Culture

Winter and Spring Stars 

By Duncan Lunan.

On a clear night around 3000 stars should be visible to the naked eye at any time, out of a total 10,000 or so visible in the course of the year.  That calculation however assumes a dark sky, and since the fainter stars are the most numerous, moonlight or street lights can very drastically cut down what’s visible. 

As the poet says, “The Moon doth with delight, Look round her when the heavens are bare” and deep-sky astronomers have been heard to long for the day when it might be shut down or at least anchored on the far side of the Earth, out of their way.  At least the Moon isn’t in the sky all the time, but ‘light pollution’ from street lights and displays is an increasing problem for amateur and professional astronomers alike: in the USA one can buy car bumper stickers saying “I would rather see starlight than street lights”.   With practise, however, it’s surprising just what subtle features can be picked out even in what are now ‘ordinary’ conditions for most people.

But it isn’t necessary to know them all to become familiar with the sky.   There are extremely skilled observers who have memorised the entire star-field visible from their home latitudes, down to the limit of naked-eye visibility and even beyond:  they are dedicated seekers for transient events such as comets and exploding stars, and such names as Alcock  (UK)  and Seki  (Japan)  are attached to many such discoveries – often sharing them, meaning that two or more observers reported the same discovery on the same night.   The rest of us can be content with knowing the constellation figures by sight, the brighter stars in them by name, and the star maps and catalogues for further information as we need it.

The Constellations

As I said back in May last year, there are several ways to learn the constellations.  The approach I don’t favour involves drawing imaginary lines across the sky from one bright star through another to a third.  It tends to come accompanied by statements that the constellations don’t look like what they’re supposed to represent, suggesting that its advocates have little visual imagination.  Fair enough, the constellations were devised well south of the UK, and Cygnus for example looks more like a Swan from Mediterranean latitudes than it does from here – but it doesn’t make that big a difference.  If the devisers and preservers of the constellations  (who were geometers too, after all)  found it more effective to map out the sky with pictures and legends, it’s worth making the effort to see it as they did.

Pointers to Polaris

That being said, there’s an exception to every rule, and nature has given us a present which the ancients didn’t have.  When the constellations were created the Pole Star was Thuban in Draco: now, thanks to the wobble of the Earth’s axis known as Precession of the Equinoxes, it’s Polaris – which just happened to be ‘fingered’ by Dubhe and Merak, the two stars in the handle of the Plough known universally as the Pointers.  That starting point is too good to ignore, but from there on, the way to learn the constellations is by their overall shapes. 

Jan. blank map, after a drawing by Jim Barker

As winter closes in, look for the Winter Triangle which Betelgeuse makes with Procyon and Sirius.   It’s easy to find on Jim Barker’s star maps from December on, and it dates back to at least c.270 BC, when it was described by the astronomer-poet Aratus:

“Let Procyon join to Betelgeuse and pass a line afar,

 To reach the point where Sirius glows, the most conspicuous star.

 Then will the eye delighted view a figure fine and vast,

 Its span is equilateral, triangular its cast.”

Winter Triangle colours, public domain

All three of the Triangle stars are brilliant, and distinctively coloured.  Betelgeuse is red, Procyon yellow, and there’s an ongoing wrangle as to whether the dominant shade of Sirius is blue or green.  

The Chandra X-ray image of Sirius A & B, a double star system located 8.6 light years from Earth, shows a bright source and a dim source. The central bright source is Sirius B, a dense white dwarf star with a surface temperature of about 25,000 degrees Celsius. The dim source (slightly above and to the right of Sirius B) is Sirius A, a normal star more than twice as massive as the Sun. The spoke-like pattern of light is an instrument artifact due to the transmission grating. The white dwarf, Sirius B, has a mass equal to the mass of the Sun packed into a diameter that is 90% that of the Earth. The gravity on the surface of Sirius B is 400,000 times that of Earth!

Sirius is the brightest star in the sky, not just because it’s relatively close to us but because it’s a class A star, more massive, brilliant and short-lived than our Sun or Procyon.  In ancient Egyptian times, its rising just before the Sun marked the onset of the Nile floods;  and its prominence in the legends of the Dogon tribe, in Mali, has led to the suggestion that it might be the home of extraterrestrial visitors.

There are many problems with that suggestion, and other reasons why Sirius is special.  When I was asked in 1978 to design a working stone circle for a Glasgow park, I wanted to include a star alignment which would allow the date of the monument to be calculated, by correcting for Precession of the Equinoxes, which I wrote about last July.  Sirius seemed a good choice because it might be bright enough to be seen rising, even in the city.  But I found to my surprise that Sirius has moved almost parallel to the equator, and still rises almost where it did in ancient times.  That chance effect was partly what made Sirius so valuable as a calendar star, over so long a time; but in my megalith design I had to use Rigel, at the foot of Orion, to fix the construction date for future archaeologists.

Betelgeuse itself marks Orion’s right shoulder, on the top left of the constellation as the giant faces towards us in the northern hemisphere.  His feet are marked by brilliant blue-white Rigel and the fainter Saiph. 

Orion

Alnitak, Alnilam and Mintaka are the three stars of his belt, and Mintaka, the right-hand star of the Belt, lies at present almost on the Celestial Equator.  On the Earth’s equator, it would pass directly overhead.  Below them his sword is jewelled with the Orion Nebula.  Only faint stars mark Orion’s head;  but as a hunter it wasn’t brains, but strength that he needed.  Up and to the right of Orion the bright star Aldebaran marks the eye of his charging enemy, Taurus the Bull. 

Taurus, Pleiades in mane, wrong star coloured red flr Aldebaran

Aldebaran is a red giant star like Betelgeuse, though not as huge in size;  it lies much nearer to us than the V-shaped Open Cluster of the Hyades, against which we happen to see it.  The Pleiades represent the curling mane of the Bull, while the second brightest star in Taurus, Elnath, is one of the two horn tips bearing down on Orion out of the Milky Way.  Old maps represent the curve of faint stars to the right of Bellatrix, on Orion’s forward shoulder, as a shield raised to resist the charge, while those above Betelgeuse to the left are his club poised to strike.

Orion’s two dogs are at his heels, with Procyon and Sirius for their brilliant eyes.  Canis Minor containing Procyon is one of the smallest constellations in the sky, but the Greater Dog is prancing, with his hind feet well below the horizon from the latitude of Britain.  Orion’s favourite game, Lepus (the Hare)  lies at his feet.  But to complete the mythological layout we have to let half the year pass:  in some versions of the story it was the Scorpion who put Orion to death, and that’s why the pitying gods put the Hunter high in the winter sky and the Scorpion low down in the summer, to set Orion as far away from his killer as he could get.

Procyon is an F5(4) yellow star only 10.4 light years from here, with just one companion star, a white dwarf which was first observed by Schaeberle at the Lick Observatory in 1896.  Given its luminosity, 5.5 times the Sun’s, Procyon’s not very likely to have habitable worlds, though the companion star has only a quarter the mass of the Sun, and with an orbital period of 40 years, it’s far enough out to allow the possibility of planets.  Ground-based observations suggested that Procyon’s surface vibrates in the same way as the Sun’s.  But in June 2003 Canada launched MOST  (Microvariability and Oscillations in Stars), promptly nicknamed ‘the Humble Space Telescope’, and it came as a surprise when MOST discovered that Procyon apparently had no surface oscillations after all.  They have since been discovered, but indications are that it’s not as much like the Sun as we thought.  

Sirius is the brightest star in our sky, 8.6 light-years away in Canis Major, with a superdense white dwarf companion.   The idea of intelligent life there was strongly promoted in the 1970s by my friend Robert Temple in his book The Sirius Mystery.   Controversially, he claimed that the rituals of the Dogon tribe in West Africa preserved the memory of visitors from Sirius to the Middle East, more than four thousand years before.  These were the Annedoti, the patrons of learning in ancient Mesopotamia, also found as seven sages in Egypt and as seven rishis, holy spirits, in ancient India.   In all accounts they are amphibious or aquatic, but also associated with the sky – in India, with the stars of the Plough, and in Egypt with the Moon and with Ursa Minor or Boötes.  Robert explained that by saying that intelligent life on a planet of Sirius had to be amphibious, nocturnal or both, because of the star’s intense output of ultra-violet radiation and x-rays.

His analysis has been challenged on many counts.  One big problem is that the Dogon are at the crossroads of western Africa and while they do have long-standing links with other parts of the continent, they were also subject to modern influences long before their Sirius beliefs were first recorded in the late 1920s.  Not only were there trading posts and schools, but there was a Dogon regiment in the First World War!  Perhaps crucially, the Dogon were recorded as believing that Sirius had a third, unseen component.  That’s not believed to be the case now, but it was when the Jesuit schools in the area were first set up.  In the second edition of his book in 1998, Robert reports modern astronomers who think there may be another companion star, but their work hasn’t won universal acceptance.

The age of the star is a bigger problem.  Its name comes from the Greek seirios, scorching, because in classical times the hottest part of the year was the ‘Dog Days’ when Sirius was invisible behind the Sun.  But the name is also apt because Sirius A, the bright star we see, is a type A0 star with a stable lifetime of less than one billion years.  Our own Solar System formed 4.6 billion years ago, but the final bombardment phase which re-melted the surface of the Moon ended only 3.9 billion years ago.  On that timescale, there would seem to be little time for an earthlike planet to form a stable surface before Sirius exhausts the hydrogen in its core and leaves the Main Sequence of the Hertzsprung-Russell diagram, expanding into a red giant star before collapsing into a white dwarf.

Hertzsprung-Russell diagram with stars, Sun below centre

However, there are some slight possibilities.  Two mysteries about the early history of the Earth are, first, that the oldest known rock fragments are zircon crystals, formed about 4.2 billion years ago but apparently in the presence of water, though during the final bombardment phase;  second, that the first living cells were highly active on Earth less than a billion years after the final bombardment ended.  It seems possible that Earth had temporary oceans, at least, even during the bombardment, and that life either evolved there during it or arrived on the last of the impacting comets, as Sir Fred Hoyle maintained with Prof. Chandra Wickramasinghe.  The late Prof. Fred Whipple believed that the final bombardment had been triggered by the formation of Neptune in the outer Solar System, 800 million years later than the other planets.  

In the Sirius system, with two suns, such distant planets might never form, and life might have had an even earlier start in consequence.  Sirius B, which is now the white dwarf companion  (‘the Pup’), must originally have been more massive than Sirius A is now, and must have expelled much of its substance into interstellar space, or to Sirius A, as it reached the red giant stage.  Some ancient writers describe Sirius as ‘red’, and it’s been suggested that Sirius B was then in its giant phase, but it’s almost inconceivable that the subsequent planetary nebula could have disappeared so completely in only 2000 years.  Even today Sirius is so bright that it flashes blue, green and red due to atmospheric refraction, and that probably explains the ancient descriptions.  And it’s hard to see how earthlike conditions on a Sirius A planet could have survived the explosion when Sirius B went supernova. 

But Sirius A could just possibly have a habitable earthlike planet, now, with aquatic, amphibious and nocturnal life all dodging the ultraviolet and x-radiation of the daytime.  For that life to be intelligent it would have to have come from outside.

Betelgeuse is a dying star now, a ‘red giant’ expanded to huge size as it uses up the last of its energy, and pulsating with a period of about five years. 

Betelgeuse-dimming due to hot plasma cloud from southern hemisphere, forming dense dust cloud

Betelgeuse was the first star, and for a long time the only one, to have its disc resolved and photographed by a technique called ‘speckle interferometry’, which synthesises the fragmented images formed as the star twinkles;  essentially the same principle by which multiple images of the eclipsed Sun appear through the leaves of a tree.  The complete disc of Betelgeuse could be resolved because of its huge size – with a diameter of 250 million miles, if the star replaced our Sun, it would take in all the planets out to Jupiter and extend 100 million miles beyond.  The first photos showed huge flares extending out from the tenuous disc of the star, and a shading covering a large part of its surface.  Everyone assured me that it must be a processing fault, but it’s now confirmed that Betelgeuse has  (or has again)  a sunspot nearly as far across as the Earth’s distance from the Sun.  In 2020, sensationally, Betelgeuse dropped to a quarter of its normal brightness, changing the whole appearance of the constellation, and speculation was rife that the supernova was about to occur.  In the event, it turned out that the dimming was caused by a huge cloud of dust, expelled from the atmosphere of the star by a huge stellar flare – but despite the instability, it looks as if the supernova might be a while yet.  It will be easily visible even in daylight, when it happens, but far enough away (643 light-years, plus or minus 46, according to the Hipparcos space telescope)  for us not to be harmed.

Betelgeuse at 643 ly plus or minus 46 (Hipparcos)

Most of the stars in Orion are by contrast young and hot: the constellation is filled with clouds of dust and gas where star formation is continuing, and one particularly intense region is visible even to the naked eye:  the Orion Nebula  (marked on the inset in the January ‘Sky Above You’ chart)  is in the middle of a line of stars which look like a sword hanging from Orion’s Belt.

Trapezium-star-cluster-Hubble Space Telescope, 2017

To the naked eye the nebula looks like a star, a jewel in Orion’s Sword, and it is even designated as one on classical star maps.  But binoculars or a small telescope will show it to be a glowing cloud of gas and dust, including the beautiful cluster of new stars known from its shape as the Trapezium.  Photographs show a great deal of detail.  As stars form in the dark clouds, their light and heat forces material away from them and makes it fluoresce.  The Sword region is just a particular concentration of material which extends over the whole of Orion, and the IRAS  (Infra Red Astrronomical Satellite)  mapping shows many expanding shells around new stars, including the Horsehead Nebula, a dark cloud silhouetted against the luminous ones beyond, as well as the shockwaves from massive stars which have already exploded as supernovae.

Horsehead Nebula, NASA

The Horsehead Nebula is on the edge of one of the clouds of dust and gas, in Orion, where star formation is taking place, around 1500 light-years from us.  The dark cloud was discovered by Williamina Fleming in 1888;  it’s two light-years across and silhouetted against an emission nebula irradiated by Zeta Orionis.  Already, erosion of its dust and gas by radiation from the stars behind is beginning to change its distinctive shape, as seen from here.  Orion is a very dynamic region and more recent ultraviolet satellite studies have shown it to be peppered with hot new stars.  

Such observations are beyond the scope of amateurs, but the two brightest stars of Orion illustrate the processes at work.   Rigel, the bright star marking Orion’s left foot (bottom right, as we look at him), is a superluminous giant which will burn out in less than a billion years.  Hydrogen-burning stars form a diagonal band across the ‘Hertzsprung-Russell Diagram’, in which stellar mass and luminosity are graphed against surface temperature and spectral type;  when a star ‘leaves the Main Sequence’, it moves into a different part of the diagram in only a few million years.   When a star like the Sun leaves the Main Sequence, its core contracts;  the outer layers expand to take up and re-radiate the energy released, after which the star collapses for good into a white dwarf of condensed matter.  But with initially more massive stars such as Rigel or Betelgeuse, the collapse triggers more violent, less stable fusion reactions.  Betelgeuse is an extreme case, with little time to go, astronomically, before tearing apart;  the Crab Nebula in Taurus, between the horns of the Bull, is the remnant of a supernova such as Betelgeuse is doomed to be.

Orion Nebula (NASA)

Information about individual stars in popular works has to be treated with caution.  For example, Sidgwick’s Introducing Astronomy gives Rigel’s distance as 460 light-years, brightness 14,000 times the Sun’s.   Patrick Moore’s Observer’s Book of Astronomy gives 900 light-years, but the Skalnate-Pleso Atlas Catalogue puts it at over 1700 light-years – scale up the brightness accordingly!

(To be continued).

You may also like: The Sky Above You – January 2022

Leave a Reply