By Duncan Lunan

When I was in my teens, the standard work on the galaxy in which we live was Bart J. Bok and Priscilla F. Bok, The Milky Way, Harvard University Press, 1957.  There was at least one later edition, but it couldn’t match the beauty of the one I had, written when radio astronomy was beginning to chart something like the true shape of the spiral arms, beyond the reach of visual astronomy.  

By that time we did at least know the general shape of the Galaxy.  The first attempt to determine it was made by William Herschel, modified by his son Sir John Herschel, whose sky survey from the Cape of Good Hope was considered, in the UK at least, to be the best of its kind in the mid-19^th century.  Counting the stars visible in the telescope at many small areas around the sky, he concluded that we were near one end of a disc of stars, not far from where it split into two.  What he didn’t realise was the obscuring effect of interstellar dust;  he thought the dark nebulae such as the Coal Sack behind the Southern Cross were true holes in the starry sky, looking out to total darkness beyond.

As discussed here recently weeks, the realisation of the true shape of the Galaxy came from study of the globular clusters, roughly spherical clumps of hundreds of thousands of very old stars.  As we now know, many of them date back to 12 billion years or more, far back towards the origins of the Universe 13.8 billion years ago in the Big Bang.  Their ancient stars are called ‘Population II’ because they were discovered after the Population I stars of the galactic disc, where the Sun resides.  The centre of the Galaxy lies at the heart of a bulging nucleus of Population II stars, which are also dominant in the spherical halo which surrounds the entire structure.  As that originally spherical structure of dust and gas continued to contract, it was thought, when I was a student, that the bulge of the nucleus had been formed by collisions of globular clusters, and the supernova explosions within it had seeded the disc with heavier elements as it took shape, giving rise to steady formation of Population I stars in it from that day to this.

After the formation of the Population II stars, supposedly, the pull of the flattened galactic nucleus drew the remaining dust and gas into what became the plane of the disc.   Within that, as shorter-lived massive stars became supernovae, the heavier elements synthesised in those explosions seeded the galactic disc and it became possible for new condensations of stars to form clusters containing stars like the Sun, potentially having planets like the Earth.   Our Sun formed 4.6 billion years ago, more than half way through the present history of the galactic disc;  yet it still consists of 80% hydrogen and 18% helium, formed in the Big Bang, and only 2% of heavier elements.  Confusingly, astrophysicists refer to all elements heavier than helium as ‘metals’;  all the iron and nickel in Earth’s core, the silicon and oxygen which make up the crust, and the nitrogen, carbon and oxygen in living beings, were thought to have formed within exploding stars, long enough ago for those explosions to seed the interstellar clouds and give rise to new clusters of stars forming Population 1.  Recent studies, particularly those of stellar motions by the Gaia space telescope, now paint a very different picture.   Out at the Earth-Sun L2 point, where it’s now been joined by the James Webb Space Telescope, by 2020, Gaia had mapped the motions of 1.8 billion stars.

Our Milky Way is one of three large spiral galaxies making up a Local Group, swinging around the much larger Virgo Cluster at a distance of 78 million lightyears, linked to it by a bridge of dark matter, and under the influence of much larger concentrations beyond.  The other two nearby spirals in the Local Group are M31, the Great Nebula in Andromeda, and M33 in Triangulum, but there’s also a large population of smaller objects.  More dwarf galaxies and remains of dwarf galaxies are being discovered within the Local Group all the time.  There are also the two irregularly shaped Magellanic Clouds, satellites of our galaxy, and ten or more elliptical ones, two of them orbiting M31.  All of these seem to be products of collisions and interactions in the past.  As previously noted, M31 will collide with the Milky Way in three to five billion years and change the size and shape of both forever, possibly forming a single giant elliptical galaxy. 

