron stars cannot have planets but here is one that clearly does. A lot of people demanded to look more closely, and a lot of telescopes got pointed it, so by 1994 it was settled: these planets are definitely real.

This is where the IAU shows just how deep their geek cred (and the Italians who recommended the name) runs, because for some people a Lich isn’t just a corpse.

Since you’re reading an astronomy blog, you’re probably heard of a game called Dungeons & Dragons. In it, the word “Lich” is used to describe a malevolent walking corpse which has magical powers and is able to summon other ghosts to its service. These three planets are named after words for ghosts: the small one is called Draugr (a Norse ghost), and the two larger ones are Poltergeist (a German ghost) and Phobetor (a Greek ghost.)

Nerds, amirite?

So where do these planets come from? Well, they’re pretty accurately named, as it happens. Let’s talk about where planets come from.

Planets form from the leftover matter orbiting stars. Over hundreds of millions of years this matter clumps together, clears its orbit and forms planets. If the matter is just hydrogen, then it will form gas planets. However, a lot of stars aren’t made entirely of fresh hydrogen, but have heavier elements in them, elements which have been produced in the core of a star and then blasted outwards after a star dies. Silicon and iron, which rocky planets are made of, come from such places. The Earth is made of the remnants of ancient dead stars.

When Lich went through its supernova, we think that some of the matter it blasted outwards didn’t get very far away, and started to orbit its old core. As that core turned into a neutron star, the matter formed into planets.

The riddle of how these planets managed to survive the supernova is solved: they didn’t survive it. Nothing survived it. They were formed afterwards, out of the wreckage. These planets are literally made of parts of the body of the old star, orbiting the corpse of its core. Naming them after ghosts – and the core after a sorcerous ghost capable of summoning and controlling others – seems pretty appropriate.

Neutron star planets (“pulsar planets” as they’re alliteratively known) are fantastically rare. Only four are confirmed to exist, and we’ve just seen three of them. We think that this is a phenomenally unlikely situation. But space is big, which means that even the least likely things may happen somewhere.

Tomorrow we’re going to continue discussing scary topics, and are going to mention witches. Or possibly necromancers.


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