(Image: William Church/Getty)
It鈥檚 hard to get a good view of our galactic home, not least because we鈥檙e inside it. So how exactly do we know what we know about it?
HUMANITY has made great strides in probing the grand mysteries of the cosmos. We have observed the afterglow of the big bang, spied on galaxies so distant we see them as they were when the universe was an infant, monitored far-off supernova explosions and penetrated to the very edge of a black hole.
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One corner of the cosmos, though, has proved resistant to our attempts to get to grips with it: the bit we call home, the Milky Way.
With the Milky Way, we are inside the thing we are trying to observe. We can only dream of zipping outside our galaxy in a spaceship to get another perspective. So we must make do with our view from an insignificant blue blob orbiting one star out of many billions in a sprawling conurbation of stellar neighbourhoods.
It鈥檚 not the best of views: from here the cosmic metropolis is obscured by a smog of dust and gas that fills up the space between the galaxy鈥檚 stars. What鈥檚 more, its districts are in constant motion, rotating at different speeds around the galactic centre.
Put that way, it seems miraculous we know much about the Milky Way at all. Yet slowly but surely we have begun to sketch a map of our galactic home.
Picture of a galaxy
On a clear night, away from bright lights, you can see a hazy band of light bisecting the sky. The Romans called it 鈥淰ia Lactea鈥�, or 鈥淩oad of Milk鈥� 鈥� the origin of our own Milky Way.
This strip of light gives us our first clues as to what our galaxy must look like from outside. It tells us we鈥檙e not part of an unsculpted blob of stars, but instead somewhere inside a flattened disc looking towards the galaxy鈥檚 dense, crowded heart (see 鈥淏right lights, big city鈥�).
The Milky Way鈥檚 closest galactic neighbour, Andromeda, is a flattened disc, too, with a distinctive spiral-armed appearance. Such structures form as clumps of stars and dust and are drawn together by gravity and begin to spin faster and faster around each other. As they do so, they are flattened down into a disc, rather as a pizza chef spins a dough ball to flatten it out. Any small disturbances in the motion or density of the disc鈥檚 material bunch together over time, forming the spiral arms.
The advent of radio astronomy in the 1950s allowed astronomers to penetrate the fog of interstellar dust and begin to map the location and motion of similar denser structures within the Milky Way.
Over time, radio and infrared observations combined to create a picture of a galaxy not just with spiral arms, but also 鈥� unlike Andromeda, though like many other galaxies we see 鈥� a central, bright bar-shaped structure of stars.
The Milky Way鈥檚 dimensions have been extrapolated by mapping groups of stars called globular clusters that hang out in the outermost galactic suburbs. At 100,000 light years across but only 1000 light years thick, our galaxy has about the same relative dimensions as a CD. We are now reasonably sure that we reside about halfway out 鈥� 25,000 light years from the galactic centre 鈥� in a region known as the Orion Arm.
Weighing the Milky Way
Our galactic neighbours can tell us something about how much stuff our galaxy must contain. Like all the nearby galaxies we see, the Milky Way is surrounded by a coterie of dwarf companions. The speed at which they circle depends on the size of the central mass that swings them round.
Measuring dwarf galaxy speeds confirms an effect seen in many other galaxies and clusters: the Milky Way鈥檚 mass is far greater than can be accounted for by visible matter. About 90 per cent seems to take the form of some inscrutable 鈥渄ark鈥� matter.
鈥淢easurements confirm that the Milky Way鈥檚 mass is far greater than can be accounted for by visible matter鈥�
The earliest estimates suggested the Milky Way鈥檚 mass was about the same as, or perhaps even greater than, that of our nearest neighbour Andromeda 鈥� a puzzling result, as the Milky Way doesn鈥檛 seem to have quite the number of dwarf satellites expected for that gravitational heft.
Last year, however, a new 鈥渂ig picture鈥� analysis by Jorge Pe帽arrubia at the Royal Observatory in Edinburgh, UK, and his colleagues took into account motions within the Local Group of galaxies, the imaginatively named cluster containing the Milky Way, Andromeda and a few others. Its conclusion was that the Milky Way has perhaps only half the mass of its neighbour 鈥� although at around 800 billion suns, it is still no lightweight ().
What鈥檚 where?
Observations of the Milky Way strip and our interactions with our galactic neighbours give us a reasonable idea of our galaxy鈥檚 outward appearance and size. But how do we find out what鈥檚 where within it? It鈥檚 not easy: the night sky is a 2D projection of a 3D picture, with little indication of how far away different objects are.
To get round this, astronomers turn to hydrogen, the most abundant element in the cosmos, accounting for 90 per cent of its non-dark-matter mass. Much of it lurks as vast, diffuse, faintly glowing clouds of neutral gas in the frigid space between stars.
