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.
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Abstract In this Blog, I examine the properties of four well known stars – the Sun, Betelgeuse, Sirius, and Canopus. Despite the distances to the last three stars, we do know quite a lot about them. I examine how we can measure stellar characteristics, and equally important, what the uncertainties are in such measurement. In historical context, I identify the key astronomers and physicists whose discoveries have led to our present-day understanding. Introduction We can't visit other stars to make measurements. Any notion of inter-stellar travel is way beyond any technology that we possess today. Even the closest star to us, Proxima Centauri is approximately 4.25 light-years (lyr) distant. And yet we do know a lot about stars. In this Blog I will discuss some ways in which this has been achieved, and also see some of the limitations and uncertainties surrounding our observations and measurements. The key discoveries underlying what we now know were made over a century ago. Today, astronomers and physicists are still investigating ways to improve the accuracy and precision of measurement in these areas. In this Blog, I describe two key stellar parameters that enable us to derive other stellar parameters. These parameters are the luminosity of stars and the surface temperature of stars (known as the photospheric temperature). I show how we can use these to calculate the size of a star and the radiation environment the stars produce. Finally, I discuss how the radiation would affect any Earth-like lifeforms on exoplanets in orbit around the star. From here on, we consider a small sample of four stars:
Spoiler alert: there are some equations in here (GCSE level), but don't worry we're just going to use them, not derive them from scratch. Luminosity of stars Let's ask "how bright is a star?" Scientifically, luminosity is a measure of the power of the radiation emitted by a star. The SI unit of luminosity is the Watt. The luminosity of the Sun is called one solar luminosity, quantitatively: Luminosity of other stars is often expressed in units of solar luminosity as we shall see later. We distinguish luminosity, which is independent of its distance from us, and the brightness we actually see which depends very much on how far away the star is because of Newton's inverse square law: By measuring the brightness that we observe, we can work back from there to determine the star's intrinsic luminosity, so long as we know the distance. Figure 2 below shows the intrinsic luminosity of stars in our four-star sample: But, how precise can we be? Let's take the case of the star Betelgeuse in Orion. The luminosity of Betelgeuse is stated as: Perhaps this is a shorthand way of saying "we don't know the luminosity of Betelgeuse to high precision". The reason is that we don't know the distance to Betelgeuse to high precision. We can see both Betelgeuse and Canopus are much more luminous than Sirius, so how can Sirius be the brightest star in the sky? The answer again is distance. Distances of our sample are shown in Figure 3 below: We see from this plot that Betelgeuse is by far the most distant of our 4-star sample, while Sirius is the closest star to us, apart from the Sun of course. Sirius is only 8.6 lyr distant whereas Betelgeuse is 548 lyr distant. Sirius appears so bright to us because it is close to us. But there are uncertainties in distance measurement. Being further distant makes the distance measurement less precise. We can see in the chart above that the error bars on Sirius' distance are very small. The error bars on Canopus' distance are wider. The error bars on Betelgeuse's distance are much wider – which means our distance measurement is far less precise (Joyce et al, 2020). In fact, Betelgeuse is just about as far distant as we can measure using the parallax method. How large are stars? The Stefan–Boltzmann Law is named for Slovene physicist Josef Stephan (1835–1893) and Austrian physicist, Ludwig Boltsmann (1844-1906) – see Figure 4. The Stefan–Boltzmann Law describes the power radiated by a body that absorbs all radiation that falls on its surface in terms of its temperature. This type of object is called a black body. Any object at a stable temperature above the absolute zero is a black body. We assume that stars are examples of a black body, and that they are spheres; as a first approximation these assumptions are true. With those assumptions, we may use Stefan–Boltzmann law, in the form of the equation: where the Stephan-Bolzmann constant, σ = 5.67 *10-8J s-1m-2K-4, is the stellar radius, which is the size of the star that we want to determine; and T is the temperature of the star’s surface. Because the Sun is so close, we can measure its luminosity directly. We can also measure its surface temperature. Re-arranging the Stephan-Bolzmann equation, we get: Which enables us to derive the Solar radius. Note also that this measure of Solar radius can be verified by other techniques. The solar parameters we need are: Remember we said that parameters of distant stars are often expressed in terms of Solar units. Why would we want to do this? Suppose that we have determined luminosity of a distant star, L*, and we have measured the star’s photospheric temperature, T*. using spectroscopy. Now, using the Stephan-Bolzmann equation for the Sun and for the remote star respectively, and dividing one by the other, we see all the constants cancel out: We now re-arrange this equation to give us an equation for the remote star’s radius: This equation is in terms of things we can measure. At first glance, it may look a bit ugly, but it’s easily solvable with a scientific calculator (or Excel, our whatever your preference may be), and gives us the data shown in Figure 5 for our four-star sample: But what about the precision of our measurements? Returning to the case of Betelgeuse, which