Orion the Hunter

Ken Tapping, January 15th, 2014

In the sky this week…

  • In the next week or two Venus will appear low in the sky before dawn.
  • Jupiter dominates the sky overnight, Mars rises around midnight and Saturn at 3am.
  • The Moon will be Full on the 15th.

On a clear night at this part of the year, the southern sky is dominated by what is probably the most spectacular of all the constellations, Orion, "The Hunter". Find Sirius, a brilliant, flashing, blue-white star, and then look up and to the right. What you will probably notice first is three stars, equally spaced and in a row, pointing very roughly back in the direction of Sirius. These stars represent Orion's belt. From there it is easy to see that the belt lies in a rectangle of stars, defining his shoulders and knees. His left shoulder is marked by Betelgeux, an orange-red giant star, and his right knee by Rigel, a bright, white star. Now head back to his belt. From the middle star there is a streak of faint stars and elusive, silvery fuzziness heading obliquely downwards, where his sword would be. His head is marked by three faint stars.

Orion is such a spectacular constellation that it is not surprising that it turns up in star maps made by many cultures. Moreover, in most cases they assigned a warrior identity to it. On the other hand, unlike other superheroes like Hercules, there are few complete stories about him. The fragments that do exist give us the impression of a strong, brave and handsome man, but not a modest one. His arrogance and boastfulness upset the gods so much that in one story they set a scorpion to chase him. That scorpion is represented in the sky by the constellation Scorpio, or Scorpius. Orion stays as far away as possible, on the opposite side of the sky, which is why we see Scorpius in summer skies and Orion in the winter.

Apparently his arrogance did not deter Diana, the goddess of the hunt, and she became quite enamoured of him. Unfortunately, she was also the focus of interest by the god Apollo. One of the problems with the Greek gods was that they were highly immoral and did not care much about the impact of their deeds on others. One day Apollo was watching Orion swimming, way out at sea. His head was a small black dot in the distance. Then Diana came along, carrying her bow and arrows. Apollo pointed out over the water and challenged her "I bet you can't hit that..."

The ancient Egyptians had certainly noticed the constellation, and assigned great importance to it. Either by design or coincidence the three large pyramids at Giza, located near Cairo, are arranged in exactly the same in relative size, spacing and orientation as the three stars of Orion’s Belt.

If you take a pair of binoculars or a small telescope and scan down Orion's sword, starting at the belt, you will see a silvery blob. A larger telescope will show it to be a beautiful cloud of glowing gas. There are newly born stars in the cloud, and their radiation, rich in ultraviolet light is making the cloud material glow. That cloud is known as the Great Nebula, where nebula means "cloud".

However, the impression one gets of an isolated cloud of glowing gas is wrong. There is a huge cloud of dark, cold gas in that region of the sky, known as the Orion-Taurus molecular cloud, because over billions of years chemical reactions in the cloud have built a witches brew of organic molecules, many of which are the precursors for life. We can observe this cloud because its material gives off infrared and radio emissions, including the signatures of all those molecules. There are so many signatures that the problem is not in detecting molecules, but in identifying them.

The glowing cloud we see in Orion is a place in the cloud where the material has collapsed to form new stars, and probably planets too. The radiation of those stars has evaporated a hole in the cloud, and what we see in the sky is the glowing inner surface of a huge cavity with newly-born stars in it.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

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