At the start of March the sight of Venus and Jupiter together was rather special but I was thwarted from seeing the planetary alignment at the end of the month due to too much cloud cover. Let’s hope we have clearer skies this month. I’ve set a precedent in the past years by describing, in the April blog, the celestial sphere as a help to understanding the night sky and I’ve decided to continue with that in order to help anyone who is taking up astronomy for the first time. Also the constellations being observed this month are circumpolar so it is quite appropriate. If you feel you know all this just skip to the observing section.
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. 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. 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 recent equinox marks the point where the path round the ecliptic crosses the celestial equator. This is when the Sun is overhead at the equator and it continues to travel further north until the summer solstice when it is overhead at the Tropic of Cancer. We see from the diagram that the ecliptic is north of the celestial equator during this period of time.
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 (i.e. 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 (i.e. 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.
The following chart represents the night sky at 11.00pm BST on the 8th of April and at 10.00pm BST on the 23rd April. To use the chart, face south at the appropriate time with the bottom of the chart towards the horizon and you will see the stars in the chart. If you are observing earlier in the evening just turn eastwards by 15 degrees for every hour before the stated time but objects will be lower in the sky. Actually this month the emphasis is on circumpolar stars so you will be looking north so constellation orientations will be upside down!
Like last month the chart features the constellation Ursa Major- The Great Bear with the even better known asterism, the Plough, outlined in red. Remember the pointer stars Merak and Dubhe in the Plough lead you to Polaris the Pole star. The distance to Polaris is about five times the distance between the pointers and although only of magnitude 2 Polaris is easy to identify because there are no other bright objects in that region of the sky. As mentioned earlier the Plough is near your zenith so you need to face north to see the stars beyond that and then the Plough will appear to be upside down. The Pole star is at a point in the sky close to the north celestial pole which is where the Earth’s axis of rotation points so it occupies a more or less fixed position in the night sky and marks ‘true north’ as seen from anywhere in the northern hemisphere. In previous centuries it was an important tool for finding your latitude and maintaining your direction. As the Earth spins on its axis all the stars in the sky appear to rotate about the Pole star on a daily basis. Also stars relatively close to the Pole star never set and are visible all year round when the skies are dark and are said to be circumpolar. Polaris is in the constellation Ursa Minor- The Little Bear and it mimics the brighter and larger shape of the Plough.
A line from Alioth in the Plough’s handle through Polaris and extended the same distance again leads to another circumpolar constellation Cassiopeia- Queen Cassiopeia wife of King Cepheus in Greek mythology. It’s asterism in the shape of large ‘W’ is very conspicuous looking north but as it rotates around the north celestial pole over the coming months it will rise higher in the sky until you see its ‘W’ shape looking south. It is always reassuring at any time of the year to locate the Pole star and observe the orientations of the Plough and Cassiopeia as they rotate about it.
From Cassiopeia towards your eastern horizon in the night sky 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 in Greek mythology and husband of Cassiopeia. This constellation has given its name to a group of stars known as cepheid variables which have played an important role in establishing accurate distance measurements to other stars and galaxies.
Finally wrapping itself around Ursa Minor and the Pole star is the large constellation Draco- The Dragon, but it suffers from a lack of any bright stars.
Something to look out for
Last month we discussed some terms associated with the ‘inferior’ planets Mercury and Venus and how they are only visible close to sunrise and sunset. Venus has been lovely to observe in the evening sky but this month the focus is on Mercury. It is the smallest of the planets and closest to the sun so can be tricky to observe close to the horizon but the greater the satisfaction when you manage to do so. Also it orbits the Sun in only 88 days so it doesn’t stay long in our twilight skies before it passes through inferior conjunction and disappears from view. Mercury will be at its highest altitude and greatest elongation east on Tuesday 11th April so this gives us our best chance of observing it. Of course it has been visible weather permitting recently but close to the horizon and setting quite quickly after sunset. You will need a clear view to your western horizon just after sunset and Mercury will be at its brightest early in the month. On the 11th April it sets two hours after sunset.
On Sunday 23rd April there will be a close approach of a three days old Moon and Venus from 7.30pm BST onwards then on Wednesday 26th April there will be a close approach of the Moon and Mars visible from 9.30pm BST. Remember Venus will be at its highest altitude towards the end of April and reaches greatest elongation west on the 4th June.
We have had some clear evening skies recently giving us a good opportunity to see the winter sky in all its glory. As I suspected comet C/2022 E3 ZTF didn’t turn out to be as good a naked eye observation as suggested by some but I did see it with the aid of binoculars and my deckchair! There has also been the pleasure of seeing the planets Jupiter, Mars and Venus in the evening sky.
The following chart represents the night sky at 10.00pm GMT on the 8th of March and at 9.00pm GMT on the 23rd March. To use the chart, face south at the appropriate time with the bottom of the chart towards the horizon and you will see the stars in the chart. If you are observing earlier in the evening just turn eastwards by 15 degrees for every hour before the stated time but objects will be lower in the sky.
The chart has some readily recognisable features this month viz. the constellation Ursa Major- The Great Bear with the even better known asterism, The Plough, outlined in red, and the stars Castor and Pollux in the constellation Gemini- The Twins which we highlighted last month together with the faint zodiacal constellation Cancer- The Crab. So no problems with navigation this month. The constellation Lynx- The Lynx is also shown but although quite large it consists of only a chain of faint stars so not much to see.