was the first star ever to be observed as a disk (as opposed to just a point of light). Here we come to some further uncertainties. Looking at the Figure 6 below, we can see that Betelgeuse is not a black body – if it was, it would appear with uniform brightness over its entire surface. Also, Betelgeuse is variable in brightness over quite short periods. Figure 6 shows the recent and unusual dimming of Betelgeuse over a period of ~ 1 year However, like many red supergiants, Betelgeuse has more than one periodicity of variability (Joyce et al, 2020). This variability over a period of approximately 14 years is shown by the observed light curve in Figure 7 below: We can see two things in Figure 7. First there is a long-term pulsation in brightness with a period of approximately 420 days, indicating Betelgeuse is in an unstable state. Second, superimposed on the long-term cycle, there was an unusual dip in brightness in 2019 (see Figure 6). Thirdly, it's very difficult to measure the radius of Betelgeuse by astrometry because what we see is a fuzzy view of the star's atmosphere. There is no sharp looking outer limit such as we see at the top of the atmospheres of gas giant planets in the Solar system. We must accept that we don't know the stellar parameters of Betelgeuse with high precision. Allowing for these uncertainties, we can say that Betelgeuse is approximately 3 orders of magnitude larger than the Sun - in this case, orders of magnitude being in the mathematical sense rather than astronomical magnitudes. Canopus is ≈2 orders of magnitude larger than the Sun. Sirius is of similar size to the Sun (1.71R⊙). What part of the spectrum do stars radiate at? If you've read this far, you'll appreciate that nowhere have we yet mentioned wavelengths of the radiation. The wavelengths at which a star radiates is a key parameter in determining whether any of its planets might harbour life. For example, if an exoplanet is bombarded with ultra-violet radiation, it certainly won't harbour life anything like human beings. Wien’s Law is named after the German Physicist Wilhelm Wien (1864–1928), who published his Law in 1893 and won the Nobel Prize for Physics in 1911. Wien’s Law shows that objects of different temperatures emit radiation that peaks at different wavelengths. The peak wavelength is given by the empirical equation: Once again assuming the stars in our 4-star sample to be black bodies, the radiation curve at various temperatures has a form shown in Figure 9 below: Examining the green curve, which shows the peak wavelength of the Sun, we see that it peaks in the visible light spectrum. The Sun does also radiate in the near ultra-violet but, fortunately for us, Earth's atmosphere blocks most ultra-violet radiation. In contrast, examining the black curve, we see that the peak wavelength of Sirius is in the ultra-violet (UV). Sirius is a binary system (Bond et al 2017). Sirius A is a spectral class A star, and its companion Sirius B is a white dwarf. No planetary system has been found orbiting around either or both stars. If there are any planets in the Sirius system, the UV radiation from Sirius A would mean that any lifeforms on those planets would of necessity have evolved differently to life on Earth Energy radiated by stars We can get a good idea of how damaging the UV radiation from Sirius A is. If we look at the black (Sirius) curve, and compare it to the green (Sun) curve, we can see straight away that the area under the Sirius curve is dramatically larger than that under the Sun curve. Also, there is a lot more area under the Sirius curve in UV wavebands than in the case of the Sun. Simply put, there is not only a lot more energy being radiated by Sirius, but also a greater proportion of Sirius’ energy is radiated as UV. Starting from Wein’s Law, the peak emitted wavelength of Sirius A is: We quantify the radiation by starting from the Planck-Einstein equation, which gives us the energy per photon: Where is Planck's constant, is the speed of light in vacuum, is the frequency of the radiation, and is the corresponding wavelength of the radiation. Hence, the photon energy of Sirius A at the peak wavelength is: If we do the same calculation for the Sun, the peak photon energy In other words, every photon radiated from Sirius A at its peak wavelength carries almost twice the energy of a photon radiated by the Sun at its peak wavelength, as we see in the Figure 10 below: How dangerous to Earthly lifeforms would this be? UV radiation is divided into three wave-bands: Sirius-A's radiation is mainly in the UV-B waveband, and an appreciable amount is in the UV-C waveband. Any advanced Earth-like lifeforms would be at high risk of skin cancer – unless those lifeforms had evolved differently (for example so that UV was actually required for their life), or their planet's atmosphere blocked the UV-B out, as does Earth's atmosphere. Conclusion Figure 11 shows a summary of the characteristics of our four-star sample. This indicates: Betelgeuse is both by far the most luminous star in the sample, and also by far the largest star. Even the next largest star in the sample, Canopus, is much smaller than Betelgeuse. The Sun is comparable in size to Sirius, but has a much lower photospheric temperature. The Sun does radiate UV, but not nearly to the extent as Sirius does. No planets have been discovered in the Sirius system but if planets did exist, the UV radiation created by Sirius if potentially harmful to Earth-like lifeforms. Due to its size and its high photospheric temperature, Canopus has a much higher luminosity than either the Sun or Sirius. The most well-known and important diagram in astronomy is the Hertzsprung-Russel diagram, which helps us study stellar evolution, whereas the diagrams in Figure 11 show stellar parameters of out four-star sample as they appear today. I won't go into H-R diagrams here as I've already exceeded my word count - maybe in a future Blog.
