The favourite constellation of the Australian summer, Orion, is still prominent in the evening sky of autumn
The constellation of Orion imaged on the evening of 12 March 2012 with the main stars labelled. Image and copyright Nick Lomb ©, all rights reserved
There is a lot to see in the evening sky this March 2012. The Moon is out of the way until near the end of the month and the two brightest planets Venus and Jupiter are close together in the north-west. On the other side of the sky Mars is still bright in the east after its recent opposition. The planets are augmented by the International Space Station making bright evening passes and there are occasional bright flashes from Iridium satellites.
Among all this activity it is also worth looking at some old favourites such as the constellation of Orion that is prominent high in the western sky. Orion is one of the easiest constellations to recognise with four bright stars in a rectangle and three stars in a row in the middle. In Australia many people refer to part of the constellation as the Saucepan – the three stars of the belt form the base and the dagger with the Great Nebula of Orion in the middle represent the handle.
The international GLOBE at night project wants people to observe one of two or three constellations in the evening sky to report on the brightness of their sky. Orion is one of the two southern hemisphere constellations available for this purpose. There are two opportunities left this year to contribute, until 22 March 2012 and 11 to 20 April 2012. Contributing an observation is easy to do and there is a cool webapp so that observations can be submitted in real-time.
The brightest star in the constellation is the blue supergiant Rigel that represents the left foot of the giant Orion according to Greek mythology. Strangely, the star is named Beta Orionis even though it is the brightest star. It is at a distance of 860 light years from us and radiates 85 000 times as much energy as our Sun. There is a faint companion that is itself double and is so far from the main star that it takes over 20 000 years to make one circuit.
The Alpha star in the constellation and the second brightest star is the huge red supergiant Betelgeuse. It is so huge that if it replaced the Sun it would engulf all the four inner planets, Mercury, Venus, Earth and Mars. It is at a distance of about 570 light years and radiates 85 000 times as much energy as our Sun.
Another one of the four stars forming the outer rectangle of Orion is Saiph. This star is a blue supergiant like Rigel, but even hotter. This high temperature means that more of its energy is radiated as ultraviolet and so it appears fainter than Rigel to our eyes.
The fourth of the stars forming Orion’s rectangle is Bellatrix. This is again a hot blue star that at a distance of 240 light year is closer to us than most of the other stars in Orion.
Finally, let’s mention Meissa that according to the old mythological drawings is the head of the giant Orion. This is a double star with one component being a rare O-class star with the extreme temperature of about 35 000 Kelvin while its companion is a little cooler 27 000 Kelvin. (The Kelvin temperature scale used by astronomers is the same as the ordinary Celsius scale, but with 273 added so that the freezing point of water is at 273 K.)
So on these dark autumn evenings once you had your fill of the bright planets and satellites, have a look at the giant Orion and then maybe report what you see to Globe at Night.
Reference: Stars by Jim Kaler