WHAT'S UP THIS MONTH - FEBRUARY 2021
(Link to What's Up March 2021)
(Link to What's Up January 2021)
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THESE PAGES ARE INTENDED TO HELP YOU FIND YOUR WAY AROUND THE SKY
The chart above shows the whole night sky as it appears on 15th February at 21:00 (9 o'clock) Greenwich Mean Time (GMT). As the Earth orbits the Sun and we look out into space each night the stars will appear to have moved across the sky by a small amount. Every month Earth moves one twelfth of its circuit around the Sun, this amounts to 30 degrees each month. There are about 30 days in each month so each night the stars appear to move about 1 degree. The sky will therefore appear the same as shown on the chart above at 8 o'clock GMT at the beginning of the month and at 10 o'clock GMT at the end of the month. The stars also appear to move 15º (360º divided by 24) each hour from east to west, due to the Earth rotating once every 24 hours.
The centre of the chart will be the position in the sky directly overhead, called the Zenith. First we need to find some familiar objects so we can get our bearings. The Pole Star Polaris can be easily found by first finding the familiar shape of the Great Bear ‘Ursa Major' that is also sometimes called the Plough or even the Big Dipper by the Americans. Ursa Major is visible throughout the year from Britain and is always quite easy to find. This month it is in the North East. Look for the distinctive saucepan shape, four stars forming the bowl and three stars forming the handle. Follow an imaginary line, up from the two stars in the bowl furthest from the handle. These will point the way to Polaris which will be to the north of overhead at about 50º above the northern horizon. Polaris is the only moderately bright star in a fairly empty patch of sky. When you have found Polaris turn completely around and you will be facing south. To use this chart, position yourself looking south and hold the chart above your eyes.
Planets observable in the evening sky: Neptune (early evening), Mars and Uranus.
EXPLORING THE NIGHT SKY THIS MONTH
The Southern Night Sky 15th February 2021 at 20:00 GMT
The chart above shows the night sky looking south at about 20:00 GMT on 15th February. West is to the right and east to the left. The point in the sky directly overhead is known as the Zenith and is shown (in red) at the upper centre of the chart. The curved brown line across the sky at the bottom is the Ecliptic or Zodiac. This is the imaginary line along which the Sun, Moon and planets appear to move across the sky. The brightest stars often appear to form a group or recognisable pattern; we call these ‘Constellations'.
Constellations through which the ecliptic passes this month are Aquarius (the Water Carrier), Pisces (the Fishes), Aries (the Ram), Taurus (the Bull), Gemini (the Twins), Cancer (the Crab), Leo (the Lion) and Virgo (the Virgin).
Close to the south western horizon is the constellation of Pisces (the Fishes). Pisces is a little faint and indistinct but it is easy to find this month because the bright and orange planet Mars is located within its boundaries.
Prominent in the south west in the early evening is the constellation of Pegasus (the Winged Horse). The main feature of Pegasus is the square formed by the four brightest stars. This asterism (shape) is known as the Great Square of Pegasus. The square is larger than might be expected but once found is easier to find again. Attached to the star at the top left of the Great Square is the ‘ > ' shape of Andromeda. Andromeda contains the only ‘naked eye' galaxy Messier 31 (M31). It is visible as a small ‘fuzzy' patch of light using binoculars.
High in the south is the constellation of Taurus (the Bull). The most obvious star in Taurus is the lovely Red Giant Star called Aldebaran. It appears slightly orange to the ‘naked eye' but it is very obviously orange when seen using binoculars or a telescope. Aldebaran is located at the centre of the ‘flattened' X shape formed by the brightest stars in Taurus. At the end of the top right (upper west) arm of the ‘X' is the beautiful ‘naked eye' Open Star Cluster Messier 45 (M45) known as the Pleiades (or the Seven Sisters). It really does look magnificent using binoculars.
