Seeing ‘Through’ The Milky Way Reveals Far-Out Structure
How is our galaxy built? What does it look like? (And how do we know?)
These questions have persistence since before we properly understood what galaxies are. Once upon a time, it was thought that what we know now as the ‘Milky Way’ was everything that was. The glowing band of light across the night sky was revealed as a swarm of faint stars when Galileo Galilei first turned a telescope on the Milky Way in 1610. The Milky Way was seen as a great system of stars, which themselves must be pretty far away if a telescopes required to see them, but it was viewed as the only such system until the twentieth century.
In the eighteenth century, people began trying to systematically map those stars in order to deduce something about the structure of the system. The Anglo-German astronomer William Herschel published a map of the Milky Way in 1785, drawn from his years of meticulous observations of its stars: William Herschel’s 1785 map proposing an overall shape of the Milky Way, based on his observations of its constituent stars.
William Herschel’s 1785 map proposing an overall shape of the Milky Way.
He also correctly identified the Sun as occupying a place within the system (the prominent “star” just to the right of center on his map) rather than lying outside of or beyond it. His logic was simple. If the Sun and its attendant planets, including Earth, resided within the system,
“the heavens will not only be richly scattered over with brilliant constellations, but a shining zone or milky way will be perceived to surround the whole sphere of the heavens, owing to the combined light of those stars which are too small, that is, too remote to be seen. Our observer’s sight will be so confined, that he will imagine this single collection of stars, of which he does not even perceive the thousandth part, to be the whole contents of the heavens."
In the generation before Herschel, thinkers began to speculate on the existence of other systems like the Milky Way. Thomas Wright suggested in 1750 that the Milky Way was really a single object, a large, flattened (and spinning) disk consisting of countless stars. The German philosopher Immanuel Kant read Wright’s musings and further developed the notion into what he called the 'island universe':
"Their analogy with the stellar system in which we find ourselves, their shape, which is just what it ought to be according to our theory, the feebleness of their light which demands a presupposed infinite distance: all this is in perfect harmony with the view that these elliptical figures are just universes and, so to speak, Milky Ways, like those whose constitution we have just unfolded."
By the turn of the twentieth century, astronomers were slowly coming to understand the the ‘spiral nebulae’ seen in their telescopes were systems like the Milky Way seen at great distances. However, these objects stubbornly refused to resolve into individual stars, even in the world’s largest telescopes, and some scientists held fast to the notion that the spiral nebulae were little more than gas clouds, perhaps illuminated by unseen stars embedded deeply within.
The disagreement culminated in 1920’s “Great Debate” between American astronomers Harlow Shapley and Heber Curtis. Shapley argued the conventional position that spiral nebulae were small, nearby interstellar clouds, while Curtis advanced the notion that they were large and far away, and thus their constituent members must be stars. Curtis based his argument in part on his 1917 observations of a kind of stellar cataclysm called a “nova” in the direction of what we now know as the Andromeda Galaxy. Novae are very intrinsically bright, like extra-bright light bulbs, meaning that they can be seen at large distances within the Milky Way. Curtis looked back over the historical record and found 11 more novae in Andromeda on old plate photographs taken through telescopes. Based on these 12 objects, he computed a distance to Andromeda of half a million light years, much further away than the most remote known star.
Curtis was ultimately proven right before the end of the decade. The debate was properly resolved in the spring of 1929 when Edwin Hubble (of telescope fame) published a paper with a radical conclusion: the ‘Andromeda Nebula’ was certifiably far away. By monitoring a kind of variable star seen in the outer reaches of Andromeda’s spiral arms, Hubble was able to infer its luminosity; comparing this against the star’s apparent brightness revealed its distance: nearly 2 million light years. The controversy was settled: Andromeda must be something like the Milky Way itself.
Edwin Hubble’s famous “VAR!” photograph marking the position of the Cepheid variable star he discovered in the Andromeda Galaxy. Carnegie Observatories photo.
What about Andromeda’s apparent shape? From images of it, and other objects that came to be known as “galaxies”, we find a variety of shapes and sizes that tell us something about their origins and histories. Hubble tried classifying galaxies by shape, arranging them into what became known as the “tuning fork diagram”:
Hubble’s tuning fork doesn’t sound like much.
There are basically two kinds of galaxies: spirals and ellipticals. A subset of spiral galaxies are known as “barred spirals” because of how their arms unwind from the center: the arms of ‘ordinary’ spirals curve all the way into the galaxies’ centers, while the arms of barrier spirals project off the ends of a linear feature running through their centers. Clearly Andromeda was some kind of spiral, seen at a fairly high inclination (whereas all of the model galaxies shown in the Hubble diagram are conveniently viewed face-on).
