Science

Variable Stars

by Duncan Lunan

Perseus and the Gorgon Medusa head

As described last week, one of the great classical legends is traced in the sky by the constellations of Andromeda, her mother Cassiopeia, her father Cepheus, and Perseus, the hero who rescued her from the sea-monster Cetus.  Cetus, who was ravaging Ethiopia at the time, is found in the southern sky, suitably chastened, while Neptune (who sent him) doesn’t have a constellation at all – though much later times awarded him a planet.  Perseus slew the monster by showing it the head of the Gorgon Medusa, which could turn onlookers to stone;  in some old maps the Gorgon’s Head is represented by the beautiful double cluster of stars in the Milky Way between Perseus and Cassiopeia.

Comet Hale-Bopp by Algol in Perseus, Chris O’Kane, 1997

Perseus has another sinister component, however, often identified on old maps as the Gorgon’s head.  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.  It was known as long ago as 1000 BC, when an Egyptian papyrus listed ‘unlucky days’ at 2.9 day intervals, corresponding to the minima of Algol.  (Florian Freistetter, A History of the Universe in 100 Stars, Quercus, 2022.)  In modern times, Algol’s behaviour was noted by the Italian Professor Geminano Montanari in 1669, and explained by the deaf-mute John Goodricke in 1782.  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, we see the apparent brightness of Algol dip twice as the stars take turns to eclipse each other.  Stars like this are known as eclipsing binaries or eclipsing variables, to distinguish them from other types of variable stars.

eclipsing binary, European Southern Observatory art

I’ve mentioned several variable stars in these essays, but it’s a topic worth covering in its own right, because it’s one of the fields in which amateurs make a significant contribution to astronomy.  The scientific study of variable stars was made possible by the development of astrophotography and spectroscopy in the late 19th century, and became the particular business of the women ‘computers’ at Harvard University between 1885 and 1992, who achieved major breakthroughs in astrophysics and cosmology, described in fascinating detail by Dava Sobell in The Glass Universe, The Hidden History of the Women Who Took the Measure of the Stars  (4th Estate, HarperCollins, 2016).  The huge archive of photographic plates which they accumulated and classified is only now being digitised and made available for further study by ‘citizen science’ projects.  As Sobell points out, newer observatories and space telescopes such as TESS, GAIA and the new James Webb Space Telescope can gather similar information much more rapidly, but still they’re only taking snapshots of the stars at particular times.  The Harvard team rephotographed many of ‘theirs’ to form an ongoing record, but to keep continuous tabs on how the variable stars change from night to night requires dedicated volunteers, watching whenever weather permits.  My late friend John W. Macvey of Saltcoats was a prominent member of the Variable Stars Section of the British Astronomical Association, which for many years has collated and published the results of that small, dedicated army of observers.

There are many types of star which do vary in intrinsic brightness, for different and complex reasons.  Our own Sun is not entirely above suspicion, because not long after Galileo discovered sunspots, there was a long period in which the Sun fell quiet.  This period is known as the ‘Maunder minimum’, after the astronomer who pointed it out, and it coincided with the ‘mini ice age’ in which frost fairs were held on the frozen Thames, and the Viking colonists in Greenland were cut off and died out.  Very few sunspots were seen in this period, and the indication is confirmed by descriptions of eclipses, in which the normal corona was very subdued, and by the absence of the distinctive 11-year sunspot pattern in tree rings of the time.   Moreover, the indications are the same for other cold spells before sunspots were discovered.   

Cepheus, with Delta Cephei at top

In the Perseus-Andromeda group of legendary 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 their period of pulsation is related to the intrinsic brightness, in all stars of this class. 

This ‘Period-Luminosity Relationship’ was discovered by Henrietta Leavitt of the Harvard team, whose untimely death robbed her of an intended Nobel prize, and there are still campaigners seeking to have it officially designated ‘Leavitt’s Law’.  She first became aware of it when studying Cepheid variables in the Magellanic Clouds, satellites of the Milky Way in the southern hemisphere.  Knowing how bright they really were, their distance could be determined.  This was the discovery which first revealed the true scale of our own Galaxy, and that the Magellanic Clouds and so-called ‘spiral nebulae’ were galaxies in their own right, millions of light-years away. 

The realisation that the ones observed at such distances were a second class called ‘Bright Cepheids’ doubled those distances at a stroke, but took time to gain acceptance – I remember the coverage in Radio Three’s monthly ‘Night Sky’ programme of the early 1960s, and the stir that it caused.

From parallax to Cepheids to T1 supernovae, plotting the expansion of the Universe

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 hemisphere, 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.  After I wrote that up in ‘Winter and Spring Stars (2)’, Orkney News, January 23rd, Dr. Alan Cayless of the Open University has used it in an exercise for students.

Cetus

Cetus, the sea-monster turned to stone by Perseus, is a long, straggling constellation which begins to the west of Orion, in the region lacking bright stars which was known to the ancients as ‘the Ocean’ or ‘the Water’.  For much of the history of astronomy, in modern times, Cetus has been famous for the variable star Omicron Ceti.  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.

