Culture

Stars and Nebulae

By Duncan Lunan

Magellanic Clouds from Cerro -Tololo, Chile (NASA)

Over the last two thousand years, compilers of star catalogues have gradually become aware that not everything in the northern sky  (apart from the Milky Way)  consisted of a sharp point of light.  In 963-964 the Arab astronomer Al Sufi noted several nebulae  (the Latin name means ‘little cloud’)  in The Book of the Fixed Stars, among them M31, the Great Nebula in Andromeda.  After the invention of the telescope, Galileo drew three of the four Trapezium stars in the Orion Nebula, in 1617, though the fuzzy patch in Orion’s Sword had been known since classical times, as had Praesepe, the ‘Beehive’ open cluster in Cancer.   Likewise the paired open clusters in the constellation Perseus had long been associated with the head of the Gorgon Medusa, though the prominent variable star Algol in the same constellation was another candidate.

M13 Hercules Bareket Observatory Israel

Other misty patches in the sky, such as the constellation Coma Berenices and the open cluster of the Pleiades, could be resolved into individual stars by people with keen sight, and for a time it seemed that they all might be.  The globular cluster M13 was resolved into stars by William Herschel in 1784, having been first observed by Halley.  But others could not be explained so easily.  The Crab Nebula supernova remnant, first observed in 1731 by Bevis, was rediscovered in 1758 by Messier while searching for Halley’s Comet, and he then began compiling his catalogue of nebulae to avoid confusion in cometary searches.  Messier discovered the M3 globular cluster in Canes Venatici in 1764, and that was made up of stars, but the Trifid Nebula was discovered the same year and definitely wasn’t;  neither was the Ring Nebula in Lyra, discovered in 1799 by Antoine Darquier and labelled M57 by Messier later that year.  The very first object on Messier’s list, M1 in Taurus, was examined at high magnification by Lord Rosse in 1843 and his drawings led to its being named the Crab Nebula, but it wasn’t composed of stars.  He also discovered the spiral shapes of external galaxies such as M33, M51 and M31, though resolving them into stars had to wait for Shapley in the 20th century.

Crab Nebula, Taurus, Hubble Space Telescope

Meanwhile, spectroscopists had discovered that the major difference between spirals and ‘diffuse’ nebulae was that the spirals had continuous spectra, like the Sun’s, indicating that they were indeed made up of stars, while diffuse nebulae had prominent spectral lines of individual elements, particularly hydrogen, which indicated that they were gaseous, though also containing large quantities of dust.  They could further be divided into reflection nebulae, which simply reflected the light of nearby stars, and emission nebulae, which were stimulated by the radiation from stars within them to emit light of their own.  There were others like the North American Nebula, which were reddened by absorbing the light passing through from stars beyond, but many dark nebulae, through which starlight didn’t pass –  like the ‘Coal Sack’ in the Southern Cross or the Northern Coal Sack in Cygnus  (‘the Northern Cross’)  which Sir John Herschel had thought to be holes in space. 

The term ‘Workshops of the Creator’ was still being applied to spiral nebulae as late as the 1940s, but by then it was long known that they are extragalactic, and the true workshops are the dust and gas clouds within them.  In the southern hemisphere, where the thicker parts of the Milky Way dominate the sky, the Aborigines were closer to the mark when they included dark nebulae, silhouetted against the Milky Way, in their creation myths.  In many dark nebulae we can see shapes being carved out by the light of new stars, vaporising the dust and gas remaining.  These include the Horsehead Nebula in Orion, the ‘Pillars of Creation’ photographed by the Hubble Space Telescope, distinctive dark nebulae in Ophiucus, and the Cone Nebula in Monoceros, all within our Galaxy.

The Horsehead Nebula is on the edge of one of the clouds of dust and gas, in Orion, where star formation was found to be taking place.  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.   (Neil English, ‘Hunting Down the Horsehead’, Astronomy Now, January 2005.)   Already, erosion of its dust and gas by radiation from the stars behind is beginning to change its distinctive shape, as seen from here.

