Astronomy on the Green – February 2021

This month I will concentrate on a significant patch of the winter night sky. Looking to the south west in the mid-evening you will observe two main stars; Sirius [a] (the Dog Star – blue colouration) the brightest star in the night sky, and Betelgeuse [b] a large and luminous red star.

These two distinctive stars take us into mythologically linked constellations. Sirius is found in Canis Major (the great dog) and is a bright blue star some 8.7 light years from us. It appears so bright as it is close to us, actually it is a fairly unremarkable star around 23 times more luminous than our sun, and some 1.8 times larger. This dog constellation has a partner constellation Canis Minor and the pair apparently chase a small constellation Lepus (the Hare).

The Hare nestles under another bright star Rigel [c] , which is part of the same constellation as Betelgeuse; to the right of our Hunting dogs is Orion (the Hunter). Orion is a large constellation containing some of the most visible nebula in the sky. Betelgeuse is the star at the top left of the hourglass and is a red giant, forming one shoulder of the Hunter. If this star was in our solar system it would cover the sun, and the orbits of Mercury, Venus, Earth, Mars, Jupiter and part way to Saturn. Luckily for us this giant is some 600 light years distant from us, it is around 100,000 times brighter than the sun and 1000 times its size. This star is near the end of its life, and within the next 100,000 years it will go supernova and explode, when this happens it will be as bright in the sky as a full moon.

Central Orion forms the shape of a tilted hourglass in the night sky, across the waist of the hourglass lie 3 stars forming a belt [d, f]. From that belt hangs a starry sword, and towards the bottom of the sword a fuzzy area can (on a clear night) be seen. This is the Orion Nebula [e], together with a dimmer nebula known as the running man (due to the shape in its centre). Even through a small telescope some detail (not colour) can be seen.

Now we travel back up the sword to the belt, near the first star (left) two less visible objects hide, the famous Horsehead dark nebula [f] , and above it the Flame nebula. Our noble Hunter chases the constellation to its right, one of the Zodiac signs, Taurus (the Bull). Taurus is the home of another red giant star Aldebaran [g] (13th brightest) and is the eye of bull, alongside this is a small open cluster (the Hyades [h]) completing the head of the bull. Follow the gaze of the bull up and to the right and you will find the Pleiades [I] a spectacular open cluster of bright blue stars, covered with a mist of nebulously from which they form, these are the Seven Sisters of mythology. Some 440 light years away this is one of the closest clusters to us. We end our brief trip across the sky here, noting that all we have seen is in our local group of stars in our own Galaxy the Milky Way.

Before I end I want to throw some numbers at you (great huh…).

Our sun is 93 million miles from us, the light it produces travels to us as 186,000 miles a second and it takes 8.3 light minutes to reach us. So now, consider that a light year is 186,000 x 60 x 60 x 24 x 365 miles, or (as you are dying to know) – 5,865,696,000,000 miles.

Astronomy therefore makes us into time-travellers we see the seven sisters as they were 440 years ago (so that big number times 440). The closest spiral Galaxy (Andromeda- similar to ours) is 2.5 million light years away.

Hums the tune to Dr Who… Bye.

Tony Boutle

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