Following Taurus is the constellation of Gemini (the Twins). The two brightest stars in Gemini are Castor and Pollux and they are named after mythological twins. To the north of Taurus is the odd pentagon shape of Auriga (the Charioteer). Dominating Auriga is the brilliant white star Capella which is almost directly overhead. For those with a telescope there is a line of lovely open clusters to search out in Taurus and Auriga. These are M35 in Taurus and M36, M37 and M38 in Auriga.
To the east (left) of Gemini is the rather indistinct constellation of Cancer (the Crab). The stars of Cancer are quite faint and can be difficult to discern especially in a light polluted sky. It is really worth searching out Cancer using binoculars or a telescope to see the Open Cluster M44 (the Beehive Cluster). M44 is older and further away than M45 (the Seven Sisters) so is fainter than M45 but still looks lovely. It has a group of stars that resemble an old straw Beehive with bees around it.
To the south of Taurus and Gemini is the spectacular constellation of Orion (the Hunter). Orion is one of the best known constellations and hosts some of the most interesting objects for us amateur astronomers to seek out. Orion is the constellation of the month.
The Milky Way (our Galaxy) flows down through the ‘W' shape of Cassiopeia in the high North West. Then down through Auriga, between Taurus and Gemini and on down through Orion to the South Eastern horizon.
CONSTELLATION OF THE MONTH – ORION (the Hunter)
The constellation of Orion photographed by Nicky Fleet
Orion (the Hunter) is one of the best known constellations and one of the easiest to recognise and dominates the southern sky at this time of the year. There are many depictions of Orion shown on many different star charts. Some old pictures of Orion are very beautifully drawn in fact some are so beautiful that the artists even moved the positions of some of the stars so they would fit the image they had drawn.
Orion the Hunter appears in the winter sky, with his club held over his head and his shield (sometimes shown as a lion's skin) held out in front of him. His hunting dogs, Canis Major (the star Sirius) and Canis Minor (the star Procyon) following behind him. See the chart below.
The constellation of Orion and his dogs Sirius and Procyon
Orion is one of the few constellations that does look (with a little imagination) like what it is named after. The most obvious feature is the line of three stars, called Alnitak, Alnilam and Mintaka that make up Orion's belt. From his belt we can see two bright stars called Saiph and Rigel below. These define the bottom of his ‘skirt like' tunic. Above the belt are two stars Betelgeuse and Bellatrix that denote the position of his shoulders.
Above and between his shoulders is a little group of stars that mark out the head. From his right shoulder (Bellatrix) he holds out a shield. From his left shoulder (Betelgeuse) a club is held above his head. It almost looks as if Orion is fending off the charge of the great bull Taurus who is located above and to the west (right) of Orion.
Down from Orion's very distinctive belt there is a line of stars, ending at the star Nair al Saif that looks very much like a sword attached to his belt. Here can be found the main interest in Orion, the Great Nebula known as Messier 42 (M42).
If an imaginary line is traced down from the belt for about six belt length towards the south eastern horizon, a bright twinkling star will be seen. This is Sirius, Orion's Large Hunting Dog in the constellation of Canis Major. It is the brightest and closest star to be seen from the UK at just 8.6 light years from us.
To Orion's left (east) of Betelgeuse a quite bright star in a rather large empty area of sky can be seen. This is Procyon in Canis Minor, Orion's Small Hunting Dog. Coincidentally both of these ‘dog stars' are double stars that have an invisible companion. They were normal stars that had reached the end of their lives and used all their Hydrogen fuel. They have collapsed to become very compact and dense White Dwarfs stars.
As well as being one of the most spectacular and beautiful constellations, Orion is also very interesting because we can see evidence of all the stages of the life cycle of stars from birth to death. Obviously we can't see the life cycle of an individual star but we can observe stars at various stages of their existence in Orion.
