The Telescope

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 

 

 

 

 

Constellations of the month

 

 
CURRENT MOON

Saturn

Saturn, crosses the meridian, so highest in the sky, at around 11pm BST as August begins. Then, its disk is ~18.2 arc seconds across and its rings - which are still nicely tilted from the line of sight - spanning some 42.5 arc seconds across. By month's end it will be best seen at around 9:30 pm BST when lying due south. During the month its brightness falls from magnitude +0.16 to +0.33 whilst its angular size flls to 17.6 arc seconds. Sadly, now in Sagittarius and lying on the southern side of the milky way, it is at the lowest point of the ecliptic and will only reach an elevation of ~14 degrees. As with Jupiter, an atmospheric dispersion corrector will help improve our view.

Jupiter

Jupiter, shining on the 1st at magnitude -2.41 and falling to -2.21 during the month, can be seen in the south as darkness falls. Its angular size drops slightly from 42.6 to 39.9 arc seconds as the month progresses. Jupiter, in the southern part of Ophiuchus, ends its retrograde motion in the 11th of August so will begin to move away from Antares in Scorpius initially lying some 7 degrees up and to its left. A highlight gives the times when the Great Red Spot faces the Earth. Sadly it is heading towards the southernmost part of the ecliptic so, as it crosses the meridian in twilight, it will only have an elevation of ~14 degrees (from central UK). Happily, its elevation will only have dropped by a degree and hour later in full darkness. Atmospheric dispersion will thus take its toll and an atmospheric dispersion corrector would greatly help to improve our views of the giant planet.

Mars

Mars. For 4 months Mars, which passes behind the Sun (superior conjunction) on September 2nd, lies too close to the Sun to be visible. We will have to wait until November to spot it in the pre-dawn sky at the start of its next apparition. .

Venus

Venus passes through superior conjunction on the far side of the Sun on the 14th August so will not become visible again in the evening twilight until late autumn.

Mercury

Mercury, is heading west of the Sun following its inferior conjunction last month and, during the first couple of weeks, rises higher in the sky in the pre-dawn twilight. By the 4th of August, shining at magnitude +1.2 it will have an elevation of ~5 degrees above the east-north-eastern horizon some 40 minutes before dawn. Find Caster and Pollux in Gemini and look around 10 degres below. Binoculars might well be needed to reduce the Sun's background glare, but please do not use them after the Sun has risen. Mercury gradually rises higher in the sky and brightens so, by the 13th of the month, reaches an elevation of ~8 degrees some 40 minutes before dawn at ~5:00 am BST when its brightness has increased to -0.36. From around the 10th to 16th August, a telescope might be able to show its ~7 arc second disk exhibiting a phase of ~52%. As Mercury falls back towards the horizon in the next week its brightness increases to magnitude -1.0 and will show a gibbous phase of ~77% but, by the last week of the month, it will become lost in the Sun's glare.

The Night Sky In and Around Swindon August 2019

Slide Presentation

The following presentation was given at our last meeting on the 12 May, 2019 by Rob Slack

The mornings of August 12th and 13th - midnight to dawn: look out for the Perseid meteor shower


A Perseid meteor

Compiled by Prof. Ian Morison
The planets this month
The constellation of Ursa Major

The stars of the Plough, shown linked by the thicker lines in the chart above, form one of the most recognised star patterns in the sky. Also called the Big Dipper, after the soup ladles used by farmer's wives in America to serve soup to the farm workers at lunchtime, it forms part of the Great Bear constellation - not quite so easy to make out! The stars Merak and Dubhe form the pointers which will lead you to the Pole Star, and hence find North. The stars Alcor and Mizar form a naked eye double which repays observation in a small telescope as Mizar is then shown to be an easily resolved double star. A fainter reddish star forms a triangle with Alcor and Mizar.

Ursa Major contains many interesting "deep sky" objects. The brightest, listed in Messier's Catalogue, are shown on the chart, but there are many fainter galaxies in the region too. In the upper right of the constellation are a pair of interacting galaxies M81 and M82 shown in the image below. M82 is undergoing a major burst of star formation and hence called a "starburst galaxy". They can be seen together using a low power eyepiece on a small telescope.


M81 and M82

Another, and very beautiful, galaxy is M101 which looks rather like a pinwheel firework, hence its other name the Pinwheel Galaxy. It was discovered in1781 and was a late entry to Messier's calalogue of nebulous objects. It is a type Sc spiral galaxy seen face on which is at a distance of about 24 million light years. Type Sc galaxies have a relativly small nucleus and open spiral arms. With an overall diameter of 170,000 light it is one of the largest spirals known (the Milky Way has a diameter of ~ 130,000 light years).