If the Milky Way was more or less like Andromeda, was it, too, a spiral of some sort? That seemed likely, because of how the Milky Way appears in our night sky. Keep in mind that we’re embedded within the galaxy, not flying above it somewhere in space. Rather than resembling the oval blobs of elliptical galaxies, the Milky Way looks more like a flattened disk seen along its edge. In certain cases, when we look out toward other nearby spiral galaxies, we see them in the same orientation, as in this image of the galaxy NGC 891:
A color image of the galaxy NGC 891 taken by the author.
We even have ways of deducing the presence of a bar in the Milky Way, so we know it properly belongs on the arm of Hubble’s tuning fork with the other barred spiral galaxies.
So by the turn of the twenty-first century, humanity had a pretty good idea of the shape and extent of our home galaxy based on observations, inferences, and some crafty detective work. Here’s what we think it would look like, if we could fly high above the disk and look back:
A model of the Milky Way galaxy, viewed from over one of its poles. Illustration by Robert Hurt, IPAC; Bill Saxton, NRAO/AUI/NSF.
But how can we be sure? What’s happening in the parts of the Milky Way we can’t see directly?
In the image of NGC 891 above, note the dark stripe running down the middle of the galaxy, appearing to split it into two halves. This is a ‘dust lane’, composed of many individual clouds of interstellar dust that tend to form and settle in the galaxy’s mid plane. Dust particles are really good at scattering and absorbing light, so they transmit very little of the starlight coming from “behind” them as seen in the direction of Earth. The Milky Way has its own dust lane, just like this one:
The “Great Rift” in the Milky Way, seen during northern hemisphere summer on Earth. NASA image.
With all that dust blotting things out, it seems like we could never know what’s going in really distant parts of the Milky Way. But some kinds of light can pass through the dust relatively unscathed — long wavelength varieties of light, like infrared. Here’s a Spitzer Space Telescope view toward the center Milky Way in the infrared part of the spectrum:
Staring at the Milky Way’s not-so-dark heart. Image: NASA/JPL-Caltech/S. Stolovy (Spitzer Science Center/Caltech).
There’s still some dust, but much less than in the visible light image shown previously. To look more deeply into our galaxy, we have to resort to longer wavelengths of light: radio waves. And that brings us to this week’s news about structures seen all the way through on our galaxy’s far side.
As reported last week in Science, Alberto Sanna and co-workers used a group of radio telescopes called the Very Long Baseline Array (VLBA) to measure the distance to a star-forming region on the side of the Milky Way opposite the Sun. VLBA uses a principle called interferometry to achieve extremely high resolution of astronomical radio sources. By distributing ten radio antennas, each with a diameter of 25 meters, across the western hemisphere from Hawaii to the Virgin Islands, VLBA achieves the same resolution as a single radio dish with a radius of thousands of kilometers. VLBA does this by very careful timing the arrival of the radio signals received by each dish. Software then reconstructs the incoming signal at very high spatial resolution.
The 10 VLBA dishes and their locations. Image: SeaWiFS Project NASA/GSFC and ORBIMAGE.
The authors then used a geometric principle known as parallax to determine the distance to the source of the radio waves, finding they originated some 66,500 light years away from Earth. That’s most of the width of the galaxy itself, thought to be around 100,000 light years. And with that they found, at exactly the expected distance, the continuation of one of the Milky Way’s spiral arms known as the ‘Scutum-Centaurus Arm’.
So why is this work important? Doesn’t it just confirm what we already thought was the case?
For one thing, it’s a demonstration of a very powerful technique for teasing out details of the structure of the Milky Way. The group’s parallax estimate blows the old record of about 36,000 light years right out of the water. As Sanna commented, "Most of the stars and gas in our Galaxy are within this newly-measured distance from the Sun. With the VLBA, we now have the capability to measure enough distances to accurately trace the Galaxy’s spiral arms and learn their true shapes.”
"This means that, using the VLBA, we now can accurately map the whole extent of our Galaxy,” he added.
The Milky Way is to galaxies what the Sun is to stars: the nearest and best-understood example. As the model for other galaxies of its kind, as our knowledge of the home galaxy improves, so does our knowledge of how other galaxies work. A fully mapped-out Milky Way helps test predictions from theories about how mass is dispersed in galaxies and therefore how it affects the motion of its stars, gas and dust. That all implies how the unseen matter of our galaxy is distributed, which in turn influences our understanding of how the universe and everything in it came to be as we see it today.