The glow comes from photons emitted when slightly energised hydrogen atoms return to their natural, lowest energy. Sent out with a radio wavelength of 21 centimetres, this radiation isn鈥檛 visible to the naked eye, but unlike visible light it can pass through dust and so make its way to Earth.
It鈥檚 here that the Doppler effect kicks in. This effect, heard in the swooping tone of an ambulance siren as it passes us, describes how radiation is squeezed or stretched in wavelength depending on whether its source is moving towards or away from us.
In the galactic whirl, hydrogen clouds in different places will be moving at different speeds and in different directions relative to us. By measuring how the light we receive from them is shifted away from its nominal 21-centimetre wavelength, we can build up a 3D picture of those motions. Comparing the Doppler shifts of other objects to this reference then allows us to work out where in the galaxy they must be (see 鈥淕alactic markers鈥�).
Hydrogen鈥檚 abundance means it provides only a rough 鈥渂ig picture鈥� of the galactic movements. Emissions from less prevalent gases such as carbon monoxide and ammonia can be used in a similar way to pinpoint individual structures more precisely.
Are we up or down?
The effect of gravity in a disc-shaped galaxy like the Milky Way is to compress almost everything of interest into the central plane. From the symmetry of the Milky Way in the sky, there鈥檚 good reason to believe that we are in that plane ourselves (see 鈥淏right lights, big city鈥�). That鈥檚 certainly been the official position since the 1950s: according to the International Astronomical Union鈥檚 , the sun lies directly within it. Ours is a frustrating ground-floor view of the galaxy.FIG-mg30030602.jpg
In the past decade or so, though, studies using the ammonia emissions of dense, star-forming regions to assess their distance from us have suggested a different picture. The true galactic plane seems to be several light years lower than the IAU definition, with the sun 75 light years above it. The galactic centre is also about 20 light years lower than previously thought, placing the galaxy at a slight tilt relative to the sun鈥檚 position (see 鈥淧enthouse view鈥�).
It鈥檚 a small effect in a galaxy 1000 light years from top to bottom, but it makes all the difference. Rather than looking out of a ground-floor window at the galactic city, we may have a slightly elevated view of that crucial central plane from a few tens of storeys up.
The Milky Way鈥檚 bones
Like radio emissions from neutral hydrogen, infrared radiation produced by the Milky Way鈥檚 warm stuff pierces our galaxy鈥檚 dusty fog. This has allowed instruments such as NASA鈥檚 infrared to infer the existence of networks of filaments underlying and jutting between the star-rich spiral arms of other galaxies 鈥� a skeleton of star-forming 鈥渂ones鈥�. The bones themselves aren鈥檛 visible: they are so dense the infrared radiation can鈥檛 get through. But they are surrounded by a less dense 鈥渇lesh鈥� whose glow can be picked up.
Astronomers had long assumed that the Milky Way possessed similar structures, but until recently they had evaded detection, swamped in the galaxy鈥檚 general infrared noise. But with care, the backlighting provided by the galactic centre allows us to view the bones directly 鈥� as infrared-blocking silhouettes.
鈥淣essie鈥� is one prominent example: a serpent-like dark cloud first discovered in 2010. It was initially thought to be 260 light years long by 1.5 light years wide, and contain material amounting to 100,000 suns. Last year, however, Alyssa Goodman of the Harvard-Smithsonian Center for Astrophysics found it was just part of a larger structure between two and five times as long ().
Mapping Nessie鈥檚 three-dimensional position using latitude, longitude and velocity data from its carbon monoxide and ammonia emissions, and combining this with the redefined level of the galactic plane (see 鈥淎re we up or down?鈥�), suggests it lies on the galactic plane about 10,000 light years from the sun. It also implies it lives on and right along the spine of the Scutum-Centaurus arm, the closest major spiral arm to the sun looking towards the galactic centre.
Such a long, filamentary structure would be expected to be extremely unstable and short-lived, suggesting Nessie is part of a much larger galaxy-scale structure. Even based on the most optimistic estimate of Nessie鈥檚 size, it accounts for just five-millionths of the non-dark-matter mass of the Milky Way. Thousands more Nessie-like features in the galactic plane could be waiting to be discovered.
From our newfound position overlooking the galactic plane, the next few years should allow us to unearth more and more of these bones of the Milky Way. At the same time, the European Space Agency鈥檚 , will be mapping 1 billion of our galaxy鈥檚 brightest stars 鈥� making for an unprecedented view of our galactic home.
This article appeared in print under the headline 鈥淢apping the Milky Way鈥�
