September wasn’t a particularly good month weatherwise but on the 5th, Mars and the Moon were good to see between the rolling clouds. Mars has continued its retrograde motion and is now below the lower arm in Pisces (more about that later) while Jupiter and Saturn have continued westward in the evening sky. Poor weather prevented the viewing of the setting Sun on the equinox so will have to wait till the spring equinox in March to fix due West.
I was rather dismissive of two of the watery zodiacal constellations, Aquarius and Capricornus, in last month’s blog and as a ‘fishy’ constellation features this month it is probably time to say something about stellar magnitudes. It is OK looking at a stick presentation of a constellation in a diagram but they don’t look like that in the sky! Originally the brightness of a star was classified on a scale of 1 to 6, 1 being the brightest and 6 being just visible to the unaided eye. (Note the the bigger the number the dimmer the star. In the modern scientific era measurements have shown that a difference in magnitude of 1 means the brightness differs by a factor of 2.5, ie a magnitude 2 star is two and a half times as bright as a magnitude 3 star and a magnitude 1 star is one hundred times brighter than a magnitude 6 star. Really bright objects have a negative magnitude). But that was over two thousand years ago in the Middle East with clear skies and no light pollution. What can we expect to see today at a latitude of about 50 degrees North with the associated weather that brings and the light pollution from modern towns and cities.
If we face South and look above us just past our zenith we see again the reassuring sight of the ‘W’ shape of Cassiopeia. The three brightest stars Caph, Schedar and Navi are close to magnitude 2 and clearly visible while epsilon Cas on the extreme left is magnitude 3.4 and considerably dimmer but easily visible if conditions are reasonable. (See diagram below).
If we drop down to the horizon towards the right we locate as we did last month the Great Square of Pegasus with the star Alpheratz at magnitude 2.1 and Algenib about half as bright with the other two stars of the square in between. The four stars stand out because they are in a fairly empty part of the sky. Now for the tricky part. Again as we did last month, starting from Alpheratz we look for the two curved strings of stars which make up Andromeda. The lower string isn’t too bad because from Alpheratz; delta Andromeda, Mirach and Almach have magnitudes 3.3, 2.1 and 2.2 respectively so no problem. However the higher curved string of stars from Alpheratz; pi Andromeda, mu Andromeda and 51 Andromeda have magnitudes of 4.3, 3.9 and 3.6 respectively. We are now in a situation where poor atmospherics and light pollution become critical if the stars are to be visible to the unaided eye. For comparison, if you are trying to locate the Andromeda galaxy, M31, it has a magnitude of 3.4.
If you are struggling to see the fainter stars even in a clear sky you need to leave the comforts of your home and find a more rural dark sky site. Sorry!
That’s all my excuses made now so we can return to sky gazing. Below Andromeda and to the south east of the Great Square of Pegasus lies the constellation Pisces- The Fish. Supposedly two fish, one the Circlet and the other the group of stars to the East of Alpheratz, tied together with a ribbon. I use the word ‘lies’ advisedly because unfortunately only two stars in Pisces are brighter that magnitude 4 and even then, only just, so it is unlikely that you will see anything if you are in your back garden! If any readers follow their ‘stars’ in the newspapers or were born under the star sign Pisces perhaps now is the time to consign astrology to the rubbish bin. Why did I bother to mention Pisces? At present the planet Mars is in Pisces and at magnitude -2.3 it is more than a hundred times brighter than a magnitude 3 star and outshines anything nearby. It will have a close approach with the Moon on the 3rd October just three days after the full Moon. It will be at its closest to the Earth on the 6th October and at opposition (on the far side of the Earth from the Sun) on the 14th October so visible all night. Now that is something to look forward to.
The diagram below has more named stars than usual not because they are bright but because I used them in the text to explain the variation we see in stellar magnitudes and again I have omitted some minor star groupings to help with clarity.
With the idea of stellar magnitudes firmly in mind let us look at three further constellations. To the southeast of the lower string of Andromeda and due East of the Great Square of Pegasus is another zodiacal constellation, Aries- The Ram. (Remember you probably cannot see anything in Pisces apart from Mars). Aries contains two brightish stars, Hamal at magnitude 2.0 and Sheratan at magnitude 2.7 which are readily seen but there is not much more. How you make the shape of a ram from that I do not know. However Aries has a claim to fame in that it was the location of the spring equinox about two thousand years ago and that event is still called the ‘first point in Aries’ even though it is now in Pisces.
Between Aries and Andromeda is the constellation Triangulum- The Triangle. It has the great redeeming feature that it is what it says on the tin- a triangle! However it doesn’t have any stars brighter than magnitude 3 but because of its compact nature it is readily recognisable if seen.
Finally to the northeast of Aries and Andromeda and southeast of Cassiopeia is the fairly prominent constellation Perseus- another hero from Greek mythology. It contains the stars Mirfak and Algol both around magnitude 2 and another five stars around magnitude 3 or brighter.
Something to look out for
As mentioned above Mars is going to be the major attraction in the night sky this month so don’t miss it and see if you can follow its retrograde motion to the first week in November. ( I used my binoculars to locate eta Pisces and epsilon Pisces).
If you want to see a ‘falling star’ your best chance will be on the 21st October when the Orionid meteor shower is at its peak.
At the end of the month there are two lunar close approaches to look out for. The Moon and Jupiter on the 22nd and the Moon and Mars on the 29th. Clear skies and happy viewing.
My attempts to see the Perseid meteor shower were thwarted by cloud cover but the close approach of the planets Jupiter and Saturn with the Moon at the beginning of the month was good to see. At the time of writing there has been a lot of cloud cover so I’m not very optimistic about seeing the close approach at the end of August.
I hope you are keeping your eye on Cassiopeia on its journey west because it is approaching its optimal viewing position and with the Plough low in the northern sky, Cassiopeia is better placed to help us find our way among the stars. Also it is just lovely to look at!
