In the present phase of the evolution of our Galaxy, stars are formed in the plane of the Milky Way within the great clouds of dust and gas known as ‘galactic nebulae‘ (as opposed to ‘extra-galactic’ ones). The Orion Nebula is an example among the winter stars. The stars form in groups termed ‘Open Clusters‘, and almost anyone can recognise the Pleiades or ‘Seven Sisters’, which form the mane of Taurus, the Bull, charging down on Orion from the right. Binoculars or a telescope will show hundreds of stars in addition to the six or eight visible normally to the eye, and the mist of dust and gas which surrounds them. We used to think that mist was the remains of the cloud out of which they formed, but it turns out to be a much larger cloud which they’re currently passing through. In 3000 BC, due to the Precession of the Equinoxes (the ongoing wobble of the Earth’s axis, with a period of 26,000 years), they were on the Celestial Equator, rising due east, and are so described in the Sanskrit Satapatha Brahmana. But, fascinatingly, the Aryan culture was not then in India but still concentrated in the Middle East – see ‘Ancient Constellations’, May 30th 2021.
As the Pleiades travel round the Galaxy, however, the gravitational pulls of other stars will gradually draw the cluster apart. This is already happening to the Hyades, a ‘V’ of stars lying between the Pleiades and Orion, marked by the red eye of the Bull, Aldebaran. As its colour implies Aldebaran too is an old red giant and is not a member of the Hyades, but much closer to us. In classical times Aldebaran was one of the four Royal Stars marking the onset of the seasons: Aldebaran for spring, Regulus in Leo for summer, Antares in Scorpius for Autumn and Fomalhaut in the Southern Fish for winter. Now, due to Precession of the Equinoxes, they all appear earlier in the year.
Most open clusters are too faint to be marked, normally, on the ‘Sky Above You’ charts, but any guide to the sky should lead you to them. (Because of the eye’s properties in darkness, the trick is to look slightly away from your target.) But one spectacular example can be seen at any time of the year and is now high overhead. Unlike most of the constellation patterns, the stars of the Plough are all members of the same stellar grouping. It’s an Open Cluster so spread out that its other member stars are all over the sky: the Sun is in fact passing through it, but the cluster is so spread out that we’re hardly aware of it.
It was thought that we wouldn’t be able to identify any stars which were formed along with the Sun, 4600 million years ago; but first one, then a number of candidate ‘sibling stars’ have been identified. (‘Our sun now has a brother star, the first one ever found’, EarthSky, 8th May 2014). By now, they may be spread all over the Galaxy. The disc of the Galaxy is in the form of a spiral 100,000 light-years across, and the Sun’s orbit is 27,000 light-years from the Galactic Centre. Each revolution takes 200-250 million years (sometimes known as the Galactic Year). The disc of the Galaxy is so vast that we see our region of it as a band of light, completely circling the sky, known as the Milky Way. Stars which we see outside that plane, therefore, are very close to us in galactic terms. Visibility in the plane of the Milky Way is much restricted by dust and gas, and the Centre itself is optically hidden from us. It was only in the 1950s that radio astronomers established that the Centre lies in Sagittarius, not in Scorpius where the stars look more numerous from our viewpoint. Some of the nearer dust clouds are visible as apparent gaps in the Milky Way (there’s a prominent one in Cygnus, the ‘Northern Coal Sack ‘).
Above Orion to the left (eastward), Gemini, the Twins, form one of the constellations of the Zodiac, within which the Sun, Moon and planets (except Mercury and Pluto) are always to be found. The Gemini themselves, the classical warrior twins, are represented by the stars Castor and Pollux, rising well to the left of the Pleiades, and before the dramatic rise of Orion, setting long after him in spring.
Perhaps the most remarkable star in Gemini is Castor, Alpha Geminorum. To optical telescopes, this is a double star, with a period of about 350 years. But when they are viewed through a spectroscope, both components of Castor show doubling of the spectral lines of the elements – indicating that each star is itself a close binary. Castor A’s companion has an orbital period of 2.9 days, and Castor B’s is 9.2 days. And there’s an outlying, faint red companion star, Castor C, in the same kind of relation to the system that Proxima Centauri has to Alpha Centauri in the nearest system to our own. (Whether our own Sun has such a distant companion remains controversial.) But Castor’s ‘Proxima’ would have to be called ‘Proximae’, since it is also a binary, with a period of 19.2 hours. The Solar System happens to lie in the orbital plane of that system, so that from here, one star is eclipsed by the other.
So we’re looking at a sextuple system, six stars tracing an elaborate interweaving pattern of gravitational interaction. On any planets, the interplays of light and shadow, would be fascinating. Sad to say, there will be nobody there to see the light show: Castor A and B are class A stars, too hot and short-lived for life to evolve on any planets they have, while Castor C is too cool. Nobody out there will call Castor C ‘Proximae’ until we get there ourselves; after all, even if there are other starfarers out there, they’re unlikely to speak Latin!
