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Constellations of the month

 

 

Saturn

Saturn precedes Jupiter into the sky, rising before sunset at the beginning of the month and crossing the meridian at 21:00 BST. It then shines with a magnitude of +0.47 with its disk 17.6 arc seconds across and the rings spanning some 41 arc seconds. By month's end, it has a reduced brightness of magnitude +0.61 with a 16.8 arc second disk. In the middle of the month it will be best seen towards the south at ~20:00 BST when it will have an elevation of just ~18 degrees.

Jupiter

Jupiter. As evening falls at the start of October, Jupiter may be seen in the southeast shining at magnitude -2.71 and having an angular size of 46.2 arc seconds. Jupiter will then have an elevation of ~22 degrees when crossing the meridian at 22:06 BST. By month's end its magnitude will have reduced slightly to -2.5 and its angular size to 42.19 arc seconds. As the month progresses it will transit sooner - by 19:10 GMT at month's end. Happily, this year Jupiter has climbed up the ecliptic somewhat so the atmosphere will not hinder our view of the solar system's giant planet as much as it has in the last two years.


Mars

Mars has its conjunction with the Sun on Oct 8th so will not be visible this month.

Venus

Venus. At the start of October, Venus, at magnitude -4.26 and having an angular size of ~19 arc seconds, will only have an elevation of ~4 degrees at sunset looking towards the southwest. It may well be lost in the Sun's glare until around 19:15. By month's end, its elevation after sunset will remain at ~4 degrees with its magnitude increasing -4.56. Venus will grace the evening sky for the rest of this year and reaches its greatest elongation east from the Sun on October 29th but will be highest in the evening sky at the beginning of December. As the ecliptic is at a shallow angle to the horizon in the latter part of the year, it will never get to a high evening elevation during this apparation.

Mercury

Mercury. This is Mercury's finest dawn showing of 2021 and may be spotted very low in the east from the 20th to the 27th of the month at ~06:00 BST having a magnitude of ~-0.4 and an angular size of ~7 arc seconds. It will be highest in the sky on the 23rd but with an elevation of just 2 degrees. Binoculars may well be needed, but please do not use them after the Sun has risen.

The Night Sky in and around Swindon - October 2021
Image of the Month


Image: NASA, ESA, HST and Judy Schmidt

This is beautiful Hubble Space Telescope image of M57, the Ring Nebula. It is termed a 'planetary Nebula' though it is, in fact, the remnant of a star like our Sun whose core has shrunk to an earth-sized object seen at its heart and called a White Dwarf star. Young white dwarf stars have temperatures of around 100,000 Celsius and their intense ultraviolet light excites the shrouds of gas expelled in the dying moments of the star to emit light; largly in the spectral lines of Hydrogen (H-alpha red and H-beta green) and Oxygen (OIII green) giving rise to the wonderful colours of the nebula. M57 lies about 2,500 light years away and is about 1 light year across.

Compiled by Prof. Ian Morison
The planets this month
October 25th - before dawn : Mercury in the East before dawn

Mercury in the dawn sky
Image: Stellarium/IM

If clear and given a very low horizon towards the east, Mercury may be seen at around 7:am BST shining at magnitude -1. Binoculars may well be needed but please do not use them after the sun has risen.

October 14th - Jupiter, Saturn and the Moon

Jupiter, Saturn and a waxing gibbous Moon
Image: Stellarium/IM
If it is clear and given a VERY low horizon towaards the West-Northwest at sunset, it should be possible to spot Mars close by Mercury. Binoculars will almost certainly be needed, but please do not use them until after the Sun has set.
The constellations Lyra and Cygnus

This month the constellations Lyra and Cygnus are seen almost overhead as darkness falls with their bright stars Vega, in Lyra, and Deneb, in Cygnus, making up the "summer triangle" of bright stars with Altair in the constellation Aquila below. (see sky chart above)

Lyra

Lyra is dominated by its brightest star Vega, the fifth brightest star in the sky. It is a blue-white star having a magnitude of 0.03, and lies 26 light years away. It weighs three times more than the Sun and is about 50 times brighter. It is thus burning up its nuclear fuel at a greater rate than the Sun and so will shine for a correspondingly shorter time. Vega is much younger than the Sun, perhaps only a few hundred million years old, and is surrounded by a cold,dark disc of dust in which an embryonic solar system is being formed!

There is a lovely double star called Epsilon Lyrae up and to the left of Vega. A pair of binoculars will show them up easily - you might even see them both with your unaided eye. In fact a telescope, provided the atmosphere is calm, shows that each of the two stars that you can see is a double star as well so it is called the double double!


Epsilon Lyra - The Double Double
Between Beta and Gamma Lyra lies a beautiful object called the Ring Nebula. It is the 57th object in the Messier Catalogue and so is also called M57. Such objects are called planetary nebulae as in a telescope they show a disc, rather like a planet. But in fact they are the remnants of stars, similar to our Sun, that have come to the end of their life and have blown off a shell of dust and gas around them. The Ring Nebula looks like a greenish smoke ring in a small telescope, but is not as impressive as it is shown in photographs in which you can also see the faint central "white dwarf" star which is the core of the original star which has collapsed down to about the size of the Earth. Still very hot this shines with a blue-white colour, but is cooling down and will eventually become dark and invisible - a "black dwarf"! Do click on the image below to see the large version - its wonderful!

M57 - the Ring Nebula
Image: Hubble Space telescope
M56 is an 8th magnitude Globular Cluster visible in binoculars roughly half way between Albireo (the head of the Swan) and Gamma Lyrae. It is 33,000 light years away and has a diameter of about 60 light years. It was first seen by Charles Messier in 1779 and became the 56th entry into his catalogue.