The diagram below seems to contain a lot this month but some of it you are already familiar with and I have omitted some minor star groupings to help with clarity.
As usual we start facing south and look up to the zenith and just short of it and to the right hand side we see the bright star, Deneb, the tail of Cygnus the swan and along with Vega and Altair we quickly pick out the Summer Triangle. From the line joining Deneb and Altair turn left by about 45 degrees to look east of south and you will spot the asterism, the Great Square of Pegasus, which stands out not because of the brightness of its stars but because it is away from the Milky Way and there are few stars visible in this area. This asterism is part of the constellation Pegasus (the winged horse in Greek mythology). Again it is difficult to imagine a horse and no obvious signs of wings. Pegasus is quite a large constellation but its other stars do not stand out as much as the square. The square isn’t actually a square and to add insult to injury the star, Alpheratz, at the top of the square isn’t part of Pegasus! However on a positive note, the Great Square of Pegasus is easily picked out and is another good signpost to help us find our way around the skies.
Alpheratz, is part of the constellation Andromeda (the princess, daughter of the mythological Queen Cassiopeia and King Cepheus) and being the brightest star is also referred to as a And (alpha Andromedae). The main features of Andromeda are two curved strings of relatively faint stars meeting at Alpheratz and it is readily found because of its association with Pegasus. The constellation Andromeda is home to one of the most famous objects in the sky- the Andromeda galaxy also known as M31. It is marked on the diagram with a red cross and labelled M31. The Andromeda galaxy is the nearest large galaxy to Earth and is similar in many ways to our Milky Way galaxy and is the only one visible to the unaided eye in the northern hemisphere. To locate it for observing, (your eyesight needs to be pretty good), start at Alpheratz and by star hopping jump to the second pair of stars along the curved strings and the Andromeda galaxy will be to the right hand side at a distance approximately equal to the distance between the two stars. There is no rush as Andromeda will be in a good position right through till November. Now some mind boggling statistics- the distance to Andromeda is about two and a half million light years which means that the light entering your eyes from Andromeda set out two and a half million years ago, about the time the first members of the genus Homo appeared on Earth using stone tools and long before Homo sapiens arrived on the scene! Andromeda is the most distant object you can see with the unaided eye but you will need a dark site with no light pollution and clear skies. Don’t expect to see something like the images shown in the gallery of the WMA website, you will have to settle for something which might be described as a smudge or fuzzy star but that doesn’t detract from the sense of achievement. Good luck!
If we drop down from the Great Square of Pegasus along the diagonal from Alpheratz to the horizon we find another zodiacal constellation, Aquarius (the Water Carrier). Unfortunately to the unaided eye Aquarius has no bright stars and is of an indistinct pattern. Since antiquity it has been seen as a figure pouring water from a jug but I am obviously lacking in imagination.
However I remember a time in the late sixties when any radio channel you switched to was likely to be filling the air with the song ‘Aquarius’ from the musical ‘Hair’ or from a version by an American pop group. It was a song to cheer you up and had the memorable line, ’This is the dawning of the age of Aquarius’. This might lead us in to discussing ‘the precession of the equinoxes’ but luckily this has already been done in a recent blog by Gordon Dennis (Dennis, July 2020).
Now follow the line from Vega through Altair down to the horizon and you will find the right hand edge of another zodiacal constellation, Capricornus (the Sea Goat, associated with many myths from ancient times) just to the south east of Aquarius. This constellation is relatively small and the second faintest zodiac constellation so doesn’t have much to offer the casual observer and I can’t recall a pop song called Capricornus. However, precession is a slow process so there might be one by the year 4750 give or take a few hundred years!
Let’s have a grand tour. Find the Great Square of Pegasus and follow the left hand side of the square upwards past Andromeda on your left till you see Cassiopeia. From the star Navi go across the top of the constellation Cepheus to the pole star, Polaris, in Ursa Minor, and continue in a straight line to the star Alioth in the handle of the Plough, part of Ursa Major. Just as before, follow the arc of the handle down till you find the bright star Arcturus in the constellation Bootes then follow round eastwards to spot the Keystone of Hercules not missing out the small but attractive constellation, Corona Borealis, on the way. One more step eastwards and you are back at the Summer Triangle comprising Vega in Lyra, Deneb in Cygnus and Altair in Aquila from where we found the Great Square of Pegasus at the start. Now give yourself a pat on the back because you have gone round half the visible night sky and identified twelve constellations.
Erratum: In last month’s blog, ’Looking to the Skies August 2020’, the line pointing to the variable star delta Cep should have gone past the first star to the top right hand one of the three stars in the bottom left hand corner of Cepheus. Sorry for the ambiguity.
Something to look out for
This is the month of the autumn equinox when as the Earth revolves around the Sun, the Sun’s apparent path round the ecliptic crosses the celestial equator and the Sun is overhead at the equator and everywhere on earth has almost equal amounts of daylight and darkness. It will happen on the 22nd of September but of course it happens at a specific time which is about a quarter of a day later each year until it is corrected for with the extra day of a leap year and so the date of the autumn equinox can vary by a day or two but it is usually on the 22nd or 23rd. Another feature of an equinox is that the sun rises due east and sets due west so note some landmark on the horizon in line with the sun at sunrise and sunset and you have your east and west directions. As far as astronomers are concerned it means we are into Autumn with more starry evenings.
There are exciting times ahead because the planet Mars is going to be a major attraction in the night sky in the coming months. Try to catch it early in the month because it rises in the east with the Moon on the 5th September at 9.30pm BST. I say catch it early in the month because there is a little project you might enjoy doing- observing the retrograde motion of Mars. By the 10th of September it stops its apparent eastward motion against the background stars of Pisces and then appears to move westwards until November. Use the stars nPsc, mPsc and ePsc on the lower branch of Pisces as your guide. It should pass between the first two of these stars about the 30th September and it will be getting brighter throughout the month. I hope the diagram clarifies the situation. The orange line is the path which Mars will follow during its retrograde motion from 1st September to the first week in November.