The next bright star above the horns of the Bull, to the right of Gemini, is the most southerly star in Auriga, the Charioteer, the constellation virtually overhead in Britain in the winter nights. The brightest star in Auriga is Capella, the Little She-goat, although more appropriately for a chariot the Arabs called it the Driver. Even there, however, the association was primarily with herding. Capella is a spectroscopic binary star, 45 light-years away. It featured as the source of an intelligent radio signal in James Gunn’s excellent novel The Listeners, presumably because its components are spectral types similar to our Sun, but actually they’re bright giants and the larger one, five times the Sun’s mass, has flared up to a luminosity of 80 times our Sun’s. In their stable lives on the Main Sequence they would have been bluish-white, intermediate between Sirius and Procyon, much hotter and shorter lived than our Sun. They orbit one another with a period of 104 days.
Capella is circumpolar in British latitudes, which is to say that in this epoch it never sets: it is the beautiful star which travels low along the northern horizon in the summer months, through night-long twilight at midsummer in Scotland. Capella’s opposite among the circumpolar stars is Vega, the brilliant blue star in the Lyre, overhead in summer, low down in the north in winter.
To the right of Capella lies a small triangle of fainter stars, the Kids, represented in old maps by three baby goats in the arms of the charioteer. Epsilon Aurigae, the apex of the narrow triangle, had a long-ongoing claim to be the most extraordinary star in the sky. In the 1950s it was thought to be the largest star known, larger even than Betelgeuse (until recently the only star to have had its disc photographed). Estimates of Epsilon’s size were reduced in the 60s, while it was recognised that the star was being eclipsed every 27 years by something invisible. Could this be the ‘accretion disc’ of matter falling into a black hole? That was the interpretation in Sydney Jordan’s ‘Lance McLane’ stories, in the Daily Record of the 1970s and 80s, where the star was the power-house of an ancient civilization.
But in the 70s it was recognised that black hole accretion discs would be more violent places – Cygnus X-1 was the first to be identified, by the intense radiation it emits. The 1982-84 eclipse revealed that Epsilon is orbited by a bright ring (hence the size overestimates) and by a black cloud which itself orbits an energy source we can’t see – but isn’t a black hole, probably a close binary of two hot blue stars. In 2009-11, images obtained by the University of St. Andrews show that explanation to be correct.
Overhead in winter, near Auriga, lies the remarkable legendary grouping of Perseus, Andromeda, Cepheus, Cassiopeia; Cetus, the sea-monster, in the south – and, in some versions of the story, Pegasus the winged horse – all well seen in the November star map attached.
Cassiopeia was queen of Ethiopia, the mother of Andromeda, and angered Neptune by boasting of her daughter’s beauty, greater than the sea-nymphs’. Neptune sent Cetus to ravage the kingdom; and Andromeda’s father Cepheus, whose attitude to women’s lib was somewhat pragmatic, had her chained to a rock in the monster’s path. She was saved by Perseus, who happened to be passing with the head of the Gorgon Medusa. (Neptune’s reaction on finding his monster turned to stone is not on record.) In some versions of the story Perseus was riding Pegasus at the time, but most say he was using winged sandals, gifted to him by Hermes (Mercury). When he returned to Greece he let fall some drops of the Gorgon’s blood on a mountain and they gave birth to Pegasus, who later carried Bellerophon to kill the Chimaera.
While it’s not easy to see them as human figures, by comparison with Orion, the constellation figures of the legend are easily learned and contain many interesting stars. Cassiopeia is one of the easiest constellations to recognise, forming a letter W, opposite to the blade of the Plough on the far side of the Pole Star. When Ursa Major is in the north in winter, Cassiopeia is high overhead, and vice versa in summer; they obey the same roles as Capella in Auriga and Vega in Lyra, marking the seasons and the ring of ‘circumpolar’ constellations which never set at Britain’s latitude. Perseus’ shape is less distinctive, but the constellation is easily found on the Milky Way between Cassiopeia and Auriga. On the side towards Cassiopeia, right on the Milky Way, there’s the famous ‘Double Cluster’ of newly formed stars, represented on old maps as the Gorgon’s head. It’s a beautiful sight, through binoculars or a telescope at low power, and you can look at it in safety: nobody’s been turned to stone by it lately. Cetus is in the southern sky, suitably chastened, in the region without bright stars known classically as ‘The Water’, while Neptune (who sent him) doesn’t have a constellation at all – though much later times awarded him a planet.
