Witch Head Nebula (IC 2118). Image credit: Jeff Signorelli

If you're somewhere that Halloween is celebrated, you may be 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.

Fortunately, it's actually a nebula some 1400 light years away in the constellation Cepheus. Nebulae are giant clouds of gas and dust in the space between the stars. This nebula is cataloged as vdB 152, and is often described as a "ghostly apparition". In the picture, it does look like the popular image of a ghost. But it's a reflection nebulae where dust is absorbing red light and reflecting blue light from nearby stars.

Cosmic witch
What is Halloween without a scary witch? The Witch Head is another reflection nebula and you can see her profile above in the header image. The nebula is more intensely blue than vdB 152, as it's closer to us and is lit by a very bright star.

Of course a celestial witch needs some celestial transport. The Witch's Broom Nebula (NGC 6960) is part of the Veil Nebula, an enormous supernova remnant. It formed from material thrown out when a massive star exploded at the end of its life. [Image credit: T.A. Rector (U. Alaska)]

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 mythology, one of the labors of Hercules was to kill the hydra and put an end to its raids on the nearby countryside. The constellation has only one head and would scarcely have needed a hero like Hercules to deal with it. However, the Hydra of myth had nine heads.

A second snake, the constellation Serpens, is quite benign. It's held in the sky by Ophiuchus, a constellation representing Asclepius, the god of medicine. His symbol is a rod with a snake twined around it.

Features that resemble spider webs are common. One example is the dramatic impact crater Apollodorus on the planet Mercury. It's nickname is Spider Crater. It's a feature in the Caloris Basin. [Image credit: NASA/Johns Hopkins University]

The fiery skull
The final stop on our Halloween tour is the most spectacular – a giant fiery skull that could be the star a horror film with the eyes of a tormented being. It turns out to be an X-ray picture of the Perseus Cluster of Galaxies. [Image credit: A. Fabian (IoA Cambridge) et al., NASA]

We've seen some pretty big things on our tour, but nothing to compare to the Perseus Cluster. It 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. 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.