Finally there will be a conjunction, I use the term loosely to mean a coming together rather than the more technical definition, of the Moon, Jupiter and Saturn on the 25th September, best viewed to the south just after 8.00pm BST.
In July’s WMA webinar, we looked at Hertzsprung-Russell diagrams. H-R diagrams are one of the most important tools available in stellar physics, since they indicate a great deal about the characteristic of stars. The H-R diagram below, known as a theoretical H-R diagram, is a scatter plot of stars photosphere temperature vs. luminosity.
Brian Davidson’s last couple of Blogs have mentioned the Summer Triangle asterism, consisting of Vega, Deneb, and Altair. Taking a sample of ‘nearby stars’ as shown below we have marked the Summer Triangle stars on the H-R diagram, showing how different these three stars are:
The spectral classes of the stars are indicated by the familiar ‘OBAFGKM’ legend. Before looking at the three stars, look at the track running from Alnitak at the top left to Proxima Centuri at the bottom right. This track is the Main Sequence, where in the stars interior hydrogen is converted into helium by nuclear fusion. Stars leave the main sequence once about 11% of the hydrogen-mass has fused to helium and the core of the star becomes unstable.
About 90% of stars are on the main sequence. It may not appear like this from the diagram, but that’s because of our sample, which is ‘nearby stars’.
Altair is a main sequence star, larger and hotter than the Sun. It will therefore have a shorter main sequence life than the Sun. Altair’s luminosity is 10.6 L⊙. Recall that luminosity is a measure of the power radiated by a star; the unit of luminosity is the Watt.
Vega has a photospheric temperature similar to that of Altair. But Vega’s hydrogen burning phase is now over. The star has entered the main sequence turnoff, on the way to becoming a red giant, before shedding its outer layers to form a planetary nebula. At that time, what will remain of Vega is a hot white dwarf star at the bottom left of the H-R diagram. This sequence of events will also be what happens to the Sun when main sequence turnoff occurs.
Deneb is a super-giant star. Although Deneb’s Photospheric temperature is comparable to both Vega and Altair, its luminosity is very much greater. Deneb has a luminosity of more than 104 L⊙, a mass of 19M⊙ and is destined to end its life in a Type II supernova event. This will leave a supernova remnant perhaps like the Crab Nebula, M1 and a neutron star which is way off scale at the bottom of the H-R diagram.
Open clusters and globular clusters
For this month’s Blog, we’ll consider a question asked at the July WMA webinar, “Can we make H-R diagrams of star clusters to help determine their characteristics?” The answer is yes.
Astronomers are familiar with both open clusters and globular clusters. Their main characteristics are shown in the table below.
How can we conclude that open clusters consist of young stars and globular clusters consist of old stars? The amount of hydrogen that a star has available for fusion is directly proportional to the star’s mass. In simple terms, the greater the mass of hydrogen packed in, the faster the reaction rate, and the higher the luminosity. The star’s luminosity determines how quickly the star will fuse the hydrogen into helium, and hence how long the star lives on the main sequence according to the relation:
Since from the mass-luminosity relation we know that:
The diagram below summarises how stars of different spectral classes leave the main sequence - the “main sequence turnoff” - as they evolve:
Plugging the numbers into the equations, this means that a star of 10M⊙ will have a lifetime of only about 13 million years.
Bear in mind that we know that about 80% of stars are red dwarfs, smaller than the Sun. A low mass star of about 0.6M⊙ has a life of ~34 billion years. That time is much greater than the age of the universe which means that no low mass star has yet completed its main sequence lifetime.
H-R diagram for open cluster M45
So, let’s plot an observational H-R diagram, (also known as a “colour-magnitude diagram”) for open cluster M45, known of course as the Pleiades:
The observational H-R diagram above is a plot of absolute magnitude (VMag) vs. colour index (BMag-VMag). The scale ranges are (x: colour index 0.2 à 1.45; y: absolute magnitude 8 à -2) .
We can see at the top left of the H-R diagram, only a few of the larger, more luminous (which of course implies more massive) stars in the Pleiades have begun their main sequence turnoff. These are the dots at the top left which are turning upward and to the right. The majority of the (less massive) stars in the plot remain very much on the main sequence. They are so young that hydrogen burning has a while to progress.
It is generally thought that open clusters disperse after a short time (in cosmological terms) before the stars in them have commenced main sequence turnoff. It is also thought that then Sun formed in an open cluster which subsequently dispersed and that this accounts for the fact that the Sun is isolated and not part of a multiple star system, although that is far less certain.
H-R diagram for globular cluster M14
Now, let’s look at the H-R diagram for globular cluster M14. This H-R diagram is markedly different to that of M45. The plot is a subset of the total data of over 1,000 stars and is plotted with the same scale ranges as the M45 plot.
In the M14 H-R diagram, just about no main sequence stars are evident. The reason is that most stars in M14 are very old, and have completed hydrogen burning and moved off the main sequence. Low mass stars are either ascending the red giant branch or have already become red giants. Like Altair, Vega, and the Sun, they will end their lives as white dwarfs. A few at the top right of the H-R diagram are supergiants and, like Deneb, will finish their lives in Type II supernova events.
The fact that the majority of stars in this globular cluster are grouped at the red end of the colour index confirms the generally red appearance of the globular cluster.
Contreras Pena, C et al (2013). The globular cluster NGC6402 (M14). A new BV color-magnitude diagram. ApJ, September 2013. DOI: 0.1088/0004-6256/146/3/57 Accessed August 10th 2020
Australia Telescope National Facility
As promised Jupiter and Saturn are now in the late evening sky and they are on the diagram below but with Jupiter shining so brightly in the southern skies it doesn’t need a signpost. The highlight of last month was the appearance of the comet Neowise visible to the unaided eye in the northern sky and Josh Dury gives a description of where to look for it in his recent e-mail, ‘Identify the constellation, Ursa Major, and use the two stars marking the edge of the saucepan to draw a line at about a similar distance until you come across a faint, smudge patch in the sky. This is Comet Neowise’. It is not usually wise to predict when a comet will appear because over the years there have been many disappointments because of unfulfilled promises of a spectacular sight. One comet which did live up to and even exceed expectations was comet Hale-Bopp back in 1997 and I remember it well. I mention it because it was discovered by amateurs.