Stars are not actually living things like us but they do exist in various stages of evolution that resemble the life cycle of living entities. We can see how stars are being ‘born' in the stellar nursery in Orion's sword in the Great Nebula (M42). These stars are still very young and at this stage are very active. After a few tens or hundreds of millions of years they will settle down to live the longest stage of their lives as normal ‘Main Sequence' stars. Our Sun is about 4.3 billion years old and approaching half way through its ‘life' as a normal main sequence star. We can see many stars at this stage in Orion and others older and many much younger.
There is another factor that affects the life span of a star and that is its mass (the amount of Hydrogen gas it is made of). Large stars have denser and hotter cores and their nuclear fusion process produces more energy and consumes the Hydrogen at a much faster rate. These large stars use up the fuel supply very quickly and therefore have short lives (some less than 5 million years). There are two bright stars in Orion that demonstrate this very well. The first is Rigel at the bottom right of Orion. This is a star about 30 times the mass of our Sun and about 120,000 times more powerful than our Sun.
A star like our Sun will fuse Hydrogen into Helium and towards the end of its life will begin to fuse some of the Helium. A larger star like Rigel will be able to fuse the Helium and then manufacture other heavier atoms until Iron is produced. At this point it will have become a Red Giant and will eventually explode as a Super Nova.
Rigel as seen using a telescope
Rigel is currently a star in its prime, very powerful and mature and perhaps just moving towards the end of its middle age. Another star Betelgeuse, located at the top left of Orion is much further along its pathway of ‘life'. It is approaching the last phases of its existence as a normal star and has grown into a Red Giant with a diameter greater than the orbit of Jupiter in our Solar System.
Betelgeuse is so big and unstable that it pulsates and wobbles rather like a water filled balloon. By carefully observing the brightness of Betelgeuse it can be seen to brighten and fade. At its brightest it can be as bright as magnitude 0.2 and at its dimmest only magnitude 1.2. It is quite difficult to determine the cycle of the pulsations and brightening because there seems to be a number of intertwined cycles. So it appears to vary at different rates of between 150 to 300 days.
Betelgeuse as seen using a telescope
Betelgeuse appears to be edging towards the end of its life. In fact it is the closest star to us that might explode as a super nova at any time in the near future (astronomically speaking). It could explode and destroy itself sometime in the next million years (maybe as soon as tomorrow). For all we know it may have already exploded but the light from the explosion will take 650 years to reach us.
These two stars are close enough to us and so bright that we can even see their nature with our naked eyes. Rigel is obviously very white in appearance which is even more obvious using binoculars or a telescope. This is because it is very hot with a surface temperature of about 12,000ºK compared to our Sun at about 6400ºK. This is in contrast to Betelgeuse which is only 3500ºK and looks distinctly orange to the naked eye and again more so when viewed using binoculars or a telescope.
Betelgeuse looks red (orange) because it is more advanced in its life cycle than Rigel and has already moved into its Red Giant Phase. The nuclear fusion process is fusing the heaver atoms it has produced into even heavier elements, with each fusion stage contributing additional energy to power the star. All this additional energy pushes out against the force of gravity pushing inwards. The additional energy has forced the outer regions of the star to expand outwards to produce this huge (in volume) bloated and unstable Red Giant star.
An artist's impression of what Betelgeuse might look like
Giant stars like Rigel and Betelgeuse destroy themselves dramatically in a massive super nova explosion but smaller stars like our Sun reach their end in a much less dramatic way. With less pressure and heat in their core they cannot fuse atoms heavier than Helium to create the heavier elements. They consume their fuel supply a lot slower and therefore last much longer. Our Sun had enough Hydrogen, when it formed, to last about 10 billion years and has so far, in the last 4.3 billion years, used just under half of its fuel.
