If you're in a place where Halloween is celebrated, you're used to seeing lots of small ghosts and witches about, not to mention images of spiders and snakes and skulls. But if you knew that the sky contained such things, would you stay at home and hide? I hope not, because they represent interesting astronomical objects and I can assure you that our virtual tour will be quite safe.
Cosmic ghosts
I admit that I wouldn't want to meet this cosmic ghost in a dark and lonely place. And certainly not this either.
Fortunately, we can only see them via large telescopes, for they are nebulae, giant clouds of gas and dust in the space between the stars. The first one (catalogued as vdB 152) is often described as a "ghostly apparition" and the picture shows it looking like the common image of a ghost. The second one (vdB 141) is pale with brownish areas suggesting dried blood. It is known as the Ghost Nebula.
Our two cosmic ghosts are reflection nebulae lying about 1400 light years away in the constellation Cepheus. The dust in the first one (vdB 152) is absorbing red light and faintly reflecting blue light from some stars in the neighborhood. In the second nebula the reddish-brown tinges come from young stars embedded in the dust, for the Ghost Nebula is a star-forming region.
Cosmic witch
What is Halloween without a scary witch? You may be able to make out her profile looking towards the bright star in this photo from the Star Shadows Remote Observatory. It's the Witch Head Nebula, a reflection nebula about a thousand light years away. Being closer to us, it's brighter than the two "ghost" nebulae and it's also lit by a very bright star - the beautiful blue Rigel in Orion. However the nebula isn't blue because of the star color. As with the other reflection nebulae, it reflects blue light more efficiently.
Of course a celestial witch needs some celestial transport. Here is NGC 6960, the Witch's Broom Nebula. It's part of the Veil Nebula, which is an enormous supernova remnant, formed from the gases thrown out by a massive star exploding at the end of its life.
Snakes and spiders
There are two snakes among the 88 constellations. In fact, the largest of all the constellations is Hydra the water snake. In Greek myth one of the labors of Hercules was to kill the hydra and put an end to its raiding the nearby countryside. The constellation has only one head and would scarcely have needed a hero of the stature of Hercules to deal with it, but the Hydra of myth had nine heads.
A second snake, the constellation Serpens, is more benign. It's held in the sky by Ophiuchus, a constellation representing Asclepius the god of medicine. His symbol, the rod of Asclepius, shows a snake twined about it.
Features that resemble spider webs are common, but here are two that look rather like spiders waiting in their webs. The first is Spider Crater in the Caloris Basin on the planet Mercury. NASA's Messenger probe took the picture of this dramatic impact crater.
The second one could almost be a space spider. It is, in fact, a spiral galaxy about ten million light years from us in the constellation Camelopardalis the giraffe. It can't be seen in visible light because it's hidden by dust. However infrared light can penetrate the dust, so the Spitzer Space Telescope was able to get this striking picture. A bit surprisingly, the galaxy, which is cataloged as IC 342, is not known as the Spider Galaxy.
The fiery skull
The final stop on our Halloween tour is the most spectacular. This image would be great for a fantasy or horror film. It really looks like there are eyes in the skull of something tormented or terrible. Yet it is simply an X-ray image of the Perseus Cluster of Galaxies.
We've seen some pretty big things on our tour, but nothing like this. The cluster contains thousands of galaxies and is about a hundred thousand light years across. In fact, the image isn't of the galaxies themselves, but of the X-rays given out by the gas between the galaxies. So bright areas, such as the one at the center, are regions of strong X-ray emission. Scientists think this one is likely to be a supermassive black hole.
