Orion, they mythological hunter, is a constellation which dominate the winter night sky. Instantly recognisable, the three stars of its belt form a straight line which is visible towards the south on every clear night. They are surrounded by four bright stars in a large rectangle – one of which (at the top left corner) is a reddish star. It is a star called Betelgeuse, a giant star, hundreds of times larger than our own Sun.

The name Betelgeuse is derived from Arabic, meaning ‘armpit of the giant’ and pretty much describes the location of the star in old renditions of Orion. Rigel is another star, diagonally opposite Betelgeuse, but it gives off a bluish-white colour. It is nowhere as large as Betelgeuse. However, it stands at the foot of Orion (hence its Arabic name). It lies around 775 light years away, meaning that the light we see now from this star has taken 775 years to travel to the Earth. Or we can also say that it left sometime in the 13th century.

Rigel is an interesting star as it is slowly reaching the end of its lifetime

Rigel is an interesting star as it is slowly reaching the end of its lifetime. Unlike our Sun (which is more or less midway through its lifetime), Rigel is a star which is fairly late in its lifecycle. The Sun is converting hydrogen into helium but Rigel has passed that stage already and is converting helium into carbon and oxygen.

Stars can only use that kind of fuel very late in their lifetimes. It also has a very high surface temperature – approaching 11,000°C, as compared to that of the Sun at 5,500°C. This accounts for the difference in the colours as well; that is why Betelgeuse, which appears reddish, shines at a lower temperature of 3,500°C.

If we extend an imaginary line to the left and downwards from the three stars of Orion’s belt we reach the brightest star in the sky – Sirius. It is the brightest star in the constellation Canis Major (the Great Dog) and sometimes it is also called the Dog Star. Only planets like Jupiter or Venus can become brighter than Sirius.

Sirius is accompanied by a companion star called Sirius B. This star is what we call a white dwarf. It is a star with its mass compacted to a relatively small size about the size of the Earth. This makes its density very high, such that a teaspoon-full of white dwarf material would weigh several tonnes.

During January try to follow also giant planet Jupiter which starts appearing in the evening skies as the Earth makes its close approach early next month. It rises around 8pm in the constellation Leo and by dawn it shines in the west-southwest.

Astronomical events during January

Alexei Pace is president of the Astronomical Society of Malta.