In another 4 billion years our Sun will begin to fuse the Helium that has accumulated in its core. The additional energy produced by the fusion of the Helium will cause the Sun to inflate to become a red giant. It is thought the Sun may reach a diameter equivalent to the orbit of Earth before the fuel supplies are exhausted. The outer layers will drift away into space to create a beautiful bubble called a Planetary Nebula. As fusion, in the core, begins to break down and eventually stop, the radiation pressure that had been overcoming the force of gravity also disappears and the star will begin to slowly collapse inwards. It will collapse into a dense sphere called a White Dwarf about the size of Earth ~12,000 km but still weighing about the same as the Sun.
In mythology, Orion the Hunter has two hunting dogs so the constellation of Orion also has hunting dogs in the form of the stars Sirius in the constellation of Canis Major (the large dog) and Procyon in Canis Minor (the little dog). Sirius can be found by following a line down from Orion's belt and Procyon can be found to the left (east) of Orion. See the chart of the constellation of Orion at the beginning of this article.
The White Dwarf star Sirius B compared to Earth
Both Sirius and Procyon are double stars with their companions both being White Dwarfs. The companions must have been slightly larger than the main stars we see now when they formed and used up their fuel supply quicker. The White dwarfs are too small to see but their great mass causes the stars to appear to wobble.
A special image that shows Sirius B (The ‘Pup')
There are many other interesting stars in Orion beside the more well known stars like Rigel and Betelgeuse. It is always interesting to ponder about the true nature of these specs of light that are so far away. The stars of Orion's belt are very easy to see with our naked eyes and the distinctive line of the three stars is one of the most recognised star formations so we should consider what interesting features they may be hiding from us.
The stars of Orion's belt
The star at the east (left) of Orion's belt is called Alnitak and is a triple star system. The system is comprised of a pair of stars of magnitude 1.9 and 5.5 orbiting around a common centre of gravity. They appear to be separated by 2.6" (arc-seconds). The third star is a fainter magnitude 10 companion orbiting 57.6" from the pair. This triple star system is thought to be about 820 light years from us.
Alnilam is the middle star of Orion's belt and is the 29th brightest star in the sky (the 4th brightest in Orion). It is a blue-white supergiant with a mass 34.6 times that of the Sun, a radius 24 times that of the Sun and 275,000 times more luminous than our Sun. It is estimated to be 2000 light years from us and relatively young with an estimated age of 5.7 million years. It is expected to develop into a Red Giant within the next million years.
Mintaka (in Orion's head) is a multiple star system with an overall magnitude of +2.23 but can vary between +2.50 and +3.90. This is because a 7th magnitude star that is currently about 52 arc-seconds away from the main component sometimes eclipses the main star. There is an even fainter star in between these two stars. The main component itself is triple star system comprised of a bright giant and a rare B class main sequence star orbiting every 5.73 days and another B class sub-giant 0.2 arc-seconds away.
There is another 7th magnitude companion that is an unusual B type main sequence star and is itself a spectroscopic binary with a faint A type companion in a 30-day orbit. This 14th magnitude star is thought to be at the same distance but it is not clear whether it is physically bound to the primary star and little is known about it.
Mintaka may be a seven star system but it is also thought to be surrounded by a cluster of faint stars that may be surrounding the whole system. The main star, Mintaka, is estimated to be 1200 light years from our Sun. It has a surface temperature of 30,000°K that means Mintaka is 90,000 times more luminous than our Sun and has a mass of 20 times that of our Sun .
Those who have a telescope should also have a good look at Rigel because it is actually an interesting double star. As the main star is a giant and very bright it is quite difficult to see the fainter companion star. First centralise Rigel using a 10mm eyepiece then carefully remove the 10mm eyepiece and insert a Barlow lens into the focuser. Insert the 10mm eyepiece into the Barlow. Gently turn the focus knob to move the focuser barrel in (towards the telescope) to refocus the image. If Rigel is not visible it may be necessary to use the finder to recentralise the image of Rigel. Adjust the focus until Rigel looks as small as possible (it may be sparkling due to air turbulence and the high magnification). Look carefully and it may be possible to see Rigal's smaller companion somewhere close to the very bright main star. See the image below.