We talked about the Summer Triangle of Vega, Deneb and Altair last month so we will start from there. From Altair in the Summer Triangle, looking southerly, drop down the Milky Way close to the horizon and to the right is the constellation Sagittarius (the Archer) which lies on the ecliptic to the left of Scorpius. Yet again it is hard to distinguish the archer of mythology but what is easily recognisable is the asterism ‘the Teapot’. The planets Jupiter and Saturn are on the diagram and in fact Jupiter is by far the brightest object in that part of the sky and you cannot miss it. Sagittarius lies in the direction of the centre of our Milky Way. There are dense clouds of gas and dust along the plane of the Milky Way which obscure our view to the centre. See the recent picture of the Milky Way sent back by my granddaughter from New Zealand.
To the right of Sagittarius and also close to the horizon you will see the star Antares which we mentioned last month.
Now let us go to the other end of the Summer Triangle with Vega and Deneb and look at two circumpolar constellations. Face south and look up to find Polaris- the pole star. Obviously it is above your zenith so you need your deckchair again! If you look to the east you should recognise the ‘W’ shape of the constellation Cassiopeia which we found previously from the Plough via Polaris. So do the reverse trip from Navi through Polaris and you come to the Plough. You will see it is almost upside down now. Just as we have watched the Plough change its orientation so we can enjoy watching Cassiopeia continue on its anticlockwise journey around the pole star gradually taking on the proper ‘W’ shape we are accustomed to during the rest of the autumn as it heads south. Just east of Cassiopeia is a group of not very bright stars forming a shape roughly similar to the gable end of a house. This is the constellation Cepheus (King Cepheus of Ethiopia in ancient mythology and husband of Cassiopeia). Perhaps its claim to fame is that it contains the prototype of an important group of variable stars called ‘cepheid variables’ which have been fundamental in establishing a ‘standard candle’ for the measurement of intergalactic distances and the rate of expansion of the universe- a key area of research in cosmology at present. The prototype was delta Cep in the bottom left hand corner of the house shape.
I guarantee you will enjoy seeing Cassiopeia in the southern skies for the rest of the year.
Something to look out for
At the beginning of the month on Saturday 1st there is a close approach of a near full moon and Jupiter with Saturn just to the east. There is another close approach on Friday 28th August. We cannot all be together for the Perseid meteor shower as usual but if you want to see some shooting stars look out on the nights of 11th and 12th August and be prepared to stay up a little longer than usual to give yourself the best chance in spite of a Last Quarter Moon.
Precession is a phenomenon that occurs when massive bodies move, due to angular momentum being affected by other masses in space-time. In the words of John Archibald Wheeler, “mass tells space-time how to curve, space-time tells mass how to move”.
Precession of Earth’s rotational axis
The most familiar example is the precession of a gyroscope; its rotational axis appears to describe a circle under the influence of Earth’s gravity. Exactly the same applies to the rotational axis of the Earth under the influence of the Sun's (and to a lesser extent, the Moon's) gravity:
As most people are aware, Earth’s rotational axis is inclined ~23.5° to the plane of the ecliptic, which accounts for the seasons. Currently, the Earth’s rotational axis points almost exactly at Polaris, which is therefore called the ‘pole star’. However, the precession of Earth’s axis has a period of ~26,000 years, so that in around 13,000 years time, Earth’s axis will point at Vega, which will then be the ‘pole star’. Then, in about 26,000 years time, Polaris will again be the ‘pole star’. This is an example of rotational axis precession.
The precession of Earth’s rotational axis also accounts for the phenomenon of precession of the equinoxes. The First Point of Aries is one of the two points where the plane of the ecliptic intersects the celestial equator (Davidson, 2020). These are called vernal equinoxes. The first point of Aries was recognized in antiquity in the constellation Aries, but due to precession of Earth’s axial rotation is today located in the constellation of Pisces. Exactly 180° around the celestial equator is the first point of Libra, which today lies in the constellation Virgo.
Let’s put that precession cycle into context. The period of precession of Earth’s rotational axis is:
Human civilisations are known to have started ~6,000 years ago. The number of precession cycles during that time is not yet one quarter:
Modern Homo sapiens are believed to have emerged ~200,000 years ago. The number of precession cycles during that time is almost eight:
Earth formed ~4.5 Bn years ago. The number of precession cycles during that time is more than 170,000:
Precession of planetary orbits
As was discovered by Kepler, a planet follows an elliptical path as it orbits the Sun. The point at which the planet makes its closest approach is known as periastron. For many years, it could not be explained by Newtonian theory that the periastron of Mercury does not always occur at the same place in the Mercury’s orbit. This is because the orbit itself is subject to precession, so that over a period of time periastron occurs at a point further around the orbit. This was established by careful observation in the nineteenth century.
Since Mercury is the planet orbiting closest to the Sun, the precession of Mercury’s orbit is higher than any of the other planets.
How orbital precession works is illustrated in the diagram below.
PLEASE NOTE that a) this diagram is looking at the solar system from ABOVE; b) the diagram is emphatically NOT TO SCALE ; c) also, the orbital eccentricities are GREATLY exaggerated; and d) the angular precession angle is GREATLY exaggerated.
Newtonian gravitational theory predicts that the magnitude of the orbital precession of Mercury should be slightly more than half what is actually observed. Although many explanations were produced to account for the observations, none were considered conclusive. Einstein’s General relativity (GR), published in 1917, predicted the rate of orbital precession to be 43 arc-seconds per century. This matched the observations exactly.