Thanks for your interest. Acknowledgement The author gratefully acknowledges Dr. Ovidiu Borchin and Mr. Bob Merritt for reviewing this paper. Their suggestions have resulted in significant improvements to this work. References Bond, H et al (2017). The Sirius System and Its Astrophysical Puzzles: Hubble Space Telescope and Ground-based Astrometry. ApJ, , 840:70 (17pp), 2017 May 10. https://iopscience.iop.org/article/10.3847/1538-4357/aa6af8/pdf Accessed July 11, 2022 Joyce, M et al (2020). Standing on the Shoulders of Giants: New Mass and Distance Estimates for Betelgeuse through Combined Evolutionary, Asteroseismic, and Hydrodynamic Simulations with MESA. ApJ, 902:63 (25pp), 2020 October 10. https://iopscience.iop.org/article/10.3847/1538-4357/abb8db/pdf . Accessed July 11, 2022 The July evening sky didn’t disappoint with lots of comments about the full Moon and the nice clear skies. The striking images released from the James Webb Space Telescope were an added bonus. Observing The following chart represent the night sky at 11.00pm BST on the 8th of August and at 10.00pm BST on the 23rd August. To use the chart, face your northern horizon (we normally face south) at the appropriate time and you will see the stars in the chart. Our focus this month is a few of the circumpolar constellations. These are the ones which rotate anti-clockwise about the north pole star (Polaris) and never drop below the horizon and are visible all year round during the hours of darkness. So facing north you will immediately see towards your lefthand side, The Plough, the well known asterism in the constellation Ursa Major- The Great Bear. It is one of the larger constellations but it is difficult to make out the shape of a bear whereas The Plough, outlined in red on the chart, is instantly recognisable. Names are shown for three of its stars and we shall use these to navigate our way around the sky. First extend the line from Merak to Dubhe (known as the pointers) about x5 the distance between the two stars (shown by a yellow line in the chart) and you arrive at Polaris- The North Pole star. It is a yellow supergiant, not particularly bright at magnitude 2 but easy to locate as it is on its own. It lies about half a degree from the north celestial pole and has been used by mariners for centuries to find their way about the oceans particularly in the days of sailing ships. It is the brightest star in the constellation Ursa Minor- The Little Bear. This small constellation mimics the shape of The Plough and so Polaris sits at the end of its handle.
Now take a line from Alioth in The Plough and extend it through Polaris about the same distance again (shown by the second yellow line in the chart) and you see a bright star, the central one of a ‘W’ formation. This ‘W’ shape is an asterism in the constellation Cassiopeia- Queen Cassiopeia in Greek mythology and wife of King Cepheus. Most people see the ‘W’ shape and call it Cassiopeia. The bright star was never given a name but it was apparently used for navigation in early space missions and has been given the name Navi, an anagram of the middle name of Virgil (Gus) Ivan Grissom who lost his life in the pursuit of space exploration. This is probably the best time of year to see The Plough and Cassiopeia because neither one is close to the horizon. These two always keep their same shape but change their orientation as they rotate about Polaris throughout the year. Maybe it’s just me but I find it a great pleasure to watch them as they change positions in the sky. It is also a great help to have two such easily recognisable star formations to find your way about the sky. For the next few months Cassiopeia will rise higher in the sky and you will be able to enjoy it at its best while the plough sinks closer to the horizon. Still facing north you will see to the west of Cassiopeia and towards your zenith a group of not very bright stars forming a shape similar to the gable end of a house. This is the constellation Cepheus- King Cepheus in Greek mythology and husband of Queen Cassiopeia and father of Princess Andromeda. 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. Another large constellation, Draco- The Dragon, has no stars brighter than magnitude 2 but its tail is to be found wrapping itself around Ursa Minor. It has one star of particular significance, Thuban, due to the fact that 5,000 years ago it used to be the north pole star but due to the precession of the earth’s axis (like the wobble of a spinning top) the celestial pole traces out a circle over a period of about 26,000 years and over the last 5,000 years it has moved from Thuban to Polaris. Finally, and just for completeness, above Cassiopeia lies the small and obscure constellation, Lacerta- The Lizard. Even its brightest star is just brighter than magnitude 4 so unaided observation becomes a challenge. Something to look out for There will be a close approach of the Saturn and the Moon on Friday 11th August, being visible as they rise about 10.00pm BST above your south-eastern horizon and reaching their highest point about 1.30am above your southern horizon. Saturn will be at opposition on Sunday 14th August so we will soon be able to enjoy it in the evening sky for some time. (Sorry for my mistake last month, I inadvertently put in the date for 2021.) We can also look forward to Jupiter being at opposition on the 26th September. Clear skies. The summer solstice is past and while most people are enjoying the long summer evenings amateur astronomers can look forward to the evening skies becoming darker a little earlier! I hope some of you managed to see the morning planetary alignment at the end of June. Observing The following charts represent the night sky at 10.00pm BST on the 8th of July and at 9.00pm BST on the 23rd July. To use the chart, face south at the appropriate time with the bottom of the chart towards the southern horizon and you will see the stars in the chart. Our focus this month is The Summer Triangle. It is not a constellation but an asterism formed by three bright stars from three separate constellations. It is easy to find the brightest star by facing south and looking up to just below your zenith and there you find a very bright star. It might be easier getting a deckchair and lying down. This star is Vega, the 3rd