WHAT'S UP THIS MONTH - JANUARY 2021
(Link to What's Up December 2020)
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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 January 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, Mars and Uranus.
EXPLORING THE NIGHT SKY THIS MONTH
The Southern Night Sky 15th January 2020 at 20:00 GMT
The chart above shows the night sky looking south at about 20:00 GMT on 15th January. 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) and Leo (the Lion).
Close to the south western horizon is the constellation of Pisces (the Fishes). Pisces a little faint and indistinct but it is easy to find this month because the bright and orange coloured planet Mars is located within its boundaries.
Prominent in the south west 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. The Great Square can be used to judge the condition of the sky for observing. If stars can be seen within the square there seeing should be good. If no stars can be seen then seeing will not be good.
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. See the images below.
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 (right) of Taurus 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. We will be having a closer look at Orion in the February issue of this magazine.
The Milky Way (our Galaxy) flows through the pentagon shape of Cepheus and down through the ‘W' shape of Cassiopeia then on through Auriga and Orion to the south eastern horizon.
CONSTELLATION OF THE MONTH - TAURUS
The constellation of Taurus
The chart above shows the constellation of Taurus the Bull. There are many different representations of Taurus but he is generally shown with his horns tipped by the stars at the end of the obvious ‘>' shape. The bright red star Aldebaran is normally used to show the bull's eye.
An illustration of the constellation of Taurus
With a little imagination Taurus appears to be charging Orion in the illustration. Taurus sits on the Ecliptic and is one of the star signs of the Zodiac. The asterism (shape) used to identify Taurus resembles a stretched ‘X'.
The bright red star Aldebaran is located at the centre of Taurus. It is easy to find and therefore helps to identify the constellation of Taurus. It is in fact a Red Giant Star and that is why it appears distinctly orange. A Red Giant is a star similar to our Sun (perhaps a little larger) that is approaching the end of life as a normal star. It has used up most of its Hydrogen fuel and has swollen into a giant . Its outer layers are now stretched over a larger area so the available heat is also spread over a bigger area so the surface is cooler and appears orange in colour . Aldebaran is not a true member of the Hyades cluster it is located at about half the distance to the Hyades and is just in the same line of sight.
Surrounding the bright red star Aldebaran is an Open Cluster of Stars known as the Hyades. It is an older cluster so its stars have begun to disperse. The Hyades star cluster is estimated to be between 400 million and 700 million years old. It is also quite far away from us so the stars appear quite faint but it does look lovely to see when using binoculars. In a dark Moonless sky the cluster can be seen with the naked eye but is best seen using binoculars. The cluster is large, at 3.5º in diameter (about 7 Moon diameters) and well dispersed.
The Open Star Clusters Hyades and Pleiades
The real jewel of Taurus is without doubt the beautiful Open Cluster, Messier 45 (M45) also called the Pleiades or the Seven Sisters. An Open cluster is created as stars form in a giant cloud of gas and dust called a ‘Nebula'.
M45 is visible to the naked eye initially looking like a ‘fuzzy' patch of light. Closer observation will reveal a cluster of up to seven stars. Using a good pair of binoculars many more stars will be seen. There are in fact about 300 bright young stars in the cluster and possibly another 1000 smaller stars. The cluster is estimated to be about 100 million years old. M45 is one of the closest open clusters to us at 440 light years.
The Pleiades look brighter than the stars of the Hyades because they are very bright large young stars and are relatively close to us. The largest is Alcyone which is 10 times the mass of our Sun and 1000 times brighter. The larger and brighter stars of the Pleiades are also rotating very fast. The fastest is Pleione which is rotating 100 times faster than our Sun.
Messier 45 (M45) the Pleiades (Seven Sisters)
The stars of the Pleiades cluster have formed from the gas and dust of a Nebula. Gravity draws the atoms of the Nebula together to form denser clumps of gas that become ever denser. Eventually the gas is squeezed into dense spheres where the pressure and high temperature at the core causes atoms to combine through Nuclear Fusion. As Hydrogen atoms are fused into Helium heat is produced and the sphere becomes a shining star. Any left-over gas and dust is blown away by intense radiation from the young stars and a cluster of new stars is revealed. This type of star cluster is called an ‘Open Cluster'.
The biggest and brightest stars of M45 (the Seven Sisters) have been named after seven Pleiades sisters from Greek Mythology. They were the seven daughters of the Titan called Atlas and the sea-nymph Pleione and were born on Mount Cyllene.
Impressive as they are, the Seven Sisters are just the brightest (naked eye) stars in a cluster of around 300 young stars. In the images above the Seven Sisters appear to be surrounded by gas remaining from the original nebula. However it is now thought the cluster is just passing through a cloud of Hydrogen gas in space.
It is thought that all stars originated and formed in a Nebula, including our own star the Sun. The stars are relatively close together when they form but over a period of many hundreds of millions of years the stars disperse and the cluster will disperse. Our Sun would have formed in a Nebula about 4.5 billion years ago so all the Sun's siblings would have dispersed throughout the Galaxy at least 3 billion years ago. If our Open Cluster had survived until today we would have a sky full of dazzling bright stars. Every night would be as bright as our nights when there is a Full Moon and the hundreds of stars would be bright enough to cast shadows.
