If you Google “Milky Way images”, you will be treated to an assortment of pictures which can be divided into two categories:
- “Nightscape” photos from Earth, showing a mottled band of silvery light rising from the landscape.
- Pictures showing the entire Milky Way face-on, or from some angle, often labeled to indicate the position of our solar system.
The origin of the first type of image is no mystery. Under dark skies and at certain times of the year, the Milky Way presents glorious photo ops over a variety of terrain and photographers from around the world seek out prime locations to capture the celestial splendor of the Galaxy.
The second category is more perplexing. How can we possibly have obtained an image from outside of our galaxy, considering humankind has yet to even venture outside our solar system?
The answer, of course, is we don’t have any such images, at least not photographic images. All pictures you see of the Milky Way in its entirety are artistic impressions or computer renderings, like this one from NASA.
But this just leads to another question: How do we know what our galaxy looks like from the outside?
Viewing From the Inside Out
To the best of our knowledge, our solar system resides some 27 thousand light years from the galactic core, roughly halfway between the centre and edge of the Galaxy. Our galaxy is enormous, around 100 thousand light years in diameter and 4000 light years thick according to current estimates.
From our vantage point we see the actual Milky Way, that silvery arch which seems to encircle the night sky. We also see regions that are sparser, more barren, depending on the direction and time of year we look. These dense and sparse regions correspond with gazing into and away from the galactic plane.
Below is a 30 second image of the Milky Way that I took while on a trip to Wales.
In pre-telescope times, philosophers could only gaze at this milky swath and guess at its composition, some more accurately than others. Galileo was the first to resolve it into individual stars with his refracting telescope, and this led some great thinkers to later speculate that our universe might be a gigantic disk of stars.
But speculating about a disc-shaped universe is a long way from being able to draw the Milky Way with any degree of accuracy. The next big leap came when Edwin Hubble identified “spiral nebulae” as other galaxies outside of our own. For the first time, we had something to compare ourselves to.
Advancements in professional telescopes allowed astronomers to study these newly-identified galaxies in greater detail until three distinct categories emerged: spiral, elliptical, and irregular, with numerous sub-types in each category. From there, it became a matter of seeing where we fit into this picture and filling in the details.
Sky Surveys
The distribution of stars in the night sky gives a good clue as to the shape of our galaxy. We see a bright band of stars and glowing gas which becomes concentrated in Sagittarius. This strongly suggests a flattened disc with a central bulge.
To fine-tune this basic shape, we need to conduct a sky survey. This involves plotting the positions and velocities of as many stars as possible to create a sort of dynamic, three-dimensional map of the sky. This is even more challenging than it sounds, because the gas and dust of the interstellar medium obscures a large part of the galaxy. Astronomers get around this by surveying the sky at non-visible wavelengths, such as infrared, ultraviolet, and radio, in addition to visible light, allowing them to penetrate some of the obscuring material.
Changing Perspectives
The first attempt at such a survey was conducted by William Herschel in the 1780s. While Herschel’s methods of counting stars were comparatively crude by today’s standards, he did arrive at the correct conclusion of a disc-shaped galaxy, though he erroneously put our solar system near the centre.
A century and a half later, American astronomer Harlow Shapley refined this picture by measuring the distances of globular clusters using Cepheid variable stars. He found that Herschel’s flat disc is embedded in a spherical halo of clusters, and that our solar system is nowhere near the middle of this halo.
Once again, Earth had been removed from its privileged place at the centre of everything. Throughout the next century, astronomers continued to improve their measurements using larger telescopes and more sensitive detectors to examine the distribution of galactic gas and dust, the locations of star-forming regions, and the positions and velocities of millions of stars. By comparing these findings with detailed studies of thousands of other galaxies, astronomers continue to fill the gaps in our knowledge.
One such advancement came in the mid-nineties with the realization that our galaxy probably has a large central bar structure from which the spiral arms originate, rather than the assumed smaller bulge. The number and composition of spiral arms has also been revised numerous times. For the last few decades the Milky Way had been drawn with four arms, but newer studies suggest a two-armed galaxy from which smaller arms or “spurs” split. Our solar system is believed to reside on the Orion Spur, between the Perseus and Sagittarius arms.
More recent three-dimensional renderings of the Galaxy may depict it with a corrugated or rippled structure rather than the typical flattened disc. Observations in the last decade suggest that the Milky Way might be much larger and more massive than previously thought. There are even suggestions that a collision and merger with a dwarf galaxy is ongoing as I type this.
And the story is far from over. The European Space Agency’s Gaia spacecraft is currently mapping the Milky Way to a degree of unprecedented accuracy. It’s quite likely that in a few years, pictures of the Milky Way will look quite a bit different than they do now.
As sky surveys become more detailed and precise, we can expect new discoveries which will revise and refine our picture of this “island universe” we call the Milky Way.
