WHAT'S UP THIS MONTH - NOVEMBER 2018
(Link to What's Up October 2018)
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THESE PAGES ARE INTENDED TO HELP YOU FIND YOUR WAY AROUND THE SKY
The chart above shows the night sky as it appears on 15th November at 21:00 (9 o'clock) in the evening 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 10 o'clock GMT at the beginning of the month and at 8 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 easy to find. This month it is in the north. 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 this month: Mars, Uranus and Neptune .
EXPLORING THE NIGHT SKY THIS MONTH
The Southern Night Sky during November 2018 at 21:00 GMT
The chart above shows the night sky looking south at about 22:00 GMT on 15th November. West is to the right and east to the left. The point in the sky directly overhead is known as the Zenith or Nadir and is shown at the 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 Capricornus (the Goat), Aquarius (the Water Carrier), Piscis (the Fishes), Aries (the Ram) Taurus (the Bull) and Gemini (the Twins).
The summer constellations are still prominent in the western night sky. We still have the familiar Summer Triangle with its three corners marked by the bright stars: Deneb in the constellation of Cygnus, Vega in Lyra, and Altair in Aquila. The Summer Triangle is very prominent and can be used as the starting point to find our way around the night sky.
The Milky Way (our Galaxy) flows down through the Summer Triangle passing through Cygnus and Aquila to the western horizon . From Cygnus the Milky Way loops up through Cassiopea which is overhead see the Zenith ( point in the sky directly overhead ) marked on the chart above. It continues on through Perseus, Auriga and down to the eastern horizon as it passes through Orion and Gemini.
Mars is still in a good position for observing for the next couple of months.
At the top, centre of the chart above is the fairly faint constellation of Ursa Minor (the Little Bear) also called the Little Dipper by the Americans. Although Ursa Minor may be a little difficult to find in a light polluted sky it is one of the most important constellations. This is because Polaris (the ‘Pole' or ‘North Star') is located in Ursa Minor.
Polaris is the star that is located at the approximate position in the sky where an imaginary line projected from Earth's North Pole would point to. As the Earth rotates on its axis, the sky appears to rotate around Polaris once every 24 hours. This means Polaris is the only ‘bright' star that appears to remain stationary in the night sky as Earth rotates every 24 hours.
To the East of the Summer Triangle 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.
Along the Ecliptic is the constellation of Taurus (the Bull). The stick figure representation of Taurus resembles a squashed ‘X' with the bright orange coloured Red Giant star Aldebaran at its centre. This is a lovely star to look at especially using binoculars or a telescope and does look noticeably orange in colour.
Following the western (right), Northern (upper) arm of the ‘X' shape of Taurus guides is to the beautiful Pleiades Open Star Cluster. This is a cluster of seven bright ‘naked eye' stars known as the ‘Seven Sisters'.
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. It 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 about 400 billion years old. It is also quite far away from us so the stars appear quite faint. 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 3.5° 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 young stars in the cluster that is estimated to be about 100 million years old. M45 is one of the closest open clusters to us at 400 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 would 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 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 250 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 disappear. 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.
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 three times the mass of our Sun. Giant stars consume their Hydrogen fuel at an experientially faster rate than 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.
Stars that are three 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 massive 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 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' .
A QUICK LOOK AT SOME TYPES OF STAR
We have examples of all the types of star mentioned in the article above on view in the sky at the moment. The bright red star Aldebaran, located at the centre of Taurus is a Red Giant about 1.16 times the mass of our Sun. So it is about the same mass as our Sun but has progressed further along its line of development (life time). It has used up most of its Hydrogen fuel and has started to fuse some of the Helium that has accumulated in its core. The additional energy from the Helium has pushed the outer layers of the star outwards. The diameter of Aldebaran is now 44 times greater than our Sun. Our Sun is 1.4 million kilometres in diameter so Aldebaran is something like 61.6 million kilometres in diameter.
Later in the night the beautiful constellation of Orion will be rising over the eastern horizon. The bright orange star at the top left of Orion is Betelgeuse a Red Super Giant. It has a mass 11.6 times that of our Sun but its diameter is about 900 times the diameter of our Sun. It will finally explode as a Supernova to produce a Supernova Remnant like Messier 1 (M1) the Crab Nebula. At the centre of the Supernova Remnant will be a Neutron Star (Pulsar) similar to the one in M1.
The star Rigel at the bottom right of Orion is a Super Giant star that is in the prime of its ‘life'. It has a mass 23 times greater than our Sun and shines about 250,000 times brighter than our Sun. It will also become a Red Super Giant and will eventually explode as a Supernova to produce a Neutron Star or perhaps even a Stellar Black Hole.
