This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Nebulae run the full gamut of a star's life, from conception to death. Emission nebulae are stellar nurseries, in which stars and even planetary systems form. Planetary nebulae and supernova remnants, however, mark the spectacular end of a star's life. Nebulae come in all shapes and sizes, and some even resemble familiar objects more often seen in a horror story or fancy dress shop than the night sky...
The Tarantula nebula is an extremely bright emission nebula in the nearby Large Magellanic Cloud — a dwarf galaxy that orbits our own Milky Way. The Tarantula nebula measures a gigantic 1000 light years across, making it the largest star forming region in our Local Group of galaxies. Were it at the same distance away from us as the Orion nebula, it would take up a large chunk of the sky and cast shadows. Luckily for northern arachnophobes like myself, this nebula lies in the southern constellation Dorado.
The Ghost Head nebula is another star forming region located in the constellation Dorado and also belongs to the Large Magellanic Cloud. This one only spans fifty light years, though. The two bright regions — the "eyes" of the ghost — are actually glowing lumps of hydrogen and oxygen made by intense radiation and fast stellar winds from massive, young stars at the heart of the nebula. The image was made using representative colours: red and blue regions show hydrogen gas that's been heated by nearby stars, and green shows oxygen gas.
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The Little Ghost nebula is a planetary nebula in the Ophiuchus constellation. It looks more like an all-seeing eye than a ghost, in my opinion, but either works for Halloween. Planetary nebulae aren't anything to do with planets at all, but got the name because they looked similar to gas giants through the small, optical telescopes of the 18th century, when they were discovered. They form at the end of a Sun-like star's life, when it throws off its outer layers while its core shrinks into a white dwarf. Our own Sun will go through something similar in five billion years... Scary stuff.
The Cat's Eye Nebula, three thousand light years away, was one of the first planetary nebulae to be discovered but still holds many mysteries. Concentric rings surrounding the inner nebula are actually bubbles of ejected mass and appear to have been thrown off in 1500 year intervals. There are several possible explanations for this, but none have yet been confirmed. The nebula is so complex that astronomers think the bright central object may not be just one star, but two — in a binary system. If that doesn't scare you, have a look at some pictures of the Eye of Sauron from the film adaptation of Tolkien's Lord of the Rings — see the resemblance?
The Snake nebula is made up of a series of dark absorption clouds of molecular gas and interstellar dust, and can be seen in the constellation Ophiuchus — but only in areas with very little light pollution. Dust grains made mainly of carbon absorb visible light and reradiate it as infrared light, which is invisible to humans, blocking out stars that lie behind the Snake nebula in the sky.