There have been more members of the Local Group in the past, because both the Milky Way and M31 have absorbed small galaxies and are still doing so.  Observations of the Andromeda spiral by the Spitzer Space Telescope and the Galaxy Evolution Explorer revealed irregularities caused by satellite galaxies passing through it.  (Kulvinder Singh Chadha, ‘Andromeda Reveals All’, Keith Cooper, ‘Ultraviolet Astronomy’, Astronomy Now, February 2006.)   It was thought that M31 had swallowed a large spiral, because it seemed to have two supermassive black holes within its nucleus.  That turned out not to be the case, when the Hubble Space Telescope resolved one of the apparent ‘cores’ into a ring of old red stars.  M31 has only one supermassive black hole, surrounded by newly formed hot blue stars  (‘Can Black Holes Create Stars?’, Astronomy Now, November 2005).  The configuration of M31’s nucleus suggests such events are recurring.

There’s a similar ‘invasion’ of the Milky Way in progress:  the Sagittarius Dwarf Spheroidal Galaxy has orbited the Galaxy at least ten times, each taking a billion years, and is currently passing through the disc at roughly the same distance from the Centre as our own.  Eight more dwarf companions of the Milky Way have the potential to do the same  (‘Invaders Target Milky Way’, Daily Telegraph, 14^th February 1998).  Dwarf galaxies go through powerful episodes of ‘starburst’ generation of new stars, as they engage in collisions with the larger spirals and with one another, but that’s not the half of it.

What we now know, from Gaia and related studies, is the nucleus of the Milky Way actually formed from collisions of dwarf galaxies rather than the smaller globular clusters.  At least five such collision events have now been identified, with indications of at least six more.  Five of the most recent have been named the Kraken Event, the Helmud Streams Event, the Sequoia Event, the Gaia-Enceladus Event  (aka the Gaia Sausage), and most recently the Sagittarius Event, still ongoing.  Debris from the Gaia-Enceladus Event, 10 to 11 billion years ago, is still out beyond the main body of the disc, falling back.  Each of these events was accompanied by a massive burst of star formation, in the nucleus and beyond, and the resulting instabilities in the nucleus probably led to the formation of the bar which now crosses the nucleus, extending up to 13,500 light-years to either side of the Centre, with the spiral arms trailing from its ends, as density waves in the disc.  There’s a good and more detailed account of all this in Gemma Lavender, Milky Way Owners’ Workshop Manual, Haynes Publishing 2019.

After 10 billion years ago star formation in the disc fell to a low ebb.  Between 6 billion years ago and 1 billion, there was another intense burst, and it’s not yet known whether that was due to still another collision or a close graze by another galaxy.  But we also now know that the heavier elements in the disc came not from supernovae but from more frequent colliding neutron stars, as I reported last week.  The remarkable implication is that Population I stars with earthlike planets will almost all be either roughly contemporary with our Sun  (give or take a couple of billion years), or else 10 billion years or more older.  Without explicitly mentioning this reasoning, Stephen Baxter’s new novel Galaxias  (Gollancz, 2021)  concerns the suppression of cultures like ours by a very much older rival from that period.

At the heart of the Milky Way is a high-energy, violent region dominated by a supermassive black hole, designated Sagittarius A*, surrounded by evidence of still more violent events in its past.  For instance it’s thought that the supermassive black hole at the Centre swallowed a planet the size of Mercury about 50 years ago, releasing a flood of x-rays, and the orbiting Chandra X-Ray Observatory has photographed the echoes from surrounding clouds reflected in our direction.  (Chandra News Release, ‘Light Echoes from Our Supermassive Black Hole’, Universe Today, 12^th January 2007.)   If there is any life down there, and remarkably enough some scientists think that there may be, it would as alien to us as the living nebula of Fred Hoyle’s novel The Black Cloud and possibly of still greater size.  (Gregory Benford, ‘A Scientist’s Notebook:  Life at the Galactic Centre’, Fantasy & Science Fiction, August 1995.)  These might include huge, unexplained arcs of material near Sagittarius A, the central radio source.   (Amarenda Swarup, ‘Is There a Monster Hiding in the Galactic Centre?’, Astronomy Now, August 2005.)  Very recently it’s been found that the nucleus is full of such filaments, averaging about 150 light-years in extent.  (Chris Q. Choi, ‘New Milky Way mosaic reveals nearly 1,000 strange ‘filaments’ at the heart of our galaxy’, Space.com, 1^st February 2022).