Perseus has another sinister component, however. The star Beta Persei is known by its Arabic name of Algol, because it marred the supposed perfection of the heavens by varying in brightness – hence Al Ghul, the demon or mischief-maker, marking the Gorgon’s head in alternative maps. Algol is in fact a double star, and the plane in which the two companions orbit one another happens to pass through the line of sight from our Solar System. As they complete a revolution, with a period of 2 days, 20 hours, 49 minutes, the apparent brightness of Algol dips twice as the stars take turns to eclipse each other, fading in brightness over 5 hours, rising again over the next five. Stars like this are known as eclipsing binaries or eclipsing variables, to distinguish them from other types of variable stars. Algol’s behaviour was noted by the Italian Professor Geminano Montanari in 1669, and explained by the deaf-mute John Goodricke in 1783.
At first sight Algol isn’t likely to have habitable planets, although as the two stars are only six million miles apart it might be possible for planets to orbit stably around both of them. The brighter star is type B8, much larger, hotter and shorter lived than our Sun, while the dimmer companion is G4, cooler than the Sun but presumably much younger. The intervals between eclipses are gradually increasing, suggesting a mass flow between the stars, though it could be due to the pull of a third, class F companion. With a history of only a few hundred million years, it’s not likely that an Earthlike, life-supporting planet could have evolved – but if it did, that life would have to be aquatic, to survive the x-ray and ultra-violet emission from the hot primary sun.
But in the same legendary group of constellations there are stars which genuinely vary in brightness. A major example is Delta Cephei, the fourth brightest star in the constellation representing the father of Andromeda. ‘Cepheid variables’ are pulsating stars, powered by unstable nuclear reactions not quite powerful enough to blow them apart. Cepheid variables are of great scientific importance, because the period of pulsation is related to the intrinsic brightness, in all stars of this class. Knowing how bright they really are, their distance can be determined. This was the discovery which first revealed the true scale of our own Galaxy, and that the so-called ‘spiral nebulae’ were galaxies in their own right, millions of light-years away.
Delta Cephei, which happens to be the north polar star of Mars, plays a similar role to the Arab Al-Ghul in Frederik Pohl’s short story ‘The Martian Star Gazers’, first published under a pen-name in 1962 but later reprinted in his short story collection The Abominable Earthman. In the story, Pohl has civilisation on Mars concentrated in the northern hemiphere, because the southern sky is thought to be the territory of a demon whose hands can be seen as the dark lanes in the Milky Way. (One of the Martian constellation names translates as ‘0l’ Grabby’s Other Mitt’.) Orion is seen as a guardian holding him off, though mortally wounded; the wound is the Nebula in Orion’s Sword. The northern sky also has its demon, but this one is asleep: the ‘W’ of Cassiopeia is his smile, and Delta Cephei’s variations show his breathing. That brings about the end of Martian civilisation in mass panic, in 1572 AD (our time), when the supernova known as ‘Tycho’s Star’ flared up above Cassiopeia. On Earth, this was a turning point for Renaissance astronomy, but on Mars, it seemed that the Sleeper had woken up.
Cetus the sea-monster is a long, straggling constellation which begins to the west of Orion, lying partly below British horizons. Tau Ceti, which is one of the nearer stars to our own, is third nearest of the stars most like our Sun. It’s 10 billion years old, twice the age of the Sun, and has a huge entourage of comets, at least 20 times as many as we have. Four planets have been disovered, between 2 and 5 times the Earth’s size, two of them in the ‘ecosphere’ where conditions could be habitable. But despite its age, it has ten times as much dust in orbit round it as we have, apparently due to collisions between an enormous retunue of comets, 25 times more than the Solar System has. Large impacts on the planets must be frequent, and David A. Hardy has kindly given me permission to reproduce his dramatic painting on the subject.
But for much of the history of astronomy, in modern times, Cetus has been famous for the variable star Omicron Ceti. Unlike Algol’s, Omicron Ceti’s variability was discovered in the 16th century and rediscovered in the 17th, when the astronomer Hevelius gave it the Latin name Mira, ‘the Wonderful’. Mira is in the class known as Long Period Variables: its cycle of brightness takes 331 days, and the range is from just above second magnitude down to 9th, so the star is invisible for five months of the Earth year. In 1923 Mira was found to be a double star. The companion is a very faint white dwarf of condensed matter, much too small to hide the main star from us, so Mira is not an ‘eclipsing variable’ like Algol. Mira A is itself pulsating, like Delta Cephei, but with a much longer period and greater force. As it happens, Mira A is swinging away from us when at its brightest and towards us when faintest, but that’s just a coincidence: as Mira is more than 650 light-years away, and the two stars are separated by a full second of arc, the two stars are too far apart for the violent exchanges of matter which account for x-ray stars, novae, and other classes of stars which are so variable that they’re tearing apart. Nevertheless, in 2006 the ultraviolet satellite observatory Galaxy Evolution Explorer discovered that Mira had shed material into a cometary tail 13 light-years in length, so something dramatic is going on there.
