Just what is the Milky Way? A faint misty arch of light across the night sky, it has fascinated people from all times and places. Many cultures have traditionally seen it as a river or road in the sky while to the !Kung people of southern Africa it was the ‘Backbone of the Night’. In the Classical Period, the Egyptians and Greeks saw it as milk split across the firmament. The Greeks incorporated it in to their mythos, telling how the philandering god Zeus tried to feed the baby Hercules (who Zeus had fathered in a liaison with a mortal woman) from his sleeping wife Hera, but she awoke and in the rumpus which followed Hera’s milk was sprayed across the heavens. The word “Galaxy” is derived from the Greek expression for “milky circle”.
Greek philosophers disagreed as to what the real Milky Way was. In the 4th Century BC, Aristotle (384–322 BC) reported than some believed the Milky Way to be a band of faint stars, but he instead proposed the theory that it was a phenomenon of the Earth’s atmosphere. Aristotle’s explanation of the nature of the galaxy was still popular in Europe right up to the Renaissance. Elsewhere, opinions differed; the astronomers of Persia and the Islamic civilization rejected Aristotle’s idea, pointing out that planets pass in front of the Milky Way so it clearly lay beyond their orbits. The Muslims and Persians were sure that the Milky Way was composed of faint stars but could not prove this, limited as they were to unaided human eyesight.
It was Galileo Galilei, who finally demonstrated the nature of the Milky Way in 1611 when based on his early telescopic observations he declared “It is nothing else but a mass of innumerable stars planted together in clusters.” Aristotle had been wrong. By 1755, Immanuel Kant (1724 -1804) was speculating that the Milky Way might be a huge number of rotating stars (one of which was our Sun), held together in orbits around a common centre by gravitational forces. This is absolutely correct. In the late 1700s, William Herschel (1738 -1822) tried to map the structure of the Milky Way (which at the time was believed to be the whole Universe) but was handicapped by not knowing the distances to the stars accurately. By the early 20th Century astronomers Jacobus Kapteyn (1851-1922) and Harlowe Shapley (1885 – 1972) had separately refined Herschel’s method to estimate that the Milky Way was a disc containing billions of stars and was tens or hundreds of thousands of light years across.
About this time, Edwin Hubble (1889 – 1953) showed that the Milky Way was not after all the entire Universe. The enormous Milky Way was merely one of a myriad of galaxies expanding through a Universe vastly larger than previously suspected.
If you could view our galaxy, the Milky Way, from outside, what would it look like? We found out in the latter half of the last century. To observers, the Milky Way is most dense in the direction of Sagittarius so presumably the centre of the Milky Way was in this direction. However our view of the galactic centre is blocked by the vast clouds of dust and gas lying in Sagittarius.We could not actually see the centre. Unexpectedly, more than eighty years ago physicist Karl Jansky concluded that mysterious radio interference was not of earthly origin, instead it was coming from the Milky Way and was strongest in the direction of the centre of the galaxy, in the constellation of Sagittarius. He had invented radio astronomy, a field which took off after the Second World War.
Even objects we could not see emitted radio waves, including the great clouds of neutral hydrogen spread throughout the Milky Way like some ghostly skeleton. Charting the emissions of hydrogen enabled us to map the Galaxy, a process which continues to this day. Recently astronomers used the Very Long Baseline Array (VLBA) radio telescope to measure the rotation rate of the Milky Way by mapping the positions of radio-emitting star forming regions. By observing how far these regions moved in three dimensions over time, the astronomers were able to calculate both the rotational speed of the entire galactic disc and the mass of the Milky Way more accurately than ever before. What is the modern view of the Galaxy?
The Milky Way Galaxy is a flat disc of stars about 100 000 light years across.It contains about 200-400 billion stars, one of which is the Sun. Our Solar System is about 26 000 light years from the centre and takes about 220 million years to orbit the centre of the galaxy once. Most of the Galactic Disc is about 1000-3000 light years thick, widening to about 15000 light years deep at the centre. The stars of the Milky Way are not scattered evenly. The central bulge of the Galaxy glows with the orangish-yellow hue of older stars and these are relatively tightly packed together, separated by less than a light year, compared to about ten light years apart on average near the Sun. These old stars date back the earliest days of the Milky Way, which formed some 13.2 billion years ago. The central bulge was long assumed to be approximately spherical but in the 1990s astronomers began to suspect that the Milky Way was a barred spiral galaxy, rather than an ordinary spiral galaxy.This was confirmed by Spitzer Space Telescope observations in 2005 which indicated that the central bulge is actually a bar of stars some 20 000 light years long.
Radio astronomers have mapped the centre of the Galaxy with great accuracy showing that the galactic centre contains a compact object of very large mass named Sagittarius A* (pronounced Sagittarius A-star) .This is an intense source of radio waves (as detected by Jansky some 80 years ago) marking the location of a supermassive black hole, four million times as massive as the Sun.This a violent and strange place compared to the region around the Solar System, where the stars are crowded together as they hurtle around Sagittarius A* through space warped by the extreme gravitational forces from the supermassive black hole. Asteroids and dust clouds fall into the black hole (occasionally perhaps every 100 000 years an unlucky star falls victim to the black hole’s relentless appetite), and are torn to shreds as they spiral to their doom.The region is awash in intense radiation, a cacophony of cosmic deathcries. The violently hostile environment of the Galactic Core would seem an unlikely place to find stellar nurseries but massive star formation is occurring both within a few light years of the Milky Way’s centre, and in a number of clusters within the inner 300 light years of the Galaxy. These central regions of the Galaxy would be spectacular sights but out there the radiation levels would be lethal to humans.If there are any living beings there, they must be very alien to us.