At first the Milky Way’s supermassive black hole was thought to be about 300 million times the mass of the Sun, though it’s turned out to be ‘only’ four million solar masses and surrounded by orbiting stars, several dozen of them with masses up to 30 times the mass of the Sun, which are delaying the infall of matter for the moment.  (Keith Cooper, ‘Is the Galactic Centre a Starved Black Hole?’, Astronomy Now, December 2005.)  Stars and gas clouds have been imaged making close slingshots around it, and we may get an image of it soon courtesy of the Event Horizon Telescope, which has already produced a radio image of the much larger one in the M87 galaxy\  There’s evidence for more violent events in the Galactic Centre, including an antimatter jet for which there’s no satisfactory explanation, and recurring violent explosions over the last million years, including two huge bubbles, 1400 light-years across, emanating from the Centre, and two even larger ‘Fermi Bubbles’, 75,000 light-years across, 1 or 2 million years old and powered by a very powerful gamma-ray source at or near the Centre.  

In the early 1960s it was estimated that the Milky Way contained around 150 billion stars.  Current estimates have increased that to at least 200 billion, and quite possibly twice that number if we knew the true number of red dwarf stars.  Stars in the galactic disc form in clusters which are less regular in shape than the Population II globular ones.  The best known and nearest example are the Pleiades, the ‘Seven Sisters’ in the constellation Taurus.  Because of the cultural significance of the Pleiades, UFO writers often insist that they are the home stars of the aliens, but physically that’s impossible, because the stars are only a few hundred thousand years old.  Groups like the Pleiades are ‘Open Clusters’, stars with a common origin, formed in the disk out of the clouds of dust and gas still found here – although remarkably enough Tennyson’s ‘silver braid’, which is only visible to the eye with averted vision, is not the gas and dust from which they formed but another cloud which they’re passing through and blowing apart with the intensity of their starlight.  After a few circuits of the Galaxy, the pulls of other stars will have separated them into independent orbits like the Sun’s.  (One of the big discoveries around the end of last century was that we are passing through a widely dispersed Open Cluster, about two hundred light-years across, whose members include most of the bright stars in Ursa Major, also Sirius and Regulus.    H.H. Turner, A Voyage in Space, Society for Promoting Christian Knowledge, London, 1915, includes two maps showing the Sun’s position within the cluster from different angles.)   Space-travellers might be based in the Pleiades now, having settled them from elsewhere, but there hasn’t been time for them to have originated there.      

In the 1930s, it was seriously supposed that this might be the only planetary system in the Galaxy.  The popular theory was that the Solar System had formed from a filament drawn out from the Sun by the pull of a passing star.  Even near-collisions between stars probably occur once or twice on average in the history of a galaxy, and when Sir James Jeans popularised his idea that the Solar System was formed by such an encounter, it would have meant that we would be alone in the Milky Way if not in the Universe, because there would be no other planetary systems.  Science fiction writers and readers ignored him, rightly, as it turned out, because we know now that planetary systems are common.  SF writers had just discovered the interstellar story with E.E. Smith’s The Skylark of Space  (1928), and went ahead regardless.  We know now that Jeans had it totally wrong and more than 100 planets of other stars had been discovered by September 2003;  the number is now in thousands and growing daily  (more on that next week). 

The Sun’s distance from the Galactic Centre has recently been re-determined, and confirmed to be approximately 27,000 light-years.  After its encounter with Pluto in 2015 the New Horizons probe will leave the Solar System in that direction, towards the Galactic Centre, but it isn’t going to reach the heart of the Galaxy.  At Solar System escape velocity it would take two to three million years to reach even the nearest star, while the Galactic Centre is 10,000 times further away.  Even to get there in 10 million years would need a launch speed of 0.003c, about 560 miles per second.