brightest star visible from the northern hemisphere and you cannot miss it because of its brilliance. To its bottom left hand side there are four much fainter stars in the form of a parallelogram and together these stars make up the constellation Lyra- The Lyre or Harp. Vega is a white star of magnitude 0 at the relatively close distance of 25 light years. It is 50 times more luminous than our Sun. Vega forms a triangle with the top right hand star of the parallelogram and another star to its top left. This other star is called epsilon Lyra and those with good eyesight may be able to detect that it is a double star. But observation through a small telescope reveals that each of these is a double and they have become known as the celebrated Double Double. Vega lies on the edge of the Milky Way represented by the lighter region in the chart to the left of Lyra. Here to the east you find a giant cross in the sky and this is the constellation Cygnus- The Swan. The bright star Deneb represents the tail of the swan which is flying down the Milky Way. Deneb is a supergiant but at a distance of 1,500 light years its magnitude is 1.3 and it is outshone by Vega. The star Albireo, representing the beak of the swan is another double star beautiful to see through a telescope. Now from a line joining Deneb and Vega, look down about halfway to the horizon and you find another bright star Altair in the constellation Aquila- The Eagle. Altair is the 8th brightest star visible from the northern hemisphere shining at magnitude 0.8 and at just 17 light years away it is one of the closest bright stars to Earth. The three stars Vega, Deneb and Altair form what is called the Summer Triangle, outlined in red on the chart. It is a great sight to see in the summer sky and a help in locating other objects. Contained within the Summer Triangle near the bottom vertex is the constellation Sagitta- The Arrow. It is the third smallest constellation and with its brightest star at magnitude 3.5 it is a bit o a challenge for the unaided observer. To the left of Sagitta and outside the Summer Triangle is another small but distinctive constellation Delphinus- The Dolphin. With stars similar in brightness to Sagitta you will need clear, dark sky conditions for observing. From Altair, drop down the Milky Way and close to the horizon is another zodiacal constellation Sagittarius- The Archer. (See the second chart.) It is difficult to see anything resembling an archer but what is much clearer is the asterism- The Teapot, outlined in red on the chart. Lying in the Milky Way as it does, Sagittarius is an area of the sky rich in star clusters and bright nebulae which hide the centre of our galaxy. As recently as May this year a supermassive black hole at the centre of our galaxy, named Sagittarius A*, was imaged and made headline news. No need to be alarmed, it is some 26,000 light years away. Something to look out for On Monday 4th July the Earth is at aphelion, its furthest distance from the Sun, and it is a reminder that our seasons are not governed by our distance from the Sun but by the tilt of the Earth’s axis to the plane of its orbit. During the summer months in the northern hemisphere the north pole is tilted towards the Sun meaning that the Sun is higher in the sky and we have more hours of sunshine compared to the winter months. On a similar theme on Wednesday 13th July the Moon is at perigee, its closest to the Earth and the full Moon will be its biggest and brightest. It has become the norm to give a full Moon a name and this month it is the Buck Moon. Apparently this comes from the fact that the antlers of male deer (bucks) are growing fastest at this time.
On Friday 15th July the Moon and Saturn rise together just before midnight. Saturn will become better to view as time passes and it reaches opposition on the 2nd August. Similarly Jupiter reaches opposition on the 20th August so we will soon be able to enjoy both of them in the evening sky. Mars travels faster than both of these so it takes longer for the Earth to catch up and Mars will be at opposition on the 14th October. Finally keep a look out for the first images from the James Webb Space Telescope due shortly. Exciting times! Clear skies. This is not the best time of year for viewing the night sky because we are approaching the summer solstice when we have the maximum amount of daylight. However we have had some clear starry nights so let us not complain too much. Observing Okay, it is time to look at the stars. The following charts represent the night sky at 10.00pm BST on the 8th of June and at 9.00pm BST on the 23rd June. We looked at the constellation Bootes- The Herdsman, last month and found its brightest star, Arcturus, by following the arc of The Plough handle downwards. As the night sky continues its westward journey we find two new constellations to the east of Bootes. First is the small but distinctive constellation, Corona Borealis- The Northern Crown. It consists of seven faint stars, the brightest Alphekka being of magnitude 2.2, in a horseshoe shape if you prefer that to a crown. Secondly, and about 30 degrees (three clenched fists at arm’s length) to the left of Bootes you will see four relatively faint stars in the shape of a quadrilateral. This is an asterism called The Keystone (outlined in red on the chart) and is part of the constellation Hercules- the strong man in Greek mythology. Again, it is difficult to see any resemblance to a strong man but the Keystone asterism is another good signpost in the sky. The second chart shows three constellations lying between Hercules and the horizon as you face south. They are named Ophiuchus- The Serpent Holder, Scorpius- The Scorpion and Libra- The Scales. The stars are mostly faint but these three constellations all lie on the zodiac; a region of the sky either side of the ecliptic and so by definition the sun passes through them. Historically the zodiac was divided into twelve equal regions each with its own zodiacal sign but in modern times the boundaries of the constellations were defined in terms of sky coordinates irrespective of the star patterns and the zodiacal constellations were no longer of equal size and there were thirteen rather than twelve. The odd one out is Ophiuchus which is a constellation but not a sign of the zodiac.