Hubble Space Telescope Horsehead Nebula (NASA)

However, as noted above, the nebulae known as globular clusters were clearly made up of stars.  Along with the stars of the galactic nucleus and halo, the stars of the globular clusters are the oldest in the Milky Way, confusingly called Population II because they were discovered after the metal-rich Population I stars of the galactic disc.  We now know that that most of them are very old, older than our Galaxy – typically 10 billion years old, but some have been found, within the Galaxy but up to 13.5 billion years old.  When I was a student, it was thought that the bulge of the galactic nucleus had formed through collisions of globular clusters, but now it seems that colliding dwarf galaxies played a larger part.

In each globular cluster the stars are mostly the same age, and stars of particular spectral types can be taken to be of roughly the same brightness, so from that a rough idea of the clusters’ distribution in space could be obtained.   It turned out to be a sphere, with the Solar System well out from the centre.   It had been known since Galileo’s time that the Milky Way that surrounds us was a disc of stars, seen from within, so large that the more distant parts of it were seen edge-on.  ‘Spiral nebulae’ had been discovered by Lord Rosse with his giant reflecting telescope at Parsonstown in Ireland, and spectroscopic observations indicated that they were composed primarily of stars.  So were we living within one?

Some of the oldest giant stars go through a phase of pulsations in which they’re classed as ‘Cepheid Variables’, after Delta Cephei which is considered to be the standard example.   Henrietta Levitt made the great discovery that the period of pulsation was related to the mean absolute magnitude of the star, meaning that Cepheids could be used as ‘standard candles’ to determine interstellar distances.  Her discovery enabled the construction of the Hertzsprung-Russell diagram, relating the absolute magnitudes, spectral types, luminosities, masses and surface temperatures of the stars in a way that led to understanding the rôle of nuclear fusion, the evolutionary history of stars, and the true distances of the globular clusters.  When Harlow Shapley discovered Cepheids in the outer reaches of the nearest spiral to us, M31 in Andromeda, they were so far away that the spiral nebulae had to be separate galaxies, or ‘island universes’ as they were called for a time.  At first it seemed that our galaxy was larger than all the rest, but as the distance scale was refined, the other galaxies turned out to be further away and therefore bigger than was first thought.  Since my early teens the apparent distance to M31 has tripled, from 750,000 light-years to 2.5 million. 

M31 from Mirach

M31, the Great Nebula in Andromeda, is therefore the most distant object visible to the unaided eye.  Struggles to find it, in my early teens, finally prompted me to learn the constellation figures and find out where things really were.  (It turned out that I’d been looking off the wrong corner of the Great Square of Pegasus.)  Photographs are no help, because the nebula is faint and diffuse, so a time exposure to bring it up will also show too many stars to find it by.  For the record, my way of finding M31 was to start with the ‘W’ of Cassiopeia, at the bottom right of which, Schedar  (Alpha Cassiopeiae)  there are two ‘extra’ stars, Zeta and Lambda.  The line from Schedar through Zeta leads straight to M31, but in those days I found it easier to follow the line through Lambda to Alpheratz  (also called Sirrah), on the northeast corner of the Pegasus Great Square.  Alpheratz is shared between Pegasus and Andromeda;  from Alpheratz, the Andromeda constellation consists primarily of a line of three stars leading up towards Perseus in the Milky Way.  The middle star of the three is Mirach  (Beta Andromedae).  Mirach is the brightest of another three-star line, like a miniature image of the constellation.  The middle star is Mu Andromedae and the last one, Nu, is next to M31 itself.  Mu and Nu have no proper names, even in Arabic, at least on Wikipedia, and even in Chinese they have only positional names within an 8-star asterism called ‘the Legs’. 

Having found M31, you have to look away to see it properly, because the rods on the periphery of the eye are more sensitive in darkness than the cones in the centre.  ‘Averted vision; is a trick worth mastering for all faint astronomical objects.  ‘Dark adaptation’, allowing the eyes time to adjust to the lower light level, is another.  The first phase generally takes about five minutes, but can be speeded up by shutting your eyes and covering them with your hands, for about a minute. 

