Cosmic Ghosts Ghouls and Vampires
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.
NGC 6369 got its nickname because of its ghostly faintness. Yet it is a ghost in a way, for it's 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 around 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.
Traditionally, 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 horrific names, e.g., Satan's Head, Blinking Demon, or Piled-up Corpses. Yuk!
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. The reason for the regular changes in brightness is that 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, but 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
A colossal explosion that for a time is 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 created by nuclear fusion, the star collapses and then explodes. But there is also a Type Ia supernova which originates with a white dwarf.
A white dwarf is a dead star. Its nuclear fusion is over and it has collapsed into an object the size of the Earth, but with the mass of the Sun. This is the end of Sun-sized stars, and they simply gradually cool.
However sometimes a white dwarf is brought back to life. This needs additional mass to get the temperature high enough for nuclear reactions to restart. It can happen when a white dwarf in a binary system sucks material from its companion. If it gets enough material for nuclear fusion to occur, rather than proceed steadily, all the energy is released in a few seconds in a violent chain reaction. This is the Type Ia supernova.
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, for example, you wouldn't expect old clusters to contain massive blue stars, because these should be young stars.
Yet star clusters do contain a certain number of unexpectedly massive blue stars. 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, mass transfer is rather humdrum. Yet the transfer is a natural consequence of the way some pairs of stars evolve.
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