February 2021

by Roger Davies

T he long, cold, Winter nights present us with much to see in the southern sky guided by the spectacular constellation of Orion, the hunter. Due south at 9pm in February, it is our signpost to the Winter skies.

The constellation of Orion, the hunter.

Top left is the red supergiant star Betelgeuse, bottom right is the blue supergiant star Rigel. If you have trouble seeing the colours of stars these two are quite distinctive. Immediately below the central star of the belt is the Orion nebula, one of the nearest regions where stars are forming today. It can be seen with the unaided eye but is beautiful through binoculars or with a small telescope. The seven principal stars can be used to find your way around the Winter sky.
Image References:
Orion chart from the Science News Journal here and 'Orion: a signpost' from Wikipedia here.
The stars of Orion are invaluable for finding your way around the Winter constellations. Follow the three stars of the belt upwards towards the red giant star Aldebaran, it is the brightest star in the constellation of Taurus. Further in the same direction is the famous star cluster the Seven Sisters or Pleiades (Subaru in Japanese – look at the car badge!). It is spectacular through a small telescope. Follow the line of the stars of the belt in the opposite direction towards the blue star Sirius, (in the constellation of Canis Major – the big dog) the brightest star in the sky. A line pointing eastward through the top two stars of Orion takes us to the star Procyon in Canis Manor. Procyon & Sirius are amongst the closest bright stars to the Sun, being and 11.5 and 8.6 light years away from the Sun respectively. Starting from Rigel in lower right a line through Betelgeuse (often pronounced beetle-juice) points to Castor & Pollux, the brightest two stars in Gemini. By familiarising yourself with Orion you can quickly become familiar the southern Winter sky.

The top left star of Orion, Betelgeuse, is a young, massive star, 550 light years distant. It is 10-20 times more massive than the Sun and formed just 8 million years ago - the Sun is more than sixty times older. It is a red supergiant, with a diameter larger than the orbit of Jupiter, it is more than 750 times the size of the Sun! So big in fact that the most sophisticated telescopes can image its surface, a feat only possible for a few stars. In the Winter of 2019/20 Betelgeuse was seen to fade, much more than expected for its normal cycle of variability, a change easily noticed by the casual observer. A picture showed that one side of the star was dimmer than the other, leading to the idea that a dust cloud, probably formed from material ejected from Betelgeuse, was dimming the light from one side of the star. There was even speculation that Betelgeuse was about to explode as a supernova, an idea now largely discounted. Betelgeuse is one of the easiest variable stars to observe, fading and brightening by ~ ±25% over 400 days, these changes arise because of changes in its size – it is brightest when biggest. Betelgeuse is a typical massive, young star – constantly changing.

Orion is home to one of the nearest regions of our galaxy where new stars are forming. The glowing patch of light found below the middle star of Orion’s belt is home to a stellar nursery of new stars forming from the gas a dust in interstellar space. The nebula glows because the gas is excited by ultra-violet radiation from the young hot stars, most notably those of the Trapezium that can be picked out by a small telescope.

The Orion nebula.

Image References:
Above, the constellation of Orion — the nebula is the small patch of glowing gas found below the middle star of Orion's belt (image from Wikimedia here), followed by the colourful image of the central part of the Orion nebula showing, at its heart, the stars of the trapezium (image from the European Southern Observatory here).
There is a reasonably good chance of seeing Mercury in the south west just after sunset (16.46) on 28th January when it will reach its highest point in the sky, a peak altitude of 13° above the horizon in the constellation of Capricorn. But you’ll need clear skies and a clear SW horizon.

We have enjoyed Mars high in the southern sky for the last few months. It is still with us in the first half of the night. On February 18th NASA’s latest Mars rover, Perseverance will land on the red planet. The van sized vehicle carries a suite scientific experiments, including microphones so that we can listen to the Martian winds. The landing involves a decent by parachute through Mars’ thin atmosphere. At an elevation of 2000m a series of eight small engines slow the decent until, about 20m above the surface, a skycrane will lower the rover, suspended by cables, onto the surface. Once the rover touches down the cables are released and the descent stage flies off to make its own uncontrolled landing on the surface, a safe distance away. The whole thing takes 7 minutes. Keep your fingers crossed for success. This unique feat of engineering will be relayed live back to Earth by TV cameras on the rover starting at 7.15pm GMT on Thursday Feb 18th. It takes 11 minutes for the signal from Mars to reach Earth, so by the time we get the first pictures it will all be over on the Martian surface. Don’t miss it! (on the NASA YouTube channel).

Happy stargazing!

SEE ALSO by Roger Davies:
Stargazing • February 2021
Stargazing • November 2020
Stargazing • September 2020
Comet Neowise • July 2020
Stargazing • July & August 2020
Stargazing • June 2020
Stargazing • May 2020
Stargazing • April 2020

Roger Davies is a resident of the village of Kidlington, Oxfordshire.

Roger is the Philip Wetton Professor of Astrophysics and Director of the Hintze Centre for Astrophysical Surveys at the Department of Physics, University of Oxford,

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