In turn, let’s put that into context. How long does it take Mercury’s orbit to precess a full 360 degrees? Based on angular measure (Helps, 2020), the answer is approximately 3 million years:
Or, looked at another way: Mercury is estimated to have formed 4.5Bn years ago. That would imply that Mercury’s orbit has completed
precessions since Mercury’s formation.
This accurate prediction of 43 arc-seconds per century was the first major observational proof that General Relativity is a valid theory. Note that we say a “valid” theory rather than a “true” theory. A scientific theory cannot be proved to be true; it can be showed to accurately account for observations. A scientific theory can only ever be “proved” to be untrue. Later, GR was also able to exactly predict the much smaller orbital precession of Venus (8.6 arc-seconds per century).
The second observational evidence pointing to the validity of GR was that gravity of a large mass would “bend” light rays passing close by it - recall John Archibald Wheeler’s ‘mass tells space-time how to curve’ above. This was verified by an expedition lead by Sir Arthur Eddington to observe a total solar eclipse in 1921. But that’s another story.
John Archibald Wheeler: https://phy.princeton.edu/department/history/faculty-history/john-wheeler
Mathematics of precession: https://en.wikipedia.org/wiki/Precession
Angular size: Helps, L; WMA Blog, May 2020
Celestial equator and plane of the ecliptic: Davidson, B; WMA Blog, May 2020
The summer solstice has passed now so we will gradually get improved lighting conditions for observing. The notes here apply at 11.00pm BST at the end of the first week of the month and at 10.00pm BST at the beginning of the last week in the month. However I find that at present the sky doesn’t really get dark until after midnight and this month you will need a clear view to the southern horizon with no obstructions and free from local light pollution. I did have a look out on the morning of June 19th to see the close approach of Venus and the Moon but I’m afraid the cloudy skies were against me.
Back at the beginning of April if you looked directly above you while facing south, the Plough was directly overhead (at your zenith) and looked like a plough. Now you will notice that it has moved anti-clockwise about the North Pole and is now upright on its handle. Keep checking the orientation of the Plough as the year progresses.
So while facing south, look directly above you and just before your zenith you will see a very bright star. Perhaps this is the time to get your deckchair out and lie flat on your back! This star is easily recognisable due to its brilliance and a grouping of four stars to its bottom left hand side. These stars make up the compact constellation Lyra (the Lyre or Harp) and the bright star is Vega, alpha Lyr, the 3rd brightest star visible from the northern hemisphere. The lighter region of the diagram to the left of Vega represents the Milky Way, the star filled disc of our galaxy, and there you find a giant cross in the sky and this is the constellation Cygnus (the Swan) with the bright star Deneb, alpha Cyg, representing the tail of the swan which is flying down the Milky Way. Deneb is the 14th brightest star visible from the northern hemisphere. At the head of the long neck is the star Alberio, beta Cyg, about which I have heard our chairman, Hugh, wax lyrical on more than one occasion so do look at it through a telescope if you get the chance.
Now face Vega and Deneb and drop down about halfway to the horizon till you find the star Altair in the constellation Aquila (the Eagle). Altair, alpha Aql, is identified by two fainter stars either side of it and together they point to Vega.
I hope you have been keeping count of these bright stars because Altair is the 8th brightest star visible from the northern hemisphere and you have now become acquainted with eight of the eighteen brightest stars. These three stars Vega, Deneb and Altair form what is called the Summer Triangle depicted in yellow in the diagram. The Summer Triangle is something you will be able to enjoy looking at for the rest of the summer into autumn. Like the Plough it is a big help in finding your bearings.
Now let us be a little more subtle because biggest and brightest isn’t always the best. Last month we found Arcturus by following round the arc of the handle of the Plough. Between Vega and Arcturus you find the constellations Hercules (the strong man from Greek mythology) and Corona Borealis (the Northern Crown). Hercules is a fairly faint constellation and looks more like flailing windmill blades than a strong man but the most distinctive feature is the four central stars in the shape of a quadrilateral forming an asterism known as the Keystone. Corona Borealis is small but distinctive, consisting of seven faint stars in a horseshoe shape if you cannot envisage a crown.
We are quite unashamedly going back to bright star ‘bagging’. We are doing this because the object in question is best observed in summertime. Imagine a line from Vega to Arcturus and from its midpoint follow a line to the horizon between Hercules and Corona Borealis until you see a reddish star. Remember you will need a good unobstructed view to your southern horizon. This star is Antares, the brightest star in the constellation Scorpius (the scorpion) and is the sole attraction because most of Scorpius and specifically its fish-hook shaped tail is not visible from our latitude. Antares is the 10th brightest star visible from the northern hemisphere and that is because it is a red supergiant and if it were to replace our sun, its surface would lie between the orbits of Mars and Jupiter. That is big! It is said to represent the heart of the scorpion.
Something to look out for
The major planets, Jupiter and Saturn, return to the late evening sky this month quite close together and visible all night. On July 14th Jupiter will be at opposition, on the opposite side of the Earth from the Sun, so will be at its closest and brightest. A week later on the 20th July, Saturn reaches opposition but unfortunately both planets will be quite low in the southern sky and although bright are not ideally located for good viewing.
This year, we celebrate 30 years of the history of the Hubble Space Telescope Here’s the HST itself, and one of its most famous images, taken in 1995.
The extent of the universe
The HST is named after the American astronomer, Edwin P Hubble, whose observations in the early 20th Century, lead to two, profound discoveries. Looking into these we will also meet several other important characters.
Hubble was physically large and imposing. he was a US Army boxing champion, serving at the closing stages of WW1, although his unit did not go into combat. He affected an English accent despite being very much an American.
For the first twenty or so years of the twentieth Century, there was great scientific debate about the extent of the universe. Many scientists believed at the time that the whole of the universe consisted of our Milky Way galaxy, and that what were then called “spiral nebulae” were some kind of structure within the Milky Way. Following painstaking observations at the 100 inch Hooker telescope at the Mount Wilson Observatory in California, Hubble and his assistant Humasson established in 1924 that spiral nebulae are in fact remote galaxies in their own right; they are now called spiral galaxies.