Rigel and its smaller companion
Seeing the companion is a bit of a challenge for a smaller telescope but it is possible to see using a small telescope like the author's Starwatcher Evostar 90mm refractor. It appeared a lot fainter than it appears in the image above but rewarding to see.
Saiph is located at the lower left (east) Orion. It is of a similar distance and size to Rigel, but appears much fainter. This is because although it has a very high surface temperature (26,000°C) that causes it to emit most of its light in the ultraviolet region of the spectrum. Our eyes are not sensitive to ultraviolet wavelengths so it appears fainter.
One of the stars in the small group of stars that define Orion's head is a very hot blue giant. Meissa is a giant star with a stellar classification of O8 III and has an apparent magnitude +3.54. It is an enormous star with about 28 times the mass of the Sun. It has an effective temperature of around 35,000K, giving it a characteristic blue glow of a hot O-type star.
The giant star Meissa
Meissa is actually a double star with a companion that is magnitude +5.61 and it has a stellar classification of B0.5 V, making it a B-type main sequence star. There is another outlying component, Meissa C which is an F-type main sequence star with a classification of F8 V. This star in turn may have a very low mass companion that is probably a tiny brown dwarf star.
M42 THE GREAT NEBULA IN ORION
Chart showing M42 the Great Nebula in Orion
When we look towards Orion we are looking into one of the nearest spiral arms of our galaxy the Milky Way. Our Sun appears to be located in the area between two spiral arms. Towards the centre of the galaxy from our point of view is the Sagittarius Arm and looking away from the centre is the Orion Arm.
An artist impression of our position in the Galaxy
Most of the stars in Orion are located about 900 light years or more away from us including Rigel but Betelgeuse is much closer at only 310 light years distant. Because the stars of Orion are in a spiral arm there is a lot of gas and dust around the whole area of the constellation. Huge numbers of stars are hidden by the gas and dust .
Below the line of the three stars of Orion's belt there is a vertical line of stars forming his sword (hanging below his belt , see the chart above ). In the line of stars making up Orion's sword a hazy patch can be seen using binoculars or even with just the naked eye on a clear night. The hazy patch is known as M42 (Messier 42), the Great Orion Nebula. This Nebula is part of a gigantic cloud of Hydrogen gas mixed with other gases and dust from which new stars are being formed. Through a pair of binoculars the nebula looks like a small ‘fuzzy ' patch in the line of stars.
The Trapezium cluster superimposed on M42
When seen through a telescope the ‘fan shaped' cloud like structure can been made out. Swirls of gas and dust can be seen, some are lit up but some are dark and silhouetted against the illuminated clouds behind.
This nebula actually pervades over the whole area of sky around Orion but we can only see this part around M42 because it is illuminated by the young stars forming in it. Most of the energy illuminating this nebula comes from a group of 4 stars known as the Trapezium. These stars have formed out of the gas and dust in the nebula; they are young, hot and very active. The Trapezium can be seen quite easily using a small telescope. The four stars of the trapezium (there is a fifth fainter star) are just the brightest of what is an Open Cluster in the process of forming. The Orion Nebula actually contains many more very young stars that are still hidden by the gas and dust of the nebula.
Special telescopes that can detect ultraviolet and inferred radiation can be used to penetrate the gas and dust to see the stars forming inside the nebula. The image below shows most of the stars that are normally hidden by the gas and dust clouds.
Stars forming in M42
Gravity draws the atoms of the nebula gas together and as the gas becomes denser it pulls in even more in until huge spheres of gas are formed. As the pressure in the core of a sphere increases the temperature rises to tens of millions of degrees and the Hydrogen atoms begin to fuse together to form atoms of Helium. In this process known as Nuclear Fusion a small amount of mass is lost and converted into energy in the form of a powerful flash of X-Rays. This heats the mass of gas of the sphere and it begins to shine as a bright new star.