Follow a line from the right hand side of The Keystone down close to the horizon and after passing through part of Ophiuchus you will see a bright reddish star with a magnitude of about 1.4. This is Antares, the brightest star in the constellation Scorpius, and the 10th brightest star visible from the northern hemisphere. It is a red supergiant and if it were to replace our sun its surface would lie between the orbits of Mars and Jupiter. Antares was supposed to represent the heart of the scorpion. The fish-hook tail of Scorpius is not visible from out latitude. Finally, to the right of Antares lies the small and faint constellation Libra. It is the only sign of the zodiac which depicts an object (a set of scales) rather than a living creature. Unfortunately it has little to offer the amateur with the unaided eye. Something to look out for There is always something special about the summer solstice and no doubt there will be news items about people going out early to catch the sun rise on the 21st June at Stonehenge or Glastonbury Tor. However, perhaps like me you have been missing the sight of the planets in the evening sky and a better reason for getting up early might be to catch an alignment of the planets from the 24th June to the end of the month. You will need a clear view of the horizon to the east-northeast from 3.00am BST onwards. As a bonus you might see a close approach of the Moon with Saturn on the 18th, with Jupiter on the 21st, with Mars on the 22nd and with Venus on the 26th. Clear skies. The better weather has provided more chances of viewing the night sky but my attempts to catch Mercury at the end of April just after sunset were thwarted by hazy clouds on my western horizon. Also, on the 24th April, the weather wasn’t favourable for seeing the pre-dawn planetary alignment. Observing Okay, it is time to look at the stars. The following charts represent the night sky at 10.00pm BST on the 8th of May and at 9.00pm BST on the 23rd May. We looked at The Plough last month and we’ll start there this month as it is such a good signpost for finding our way about the night sky. You will find the middle of the handle near your zenith (directly above you) so, facing south, follow the arc of the handle of The Plough downwards to the brightest star in that region, Arcturus, which has the distinction of being the second brightest star visible in the northern hemisphere at a magnitude of about 0.2. It is an orange giant nearing the end of its life and is relatively close at 36 light years. It is the brightest star in the constellation Bootes- The Herdsman but it is difficult to distinguish such a figure whereas the Kite asterism is easy to see and is what most people recognise as Bootes. Also marked on the chart is the second brightest star in Bootes, Izar, a binary star consisting of an orange giant and a blue star, well worth a look through a telescope if you get the chance. In the second chart you should immediately recognise the constellation Leo- The Lion which we looked at in March. Between Bootes and Leo lies the constellation Coma Berenices supposedly representing the hair of Queen Berenice of Egypt but it contains no stars brighter than magnitude 4. Carry on following the arc from Arcturus for about the same distance again until you see another bright star. This is Spica the brightest star in the constellation Virgo- The Maiden, which is one of the zodiacal constellations. Spica is a blue-white star with an average magnitude of about 1 and is 260 light years from Earth. It may be easier to memorise these two star hops using the expression (Arc on to Arcturus and Speed on to Spica). Something to look out for As previously mentioned the planets are not favourably positioned for viewing at present so I am repeating the orbital diagram I showed in February. You can see that the planets Venus, Mars, Jupiter and Saturn are visible only in the morning and not visible at all during the evening or nightime. At present, as the Earth rotates on its axis, these planets are positioned such that they rise in the east just before the Sun which proceeds to outshine them when it rises. Now the outer planets orbit more slowly than the Earth and have further to travel during an orbit ( Jupiter takes about thirty Earth years to complete its orbit) so what has to happen is that the faster travelling Earth must overtake them and position itself between them and the Sun and once again they will appear in our night sky in a few months.
You can still look out for Mercury to the West just after sunset. Otherwise enjoy the Moon and the constellations. Clear skies. We passed the spring equinox on the 20th March so we are getting more daylight and the clocks went forward an hour on the 27th March so it will be later before the sky darkens. The good news is that we have had some clear nights and seeing the winter sky has been really good. Like last year, I thought that it would be a good idea to repeat the bit about the celestial sphere and how the sky changes in appearance from night to night and month to month. I hope this will prove useful to any newcomers to the subject and any youngsters who are hopefully embarking on observing the skies as a lifetime’s hobby. 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. Observing Okay, it is time to look at the stars. The following charts represent the night sky at 10.00pm BST on the 8th of April and at 9.00pm BST on the 23rd April. The fact that some stars appear in a group does not indicate that they are close together and their distances can vary by very large amounts. Some groups of stars stand out but may be only part of a constellation and such groupings are called ‘asterisms’. So we will begin with possibly the best known one- The Plough. Start by facing north east and you will readily see The Plough high in the sky standing on its handle. It contains seven stars and the chart shows three of them named. The Plough is part of the constellation – Ursa Major- The Great Bear, but it takes a lot of imagination to see a bear and that region is mostly referred to as The Plough. In North America it is called the Big Dipper and perhaps here in the UK a better name in modern times would be ‘The Pan’. We said in the introduction that the stars rotate about the celestial North Pole and stars close to there never set but are visible all year round when the skies are dark. Stars like this are said to be circumpolar and Ursa Major is a circumpolar constellation. But note The Plough’s orientation carefully because as it continues on its circular journey it will appear upside down in six months’ time as it travels anti-clockwise about the north celestial pole.
The constellations are used as signposts in the sky and enable us to engage in a fun activity called ‘star hopping’. The two stars in the Plough, Merak and Dubhe, are called the pointers and a line from Merak to Dubhe continued onwards (shown in yellow on the chart) 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. Polaris is in the constellation- Ursa Minor- The Little Bear. Now consider a line from the star Alioth in the Plough, through Polaris and continued onwards for about the same distance again (also shown in yellow on the chart) until you see a bright star. It will be the central star of a W (or M) formation, an asterism in the constellation Cassiopeia- Queen Cassiopeia in Greek mythology. 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 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 also 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. Just imagine the two of them at the ends of a long pole rotating about the North Pole. Do have a look at them from month to month so that you become familiar with their orientation. Something to look out for As previously mentioned the planets are not favourably positioned for viewing at present but on the 24th of April Saturn, Mars, Venus and Jupiter are all in a line before dawn as the Sun rises in the east. I think I might just get up early to see that! However the elusive planet Mercury is making an evening appearance and will be at its highest point in the sky on Thursday 28th April. It will be well placed and shining at magnitude 0.2 but still challenging to see close to the horizon at sunset. Clear skies. The good news about the James Webb Space Telescope continues because they have obtained satisfactory images of an object from all eighteen segments of the mirror and are now in the process of synchronising them to produce a single composite image. Roll on summertime when they start receiving data. Observing 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 southern horizon and you will see the stars in the chart. If you are observing a little earlier in the evening then the view is shifted 15 degrees eastwards for every hour before the specified time. I promised you an old favourite last month and nobody can be disappointed with the constellation Leo- The Lion. It is well known because it is a large zodiacal constellation and its outline stars do resemble a crouching lion making it readily recognisable. However, if you are not sure, follow a line from Dubhe through Merak, the two pointer stars in The Plough, down towards your southern horizon and you will find Leo. Earlier in the evening it will be towards the East. This constellation contains an asterism known as The Sickle because of its hook shape and outlined in red on the chart. It also looks like a backwards question mark. The brightest star in Leo is Regulus located at the bottom of the Sickle or the dot at the bottom of the question mark. It is a large blue-white star at a distance of eighty light years from Earth and with a magnitude of 1.4 it is the 15th brightest star in the northern hemisphere. The two other named stars, Algieba and Denebola, are not so well known but with magnitudes of 2.1 and 2.2 they help to make Leo stand out in the night sky. Algieba is a double star, consisting of two yellow giants, easily resolved into its components if you have a small telescope.