The Sun is in a somewhat elliptical orbit around the Galactic Centre, with a period of 200,000– 230,000 years.  The uncertainties are because we don’t know the exact shape of the orbit, nor how it will be changed by encounters with other stars, galactic clouds and stellar clusters it will meet on the way.  We do know that the Sun is not in the exact central plane of the Milky Way, and as a result it oscillates above and below that plane with a period of approximately 32 million years.  Mass extinctions in the history of life on Earth seem to happen on an average of every 36 million years, and in their 1982 book The Cosmic Serpent, Prof. Victor Clube and Dr. Bill Napier suggested that this might be due to a periodic influx of comets as the Sun passed through interstellar clouds in the galactic plane.  Subsequent space probe encounters with comets, starting with Halley’s Comet in 1986, seem to suggest instead that comets originated with the early Solar System, and the only interstellar comet to come our way since  (Comet Borisov, 2019)  seems different enough to confirm that.  But there are still lots more unanswered questions to resolve.  For example, given that many thousands of stars must have passed through or near the Oort Cloud of comets during the history of the Solar System, how has the cloud maintained its numbers if it hasn’t been replenished?

The Milky Way has two major spiral arms, the Scutum-Centaurus Arm and the Perseus Arm, stemming from the ends of the central bar.  There are lesser arms and spurs, such as the Norma, Orion and Sagittarius Arms, and we are currently located off a feature called the Orion spur, from the inner edge of the Perseus Arm.  Gaia has recently discovered a similar feature off the Sagittarius arm, 6000 light-years from the Centre.

The Gaia data is adding considerable detail to the maps of space around us, and it turns out that local interstellar absorption are parts of a patchy accumulation of dust around us called the Local Cloud.  (My own interpretation, published in New Worlds for Old and updated in 1981 for Man and the Planets, turns out to ‘fit where it hits’.)   In general, though, we are near the centre of a local bubble, swept clear by supernovae, about 10 million years ago.  The Sun entered it about 5 million years ago, so now we’re surrounded by the low-density bubble, extending out to 300 light-years.  It’s not clear whether that makes interstellar travel easier or more difficult.  As long ago as 1951, the late Prof. Michael Ovenden pointed out the problems that interstellar dust and gas would cause, for spaceships travelling at high speed.  In Arthur C. Clarke’s The Songs of Distant Earth the starship has a large forward shield of ice, and the designers of the British Interplanetary Society’s Daedalus interstellar probe had to build on a thick ceramic shield, even for cruise at 12% of the speed of light.  Proposed photon drives, capable in theory of reaching near lightspeed, would have big trouble protecting the very large heat radiators they would require, and Robert Bussard’s answer to that, the interstellar ramjet, turns out to be more effective as a brake at speeds above 0.05c.  Would the depleted medium of the bubble ease that speed limit, or be too impoverished to power it at all?  All those sums have to be done again.

Still, one mystery has been resolved.  It’s been known for quite some time that the Solar System was surrounded by a ring of recent star-forming regions called Gould’s Belt, extending out to 1000 light-years, but with its inner edge roughly equidistant from us.  There was no obvious reason why that should be the case, but it turns out that the inner edge lies on the inner surface of the supernova bubble, which is triggering star formation by a wave of compression in the interstellar medium as it moves outwards.  Star clusters hang on that inner surface like bunches of grapes.  That the Sun’s motion has carried us to the centre of it is a coincidence, like the dust cloud through which the Pleiades are passing.   But with Gaia’s third major data release expected this year, no doubt there will be new mysteries to take the place of that one.

See also: The Sky Above You – February 2022

This is part of a series of excellent astronomy articles by Duncan Lunan which you can find through the search button.