The names of the Seven Sisters
The Seven Sisters Cluster is close to us so the cluster has a relatively high apparent movement across the sky although it is still too slow for us to perceive. It will take 30,000 years to move a distance equal to the diameter of our Moon.
Although the cluster is moving through space the individual stars all have slightly different trajectories and relative speeds. Gradually over millions of years the stars will move further apart and the cluster will disperse, like the Hyades. Binoculars will reveal around 30 to 50 stars in the cluster and a telescope will reveal many more. However the cluster is too large to fit into the field of view of most telescopes so the outline of the cluster will be lost.
There is another very interesting object in Taurus. At the end of the lower left (eastern) arm of Taurus is Messier 1 (M1) the Crab Nebula. It can be seen using binoculars in a dark clear sky but really needs a telescope. From Aldebaran look east to the star ? (Ksi) Tauri. Just above ? Tauri is a small smudge of light, this is M1. See the chart above.
Messier 1 (M1) the Crab Nebula imaged by Hubble
This is the remnant of a giant star that exploded as a Supernova about 7000 years ago. The light from the explosion took 6000 years to reach Earth and was observed by Chinese astronomers in the year 1054 AD. It has faded now and has expanded but can still be seen in a dark clear sky as a ‘fuzzy' patch of light using a medium sized telescope.
A Supernova is the ‘death' of a star more than five times the mass of our Sun. Giant stars consume their Hydrogen fuel at an exponentially faster rate compared to smaller stars. Consequently bigger stars do not ‘live' as long as smaller stars. As stars begin to exhaust their supply of Hydrogen they develop into a Red Giant like Aldebaran. Very massive stars develop into larger Red Super Giants.
A star like our Sun and those up to about twice the mass of our Sun become Red Giants and eventually slowly collapse as their fuel eventually runs out. The outer layers of the Red Giant drift away to form a gas bubble but the core ‘gently' collapses to form a White Dwarf Star. Some of these white dwarfs are so compressed that most of the Carbon in their cores may be compressed into solid Diamond.
Stars that are five or more times the mass of our Sun come to a more dramatic end. As the fuel of a larger Red Giant Star finally runs out the star suddenly collapses and all the mass of the star falls inwards under the powerful force of its own gravity. The collapse reaches a point where the pressure and heat causes a gigantic thermonuclear explosion. The outer regions of the giant star are blown into space to create a Supernova Remnant like M1. The inner regions are compressed into a super dense Neutron Star about 10km in diameter but with a mass from 1.4 times and up to just over twice the mass of our Sun.
These tiny stars spin very fast with some spinning faster than 1000 revolutions per second. They also may have powerful beams of high energy particles emanating from their poles. The axis of spin is often not at the actual pole of the Neutron Star so the beams can sweep around the sky. If one of these beams sweeps past in our direction we will see a burst of radiation mainly of short radio waves. These are seen as regular pulses at the frequency of the star's rotation. These very regular pulses lead to the discovery of these very strange stars so they are also called ‘Pulsars' .
THE SOLAR SYSTEM - JANUARY 2021
The locations of the planets at 14:00 on 15th January
MERCURY will be close to the Sun low in the east this month. It will be difficult to see but at its best on 24th January.
VENUS is moving closer to the Sun, low in the eastern early morning sky. It is now ‘gibbous' (wider than half Moon shaped) and becoming difficult to see. It is moving back towards the Sun and will move into Superior Conjunction (behind the Sun) on 25th March 2021.
MARS is still well positioned in the evening sky. However it is getting smaller at about 9 arc-seconds as Earth pulls further away from it. It 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.
JUPITER is now past its best and just visible in the South West after sunset. It will be difficult to see in the bright twilight sky just above horizon. It will be in the thick, murky, turbulent air close to the horizon so the view will be badly distorted so very little detail will be seen. However it will be well worth searching out in the beginning of the month to see it still in conjunction with fellow Gas Giant Saturn. Jupiter moves in conjunction with the Sun on 29th January.
The planets at 16:15 on 5th January 2021
SATURN will be difficult to see in the bright twilight sky just above the South Western horizon. It will be worth going to a location with a clear view to the South West to see Saturn while it is still close to Jupiter. Saturn moves in conjunction with the Sun on 24th January.
URANUS will be easy to find as it will be close Mars. This month it will be due south 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 it will be in the south west and close to the horizon. It will be setting over the horizon at 21:15 about 19:30 and will require a telescope to see well.
The Sun rises at about 08:10 at the beginning of the month and 07:40 at the end. It sets at 16:00 at the beginning of the month and 16:30 at the end. There have been some very nice Sunspots recently like those seen over Christmas see the image below.
Sunspots imaged by SOHO on 27th December
THE MOON PHASES DURING DECEMBER
Last Quarter will be on 6th January
New Moon will be on 13th January
First Quarter will be on 20th January
Full Moon will be on 28th January
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