Science fiction writers were quick to acknowledge that the centre of the galaxy was radiating tremendous amounts of energy and was home to the oldest stars, hence in classic SF, for example Clarke’s 2001: A Space Odyssey (1968), the galactic centre is usually dominated by the most ancient and powerful alien civilisations. This was inverted in Vernor Vinge’s Zones of Thought series (notably 1993’s A Fire Upon the Deep) in which the central portion of the Milky Way comprises the ‘Unthinking Depths’ where the very laws of physics (as re-imagined by Vinge) prohibit the existence of intelligent beings. The dramatic starscapes of the Milky Way’s centre are also the setting for vast epics of interstellar conflict, notably Gregory Benford’s Galactic Center series and Stephen Baxter’s Xeelee Sequence. The centre of the Milky Way was where Isaac Asimov originally located the planet Trantor, famed as the capital of the Galactic Empire in his classic Foundation series and related stories. As science advanced, Asimov in later stories quietly altered Trantor’s galactic location so that it was still dramatically close to the galactic centre but could have an environment compatible with human survival. Advancing knowledge of high energy phenomena in other galaxies inspired a dramatic vision of a lifeless galactic centre in Larry Niven’s 1966 story At the Core. In this Known Space tale, a lone explorer finds the galaxy’s core has exploded in a chain reaction of supernovae. Eventually the lethal radiation from the Core explosion will blaze through the dust clouds exterminating life in the Milky Way. In The Inferno (1973) by Fred and Geoffrey Hoyle (yes, that Fred Hoyle) a similar cosmic disaster at the centre of the Galaxy threatens most life on contemporary Earth. The political and scientific establishment are too stupid to deal with the crisis but luckily a very Hoylesque astronomer knows exactly What Must Be Done and takes personal charge of the survivors, gleefully executing looters and malcontents.
Further out from the core, the galactic disc is characterised by the great spiral arms full of interstellar gas and dust.At the moment we believe there are at least four major spiral arms, these are the 3-kpc and Perseus Arm, the Norma and Outer Arm, the Scutum–Centaurus Arm and the Carina–Sagittarius Arm. Our Solar System ambles along in the Orion–Cygnus Arm, an offshoot of the 3-kpc and Perseus Arm. The spiral arms are believed to be regions where the interstellar medium, the thin gas and dust which pervades the galaxy, is slightly denser than average.These regions of higher density are moving around the galaxy as density waves. This higher density is enough to make the conditions ripe for the creation of new stars, so the arms are brightly aglow with the bluish light of young stars. This brightness of the arms gives the false impression that the dimmer “gaps” between spiral arms are relatively empty of stars (even science fiction authors get this wrong, for example, the apparent scarcity of stars between the spiral arms is a plot point in Niven’s At the Core). In fact stars are just as common between the arms as in them. The density waves move at a different rate from the stars and nebulae, eventually leaving the stars they formed behind.
Most people interested in the possibilities of extraterrestrial life are aware of the concept of a “Goldilocks zone”, a region around a star where the temperatures are “just right” to allow liquid water (and hence possibly life) to exist.Recently it has been suggested that there are good regions and bad regions in the Milky Way for potential life.Towards the galactic centre, supernovae are common, so common that their radiation would sterilise any planets developing life. Further out, heavy elements (anything higher than helium on the Periodic Table) are scarce, possibly too rare even for planets to form. It may be than all life in the Milky Way is found in a doughnut-shaped Goldilocks zone” centred on the galactic core.The inner edge is 23 000 light years from the core, and the outer edge is 29 000 light years from the core. Our Sun is right inside this zone (or I would not be here to write this.)
An important region of the Milky Way which is often overlooked is the Halo. This is a spherical shell of thinly spread old stars and globular clusters. The Halo seems to be about 200 000 light years in diameter. Planets orbiting isolated stars in the Halo would have dark night skies almost empty of stars – but with half the sky dominated by the glory of the Milky Way. The Halo is ancient; the halo stars were among the first inhabitants of our galaxy and collectively represent an older population of the stars which formed much earlier than the stars in the galactic disc like our Sun. Distant and relatively empty, the Halo rarely appears in science fiction, when it does it is always depicted as a region of mystery. In the grim and dark Warhammer 40K universe, the Halo is an all but unexplored region, populated by ancient and terrifying races best left undisturbed. In David Brin’s Startide Rising (1983), the crew of an Earth starship flee to the Halo to use a rare, metal-rich planet in an isolated globular cluster as a hiding place from fleets of avaricious aliens. Unfortunately for the heroes but happily for the reader, their pursuers catch up with them and widescreen space melodrama in technicolor ensues.
Everything described so far is matter, stuff we can see or (in theory) touch. But this is only a small fraction of the whole Milky Way. Most of the mass of the Galaxy is completely invisible and mysterious in nature to us! Most of the Milky Way’s gravity is due to an utterly unknown stuff called dark matter. We know it must be there because its gravitational influence is binding the Milky Way together. A spherical halo of dark matter extends three hundred thousand light years or so from the Galactic Centre, more than 10% of the distance to the Andromeda Galaxy .
So that is the Milky Way, a beautiful wonder in the night sky, our home, a vast and ancient galaxy full of exotic locations revealed from Earth by astronomers. Now all we have to do is turn science fiction into reality, in the words of e.e. cummings:
“listen:there’s a hell
of a good universe next door; let’s go”
(Article by Colin Johnston, Science Communicator)