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December heavens above

December nights are short and warm, and also give us the first sight of the bright stars of summer in our evening skies. Rising in the east is ‘Orion’s Belt’, the familiar and famous collection of three bright stars all in a line. It is one of the most recognisable ‘asterisms’ (shapes made out of stars) in the whole sky.

The Belt forms the central part of the constellation Orion, which is straddled on either side by two more bright stars, red Betelgeuse and Blue-white Rigel. Have a look at these stars through a pair of binoculars, the different colours are easy to see.

These two stars are such different colours due to their differing temperatures. The surface of Rigel is very hot, at 12,000 degrees (twice the surface temperature of the Sun) and as such shines blue. It is also far bigger than our Sun, 80 times the diameter, and shines with a brightness of 120,000 Suns. If the Earth went around Rigel instead of the Sun, there would be no life and the Earth would be a fried crisp of a planet.

Betelgeuse is very different to Rigel. It is red as it’s surface temperature is very low, only 3,000 degrees (half that of the Sun). It is also gigantic. Classified as a red supergiant star, Betelgeuse is more than a thousand times bigger than the Sun. So big in fact, that if the Sun was replaced by Betelgeuse, it would not only swallow up Mercury, Venus and the Earth, but it would also consume Mars, Jupiter and Saturn too! Most of the Solar System would disappear into it’s immense surface.

Betelgeuse is so big that despite being 1,863,817,000,000,000,000 km away from us, the biggest telescopes in the world can actually see it as a disc. In fact, if we could drive there in a car, it would take more than 2 billion years to get there.

Due to it’s immense size, Betelgeuse is highly unstable and reaching the end of its life. Stars as big as Betelgeuse explode in a gigantic explosion, called a supernova. Betelgeuse is due to go supernova sometime within the next 100,000 years. In astronomy, that is practically tomorrow.

Black hole eats star

Some stars, like Betelgeuse, are so big that when they go supernova, they form what astronomers call a ‘black hole’. This happens after the supernova when the immense pressures and gravity in the core of the star cause the core to collapse so far that it has no size at all, and so much gravity that nothing can escape it, not even light.

Black holes are so powerful that sometimes their gravity captures passing stars, and the consumed star is ripped apart and swallowed. This happened recently, and astronomers were able to calculate that the star was totally swallowed over just a matter of weeks.

Also visible this month is another red star, Aldebaran, in the constellation Taurus. Aldebaran is the same surface temperature as Betelgeuse, hence has the same colour, but is much closer to us and much smaller in size.

Aldebaran is a red giant, and is 40 times the size of the Sun. It is actually, very similar to the Sun in many properties, only much older. It is expected that the Sun will expand to form a red giant very much like Aldebaran when it enters the next phase of its life. When that happens, the Earth will be swallowed up. Don’t worry though, it is not due to happen for another few billion years.

Also visible this month are bright white Sirius, the brightest star in the whole sky, rising in the east, blue-white Canopus higher in the south-east, blue-white Achenar and white Fomalhaut.

All of these stars are marked on the map for the month. Go outside and take a look! They are there for all to see. Each one has a unique story to tell.

There are no bright planets in the evening skies this month. They are all hiding in the early morning. Early risers can see the collection of Venus, Mars and Jupiter shining brightly before the Sun comes up.

The Moon in December is at last quarter (half) phase on the 4th, new on the 12th, at first quarter phase on the 19th and full on the 26th. A Full Moon for Christmas this year!

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