Cepheid variables, Long Period variables and Irregular variables are all massive stars in the last phases of their lives.  They have exhausted the hydrogen in their cores whose fusion kept them stable during their lives on the ‘Main Sequence’ of the Hertzsprung-Russell diagram  (see ‘Winter and Spring Stars (1), and ‘Novae and Supernovae’, January 16th and 30th).  More energetic reactions are now synthesising heavier elements in those cores, in reactions powerful enough to overcome gravity and expand the stars, interrupting the reactions and causing the stars to contract again.

The processes are unstable and must eventually lead to violent ends.  Betelgeuse, the red giant star on top left of Orion, has expanded to greater size than the orbit of Mars and is surrounded by huge flares of gas which have been blown off from its outer layers.  Betelgeuse has a regular variation over an 11-day period, but in October 2019 it dimmed dramatically in brightness, and first thoughts were that it had entered its final contraction before exploding.  It turns out that a huge cloud of plasma had been ejected from deep within the star, and silicon and oxygen atoms in it had combined to create an enormous dust cloud on the side facing towards us.  The cloud has since moved or dispersed, but the regular variation has not resumed and instead the visible surface of Betelgeuse appears to be ‘bouncing’ in the aftermath.

Eta Carinae, a giant star in the southern hemisphere, brightened dramatically in 1843, and telescopic studies show that this was only the latest in a series of violent outbursts, surrounding it with clouds of dust and gas.  Betelgeuse and Eta Carinae are perhaps the most likely stars to go supernova within our view, though that could be next week or a million years hence.  The closest supernova which we’ve had the opportunity to study was the one in the Large Magellanic Cloud in 1987, and it was a big surprise because its precursor star turned out to have been blue, not red.  It had blown off its outer layers in previous eruptions, and there were fireworks when the supernova shockwaves caught up with them,

Helix Nebula NGC 7293 ‘Eye of God’, supposedly once every 3000 years

Less massive stars, though still giants, do the same, The compression wave accompanying the final detonation forms a white dwarf rather than a neutron star, and the expelled layers form circular patches which are called ‘planetary nebulae’ because of their appearance, though they’re far less bright than planetary discs.  The Ring Nebulae in Lyra is a famous example, the first to be photographed in colour, and the Hubble Space Telescope has revealed many of them to have beautiful, elaborate structures.  Many of them are actually tubular, with the open end happening to face towards us.  One example is the Helix-Nebula, NGC 7293:  there’s an Internet myth that it’s the ‘Eye of God’, visible once every 3000 years, but actually it’s there all the time and has a respectable place in the New General Catalogue of nebulae.  Another example is the Southern Ring Nebula, photographed in unprecedented detail by the James Webb Space telescope for the first batch of images to be released – revealing that a small streak on the edge of it is actually an edge-on spiral galaxy, far beyond it.

Novae and supernovae were among the discoveries made by the Harvard ‘computers’ – one of the first was in the Centaurus spiral, found by Wilhelmina Fleming in 1895.  Now we know that novae and Type 1A supernovae are not core explosions, but surface detonations of matter drawn from red giant stars by white dwarf or neutron star companions.  It turns out that Type 1A supernovae all have roughly the same intrinsic brightness, which means they can be used as ‘standard candles’ to extend the measurement of distances far beyond those revealed by the Cepheids.  They’ve verified that the ‘cosmological red shift’, generated by the continuing expansion of the Universe, is also a reliable guide to distance – and that brings right up to date with a current issue concerning the James Webb Telescope.

This new image shows the dramatic shape and colour of the Ring Nebula, otherwise known as Messier 57. From Earth’s perspective, the nebula looks like a simple elliptical shape with a shaggy boundary. However, new observations combining existing ground-based data with new NASA/ESA Hubble Space Telescope data show that the nebula is shaped like a distorted doughnut. This doughnut has a rugby-ball-shaped region of lower-density material slotted into in its central “gap”, stretching towards and away from us.
‘Red UFOs’ , Ultra-red Flattened Objects, appear in Webb images (right), but not HST (left), 10.3-12.7 billion yrs before present

Already it’s been established that the JWST, and even its finder telescope, can see further back into time than the Hubble Space Telescope.  It’s hoped that JWST will be able to see galaxies back to within 250 million years of the Big Bang, and maybe even closer to the theoretical limit of observation.  Indeed, when the first JWST deep field image was released, it was claimed that some deep red galaxies within it had to be only 200 million years after the Bang.  But these assessments were based only on colour, not on redshifted spectra.  Other scientists soon disagreed:  the images were too red, and looked to be due to dust in large quantities, not to be expected in the very earliest galaxies.  What really set the cat among the pigeons was a JWST image showing a deep red galaxy, obviously filled with dust, which simply had not been visible in the Hubble view of the same starfield.  They are reckoned to date from 3.5 to 1 billion years after the Big Bang,  (Kelly Kizer Whitt, ‘UFOs – Ultra Red Flattened Objects – Revealed by Webb’, EarthSky, August 8th 2022.)  The last I heard was that it was still hoped that far-redshifted objects will be found in the JWST images, but confirming their status will have to wait for spectral analysis – which may not be long in coming.  

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