In 1959, before the visitor centre at Jodrell Bank was open to the public, I was allowed to join a student party from Manchester University on a tour of the site, by courtesy of the late Prof. Sir Bernard Lovell.  The big radiotelescope’s resolution was then roughly equivalent to the area of the Full Moon, and one thing we learned on that visit was that nevertheless it could chart the spiral arms of M31 out to a considerably greater distance.  In fact, telescopic photographs of M31 as a whole are misleading in that respect, because their magnification is fairly low in order get it all in, and a time exposure to show the outer arms over-exposes the nucleus, which is actually a starlike point when viewed by eye through a telescope.  Allegedly, with full adaptation taking an hour or more, on a really dark site, the Great Nebula becomes truly great and can be traced out to beyond lunar size with the unaided eye.  The late James Blish described the effect in his African novel The Night Shapes, but I didn’t get the chance to ask him about it when we met, and it was only recently that I met someone who could confirm it.  Presumably, even those who go on safari are reluctant to stay away from campfires and lights for the length of time required.

collision now, 2b, 3.75, 3.85, 3.9, 4, 5.1, 7b yrs

Over the next few billion years the view of M31 will continuously become better, eventually uncomfortably so, because 3.75 billion years from now, still within the stable lifetime of the Sun, the fringes of the Milky Way will begin to collide with those of M31  Some scientists believe that the outermost gas halos of the two spirals are already beginning to interact.  Recent data from the Gaia space telescope suggests the collision may not occur till 4.5 billion years hence, initially as a graze, but even so, both spirals will be thrown into chaos, with an immense burst of star formation, before settling in 5.5 billion years into an immense elliptical galaxy which currently has the unlovely name of ‘Milkomeda’.  There will be more to say about that in future articles.

Although M31 is the only spiral galaxy visible to the naked eye, the Milky Way does have a retinue of satellites, dwarf galaxies orbiting around it within the assembly known as the Local Group.  Collisions with those are comparatively frequent and there will be quite a lot to say about that later, as well.  Only two are visible to the naked eye and both are in the southern hemisphere, known as the Magellanic Clouds because they were first discovered by Europeans on Magellan’s round-the-world expedition.  They are linked by a bridge of young stars and gas formed by a collision between them 100 million years ago, possibly one of several, and it’s possible that they once were a single dwarf spiral, pulled apart by a passage through the outer arms of the Milky Way.  There’s still a lot going on out there:  S Doradus, one of the brightest stars known, was once thought also to be the largest, though that honour now belongs to R136a1, one of a cluster of massive stars in the Tarantula Nebula within the Large Magellanic Cloud.  The closest supernova since the invention of the telescope occurred  nearby in 1987  (as seen from here – actually 167,000 years ago), and is still the subject of intensive study, while ‘light echoes’ from earlier ones are seen chasing across the Cloud from one gaseous nebula to the next.

Magellanic Bridge

The LMC will soon be in the news again because it’s expected to be the first target for the James Webb Space Telescope, which achieved orbit on January 24th around the L2 point, a million miles beyond Earth on the far side from the Sun.  Its 5-layer sunshade and its mirror components were all deployed successfully on the way out to L2, but it will take 100 days to cool the telescope down to operating temperature, and five months to fine-tune the multiple components of the mirror and the instruments attached.  That will be done by taking multiple images of one or more clusters of young stars in the Large Magellanic Cloud, chosen because they’re all of the same age and similar brightness – but more importantly because the Large and Small Magellanic Clouds are close to the south Ecliptic pole.  The Ecliptic is the plane of Earth’s orbit, so the Sun makes a circuit round it in the course of a year, and the JWST’s sunshade must always be perpendicular to it, with the telescope always on the shadowed side.  So the telescope can only see half the sky at a time  (plus a 15-degree rock), but the Ecliptic poles, and therefore the Magellanic Clouds, will always be in view.

(To be continued.)

Duncan Lunan has written a series of Astronomy articles in The Orkney News which you can find using the search button

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