What Can You See in the Night Sky This Week?

5 December 2011 by Robert Simpson

The constellation of Orion - the Hunter - is now beginning to appear in the Southeast at around 8pm. Orion is one of the most-easily recognised constellations and you can spot him by looking for the three bright stars that make up his belt. Once you have found his belt, look for his sword: a triplet of faint objects dropping down to the left of his belt. The middle of these is not a star but the Orion Nebula, a region where stars are forming about 1,300 light years away. This fuzzy blob is a good spot to try and view using a small telescope.

You can print out our custom star map here, or look at Orion using LCOGT's excellent sky viewer here.

The Orion nebula is a vast stellar nursery - it is roughly 12 light years across - where many young stars and protostars are emerging from a cocoon of dust and gas. These huge 'molecular clouds' of dust and gas are littered throughout our Galaxy, and are where the majority of star formation occurs. Such regions begin as enormous, cold, dusty blobs floating through the Galaxy. Passing stars - or even other molecular clouds - perturb them and create pressure waves that sweep up material. Sooner or later one of these interactions creates a section of the cloud that is very dense, and that's when it begins to collapse under its own gravity. When this happens, the region sucks in material from the rest of the cloud. The once cold material begins heating up and collapses into a warm core within the cloud. Eventually it become hot and dense enough that nuclear reactions begin, and a star is born.

In the Orion Nebula this process of star formation is going on right now, and has occurred multiple times in the Galaxy's history. The cloud is very large, and lucky for us it's fairly close by (in astronomical terms!). As such, we can make out stars within the nursery, and have studied this fascinating object with many telescopes over many decades.

The bright, central part of this star-forming nebula is visible to the naked eye, but the whole region actually spans 10 degrees across the sky - several times bigger than a full Moon. In early December, Orion is clearly visible by 9pm but it will rise earlier each night: it's a prominent Winter constellation. This means you have a long time to see the Orion Nebula, and to take a closer look.

Orion is also host to another example of part of the life of a star. The bright red giant star Betelgeuse can be seen up and to the left of Orion's belt, where the Hunter's armpit is located. This huge star is near the end of its life and is expected to die in a spectacular supernova explosion. Betelgeuse is only 650 light years away and thus such an explosion would be spectacular even from our distant viewing position. This supernova could happen tomorrow, or in a million years - either way that means that this massive star's death is 'imminent' in astronomical terms.

Betelgeuse is one of the brightest stars in the night sky. If you follow the line of Orion's belt downward toward the horizon, you'll find the brightest star of all: Sirius. At present Jupiter shines more brightly than both, and lies in the opposite direction along the belt. All three are good candidates to try and spot whereever you find yourself, and whatever street lighting surrounds you.

Don’t forget that if you want to know about other objects up in the night sky this week, visit the Blackett Observatory’s What’s Up page.

Categories: what's up | night sky | supernovae | star formation | stargazing