Don’t forget to enjoy Orion as it continues westwards in the evening sky to the right of Leo. Just above Leo lies the small and insignificant constellation Leo Minor- The Little Lion. Its brightest star is around magnitude 4 and even if you could see it there is no resemblance to a lion. The next constellation this month is Ursa Major- The Great Bear but better known for its asterism- The Plough, outlined in red. Ursa Major is a circumpolar constellation visible all year round as it rotates about the celestial north pole. The chart is a bit misleading because your zenith (the point directly above you) lies near the front feet of Ursa Major so you need to turn and face towards the north and you will see the plough standing on its handle at this time of year. The pointer stars have already been mentioned in locating Leo but if you go in the opposite direction you find Polaris – The Pole Star. (There will be more about that next month.) The second star along the handle of The Plough, Mizar, is a well known optical double and has been used as a test for how good your eyesight is. However, even a small telescope will reveal that Mizar is part of a true binary system in its own right. Something to look out for As mentioned last month the planets are not in favourable positions at present unless you are an early bird and want to view them before dawn. Remember it is the spring equinox on Sunday 20th March and the following Sunday the clocks move forward one hour as we start British Summer Time (BST). Clear skies. 1. Introduction The existence of what we today call Black Holes was first predicted by the Englishman John Mitchel (1724-1793) in a 1783 paper presented to the Royal Society in London. He reasoned that a star’s gravitational pull might be so strong that the escape velocity would exceed the speed of light. Albert Einstein (1879–1955) published his Special Theory of Relativity (SR), which formalised the concept of space-time, in 1905. Einstein followed this with the General Theory of Relativity (GR) in 1917, which built on SR and provided a new theory of gravity in space-time. GR predicts that gravitational collapse can lead to an object so dense that nothing - not even light - can escape. Karl Schwarzschild (1873–1916) developed a solution to Einstein’s Field Equations and in that process discovered the radius of the event horizon of a Black Hole. The event horizon is the radius at which if an object were to fall below it, the object would no longer be visible to an external observer. Note that the event horizon is not the same as the Black Hole itself. At the centre of a sphere of radius equal to the Schwarzschild Radius, Rs, as predicted in GR, there is a singularity of r = 0 and of infinite density which is the Black Hole. Today (2021), It is believed there are at least two different cases of origin for a Black Hole: • The final stage of the evolution of large stars (≥ 8M⊙) is gravitational collapse in a Type II supernova event, the final compression leading to the formation of a stellar mass Black Hole. • It is also believed that as a result of stellar coalescence, a super-massive Black Hole lies at the centre of most, if not all, galaxies in the universe. The Black Hole at the centre of our Galaxy, the Milky Way, is called Sagittarius A* (pronounced ’A star’). 2 The event horizon of a Black Hole 2.1 The event horizon and popular misconceptions The event horizon of a Black Hole can be thought of as the boundary within which the black hole’s escape velocity is greater than the speed of light, i.e. Vesc > c. Since nothing can exceed c, this means that any object falling into the event horizon will never be able to escape from it. A common misconception about Black holes is that they ”hoover up” any matter that approaches them. This is not the case. Black Holes can’t seek out material to consume any more than any other massive object such as a planet or a star. Another common misconception is that matter can be observed falling into a black hole. This is also impossible. A distant observer will witness the object moving slower and slower, while any light the object emits will be further and further redshifted. We can detect accretion disks around black holes, where material moves with such speed that friction creates high-energy radiation [4]. Some matter from these accretion disks is forced out at near relativistic velocity along the Black Hole’s spin axis. When this matter collides with the inter-stellar medium, visible jets are created which may be light years in length. An example of this is the active galaxy M87, which is emitting a relativistic jet at least 1,500 parsecs long. 2.2 The Schwarstschild metric The Schwarzschild metric, sometimes called the Schwarzschild solution, is a solution of Einstein’s field equations in empty space. The solution is valid only outside the gravitating body; for a spherical body of radius R, the solution is valid for r > R. The Schwarzschild solution describes spacetime under the influence of a massive, non-rotating, spherically symmetric object. Using the Virial Theorem, we may equate the kinetic energy of a small mass, m trying to escape the gravity of a much larger mass M: The actual Solar radius, R⊙ = 6.96 ∗ 10⁵ km is evidently very much larger than the three km Schwarstschild radius for one solar mass (by way of comparison, the Schwarstschild radius for an object of mass equal to one Earth mass is about 9 millimeters). Equation 4 provides us with a useful calibration tool, in that RS scales as 3kmM⊙−1. Putting this in the context of the solar system, we know that the mean orbital distance of Mercury is 5.79 ∗ 10⁷km. Hence we can conclude that if the Sun was somehow replaced by the hypothetical one solar mass Black Hole, Mercury’s current orbital parameters would place it well outside the event horizon. This in turn means that Mercury (or any of the other Solar system planets) would not be in danger of being ”sucked into the Black Hole” as is sometimes popularly suggested. 2.3 Other geometric representations The Virial Theorem assumes a non-rotating system. Examples of these in astronomy include globular clusters. However, it is known that Black Holes, like stars and spiral galaxies, do rotate. Whereas the Schwarstschild metric describes a non-rotating Black Hole, the Kerr metric, discovered in 1963 by New Zealand mathematician Roy Kerr (1934-), describes the geometry of empty space-time around a rotating, uncharged, axially-symmetric Black Hole with a quasi-spherical event horizon. The Kerr metric is an exact solution of the Einstein field equations of GR. Rotating Black Holes have two surfaces with the size and shape of these surfaces depending on the black hole’s mass and angular momentum. The outer surface bounds a region called the ergosphere and has a shape similar to an oblate spheroid. The inner surface marks the event horizon; objects passing into the interior of this horizon can never again communicate with the world outside that horizon. Note neither surface has a physical presence. In summary, if Q represents the body’s electric charge and J represents its spin angular momentum: • In the Schwarzschild metric, Q = 0 and J = 0. • In the Kerr metric, Q = 0 and J ≠ 0. The Kerr metric can be represented in Cartesian Coordinates as: Kerr’s mathematical discovery was almost serendipitous. Prior to that time, Black Holes were commonly regarded as interesting theoretical objects which might or might not exist in nature. However, the first quasars (3C 48 and 3C 273) were discovered in the late 1950s, as radio sources in all-sky radio surveys. They appeared mysterious in that the emission lines of quasar spectra did not correspond to any known chemical elements. In 1963, the same year that Kerr published his result, Dutch-born astronomer and Caltech professor Maarten Schmidt (1929-) was studying 3C273 and realized that one emission line was that of hydrogen, albeit very highly redshifted. In turn, that indicated 3C273 was very distant (actually ≈ 3 ∗ 10⁹lyr distant). For the quasar to be so far away and still visible; Schmidt concluded it must be of very high luminosity. (3C273’s apparent V magnitude is 12.9, so within range of amateur telescopes. 3C273 has the highest apparent magnitude of any currently known quasar and is the closest quasar to the Solar system.) Following these discoveries, Black Holes were no longer a theoretical curiosity and there has been much recent observational work done on quasars. An indication of how technology has advanced in recent years can be discerned from the fact that just one quasar was observable by the European Space Agency Hipparchus mission (launched 1989, ran for 4 years). The follow-on ESA GAIA mission is observing more than 500,000 quasars (Perriman, 2022). In fact, quasars (which are distant active galaxies, and should not to be confused with pulsars, which are neutron stars) are the intensely powerful centres of distant, active galaxies, powered by an accretion disc of particles surrounding a supermassive Black Hole. 3C273 is now estimated to have luminosity 2∗10¹²L⊙. Yet 3C273 appears to be less than a light-year across (our Galaxy is thought to be ≈ 100,000lyr in diameter). 3 Sagittarius A* Sagittarius A* is the designation assigned to the Black Hole at the centre of our Galaxy. It was first discovered in 1951 by Australians Harry Minnett (1917-2003) and Jack Piddington (1910-1997) who used radio observations at 1200MHz and 3000MHz (Winnett and Piddington, 1951) . This was confirmed by observations in 1994 by a team led by Reinhard Genzel (Genzel1, Hollenbach and Townes, 1994) of the Max Planck Inst. fur Extraterrestrische Physik. Genzel and Andrea Mia Ghez of the University of California, Los Angeles were jointly awarded the Nobel Prize in Physics in 2020 for their discovery that positively identified Sagittarius A* as a supermassive compact object, for which a Black Hole is the only currently known explanation. Genzel’s team reported ten years of observations in 2002 of the motion of the star S2 orbiting Sagittarius A*. The observations of S2 used near-infrared (NIR) interferometry at λ = 2.2m. The VLBI radio observations of Sagittarius A* were aligned with the NIR images. The rapid motion of S2 (and other stars close to Sagittarius A*) were easily distinguishable from slower-moving stars along the line-of-sight so these could be subtracted from the images. They found that S2 is in a highly eccentric Keplerian orbit (ϵ = 0.8843, one focus of which was at Sagittarius A*. From this, the team deduced the mass of Sagittarius A* to be 4.1∗ 10⁶M⊙, and that the source radius is no more than 17 light-hours (120AU) Using this result and Equation 4, we may deduce that the Schwarstschild radius of Sagittarius A* is: i.e ≈ 3 orders of magnitude larger than the Schwarstschild radius of Sagittarius A*. If you prefer. this can be equivalently expressed as ≈ 5.9 ∗ 10⁻⁴parsecs (recall that M87s jet extends at least 1,500parsecs). Alternatively this is also equivalent to 1.9 ∗ 10⁻³lyr or 120AU. 4 Hawking radiation and Black Hole evaporation In 1974, Stephen Hawking (1942-2018), determined that a Black Hole should emit radiation with a perfect black body spectrum (Hawking, 1974). Assuming Hawking’s theory is correct, Black Holes are expected to shrink and evaporate over time as they lose mass by the emission of photons and other particles via this phenomenon. The temperature of this Hawking radiation, the Hawking temperature, is directly proportional to the Black Hole’s surface gravity: TH ∝ gBH (10) and in the case of a Schwarzschild Black Hole, it’s gravitational force is inversely proportional to the mass: gBH ∝ 1/MBH (11) Hence, large black holes emit less radiation than small black holes, and so large Black