Hubble’s discovery was made possible by way of an earlier crucial discovery made by Henrietta Swan Leavitt, who worked at the Harvard College Observatory. Leavitt had the task of examining photographic plates to measure and catalog the brightness of stars.
This work led Leavitt to discover the so-called 'period-luminosity relationship' of Cepheid variable stars. Probably the best known Cepheid variable star is Polaris, the current pole star. Leavitt’s discovery was that the rate at which these stars appeared to vary in brightness was directly related to their intrinsic luminosity. This meant that measuring the period of change provided astronomers with the first "standard candle" with which to measure the distance to remote astronomical objects. Hubble used this technique to show that Cepheids in the Andromeda galaxy, M31, was too far distant to be part of the Milky Way Galaxy. It was later discovered that there different types of Cepheid variables, and this meant that M31 is actually twice as far distant as Hubble first calculated.
The expansion of the universe
Hubble’s second observational discovery was to prove equally profound. It was in fact preceded by a theoretical discovery by Georges Lemaître, a Belgian Catholic priest and professor of physics at the Catholic University of Louvain. Lemaître applied Einstein’s general relativity (GR) to cosmology deriving solutions to Einsteins field equations, giving results that implied an expanding universe.
Extrapolating back in time, Lemaître postulated an origin of the universe in what he called a 'primeval atom' – in effect, the “big bang”. This was in 1927, two years before Hubble's publication of his observational findings of expansion of the universe.
An advanced mathematician, Lemaître could hold his corner in intellectual argument with Einstein (no less!). The two met on several occasions, including at the Solvay Conference in 1931.
Albert Einstein of course needs no introduction. Einstein published his theory of General Relativity in 1917. Developed from his theory of Special Relativity (published in 1905), GR included an explanation of the phenomenon of gravity. Among other things, GR successfully accounted for variations in the precession of the orbit of Mercury which Newtonian gravitational theory was unable to explain. Einstein had believed that the universe was static, although others (including Alexander Friedman and Georges Lemaître) provided solutions to his equations that indicated that the universe must be either expanding or contracting.
In January 1931, Einstein visited Hubble at the Mount Wilson Observatory where the 100 inch Hooker telescope is located.
Einstein, perhaps rather reluctantly, conceded that the expansion predicted by general relativity must be real, added a term called the 'cosmological constant' to his field equations. In later life, he said that this was "his biggest blunder", although today the cosmological constant is now thought by many cosmologists to account for the role of dark energy.
Confirming Hubble’s discovery using modern data
Hubble's observations, published in 1929, established that the spectra of majority of galaxies exhibit a redshift, showing they are moving away from us, and that the further away they are, the faster they appear to be receding. This became what is now called Hubble's Law and is a cornerstone of modern cosmology.
The data plot below shows the plot published in Hubble's 1929 paper.
The slope of the trendline indicates the value of what is called the Hubble parameter, H₀, a measure of the velocity of recession of galaxies vs. distance. Hubble's early estimate was that H₀ ~500 km s¯¹ Mpc¯¹ . This was quickly realized to be much too high, as it implied an age of the universe of less than 2 million years, whereas it was known that Earth was much older than this.
The plot below has been constructed from modern data in the NASA Extragalactic Database (NED).
As in Hubble’s work, the plot shows recessional velocities against distances. The red trendline represents H₀. Observations since Hubble's time have refined and reduced the value of H₀ and today the value is thought to be in the range 60-90 km s¯¹ Mpc¯¹ - the exact value is still highly debated in the community. The slope indicates on this plot for this sample of 44 galaxies, H₀ ~64 km s¯¹ Mpc¯¹ .
For Hubble’s confirmation of the extent of the universe and for Hubble’s Law, the Hubble Space Telescope, which has made so many discoveries of its own in its 30 year operation, is named in his honour.
Well I’m pleased to say that the planets Mercury and Venus didn’t disappoint during the month of May. They were within one degree of each other on Friday 22nd although Mercury is challenging to spot unless you are located in a good site and your eyesight is quite sharp. They repeated with a more separated appearance on Sunday 24th but with the addition of a beautiful crescent Moon nearby. My eyesight isn’t what it used to be but I still managed to see Mercury naked eye. Seeing all three together was something special. This is a difficult time of year for astronomers as there is so little light free time and any local light pollution makes the matter worse.
We’ll start where we left off last month when we used the Plough to locate Polaris (the Pole star). You will notice that the Plough is not directly overhead anymore because Ursa Major is a circumpolar constellation and as it rotates about the Pole star, the Plough moves so that its pointer stars Merak and Dubhe keep pointing towards the Pole star and it changes its orientation in the sky so that looking north at present it appears to be standing on end. This is something to keep an eye on throughout the year until it returns to its original orientation in the sky.
Courtesy In-the-sky.org edited by B Davidson
So facing north, use the pointers, Merak and Dubhe, to find the Pole star and then from the third star in from the end of the Plough handle, Alioth, make a line through the Pole star and continue about the same distance beyond until you see a bright star. It will be the central star of a W formation, an asterism in the constellation Cassiopeia. Most people see the W shape and call it Cassiopeia. The bright star was never given a name in Western or Middle Eastern culture so is referred to as gamma (g) Cas. The convention is to name stars using the letters of the Greek alphabet and an abbreviated form of the constellation. Generally this is done in the order of brightness of the star but it is not a hard and fast rule.
However this star has been given the name Navi, allegedly by the American astronaut Virgil (Gus) Ivan Grissom as an anagram of his middle name because it was used for navigation in the early space missions. A fitting tribute to someone who made the ultimate sacrifice for space exploration. The constellation Cassiopeia is circumpolar and because it is directly opposite the Plough across the North Celestial Pole the two will have exchanged positions in six months so we will see Cassiopeia much better in November. As we will the other circumpolar constellation shown on the diagram, Cepheus, which is rather indistinct at present suffering from being too close to the horizon, the lack of proper darkness and the Bristol glow when looking north.