I still like observing the plough throughout the year as it changes its orientation. Remember the pointer stars Merak and Dubhe lead you to Polaris the Pole star while a line from Alioth through Polaris leads you to Cassiopeia which is now closer to your northern horizon. Lastly, Mizar is a double star which some people with good eyesight can resolve into its components.
A line from the Plough’s pointer stars in the opposite direction leads you down to our point of interest this month, the constellation Leo- The Lion. Like Ursa Major it has an easily recognisable asterism, The Sickle, outlined in red. The Sickle also looks like a backwards question mark. Both the constellation and the asterism bear a good resemblance to the objects they are meant to represent.
The brightest star in Leo is Regulus located at the bottom of the Sickle or the dot at the bottom of the question mark. Regulus is a large blue-white star of magnitude 1.4 at a distance of eighty light years from Earth. Like so many stars it is part of a multiple system but to the naked eye presents as a single star. The three other named stars in Leo are not so well known but with magnitudes around 2.0 they make Leo stand out in the evening sky. You will be able to enjoy it more in the coming months as it rises higher in the sky.
The Inferior Planets
We have enjoyed watching Saturn, Jupiter and Mars over the past months and although Mars is still with us Saturn is entering conjunction (on the opposite side of the Sun from the Earth) and no longer visible and Jupiter is heading that way. However we have Venus to look forward to and, along with Mercury, it behaves differently because they are inferior planets. This is not a disparaging term it simply means their orbits are entirely within the orbit of the Earth. The chart below is a plan view of the solar system looking down on the orbits of Mercury, Venus and Earth with the Sun in the centre and should be helpful in explaining some terms in common use.
Mercury (pink) and Venus (green) are never at opposition but instead are at ‘inferior conjunction’ when they are aligned with and lying between the Earth and the Sun. Venus is shown in this position on the chart. Likewise when they are aligned but on the opposite side of the Sun to the Earth they are said to be in ‘superior conjunction’. Mercury is shown in this position. It is clear from the chart that Mercury and Venus cannot be seen in the middle of the night because that view is away from the Sun and they are located in the opposite direction. Instead we can only observe these planets close to sunrise or sunset.
After inferior conjunction as they move round their orbits these planets gain angular separation from the Sun until they reach a maximum known as greatest western elongation which applies at sunrise and is exhibited by Mercury on the chart. The angle formed by the Sun, Mercury and Earth is 90 degrees so from Earth half of Mercury is illuminated and half is in shadow and this is also referred to as ‘dichotomy’.
After superior conjunction they reach a maximum angular separation from the Sun at greatest eastern elongation (also known as dichotomy) and this applies at sunset. Venus is shown in this position. Clearly when planets are at greatest elongation it is a suitable time to observe them.
The inferior planets exhibit phases similar to the Moon but with important differences. They pass through crescent and gibbous stages but at superior conjunction which corresponds to a full Moon, with the face facing Earth fully illuminated, they are at their furthest from Earth and so not at their brightest. Instead, in crescent phase close to inferior conjunction and corresponding to a new Moon they are at their closest to Earth and at their brightest. I hope this explanation of these terms will help when we discuss Venus in the coming months.
Something to look out for
Mercury is going through superior conjunction so won’t be visible this month. Remember Venus is shining at a magnitude of about -4.0 throughout the month so will be great to see.
It has a close approach with Jupiter after sunset in the west on Thursday 2nd March. This year the spring equinox in the northern hemisphere is on Monday 20th March. On Wednesday 22nd March a slender crescent Moon lies south-west of Jupiter above the western horizon about 7.00pm. There is a close approach of the Moon and Venus low in the west after sunset on Friday 24th March. Finally there is a close approach of the Moon and Mars located in the constellation Gemini on the evening of Tuesday 28th March.
Looking ahead, Venus will be at its highest altitude towards the end of April and reaches greatest elongation west on the 4th June.
I think the rule for all star gazers on a clear night during January was make sure that you are well wrapped up and have a warm drink close at hand!
The following chart represents the night sky at 10.00pm GMT on the 8th of February and at 9.00pm GMT on the 23rd February. To use the chart, face south at the appropriate time with the bottom of the chart towards the horizon and you will see the stars in the chart. If you are observing earlier in the evening just turn eastwards by 15 degrees for every hour before the stated time but objects will be lower in the sky.
Betelgeuse in Orion enables you to locate the area depicted by the chart and in contrast to the ‘big’ sights we’ve been able to enjoy on a clear night during the past two months we’ll have a look at some less noticeable objects this month.
Starting with the zodiacal constellation Gemini- The Twins, above and to the left of Orion, we are probably all familiar with the very obvious bright stars Castor and Pollux which dominate their part of the sky. Pollux is a magnitude 1.1 single yellow star while Castor is a magnitude 1.6 star which can be resolved into two stars with a small telescope and a larger telescope will reveal a faint red companion. But the story doesn’t finish there because each of these stars is itself a double making six stars altogether. You just never know exactly what you are observing when you look to the stars! However there is more to this constellation because the bodies of the twins are represented by two lines of faintish stars with their feet in the Milky Way. You should be able to pick out Alhena at magnitude 1.9, representing the feet of Pollux just over halfway between Pollux and Betelgeuse.
