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 òÀØBackbone of the NightòÀÙ.ˆà 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 HeraòÀÙs milk was sprayed across the heavens. The word òÀÜGalaxyòÀÝ is derived from the Greek expression for òÀÜmilky circleòÀÝ.
The Milky Way as it is rarely seen in our light-polluted skies.(Image credit:S.Brunier/ESO)
Greek philosophers disagreed as to what the real Milky Way was. In the 4th Century BC, Aristotle (384òÀÓ322 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 EarthòÀÙs atmosphere. AristotleòÀÙs explanation of the nature of the galaxy was still popular in Europe right up to the Renaissance.ˆà Elsewhere, opinions differed; the astronomers of Persia and the Islamic civilization rejected AristotleòÀÙs 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 declaredˆà “It is nothing else but a mass of innumerable stars planted together in clusters.” ˆà 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.ˆà By the early 20th Century astronomers Jacobus Kapteyn (1851-1922) and Harlowe Shapley (1885 òÀÓ 1972) had separately refined HerschelòÀÙs 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.
Looking towards the heart of our Galaxy, the Milky Way. The Arches Cluster is located in the centre of the image, but its stars are hidden behind large amount of dust. The bright star at the top of the image is 3 Sagittarii, while the cluster of stars seen at the bottom left is NGC 6451.ˆà “They too might look across the 50,000 light-years to the core of the Galaxy, glimpse the titanic forces flickering there among the most ancient of the stars – and marvel at the mentalities that must control them.” (quote from The Lost Worlds of 2001 by Arthur C. Clarke, image credit:ESO, Digitized Sky Survey 2 & S. Guisard)
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.ˆà 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.ˆà 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.
Compared to some “active” galaxies, the Milky Way does not possess the huge high energy jets or bright central nuclei characteristic of these galaxies.In active galaxies, matter falling into their supermassive black holes generates the bright nuclei and powerful jets.Our central black hole is “inactive”, probably because there is too little matter in the central part of the galaxy left to feed it.This Chandra X-ray Observatory image is an X-ray picture of the region near Sgr A* and shows that a bright X-ray source exists at the location of Sgr A*.The image also reveals the presences of a weak X-ray jet from Sgr A*, about 1.5 light years in length, produced by high-energy particles ejected from the vicinity of the black hole.(Image credit:NASA/CXC/MIT/F.K. Baganoff et al.)
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.ˆà Asteroids and dust clouds fall into the black hole (occasionally perhaps every 100 000 years an unlucky star falls victim to the black holeòÀÙs 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.ˆà 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