Holes will take longer to fully evaporate. A stellar black hole of M⊙ has a Hawking temperature of only 6.2 ∗ 10⁻⁸K, whereas the cosmic microwave background radiation has T ≈ 2.7K. Hence, stellar-mass Black holes receive more radiation (and thus, more mass) from the cosmic microwave background than they emit through Hawking radiation and will grow instead of shrinking. Although the theoretical evidence for Hawking radiation is very strong, the onlyblack holes we have seen in nature are the size of stars or galaxies, and their Hawking radiation is invisible. To be able to evaporate, a Black Hole must have a Hawking temperature larger than 2.7K and)mass less than the Moon. Using Equation 4, the event horizon of this Black Hole would have a RS < 10⁻⁴m. 5 Conclusions Black Holes are objects so dense that nothing -not even light can escape their gravitational field. Black Holes are at the centre of a sphere the radius of which is called the event horizon. any object passing through the event horizon can never escape out of it. The possible existence of Black Holes was first suggested in the eighteenth Century. In the second half of the twentieth Century, the discovery of Quasars prompted renewed interest in Black Holes as the only conceivable source of such vast energy in active galaxies such as M87. It is now recognised that Black Holes are located at the centre of most galaxies in the Universe. Our own Galaxy, the Milky Way, has a Black hole at the Galactic centre named Sagittarius A*. It is estimated that Sagittarius A* has a mass ≈ 4.15M⊙. This has been confirmed by observations of stars orbiting the Black Hole. 6 References R Genzel1, R Hollenbach, D and Townes, C (1994). The nucleus of our Galaxy Report on Progress in Physics, Volume 57, Number 5 https://iopscience.iop.org/article/10.1088/0034-4885/57/5/001/pdf Perriman, M. In lecture presented to AGM of Wells and Mendip Astronomers, January 23 2021. Hawking, S (1974). Black hole explosions?. Nature 248, 30–31 (1974). https://doi.org/10.1038/248030a0 Wang, Q et al (2013). Dissecting X-ray-Emitting Gas Around the Center of Our Galaxy. https://ui.adsabs.harvard.edu/abs/2013Sci...341..981W/abstract. Winnett, H and Piddington, J (1951). Observations of Galactic radiation at 1200 and 3000 Mc/s https://articles.adsabs.harvard.edu/pdf/1951AuSRA...4..459P I think I am turning into a ‘grumpy old man’ because last month I was complaining about the lack of clear skies and now that we have had some clear skies I’m complaining because it is too cold! However I was pleasantly surprised early one morning when I drew open my curtains and there was the full Moon about to set in the west- a beautiful sight. The big story in January has to be the successful deployment and positioning of the James Webb Space Telescope at its Lagrange L2 destination. It was quite tense at times as the various stages had to be completed but it all went off flawlessly. It will be exciting times when the data starts coming back. Observing 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 southern horizon and you will see the stars in the chart. If you are observing a little earlier in the evening then the view is shifted 15 degrees eastwards for every hour before the specified time. With part of Orion, Gemini and the Winter Triangle comprising the stars Betelgeuse, Procyon and Sirius on show there is no problem with navigating the skies but our focus this month is on fainter stars to the east of them. Of course, I’m sure you will still enjoy viewing Orion and the brighter stars that were mentioned last month. The constellation Cancer- The Crab, lies to the southeast of Gemini and being a zodiacal constellation its name is relatively well known although perhaps not its location. It is the faintest of the constellations with its stars typically being of magnitude 4 or dimmer so dark skies are essential. Its brightest star, the one nearest Procyon, is of magnitude 3.5. Cancer does not lie on the Milky Way so is in a dark region of the sky not containing very much apart from an open cluster known as Praesepe- The Manger, or the Beehive Cluster or M44. It is visible to the naked eye, as a small nebulous patch, under good viewing conditions and has been known of since ancient times. Of course, it would be better viewed with a pair of binoculars. Below and to the southeast of Cancer is the constellation Hydra- The Water Snake. It is the largest constellation in the night sky with the tail of the snake lying in the southern hemisphere. Unfortunately the chain of stars representing the snake is of an indistinct shape and mostly faint. The ringlet of stars representing the snake’s head is just south of Cancer while the brightest star, Alphard, sits on its own to the southeast of this. It is meant to be the heart of the snake and is an orange giant of magnitude 2. If you find this month’s constellations hard to spot just remember the night sky keeps changing and next month you will be enjoying an old favourite. Something to look out for The planets aren’t in good orbital positions for viewing at present. Mars passed through opposition in October and is now a morning object for a short time. Mercury and Venus are close to inferior conjunction while Jupiter and Saturn are close to superior conjunction. This is best seen in the orbital diagram below. On Wednesday 2nd February there will be a close approach of the Moon and Jupiter just after sunset low above your west-southwest horizon so hopefully we will be able to see Jupiter in a good setting before it reaches conjunction.
The media seems to get quite excited about full moons these days and this month the full moon is on Wednesday 16th and is popularly known as the ‘Snow Moon’. Theoretically there is an exact time for a full moon but it can be observed at any time of night. Clear skies. |
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