M56 - Globular Cluster
Cygnus

Cygnus, the Swan, is sometimes called the "Northern Cross" as it has a distinctive cross shape, but we normally think of it as a flying Swan. Deneb,the arabic word for "tail", is a 1.3 magnitude star which marks the tail of the swan. It is nearly 2000 light years away and appears so bright only because it gives out around 80,000 times as much light as our Sun. In fact if Deneb where as close as the brightest star in the northern sky, Sirius, it would appear as brilliant as the half moon and the sky would never be really dark when it was above the horizon!

The star, Albireo, which marks the head of the Swan is much fainter, but a beautiful sight in a small telescope. This shows that Albireo is made of two stars, amber and blue-green, which provide a wonderful colour contrast. With magnitudes 3.1 and 5.1 they are regarded as the most beautiful double star that can be seen in the sky.


Alberio: Diagram showing the colours and relative brightnesses
Cygnus lies along the line of the Milky Way, the disk of our own Galaxy, and provides a wealth of stars and clusters to observe. Just to the left of the line joining Deneb and Sadr, the star at the centre of the outstretched wings, you may, under very clear dark skys, see a region which is darker than the surroundings. This is called the Cygnus Rift and is caused by the obscuration of light from distant stars by a lane of dust in our local spiral arm. the dust comes from elements such as carbon which have been built up in stars and ejected into space in explosions that give rise to objects such as the planetary nebula M57 described above.

There is a beautiful region of nebulosity up and to the left of Deneb which is visible with binoculars in a very dark and clear sky. Photographs show an outline that looks like North America - hence its name the North America Nebula. Just to its right is a less bright region that looks like a Pelican, with a long beak and dark eye, so not surprisingly this is called the Pelican Nebula. The photograph below shows them well.


The North America Nebula
Brocchi's Cluster An easy object to spot with binoculars in Cygnus is "Brocchi's Cluster", often called "The Coathanger",although it appears upside down in the sky! Follow down the neck of the swan to the star Albireo, then sweep down and to its lower left. You should easily spot it against the dark dust lane behind.
October: find M31 - The Andromeda Galaxy - and perhaps M33 in Triangulum

How to find M31
Image: Stellarium/IM
In the late evenings when the Moon is not prominent, the galaxy M31 in Andromeda will be visible high in the southeast. The chart provides two ways of finding it:

1) Find the square of Pegasus. Start at the top left star of the square - Alpha Andromedae - and move two stars to the left and up a bit. Then turn 90 degrees to the right, move up to one reasonably bright star and continue a similar distance in the same direction. You should easily spot M31 with binoculars and, if there is a dark sky, you can even see it with your unaided eye. The photons that are falling on your retina left Andromeda well over two million years ago!

2) You can also find M31 by following the "arrow" made by the three rightmost bright stars of Cassiopeia down to the lower right as shown on the chart.

Around new Moon (6th October) - and away from towns and cities - you may also be able to spot M33, the third largest galaxy after M31 and our own galaxy in our Local Group of galaxies. It is a face on spiral and its surface brightness is pretty low so a dark, transparent sky will be needed to spot it using binoculars (8x40 or, preferably, 10x50). Follow the two stars back from M31 and continue in the same direction sweeping slowly as you go. It looks like a piece of tissue paper stuck on the sky just a bit brighter than the sky background. Good Hunting!

The constellations Pegasus and Andromeda
Pegasus

The Square of Pegasus is in the south during the evening and forms the body of the winged horse. The square is marked by 4 stars of 2nd and 3rd magnitude, with the top left hand one actually forming part of the constellation Andromeda. The sides of the square are almost 15 degrees across, about the width of a clentched fist, but it contains few stars visibe to the naked eye. If you can see 5 then you know that the sky is both dark and transparent! Three stars drop down to the right of the bottom right hand corner of the square marked by Alpha Pegasi, Markab. A brighter star Epsilon Pegasi is then a little up to the right, at 2nd magnitude the brightest star in this part of the sky. A little further up and to the right is the Globular Cluster M15. It is just too faint to be seen with the naked eye, but binoculars show it clearly as a fuzzy patch of light just to the right of a 6th magnitude star.

Andromeda

The stars of Andromeda arc up and to the left of the top left star of the square, Sirra or Alpha Andromedae. The most dramatic object in this constellation is M31, the Andromeda Nebula. It is a great spiral galaxy, similar to, but somewhat larger than, our galaxy and lies about 2.5 million light years from us. It can be seen with the naked eye as a faint elliptical glow as long as the sky is reasonably clear and dark. Move up and to the left two stars from Sirra, these are Pi amd Mu Andromedae. Then move your view through a rightangle to the right of Mu by about one field of view of a pair of binoculars and you should be able to see it easily. M31 contains about twice as many stars as our own galaxy, the Milky Way, and together they are the two largest members of our own Local Group of about 3 dozen galaxies.


M31 - The Andromeda Nebula
M33 in Triangulum

If, using something like 8 by 40 binoculars, you have seen M31 as described above, it might well be worth searching for M33 in Triangulum. Triangulum is

the small faint constellation just below Andromeda. Start on M31, drop down to Mu Andromedae and keep on going in the same direction by the same distance as you have moved from M31 to Mu Andromedae. Under excellent seeing conditions (ie., very dark and clear skies) you should be able to see what looks like a little piece of tissue paper stuck on the sky or a faint cloud. It appears to have uniform brightness and shows no structure. The shape is irregular in outline - by no means oval in shape and covers an area about twice the size of the Moon. It is said that it is just visible to the unaided eye, so it the most distant object in the Universe that the eye can see. The distance is now thought to be 3.0 Million light years - just greater than that of M31.


M33 in triangulum - David Malin