M101 - The Ursa Major Pinwheel Galaxy
Though just outside the constellation boundary, M51 lies close to Alkaid, the leftmost star of the Plough. Also called the Whirlpool Galaxy it is being deformed by the passage of the smaller galaxy on the left. This is now gravitationally captured by M51 and the two will eventually merge. M51 lies at a distance of about 37 million light years and was the first galaxy in which spiral arms were seen. It was discovered by Charles Messier in 1773 and the spiral structure was observed by Lord Rosse in 1845 using the 72" reflector at Birr Castle in Ireland - for many years the largest telescope in the world.

M51 - The Whirlpool Galaxy
Lying close to Merak is the planetary nebula M97 which is usually called the Owl Nebula due to its resemblance to an owl's face with two large eyes. It was first called this by Lord Rosse who drew it in 1848 - as shown in the image below right. Planetary nebulae ar the remnants of stars similar in size to our Sun. When all possible nuclear fusion processes are complete, the central core collpses down into a "white dwarf" star and the the outer parts of the star are blown off to form the surrounding nebula.

M97 - The Owl Planetary Nebula & Lord Rosse's 1848 drawing of the Owl Nebula
August 1st to 6th - after sunset: Jupiter and Antares

Jupiter and Antares
Image: Stellarium/IM
Given a low horizon looking towards the south south-west as it becomes dark one will, if clear, be able to spot Jupiter lying up to the left of Antares.
August 9th - evening: Jupiter near the Moon

Jupiter near the Moon
Image: Stellarium/IM
Looking south around midnight, Saturn will be seen over to the left of the Moon two days before full.
The constellation Virgo
Virgo, rising in the east in late evening this month, is not one of the most prominent constellations, containing only one bright star, Spica, but is one of the largest and is very rewarding for those with "rich field" telescopes capable of seeing the many galaxies that lie within its boundaries. Spica is, in fact, an exceedingly close double star with the two B type stars orbiting each other every 4 days. Their total luminosity is 2000 times that of our Sun. In the upper right hand quadrant of Virgo lies the centre of the Virgo Cluster of galaxies. There are 13 galaxies in the Messier catalogue in this region, all of which can be seen with a small telescope. The brightest is the giant elliptical galaxy, M87, with a jet extending from its centre where there is almost certainly a massive black hole into which dust and gas are falling. This releases great amounts of energy which powers particles to reach speeds close to the speed of light forming the jet we see. M87 is also called VIRGO A as it is a very strong radio source.

The Giant Elliptical Galaxy M87
Below Porrima and to the right of Spica lies M104, an 8th magnitude spiral galaxy about 30 million light years away from us. Its spiral arms are edge on to us so in a small telescope it appears as an elliptical galaxy. It is also known as the Sombrero Galaxy as it looks like a wide brimmed hat in long exposure photographs.
The Sombrero Galaxy

M104 - The Sombrero Galaxy
August 10th to 16th before dawn: Mercury at its best

Mercury at its best this year
Image: Stellarium/IM
Given a low horizon in the east north-east Mercury should be visible in the pre-dawn sky as it reaches its greatest angular separation from the Sun. As the ecliptic before dawn is at a reasonable angle to the horizon, Mercury will then have an elevation of 8-9 degrees.
The constellation Hercules
Between the constellation Bootes and the bright star Vega in Lyra lies the constellation Hercules.The Red Giant star Alpha Herculis or Ras Algethi, its arabic name, is one of the largest stars known, with a diameter of around 500 times that of our Sun. In common with most giant stars it varies its size, changing in brightness as it does so from 3rd to 4th magnitude. Lying along one side of the "keystone" lies one of the wonders of the skies, the great globular cluster, M13. Just visible to the unaided eye on a dark clear night, it is easily seen through binoculars as a small ball of cotten wool about 1/3 the diameter of the full Moon. The brightness increases towards the centre where the concentration of stars is greatest. It is a most beautiful sight in a small telescope. It contains around 300,000 stars in a region of space 100 light years across, and is the brightest globular cluster that can be seen in the northern hemisphere.

The Globular Cluster M13 in Hercules. Image by Yuugi Kitahara
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 Gygnus 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.

The Perseid Meteor Shower

If clear, these mornings should give us a chance of observing the Perseid meteor shower - produced by debris from the comet Swift-Tuttle. The early morning of the 12th/13th of August will give us the best chance, if clear, of viewing the shower, but the peak is quite broad and so it is well worth observing on the nights before and after. Most meteors are seen looking about 50 degrees from the "radiant" which lies between Perseus and Cassipeia. Sadly, this year, the Moon will be just before full so we will only expect to see the brighter meteors. NB: As we need to view a very wide area of sky, normal binoculars would be of no use but the Vixen SG 2.1 x 42 that I have reviewed in the Astronomy Digest could be useful albeit over the smaller field of view of ~27 degrees.