Now let’s go in the opposite direction. Follow the arc of the handle of the Plough round to the star, Arcturus which has the distinction of being the second brightest star visible in the northern hemisphere. Also known as alpha(a) Boo.
Courtesy In-the-sky.org edited by B Davidson
It is the brightest star in the constellation Bootes (The Herdsman) and again it is difficult to distinguish such a figure whereas the Kite asterism is easier to see. Carry on following the curve of the arc for about the same distance until you see a bright star on the Ecliptic. This is Spica the brightest star in the constellation Virgo (The Maiden). It may be easier to memorise this procedure using the expression "Arc on to Arcturus and Speed on to Spica". Now that you are on the ecliptic you can follow it round to the west (the same path followed by the Sun earlier in the day) and from last month you should recognise Regulus in the constellation Leo. So face west to Regulus then look up and you are back at the Plough.
Something to look out for
It is a challenging time for observing the skies when there is so little darkness but it is the summer solstice on June 20th so things will start to improve from then onwards. What about some daytime observation. Our favourite planet at present, Venus, is approaching inferior conjunction, the point in its orbit when it lies between the Earth and the Sun so we cannot see it during the first half of June but it soon makes an appearance in the morning sky and on June 19th it will be close to the waning crescent Moon at dawn. That would mean an early rise! It will be occulted by the crescent Moon (ie the Moon will pass between us and Venus) from 8.35am (BST) onwards but unfortunately it will not be visible to the naked eye. Perhaps some of our imaging friends will try to capture the event but great care needs to be taken as the Sun is up and in the same direction. It is a C shaped waning crescent so Venus will disappear behind the crescent then reappear about an hour later.
So what is Dark Matter?
Presumably there must be some kind of exotic particles that constitute Dark Matter (DM). The ‘Standard Model’ of particle physics looks like this:
It turns out there are candidates for DM in this model. ‘Neutrinos (the ‘e’, ‘μ’ and ‘τ’ in the leptons group) are DM candidates. Millions of neutrinos pass through the Earth and through our bodies every second, only very rarely interacting with matter. However, neutrinos have an extremely small mass and there are not nearly enough of them to account for the amount of DM required. One group of researchers postulates ‘sterile neutrinos’ which supposedly only interact with other neutrinos, arising when an ordinary neutrino morphs into a sterile neutrino. These results are highly contentious in the community, so neutrinos may only offer a partial explanation.
Another theoretical possibility is called a ‘Massive Compact Halo Objects’ (MACHO), a body composed of normal matter whilst emitting little or no radiation. Possible MACHOs include black holes, neutron stars, red dwarf stars and brown dwarf stars, or even planets not associated with any stars. These would be very faint and emit mainly at infra-red wavelengths rather than optical. Some, not completely conclusive observational evidence for MACHOs has been obtained via gravitational micro-lensing observations. Future observations by the upcoming James Webb Space Telescope, which will observe in the infra-red, may detect MACHOs, but there is still a problem. Theoretical studies indicate MACHOs cannot comprise more that 20% of the required dark matter. Add to that the 3% of normal matter we can see, and we still have the question “where is the other 77%?”
Another DM candidate is a theoretical, non-baryonic particle named ‘Weakly Interacting Massive particle (WIMP). The characteristics of a WIMP are framed such that if they exist it would answer the question as to what DM is. The theory is that WIMPs ought to interact very weakly with baryonic matter. The inferred distribution of dark matter in our galaxy (i.e. the DM halo) shows a considerable contribution in our location, so as we move through space, we ought to pass through much DM. If DM is made of WIMPs, then we could directly detect the rare interactions between WIMPs and ordinary matter. The existence of WIMPs is allowed under an extension of the standard model of elementary particles called supersymmetry. The first problem with WIMPs is that supersymmetry theory has no observational basis. And the second snag; nobody has detected a WIMP.
The last current theory for DM postulates particles named Axions. As with WIMPs, the properties of Axions are framed such that they would account for DM. Because of these properties, axions would interact only minimally with ordinary matter. Axions are predicted to be electrically neutral, have very small mass and very low interaction cross-sections for the strong and weak nuclear forces. This would require modifications to Maxwell’s Equations. Axions would also change to and from photons in magnetic fields. Quite a wish list!
Current physics assumes gravity has always acted as it does now; acts the same everywhere; and under all conditions. Suppose that isn’t the case? The leading –though by no means widely accepted - alternative theory to DM is Modified Newtonian Dynamics (MOND) which postulates that under conditions of low acceleration, gravity behaves differently. It also asserts that the inverse square law, while being true over comparatively small ranges such as the solar system, is not applicable over galactic scales. While MOND appears to account for the motions of galaxies without the need for DM, it does not account well for the observed motions within galaxy clusters – reminding us of Fritz Zwicky’s 1933 DM conclusions.
MOND also flies right in the face of Einsteins General Relativity, which has passed every experimental test that has been thrown at it since 1917.
Most physicists believe DM exists. We do know what DM does. We have little idea about what DM is. Current explanations involve serious modifications of the Standard Model of particle physics, or serious modifications to General Relativity, maybe even both. It’s uncomfortable to think that we don’t know what most of the matter in the universe is. It’s an interesting time to be involved in astrophysics.
The author wishes to acknowledge the assistance of Bob Merritt in the preparation of this article.
In these days of electronic gadgets, multiple apps and go-to telescopes it is easy to forget that the best observational devices we have are our eyes. They have a large field of view, can change direction almost instantaneously but can focus in on detail as well. They don’t involve additional cost, need little preparation and no tidying away after you have finished your observations! But do treat them well. Give yourself fifteen minutes to get accustomed to the dark and avoid the use of bright white lights outside. Red LED lights are readily available if you need them to avoid obstacles or read documents and this will enable you to maintain your night vision. Having said that many people find a pair of binoculars very useful for picking out fainter stars when light conditions aren’t optimum.