Much of the gas and dust in the nebula shines by reflecting light from the very young stars of the Trapezium in the centre of the nebula. Some gas also produces its own light because the ultraviolet radiation energy from the powerful young stars excites the gas atoms . This causes them to glow somewhat like a fluorescent light or the Aurora (Northern Lights) .
When a photon of ultraviolet light from the powerful young stars hits a gas atom it causes an electron to jump from its normal orbit around the nucleus to a higher orbit. After a very short time the electron jumps back to its original orbit and emits a flash of light. The colour of this light is unique to the type of atom that has emitted it. For example Hydrogen always emits red light. See the third image below.
The Orion Nebula can be seen with the naked eye from a dark location on a clear moonless night. It is easily seen using a pair of binoculars. The image below shows the sort of view that can be seen using a pair of 8 x 50 binoculars.
Binocular view of M42 with Orion's belt at the top
A small telescope will show a larger view and some detail in M42. Structure in the nebula can be seen with parts of the nebula illuminated and other parts appearing dark.
The sort of view seen using a small telescope
A larger telescope will show more detail in the structure with the nebula made up of wisps of gas appearing. Photographic images show much more detail including colour in the clouds of gas and dust. The red in the image below is typical of the emissions from excited Hydrogen gas in the nebula.
A photographic image of M42
THE SOLAR SYSTEM - FEBRUARY 2021
MERCURY will be rising just before the Sun low in the east this month. It will be difficult to see this month. The smallest planet will be close to Jupiter and Saturn as they rise in the bright dawn sky. Mercury will be in Inferior Conjunction (between Earth and the Sun) on the 8th February.
Mercury, Venus, Jupiter and Saturn at sunrise
VENUS is now too close to the Sun, low in the eastern early morning sky and will be very difficult to see. It is moving towards the Sun and moves into Superior Conjunction (behind the Sun) on 25th March 2021.
MARS is still well positioned in the evening sky for observing and will be in the south as the sky darkens. However it is getting smaller at about 7 arc-seconds as Earth pulls further away from it. Mars will be around until May but will be moving closer to the south western horizon and appearing smaller. After it has moved over the horizon we will not see it again for two years.
Mars, Uranus and Neptune after sunset
JUPITER will not be visible this month as it was in conjunction with the Sun on 29th January so is hidden in the glare of the sky just before the Sun rises. It will start moving far enough away from the Sun to see in the morning sky next month. From March onwards it will be worth getting up in the early hours to see it rising in the South East from about 04:00. Jupiter and Saturn will move further away from the Sun during the year and will be at their best for observing in August. Jupiter will be at opposition on 20th August.
SATURN will be even more difficult to see in the bright early morning sky than Jupiter. The ringed planet rises just before Jupiter the South West at about 04:00. It is moving into the early morning sky before sunrise after its journey behind the Sun during its conjunction with the Sun on 24th January. Saturn will be at its best this year on 2nd August when it will be at opposition so it will be due south at midnight.
URANUS will be easy to find as it will be close Mars. This month it will be in the south west and at its best at about 18:30 but it will need a telescope to see it.
NEPTUNE will not be easy to observe this month as it will be in the south west and close to the horizon. It will be setting over the horizon at about 19:00 and will be moving into conjunction with the Sun on 11th March. It will require a telescope to see in the bright sunset sky.
The Sun rises at about 07:35 at the beginning of the month and 06:50 at the end. It sets at 16:50 at the beginning of the month and 17:40 at the end. There have been some very nice Sunspots recently like those seen over Christmas, see the following image.
Sunspots imaged by SOHO on 27th December
It has been confirmed that the Sun has moved into its increasing active phase for Solar Maximum 25. We can expect the numbers of sunspots and surface activity to increase during this year and the three following years. A special solar filter must be used when observing the Sun.
THE MOON PHASES DURING FEBRUARY
Last Quarter will be on 4th February
New Moon will be on 11th February
First Quarter will be on 19th February
Full Moon will be on 27th February
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