Below and to the left of Gemini lies another zodiacal constellation Cancer- The Crab. It is the faintest of the zodiacal constellations but fairly easy to find in a dark part of the sky between the bright stars of Gemini and Leo.
Finally below and to the left of Cancer and to the left of the bright star Procyon is the constellation Hydra- The Water Snake. It is the largest constellation in the night sky but its long chain of faint stars makes it hard to trace. Its brightest star Alphard marks its heart while its tail is in the southern hemisphere.
Something to look out for
Some of you may have seen some of the hype in the Press about the comet C/2022 E3 ZTF and how it will be a naked-eye spectacle to enjoy. Forgive me if I am getting cynical in my old age but I have heard this so many times before and ended up being disappointed apart from the comet Hale-Bopp in the 90s which exceeded expectations. I like to be optimistic so the chart above shows you where to look out for this latest comet. I have just drawn the comet’s path free-hand (yellow line) so it is not accurate but should give a good indication of where to look. I suggest you locate Polaris from the Plough and Capella from the Winter Hexagon then use a pair of binoculars to scan the path between them. The comet will be at its brightest at the start of the month but unfortunately there is a full Moon on the 5th which won’t help with viewing. The numbers on the path indicate where the comet is forecast to be on that date in February so the 6th of February might give a chance of seeing it close to Capella.
On Sunday 5th February there is a close approach of Mars and Aldebaran and then on the 22nd the crescent Moon sits midway between the two brightest planets Jupiter and Venus in the west-southwest after sunset. This is prior to the two planets closing in on each other towards the end of the month for their closest approach on the 1st March. Hopefully they will be visible several evenings in succession so that their relative movement can be seen and provide an impressive display.
Some recent clear skies have given us the chance to see the wonderful winter night sky with Orion
dominating the scene and with the planets Mars and Jupiter adding that bit extra.
The following chart represents the night sky at 10.00pm GMT on the 8th of January and at 9.00pm
GMT on the 23 rd January. To use the chart, face south at the appropriate time with the bottom of the chart towards the horizon and you will see the stars in the chart. If you are observing earlier in the evening just turn eastwards by 15 degrees for every hour before the stated time but objects will be lower in the sky.
The constellation Orion- The Hunter, dominates the winter night sky this month so let us look at it in a little more detail. It doesn’t take much imagination to see why it represents a person with the four bright stars Betelgeuse, Bellatrix, Rigel and Saiph ranging in magnitudes from 0.3 to 2.1 and defining the shoulders and feet while the three distinctive central stars are his belt. Then below the belt a group of stars form his sword. This latter area contains a huge star forming region known as the Orion Nebula, M42, shown by a red X on the chart. For those of you with good eyesight and clear skies it is visible to the naked eye. However if you get the chance do try to observe it using a telescope.
The constellation Canis Major- Greater Dog, lies below and to the left of Orion and it is readily located due to the presence of the brightest star in the sky, Sirius. It shines at a brilliant magnitude of -1.4 because it is relatively close to our solar system at a distance of 8.6 light years. It is part of a binary system, the primary star being orbited by a white dwarf.
The constellation Canis Minor- Little Dog, lies to the left of Orion but is one of the smaller constellations and has little of interest apart from its main star, Procyon, a main sequence star of magnitude 0.4 and also relatively close at a distance of 11.4 light years.
The three stars Betelgeuse, Sirius and Procyon form a large triangle known as the Winter Triangle, the red triangle on the chart, which helps with navigation of the night sky.
Something to look out for
For those of you who have been observing Mars closely, its retrograde motion ends on Thursday 12th and it starts moving east again. You may remember it lay between the horns of Taurus when it started retrograde motion on the 30th October whereas it lies considerably to the west of that now.
There are two close approaches with the Moon towards the end of the month. On Thursday 26th there is a close approach with Jupiter, visible after 5.00pm above your southern horizon. Then on Tuesday 31st there is a close approach with Mars, visible after 5.30pm to the southeast and culminating around 8.00pm.
I hope you have had a chance to look at the evening sky as the Sun sets earlier and been able to pick out Mars. I’ve had some people ask me what ‘what is that bright ‘star’ in the east in the early evening?’
The following charts represent the whole night sky at 10.00pm GMT on the 8th of December and at 9.00pm GMT on the 23rd December. You will understand from this that we want to have a look at everything in the winter sky. To use the charts, face in the direction indicated by the letter at the bottom of the chart at the appropriate time and you will see the stars in the chart. If you are observing earlier in the evening just turn eastwards by 15 degrees for every hour before the stated time but objects will be lower in the sky.
You will have no problems identifying anything because we are unashamedly looking at only the clearest and brightest constellations and stars available.
So with the chart above, face your south-eastern horizon and mentally tick off the following eleven objects. We will start with my favourite constellation, Orion – The Hunter, with one of the brightest red supergiants in the sky, Betelgeuse, representing his right shoulder and Rigel, a brilliant blue-white supergiant representing his left foot. Then there is the unmistakeable asterism, Orion’s Belt, around his waist. Follow a line down from Orion’s Belt towards the horizon and slightly eastwards and you find the brightest star in our night sky, Sirius, aka the ‘Dog star’.
Now follow a line from Orion’s Belt about the same distance in the opposite direction and you should find another giant red star, Aldebaran, in the constellation Taurus- The Bull. Of course this star is currently outshone by the nearby planet Mars which is crossing the upper horn in Taurus as it continues its retrograde motion. So below and to the right hand side of Mars you should be able to pick out the red star Aldebaran if the sky is clear. A chance to spot the two together.
Continue in the same direction beyond Aldebaran and there lies a bright star cluster- The Pleiades, popularly known as the ‘Seven Sisters’. The Pleiades doesn’t form a large object in the sky but it does catch the eye.
Return to Aldebaran and on the line from Orion’s Belt turn through 90 degrees to your left and continue upwards to your zenith. The bright star you will see is Capella, a binary system consisting of two yellow giants and the 4th brightest star visible from the northern hemisphere lying in the constellation Auriga- The Charioteer.
The chart above is the same chart as before but now we are looking towards our south-western horizon. We were looking at this part of the sky earlier in the year when it was higher in the sky. You will probably have already seen it but near your zenith is the familiar shape of the constellation Cassiopeia- Queen and wife to King Cepheus in Greek mythology. We will come back to it in the final chart. To the west you can see the constellation Cygnus- The Swan, flying down the Milky Way with the bright supergiant star, Deneb, representing its tail. And further west still and nearer the horizon is the even brighter star, Vega, in the constellation Lyra- The Lyre, the third brightest star visible from the northern hemisphere. We enjoyed these in the summer months as part of the Summer Triangle but the third star Altair has dropped below the horizon.
For our final chart below we now need to face our northern horizon.
We are in very familiar territory now as close to the horizon we can readily pick out The Plough, the well known asterism in the constellation Ursa Major- The Great Bear. The two pointer stars at the top of The Plough direct us to Polaris, the pole star, at a distance of about five times the distance between the pointers. Note that Polaris is the dimmest named star in this blog. If you now follow a line from the third star in from the end of the Plough’s handle through Polaris and continue the same distance again we arrive back at Cassiopeia. It is one of my pleasures throughout the year watching The Plough and Cassiopeia rotate about Polaris in a circle and changing their orientation with the passing months.
I hope you get a chance to enjoy this trip round the winter night sky this month
Something to look out for
Mars reaches opposition on the 8th December and, excluding the Moon, is the dominant object in the night sky. Its magnitude is approaching -2.0 and at the beginning of the month reaches its highest point in the sky about midnight so it can be observed from early evening all through the night.
There will be a close approach of the full Moon with Mars on Thursday the 8th December and in fact there will be a lunar occultation with the disappearance of Mars behind the Moon at about 5.00pm GMT in the western sky with it re-appearing about an hour later. Finally, since we are concentrating on Mars this month, by the 30th December it will be north of Aldebaran and almost equidistant between that star and the Pleiades.
This is the second part of a three part series on the subject of measuring stellar parameters by observation. Using data obtained by WMA members and data obtained from the European Space Agency GAIA mission, I revisit the Hertzsprung-Russel diagram (hereafter "H-R diagram") in some detail, showing its use as a tool to classify stages of stellar evolution. I find that, although the scientific principles underlying the H-R diagram were discovered over 100 years ago, and the H-R diagram itself was first used in a paper published in 1911, it is still of fundamental importance today.
The H-R diagram is one of the most significant diagrams in astronomy. It was independently discovered by Danish astronomer Ejnar Hertzsprung (1873-1967) and US astronomer Henry Norris Russell (1877-1957) in the early 20th century. Both Hertzsprung and Russell continued prominent astrophysics careers in later years.
In 1909, Karl Schwarzschild (1873-1916), the Director of Potsdam Observatory, offered Hertzsprung the position of Senior Astronomer. Hertzsprung established the concept of absolute magnitude as a method of calibrating astronomical distances. Absolute magnitude is defined as the brightness of a star at the distance of 10 parsecs (32.6 light years). Absolute magnitude can be specified for different wavelength ranges. For stars, the absolute visual magnitude is usually expressed as that in the visual (V) band of the spectrum in the Johnson photometric system.
At Potsdam, Hertzsprung was able to establish a formal relationship between a stars observed colour, which is determined by the photospheric temperature, and its observed absolute magnitude, a measure of the stars luminosity. This laid the scientific foundation for what was to become the H-R diagram. Hertzsprung's first paper on the subject was published in 1911.
Meanwhile, working independently in the United States, Henry Norris Russell (see References) had been studying the same field as Hertzsprung, and came up with very similar conclusions (Russell, Henry N) . Russell's early version of the H-R diagram, published in 1913, included giant stars, nearby stars with distances measured using the parallax method, and stars in the Hyades open cluster.
Forms of the H-R diagram
1. Colour-magnitude H–R diagram
There are several forms of the H-R diagram. The form of the H-R diagram shown in Figure 2 below is called a colour-magnitude diagram, in this case that of M45, the Pleiades.
We can see from this diagram that the stars lie in a diagonal distribution in absolute magnitude vs. colour index.
The absolute magnitude is magnitude the star will have when viewed from a distance of 10 parsecs. Table 1 below shows the Johnson photometric classification system, the usual form of colour index, derived from various filters.
The colour index is a simple numerical expression that determines the colour of a star which directly scales the stars photospheric temperature. The lower the colour index, the more blue the star is (higher temperature); the higher the colour index, the more red the star is (lower temperature). This form of the H-R diagram is also called the observational H-R diagram.
2. Theoretical H–R diagram
Another form of the H-R diagram is shown in Figure 3 below, in this case of globular cluster M2.
This form of the diagram plots the photospheric temperature of the star on the x-axis and the luminosity of the star on the y-axis, normally as a log-log plot. This form of the H-R diagram is called the theoretical Hertzsprung–Russell diagram.
Data to construct H-R diagrams is freely available online, for example from the European Space Agency GAIA satellite, where photospheric temperature, and luminosity expressed in units of Solar luminosity, have been calculated before being added to the online database
3. H-R diagrams of star clusters
Open Clusters and Globular Clusters are distinctly different types of object. The contrasting H-R diagrams of the Pleiades open cluster M45 and the globular cluster M14 for comparison in Figure 4 below.
We can see that in the case of M45 most of the stars lie in a diagonal distribution from top left to right bottom of the plot. These stars are very young and are said to be on the main sequence, where nuclear reactions within the stellar core are fusing Hydrogen atoms into Helium atoms. This happens due to the intense pressure within the stellar core raising the core temperature to the point that fusion can commence. Note that nuclear fusion happens because the core is hot, and not the other way round. The star’s position along the main sequence is determined by its mass, with the most massive being at the top left, and the least massive at the bottom right. Note also the length of time a star stays on the main sequence also depends on its mass. The higher the stellar mass, the shorter time the star stays on the main sequence. Stars at the top left in an H-R diagram spend the least time on the main sequence and stars at the lower right of an H-R diagram the longest time.
It's important to understand that as they evolve, stars do not move along the main sequence; they move off the main sequence once the hydrogen in the star's core has been fused. After the hydrogen in the core of the star has been fused into helium, the star evolves off the main sequence, as we see in the H-R diagram for globular cluster M14 in Figure 4.
Generally, in all H-R diagrams, there is the absence of stars in the region between spectral class A5 and G0 and between absolute magnitude 1 and 3. This is called Hertzsprung gap. This is because stars in this region move rapidly through this section – just a few thousands of years.
In the M45 plot in Figure 4, there an obvious outlier - a data point at the bottom right. This is likely to be a white dwarf – the remains of the of an approximately solar mass star at the end of its life. This star too has come off the main sequence. Most likely this star is not part of M45 but just happens to be on a similar line of sight.
Globular clusters consist of very old stars - in fact globular clusters are thought to be among the oldest structures in any galaxy. In the H-R diagram of globular cluster M14, few main sequence stars are evident; most stars have left the main sequence having completed hydrogen burning.
Low mass stars in M14 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 Betelgeuse and Deneb, will finish their lives in Type II supernova events.
Globular clusters are found in the halo and nuclear bulge of galaxies. Our galaxy, the Milky Way has 147 identified globular clusters, which is thought to be around half the number that exist. Today, globular clusters have been observed in many other galaxies as well as the Milky Way.
Compared to our region in the disk of the MW, globular clusters are very compact structures. We may deduce this from the density of data points in H-R diagrams for globular clusters, bearing in mind that all he plots in this paper use the same selection parameters (cone angle and magnitude). The number density of stars in the region of the Solar system is only about 0.004 stars per cubic light year. In the dense center of a GC, the number density of stars can be between 500 to 1000 times greater. This compactness results in a very close gravitational binding between stars, which in turn means GCs are very stable. Hence It thought that most GCs will probably maintain their identity almost indefinitely.
Analysing globular cluster H-R diagrams
In Figure 5 below are three globular clusters imaged by WMA members.
We may deduce from Figure 5 that M2 consists of the oldest stars in this sample (i.e. the most highly evolved), M92 the youngest, and M10 somewhere in between. Further examples of globular clusters with stars at various stages of post- main sequence development are shown in Figure 6 below.
Examining the H-R diagrams in Figure 6 above, we can see three globular clusters where most of the constituent stars are well evolved away from the main sequence. 2Mass-GC01 is an extremely faint Milky Way GC. Effectively obscured in visible light by an estimated 21.5 +/- 1.0 magnitudes, it could only be discovered in the infrared.
M12 is approximately 5kpc distant, and has a diameter of about 75 light-years. Comparing this to the diameter of the Milky Way (~100,000 lyr) puts the size of GCs into context, Laevens 1 is the most distant Milky Way globular cluster yet known, at an estimated 146kpc.
In contrast, objects such as M92 and Willman 1 have less stars in post main sequence stages. This can lead to ambiguity in classification. For example, Willman 1 is either a globular cluster or by far the smallest galaxy yet identified (Willman et al). Either way, it is external to the Milky Way and approximately 38 +/- 7 kpc distant. On the other hand, M92 is confirmed as a Milky Way globular cluster 8.2 kpc distant.
H-R diagrams have been available as astronomical tools for over 100 years and remain of key scientific importance today. Data for constructing H-R diagrams are freely available online, for example from the European Space Agency GAIA mission (see Acknowledgements).
Using H-R diagrams we can infer some of the properties of star clusters, the study of which gives insights to stellar evolution. During the “hydrogen burning” phase of very young stars, nuclear reactions in the stellar core fuse hydrogen atoms into helium atoms, and the star is described as being on the “main sequence”. The length of time a star stays on the main sequence depends on its mass. The higher the stellar mass, the shorter time the star stays on the main sequence.
Hence, among other properties we can estimate the age of the cluster by using their H-R diagrams to judge what proportion of stars have left the main sequence. The H-R diagrams of open clusters show that most of their stars are on the main sequence, and are therefore at a young evolutional stage. Some of the stars in open cluster M45 are just a few million years old. In contrast, the H-R diagrams of globular clusters show that a high proportion of stars have left the main sequence and the stellar cores are fusing higher order elements. Globular cluster M92 appears to have less stars that have left the main sequence than does M2, indicating that M2 is the older of the two. Some globular clusters appear to more than 12 billion years old.
This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.
Russell, Henry N (1912). Relations between the Spectra and Other Characteristics of the Stars ’ Proceedings of the American Philosophical Society , Oct. - Dec., 1912, Vol. 51,No. 207 (Oct. - Dec., 1912),pp.569-579.https://www.jstor.org/stable/pdf/984021.pdf?refreqid=fastly-default%3A81ac3701b6e4316dd5da365af75ab30f&ab_segments=0%2Fbasic_search_gsv2%2Fcontrol&origin=search-results Accessed November 5 2022.
Willman, B et al (2010). Willman 1 - a probable dwarf galaxy with an irregular kinematic distribution. https://arxiv.org/pdf/1007.3499.pdf Accessed November 5 2022.
The clocks went back last weekend so we can see the evening sky a bit earlier and at present with quite mild conditions but I’m sure it will get colder.
The following chart represents the night sky at 11.00pm GMT on the 8th of November and at 10.00pm GMT on the 23rd November. To use the chart, face your southern horizon at the appropriate time and you will see the stars in the chart. If you are observing earlier in the evening just turn eastwards by 15 degrees for every hour before the stated time but objects will be lower in the sky.
Some parts of the chart are the same as last month so a little familiarity should help with navigation. I hope you have been enjoying Cassiopeia at its best, high in the sky going from autumn to winter. Locate Cassiopeia and drop down slightly towards the horizon and to the left of Andromeda you will see the prominent constellation Perseus- representing the mythological Greek hero of the same name, and containing the two bright stars Mirphak and Algol. Again a human shape isn’t obvious but the branch up from the bright star Mirphak is meant to be his right arm and sword while the lower right hand branch is his left arm with the head of Medusa, depicted by Algol, in his left hand. The third branch represents his left leg. Mirphak is a magnitude 1.8 yellow supergiant while Algol has the distinction of being the first eclipsing binary to be observed. In fact it is part of a triple system whose magnitude dips from 2.1 to 3.4 as its components pass in front of each other. With another five stars brighter than magnitude 3 Perseus stands out in the night sky.
To the left of Perseus lies the unmistakeable Capella, the fourth brightest star in the northern hemisphere at magnitude 0.1 and the brightest star in the constellation Auriga- The Charioteer. Its seven brightest stars form the rough shape of a septagon although its lowest star Alnath is actually in Taurus. Capella is ‘only’ 42 light years from Earth and is a binary system consisting of two yellow giants too close to be resolved in most amateur telescopes.
As mentioned above, Alnath is in the constellation Taurus- The Bull lying just below Auriga and extending down and towards the right. It is a zodiacal constellation lying between Aries and Gemini. Its brightest star Aldebaran is readily picked out as it is a red giant of magnitude 1 supposedly representing the red eye of the angry bull at the base of the lower horn. The ‘V’ shape to the right of Aldebaran represents the face of the bull and is an open star cluster called the Hyades. The more famous open cluster, the Pleiades or Seven Sisters, is in the constellation Taurus but lies to the northwest of Aldebaran roughly in the direction of Algol. It is worthy of observation in its own right and is one of my favourites for viewing with the unaided eye.
Something to look out for
Mars reaches opposition on the 8th December so it is becoming the planet to watch. It is currently shining at magnitude -1.2 and lies between the tips of the horns in the constellation Taurus and so enables a comparison to be made with the star Aldebaran. Mars just started retrograde motion on the 30th October so it will be moving from east to west against the starry background throughout the month.
There will be several close approaches of the Moon with different planets, starting with Jupiter on the 4th November. Visible from early evening but best viewed about 9.15pm, 36 degrees above your southern horizon. This is followed by Mars on the 11th November, again visible from early evening but highest in the sky in the early hours. Finally Saturn joins in on the 29th November about 20 degrees above your southern horizon in the early evening.
For those of you who like to see a ‘shooting star’ the Leonid meteor shower should be at its best around the 18th November from 10.30pm onwards.
Jupiter has been giving a wonderful appearance in the evening sky and those of us who attended the September meeting at Oakhill had the bonus of seeing the rings of Saturn and the Galilean moons of Jupiter with the aid of telescopes. The latter planet has just passed opposition and will provide good viewing for some time to come. With the autumn equinox behind us there will be darkess earlier in the evening to aid observation.
The following chart represent the night sky at 11.00pm BST on the 8th of October and at 10.00pm BST on the 23rd October. To use the chart, face your southern horizon at the appropriate time and you will see the stars in the chart.
Starting from the Great Square of Pegasus which featured last month, it is easy to locate the star Alpheratz which is in the constellation Andromeda- Princess Andromeda, daughter of King Cepheus and Queen Cassiopeia in Greek mythology. The main features of Andromeda are two curved strings of stars, extending to the left of Alpheratz, the lower of which stands out more with the presence of two stars, Mirach and Almach, of around magnitude 2 while the upper curve of stars is fainter with stars of magnitude 3 to 4.5. Also, Cassiopeia is just above it and to the east. Fortunately some of the brighter stars form natural pairs with the fainter stars making the latter fairly easy to locate. The constellation Andromeda is home to one of the most famous objects in the night sky- the Andromeda galaxy also known as M31 and shown on the chart by a red ellipse. The Andromeda galaxy is visible to the naked eye (apparent magnitude m = 3.4) but good conditions are required and it helps if you know exactly where to look. From Alpheratz, jump to the second pair of stars along the curved strings and extend a line from Mirach through the fainter star and the Andromeda galaxy will be at a distance approximately equal to the distance between the stars. Don’t expect to see a beautiful coloured spiral galaxy like you see in images from the Hubble or the James Webb telescopes, more of a smudge or fuzzy star. Nevertheless the fact that you are looking at the most distant object that you can see with the unaided eye at a distance of two and a half million light years (that means that the light entering your eye set out from Andromeda two and a half million years ago) should give you a sense of achievement and some wonder.
The constellation Triangulum- The Triangle is pretty insignificant but at least its name matches its shape and its compact size makes it easy to spot just below Andromeda.
The final constellation Aries- The Ram, bears no resemblance to a lamb but in Greek mythology it represents the golden lamb whose fleece was sought after by Jason and the Argonauts. It is also one of the zodiacal constellations. Its brightest star, Hamal, is easily picked out, shining at magnitude m = 2.0.
Something to look out for
There will be quite a lot happening in the skies this month both at night and during the day. Saturn doesn’t have the brightness of Jupiter but on Wednesday 5th October it will have a close approach with the Moon giving you a better chance to locate it. The sight will be best seen above your southern horizon around 9.30pm.
Not to be outdone Jupiter will do the same on Saturday 8th above your south eastern horizon at around 9.30pm.
On the morning of Tuesday 25th there will be a partial (around 12%) solar eclipse between 10.00am and 12.00 noon. Remember not to attempt looking directly at the sun but use approved filter glasses or project using a pinhole camera. Keep a watch out for any news from WMA about the location of a public viewing using the new solar telescope.
On Friday 25th there will be a chance to catch the Orionid meteor shower so put on some warm clothing and get your deckchair out. The radiant point is close to the bright star Betelgeuse in Orion.
Finally Mars will become more prominent in the evening sky before it reaches opposition on the 8th December. It is in the constellation Taurus and it starts retrograde motion on Sunday 30th . The last retrograde motion was through Pisces and was difficult to follow due to the faintness of the stars. I remember tracking the retrograde motion of Mars through Leo many years ago and I’m hoping the bright stars in Taurus will enable a similar enjoyable experience this time.
I hope you have been enjoying some of the clearer evenings we have had with the Summer Triangle prominent high in the sky and with Cassiopeia rising higher as The Plough drops lower towards the horizon.
The following chart represent the night sky at 11.00pm BST on the 8th of September and at 10.00pm BST on the 23rd September. To use the chart, face your southern horizon at the appropriate time and you will see the stars in the chart.
The chart looks a bit messy but The Summer Triangle and part of Cassiopeia are included just to help us find our bearings. This month we are concentrating on the constellation Pegasus- The Winged Horse. The constellation Andromeda- Princess Andromeda, daughter of Queen Cassiopeia and King Cepheus in Greek mythology, is included but we shall say more about that next month.
So facing south, look upwards to your zenith and to the right hand side there is the bright star Deneb, the tail of Cygnus the swan, and further to the right the even brighter star, Vega, which enables us to pick out The Summer Triangle with Altair closer to the horizon.
Enjoy that for a moment then identify Cassiopeia to your east. You should now be able to pick out the asterism, the Great Square of Pegasus, lying south of Cassiopeia and to the east of The Summer Triangle. The stars aren’t particularly bright but they are away from the Milky Way in an empty area of the sky. The Great Square is part of the constellation, Pegasus- the Winged Horse in Greek mythology, but like many constellations it is difficult to make out the image which it is meant to represent and there is nothing which looks like wings. Let us not quibble that The Square isn’t actually a square or that the star, Alpheratz, at the top of the square isn’t even in Pegasus! The important point is that The Great Square of Pegasus gives us another signpost in the sky. The stars, Markab and Scheat, are similar in brightness with magnitudes of 2.5 and 2.7 respectively, but the former is a blue-white star while Scheat is a red giant variable. The final star, Algenib, is marginally dimmer at magnitude 2.8.
Something to look out for
Planets aren’t usually shown on the chart because they move slowly against the starry background but Saturn passed opposition at the middle of last month and Jupiter will be at opposition on the 26th of this month and they are both in the area of the sky discussed above. The planets have been missing from our evening sky for some time so it is good to have them back in good viewing locations. Saturn and Jupiter are in Capricornus and Pisces respectively but these constellations have relatively faint stars so find the correct area in the sky using the Great Square of Pegasus and the Summer Triangle and with magnitudes of 0.4 and -2.9 you can pick out the two planets fairly easily any time throughout the month because they outshine anything in their locality. On Thursday 8th there is a close approach of Saturn and a bright gibbous Moon and Jupiter has a similar close approach on Sunday 11th.
For those of you who are really keen and don’t mind staying up into the ‘small hours’ there is the added bonus of catching Mars as well. It will be lying a good distance further east in the constellation Taurus and best observed after midnight but shining at magnitude -0.2 it would complete a planetary parade!
