Medusa statue by Bernini (1630) in the capitoline Museums, Rome. [Encyclopaedia Britannica]

Astronomers often use colorful language for objects in space. But unlike the ghosts, ghouls and vampires in horror stories, you can safely read about the astronomical ones late at night.

Galactic Ghosts

Little Ghost Nebula
The Little Ghost Nebula (NGC 6369) is a planetary nebula over 2000 light years away in the constellation Ophiuchus (Serpent-bearer). William Herschel discovered it in 1784, seeing it only as a dim nebula. He would be amazed to see the complex and beautiful structure that the Hubble Space Telescope shows us.

NGC 6369 got its nickname because of its ghostly faintness. Yet it is a ghost in a way, for it was formed of material from a dying star. What's left of the star is a white dwarf, which you can see in the picture near the center of the nebula.

Ghost of the Summer Sun
Arcturus, in the constellation Boötes, is a warm yellow star and the brightest star in the northern celestial hemisphere.

You can't see Arcturus from the southern hemisphere around Halloween. However, in and beyond the northern mid-latitudes it's visible before sunrise and after sunset. And for a few days around October 29, when you see Arcturus, it's where the Sun was at that time in late June and early July. That's why it's sometimes called the “ghost of the summer Sun”.

In the far north – places like Barrow, Alaska or Tromsø, Norway – you don't see the Sun dip below the horizon in the summer. So you might wonder what happens to Arcturus around Halloween. Like the midnight Sun, Arcturus drops low in the night sky, but stays above the horizon all night.

Ghouls

In folklore, a ghoul was a misshapen monster that plundered graves and feasted on corpses.

Algol, the Demon Star
Algol is a bright star in the constellation Perseus. It represents the head of the terrifying Medusa, whom Perseus slew. Although the myth is Greek, the star's name is Arabic. It means the demon, and is related to the word ghoul. Different cultures gave the star other horrific names, e.g., Satan's Head, Blinking Demon, or Piled-up Corpses.

You might say Algol is a blinking demon, because it's a variable star whose brightness dims for several hours every 2.87 days. The first clear record of its variability was made in 1667 by Italian astronomer Geminiano Montanari. These regular changes in brightness happen because Algol is actually two stars orbiting each other. From our viewpoint, one regularly eclipses the other.

The Galactic Ghoul
Far from exhibiting ghoulish behavior, the “Galactic Ghoul” – tamely cataloged as the DR 6 nebula – is a stellar nursery in the constellation Cygnus. It looks as if it has two eyes, a nose and a devouring mouth. In fact, these are cavities excavated in the nebula by the stellar winds and strong radiation of young stars. To give you an idea of its size, the “nose” is about 3.5 light years long. It contains a small cluster of newborn stars, and they are big babies. Each one is between ten and twenty times the mass of the Sun.

Zombies and vampires

Zombie stars
A colossal explosion! For a time, it's as luminous as an entire galaxy. That's a supernova. Type II supernovae occur when massive stars run out of fuel. Without the outward pressure from by nuclear fusion, the star collapses and then explodes. But there is also a Type Ia supernova which originates with a white dwarf.

Although a white dwarf is a dead star, sometimes one is brought back to life. To do this, it needs additional mass to get hot enough for nuclear reactions to restart. This can happen when a white dwarf in a binary system sucks material from its companion. It may get enough material for nuclear fusion to occur. If that happens, all the energy is released in a few seconds in a violent chain reaction. This is a Type Ia supernova.

Vampire stars
Star clusters are made up of stars formed from the same collapsing nebula, so they're of a similar age. The differences in color and luminosity are determined by their starting masses. High-mass stars mature quickly, burn brightly, and die young. So old star clusters shouldn't contain massive blue stars, because these are blue stars.

Yet star clusters may contain a certain number of unexpectedly massive blue stars. And there is evidence that they are genuinely part of the cluster – not in the foreground nor straggled in from elsewhere. But how have they kept their youth? Some do it as vampire stars in binary star systems. The more massive of the two stars evolves more quickly. As it uses up its main nuclear fuel, it expands. This leaves it vulnerable to losing its outer layer to its companion. The companion stays young by “feeding” off it.

Although vampire star sounds quite dramatic, it's a mass transfer, which sounds rather tame. The process is simply a natural consequence of the way some pairs of stars evolve.