The objective of this series of articles is to help people find their way around the night sky using only their eyes so there will not be lots of detail on individual stars or planets as that can be found elsewhere. If a star has a number after it that tells its rank in the order of brightness of stars in the northern hemisphere. The idea is that constellations and asterisms (star patterns) are like addresses and signposts for where you want to go. There are 88 constellations in all so only the main ones which are easily recognised will be looked at. As well as this it is hoped that it will be possible to highlight any unusual phenomena which may occur as the year progresses such as planets in good observational positions, planetary conjunctions, special Moon effects, meteor showers or comets.
The Celestial Sphere
Before we venture outside let us recall some helpful facts. It is useful to think of the sky as a hollow sphere which has the Earth at its centre and to which all the heavenly objects are attached. This sphere is known as the celestial sphere. Just like when you visit a planetarium. Far away objects such as stars and galaxies are in more or less ‘fixed positions’ on the celestial sphere whereas the Sun, Moon and planets continually shift their positions but stay close to a circular path on the sphere’s surface called the ‘ecliptic’ which is tilted to the celestial equator because the Earth’s axis is tilted by 23.5 degrees to the plane of its orbit. In reality of course the Earth revolves round the Sun and the ecliptic is where the plane of the Earth’s orbit cuts the celestial sphere. This makes sense because when we observe the Sun we are looking along the radius of the Earth’s orbit and hence in the plane of its orbit. The celestial sphere also has north and south poles directly above the corresponding poles on Earth and a celestial equator directly above the Earth’s equator.
For us in the northern hemisphere we see the stars rotate about the north celestial pole. Don’t worry about some of the additional information on the diagram. The yellow line is the ecliptic and it shows the signs of the zodiac (representing the constellations) and how the Sun appears to pass in front of them as the Earth revolves around the Sun. Remember we are using a model for what we see and this is governed by the movement of the Earth. The Earth spins about its axis from West to East once a day (ie 360 degrees in 24 hours or 15 degrees per hour) and that is why we see the Sun move across the sky daily from East to West. It may not be so obvious that the stars are doing the same thing at night and they move across the sky from East to West at 15 degrees per hour as well. Of course, they also do it during the day, but we cannot see them for the glare of the Sun.
The Earth also revolves about the Sun once a year (ie 360 degrees in 365 days or about 1 degree per day or 15 degrees in 15 days) which is why the sky at 10.00pm one day will look like the sky at 9.00pm 15 days later. If you wait till 10.00pm again the celestial sphere has moved on by 15 degrees or 1 hour and all the stars have moved that amount further west.
Okay, it is time to look at the stars. We have to start with the planet Venus because it has given a brilliant display over the past two months and continues to do so. Venus is breaking records at present because it reached its brightest on the 28th April and is nearest to the highest altitude in the sky at sunset that it can be. It is well worth observing in twilight when there is nothing else in the sky. Just go outside and look westwards and provided the sky is clear I defy anyone not to see it! After a cup of coffee you can go back out when the sky has darkened. Those of you with telescopes may want to observe Venus as a crescent before it enters inferior conjunction (passes directly between the Earth and the Sun). Make the most of it because by the end of May it will have ceased to be an evening star as it passes between the Earth and the Sun but it won’t be gone for long, reappearing as a morning star by mid-June.
While facing Venus turn southwards and trace a path back along the ecliptic, the path that the Sun took earlier in the evening, until you see two bright stars. You will have to go about 35 degrees ( see Lilli’s article on measuring Angular Size). These two stars are Castor (17) and Pollux (12) the dominant stars in the constellation Gemini- the Twins. The other stars are much fainter and may not be visible if lighting conditions are poor.
Now continue backwards along the ecliptic about another 35 degrees and you will find another bright star, Regulus (15), the brightest star in the constellation Leo- the Lion, which unlike many constellations does look like what it represents, a crouching lion. Above Regulus and representing the lion’s mane is theasterism known as the Sickle, looking like a backwords question-mark with Regulus the dot at the bottom.
For the final star hop all you have to do is raise your head till it is looking upwards at the zenith, the point on the celestial sphere directly above your head. You will immediately recognise the Plough, not a constellation this time but probably the best known asterism. ‘Pan’ would be a better name in modern times and in the USA it is known as the Big Dipper.
The Plough is part of the constellation Ursa Major- the Great Bear. But like many constellations it takes a lot of imagination to see a bear. In the diagram there are two stars named on the Plough, Merak and Dubhe, and these are called the pointers. A line from Merak to Dubhe continued onwards leads to Polaris- the Pole Star. The distance is about x5 the distance between Merak and Dubhe. Polaris is very close to the celestial north pole and easily found because although not very bright it is the only star visible in that area.
Now let us retrace our steps. Follow a line from Dubhe through Merak downwards and you come back to Leo. Now go westwards along the ecliptic and you come to Gemini again. Then how could anyone resist taking another look at Venus! Hopefully this hopping about the sky from one known star group to another will give you confidence to continue on the journey to other constellations in the weeks ahead. The next article will be on the website ready for the beginning of June and we’ll be starting from the Plough so you will know where you are. The same pattern will be adopted for the following months.
Something to look out for
There is a chance to see Mercury just after sunset at the end of May. Its orbit lies between the Earth’s orbit and the Sun and it will be at half phase (dichotomy) on the 29th May, so shining brightly. However it will be tricky to see as it is close to the horizon with an altitude of 16 degrees at sunset. Sunset is at 9-09 pm BST and Mercury sets at 11-17 pm BST but it is losing altitude all the time after sunset. Mercury will be located in the west to the bottom right hand side of Gemini. There is a chance to identify it more easily between the 22nd and the 24th of the month when it will be close to Venus but not so bright and both near the horizon.