Orion's Hunting Dog ‘Canis Major' is marked by the closest and brightest star in our sky and is called Sirius. It is a star similar to our Sun but twice the mass and about 25 times brighter than our Sun. It is in fact a double star but its companion was probably a little larger than Sirius and lived its ‘life' faster. It became a Red Giant like Aldebaran then collapsed to form a white Dwarf Star. It is now about the size of our planet (12,000 km diameter) but has a mass about the same as our Sun. In the Constellation of Lyra is the bright star Vega (at the top right star of the Summer Triangle). Vega is a star similar to our Sun but about twice the mass and about 40 times more luminous. Just below Vega is Messier 57 (M57) a Planetary Nebula. A Planetary Nebula is nothing to do with a planet it is in fact the remnant of a star similar to our Sun. It has passed through its Red Giant phase and has collapsed to form a White Dwarf Star. The outer layers of the Red Giant drifted off into space as the core of the star collapsed. A beautiful ‘ring' shaped bubble was formed by the outer layers that is now called the Ring Nebula Messier 57 (M57).
We will be having a closer look at Orion next month and his Hunting Dogs in January.
THE SOLAR SYSTEM SEPTEMBER 2018
MERCURY will pass through Inferior Conjunction with the Sun on 27th November. This means it will pass in front of the Sun (not across the face of the Sun but just above). So it will be too close to the Sun to be seen.
VENUS was in Inferior conjunction (passing just below the Sun) on 26th October. It is now emerging from the glare of the Sun. It rises over the eastern horizon at about 05:00 but will be difficult to see in the brightening sky before sunrise.
Venus in the east at 05:00 GMT
MARS will still be well placed this month for observing but is low over the southern horizon in turbulent and smoggy air. The Red Planet passed through ‘Opposition' on 27th July so is still relatively close to Earth. It is 10.5 arc-seconds in diameter and is still bright at magnitude -1.3.
Uranus, Neptune and Mars at 20:00 GMT
JUPITER is moving into conjunction with the Sun on 26th November so it will not be visible this month.
SATURN is moving in too close to the Sun to be seen this month and will be lost in the glare of sunset.
URANUS will be in a good observable position in the south at midnight. Uranus will be quite high in the south east as soon as the sky is dark. A good pair of 9x50 binoculars will reveal a slightly fuzzy blue, star like, object. A telescope at a magnification of 100x will show it as a small blue/green disc. See the Mars chart above.
NEPTUNE is still in a very good position this month for those who have a telescope. A telescope will be needed to show Neptune as a small blue/green disc using a magnification of 100x but it is small and difficult to find.
There have been no sunspots over the past few months so it looks as though the active phase of the Solar Cycle is now over.
The Sun rises at 07:00 at the beginning of the month and at 06:40 by the end of the month. It will be setting at 16:30 at the beginning and 16:00 by the end of the month.
THE MOON PHASES IN NOVEMBER
New Moon will be on the 7th November
First Quarter will be on 15th November
Full Moon will be on 23rd November
Last Quarter will be on 30th November
THE LEONID METEOR SHOWER
The chart above shows the night sky looking towards the east at 01:00 on 18th November when the constellation of Leo is rising over the eastern horizon. The radiant point of the Leonid Meteor Shower is shown close to the star Algieba. The paths of the meteors are shown by the straight lines emanating from the direction of the radiant point. The Radiant is a perspective effect rather like driving in a rain storm. The rain drops appear to radiate from a point directly in front of the car. Similarly the meteors appear to radiate from a point in the direction of travel of Earth along its orbital path. The meteors are debris from Comet 55P/Swift Tuttle.
During the evening of 17th November the constellation of Leo will be below the eastern horizon so any Leonid meteors will appear to originate from over the horizon. This means all the meteors will be moving up from the eastern horizon and in a ‘fan' shape across the sky. Leonid meteors tend to be fast and relatively bright so look anywhere from above the eastern horizon to overhead. Make sure you are warm and comfortable and enjoy the show .
As Comet 55P/Swift Tuttle returns to loop around the Sun every 33 years some of its ice is melted by the Sun. The mainly CO2 and water ice sublimes (turns directly from solid to gas) and can form jets on the surface of the comet nucleus. Particles of dust trapped in the ice are liberated and projected from the surface by the jets. The light gases are driven off by the Solar Wind (radiation from the Sun) but the heavier dust particles continue in the same direction as the nucleus. A trail of dust is created along the orbit of the comet that becomes the Meteoroids.If the trail of dust crosses Earth's orbit, Earth will plough into them at a speed of about 70,000 km/hr. As the Meteoroids hit the atmosphere they heat the atmosphere and create the streak of light that we call a Meteor. The last pass of Comet 55P/Swift Tuttle was in 1999 from which a very active shower was seen. Another heavy shower is not expected until the late 2020's heralding the imminent return of Comet 55P/Swift Tuttle in 2031. So our shower this year is not expected to be spectacular but any meteors that we are able to see should be worth seeing.
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