Finally on Friday 23rd the Sun crosses the celestial equator, marking the autumn equinox in the northern hemisphere, and after which there will be more hours of darkness than daylight.
I in my ignorance until very recently, believed that astrobiology started as a recognised science after the second world war possibly as late as the 1970’s. This was because detailed knowledge of the structure of the universe and detailed knowledge of the beginnings of life on this planet both started at about that time. In fact, Biology became a science in the late 1800’s mostly due to the work of Darwin and Wallace. I failed to note that detailed knowledge of a subject is not needed to start thoughts of what might be, to help stimulate investigations as occurred with Biology.
In my work at Wells & Mendip Museum library, I was investigating the books written by Alfred Russel Wallace whose principal claim to fame is to be the partner of Charles Darwin in the discovery of Biological ‘Natural Selection’. Much to my surprise, I found two books on Astronomy both on the subject of Astro-biology, and I also found that Wallace had a keen interest in astronomy throughout his life as well as being a biologist. On further investigation, it appears that discussions on ‘Man’s place in the universe’ has been in progress by philosophers throughout the whole of the nineteenth century and indeed before. Wallace’s two books were entitled ‘Man’s place in the Universe’ printed by Cambridge University press in 1903, and ‘Is Mars habitable?’ printed by McMillan press London in1907. At this age, neither book is still in copyright and so I can quote from them at will.
It appears that many 19th century philosophers considered both the purpose of the universe and the role of Man in it, as one of their key philosophical questions of the time. During this period most philosophers took what they described as the ‘Pluralist view’, meaning that there were many intelligent civilisations on different planets. Both these questions became widely public in 1859 on publication of books by Huxley and Whewell both leading philosophers of the time. Wallace did not reply to these books at the time, as both he and Darwin were deeply involved in collecting the extra the supporting evidence they felt necessary for their own Theory.
However, in his 80’s when he was no longer fit for field work, he took to astronomy which was a subject he felt able to do from his armchair. This was the first time an eminent Biologist had considered life on other planets. Wallace was taping into the philosophical tradition of life on other worlds, while trying to give a scientific view to this question.
Wallace took the view that the human mind was special and was the only one of its kind in the Universe. Darwin had taken the mainstream ‘pluralist’ view and wrote a book on ‘The Descent of Man’ to confirm his views and show what they meant to him. Wallace’s book ‘a man’s place in the Universe’, started with five chapters on the most recent discoveries in astronomy to confirm his scientific credentials. The theme of the book was Wallace’s amazement that ‘mild climates and generally uniform conditions had prevailed throughout all geological epochs; and on considering the delicate balance of conditions required to maintain such uniformity, he became convinced that the evidence was extremely strong against the probability of any other planet being inhabited’ (S728 1903b, v-vi). Very interesting of itself but more so when you note that consistent climate is not an issue considered in the Drake equation.
One of Wallace’s main problems in writing his book was the lack of a scientific definition of life. Philosophical definitions were of no use. No other biologist (and only the philosopher Huxley) had tried to make the necessary definition. Wallace needed this definition to determine the criteria for life to exist. Wallace and Huxley’s definition of life was based on the existence of protoplasm which is found in in the majority of plant, animal and fungi cells. This only exists,
In 1904 Wallace added an appendix to his book based on the theory of evolution. Wallace argued that since humanity is the result of a very long chain of modifications in organic life which only occur under certain circumstances, then the chances of the same conditions and modifications occurring elsewhere in the universe were very small indeed. Moreover, since no other animal on Earth, despite the great diversity of forms, approaches the intelligence or moral nature of humanity, these characters were unlikely to arise in any other form.
Based on what he knew in his day, (very different from what we think we know today,) Wallace had a go at estimating the chances of intelligent life arising on another planet. The numbers he came up with are quite interesting. ‘If the physical and cosmical improbabilities set out in the body of my book are somewhere about one million to one, then the evolutionary improbabilities cannot be less than perhaps100 million to one; the total chances of man or a being of equivalent intelligence arising on another planet are 100 million million to one. (S729 1904, Appendix, 334-5). The evolution of intelligence is one of the still unknown factors of the Drake equation, and perhaps there is no better person to estimate it than the person who helped create the modern view of evolution by natural selection. If the estimate of 100 million to one is anywhere near to correct then it is quite possible that we are the only intelligent life form in the milky way galaxy. We must take great care of ourselves as a species!!
Wallace’s book on ‘is Mars habitable?’ is a reply to Percival Lowell’s book ‘Mars and its Canals’. Lowell has observations which he says proves that the canals are good evidence in proving habitation by intelligent beings of the red planet. Wallace gives three separate facts in rebuttal:
Wallace in his first book sees an overall purpose to the Universe.
‘Lastly, I submit that the whole of the evidence that I have brought together in this book leads to the conclusion that our Earth is almost certainly the only inhabited planet in the solar system; further, there is no inconceivability, no improbability even-that in order to produce a world so precisely adapted in every detail for the orderly development of organic life, culminating in man, such a vast and complex universe as we know exists around us, may have been absolutely required.’(S728 1903b, 306)
Although most of us would disagree with Wallace’s conclusions today, I feel confident that this is a good scientific attempt at Astro biology as early as 1903, that is 60 to 70 years before I had thought this possible. My view of Astrobiology is changed for ever.
Note: The text in italics are direct quotes from for Wallace’s books.