Orion is the only constellation mentioned explicitly in the Bible, though it’s known that the tribes of Israel had constellation symbols, and there are hidden references such as the four faces of the angels in the vision of Ezekiel. But as Orion disappears with the winter stars into the west, there is a theory that links him with the Bible and also with the stars o£ spring. Orion is mentioned in the Book of Job (9:9 and 38: 31-32), and it’s interesting that both times Arcturus and the Pleiades are also mentioned. Dr. Charles Herbeger suggests that the relationship with the Pleiades at least is no coincidence.
Dr. Herbeger suggests that many of the mysteries of the Book of Samson can be explained if he was originally a Cretan Sun-king, identified with Orion, whose legend was brought to the Middle East by the Philistines. The lion killed by Samson would be the summer constellation Leo, and the bees which afterwards nest in the lion’s body would stand in Cretan mythology for the soul of the previous Sun-king whom Samson had deposed. The 300 foxes with flaming tails, with which Samson destroys the crops of the Philistines, may be the Orionid meteors from Halley’s Comet, which the Earth encounters 300 days after the winter solstice.
For the bizarre incident of the jaw-bone of the ass, used to slay the Philistines and afterwards to provide water, Dr. Herbeger suggests that the captivity in Egypt may be responsible rather than the later one in Babylon. Egyptian mythology describes the god Set as having an ass’s head on earth, but a bull’s head and leg in the sky, identified with the stars we call the Plough. In the constellation of the Bull, however, the open clusters of the Hyades and Pleiades have a long-established association with rain. Delilah, Dr. Herbeger suggests, was a Moon-goddess; and the pillars which Samson breaks are the solar pillars found in Philistine and Jewish temples.
Carl Sagan suggested that an exploding star in the open cluster called ‘Praesepe’, the Beehive, might have been part of the Samson legend. Praesepe lies in Cancer between Leo and Boötes, the Herdsman, the summer counterpart of Orion – and the brightest star in Boötes is Arcturus. Arcturus is one of the brightest stars in the sky, the very brightest north of the Ecliptic, and was accorded a special role in the astrology of the ancient world. It’s one of the few stars mentioned in the Bible, and often was regarded not as part of the constellation but as an entity in its own right.
In Britain Arcturus is prominant during spring and summer – especially in Scotland, where twilight lasts throughout the night in June and July. Boötes, the Herdsman, guards the North Pole as Orion guards the Equator and the southern sky. Arcturus is so bright because it’s only 41 light-years away, and as a result it has one of the largest Proper Motions known, crossing the apparent diameter of the Full Moon in only 800 years. In Punch, 1907, a poet wasn’t impressed by that – “For a star of your parts, you’re confoundedly slow” – but it’s one of the few stars to have changed its position in the sky significantly, in the 13,000 years since Vega in Lyra was the nearest bright star to the Pole, half-way round the processional cycle from when Polaris stood there last. As a red giant 26 million miles across, Arcturus must already be frying its inner planets; although in David Lindsay’s A Voyage to Arcturus (1920) it’s described as blue, apparently for some atmospheric reason.
Following the Plough up the sky in winter and spring, the stars of Leo are not particularly bright, but are prominent because they lie in a sparse region of the sky, far from the Milky Way. The North Galactic Pole lies in the small constellation Coma Berenices, which is represented on some old maps as the tuft on the upraised tail of the lion. It marks the perpendicular to the plane of the galactic disc, from our position within it.
The head of the lion is marked by stars known as the Sickle, resembling a question mark inverted left to right. The brightest star in Leo is Regulus, the Little King, at its foot – one of the few stars with a proper name in Latin. With a magnitude of only 1.3, its status is due to its isolation from other bright stars. It lies almost exactly on the Ecliptic, the Sun’s path against the stars through the year, and is a double star, with a third companion visible only in large telescopes. Gamma Leonis, Algieba, at the second bend of the Sickle going up, is also triple, but the two stars seen in binoculars are probably an ‘optical double’, not physically connected. Telescopes show the brighter one to be a true double, which Sidgwick’s Introducing Astronomy describes as ‘very fine’. Epsilon Leonis, Algenubi or Ras Elased, at the other end of the Sickle from Regulus, is an optical triple.
The lion’s paws are marked only by faint stars, but his body is a bright right-angled triangle. Furthest from Regulus is Denebola, whose name is a condensed form of the Arabic for ‘the tail of the lion’, though to give it the proper length it should indeed be extended to the stars of Coma.
(To be continued).
See also: Winter and Spring Stars
Duncan Lunan has written a series of Astronomy articles. If you wish to check them out just use the search button on the website.