The idea that each star is a sun, many with their own solar systems, is a powerful reminder of the immense scale of the cosmos. However, the distances to stars in our galaxy are tiny in comparison to distances to other galaxies.
Since antiquity, observers have noted the existence of nebulous stars; diffuse smudgy or cloudy looking stars. Some of them turned out to be what we now know as nebulae, the places where stars form. Many turned out to be something else entirely. It wasn't until the 1920s when it was confirmed that many of these nebulous stars were in fact completely different galaxies, whole other sets of billions of stars like the Milky Way, far beyond our own.
We now know the Milky Way is but one of the billions of galaxies in the universe. Looking back at how astronomy developed this concept over time one can see how philosophers and scientists struggled with comprehending the nature of galaxies, and thus the enormity of our universe.
The Milky Way Resolves into More Stars
To the naked eye it is unclear exactly what the Milky Way is. In ancient Greece, the atomist philosopher Democritus had proposed that the bright band of light might consist of distant stars. The atomists' views were eclipsed by Aristotle's perspectives on the universe.
In Aristotelian Cosmology, the Milky Way was understood to be the point where the celestial spheres came into contact with the terrestrial spheres. One of the important observations Galileo noted in his 1610 Sidereus Nuncius was that, under the view of a telescope, parts of the Milky Way resolved into a cluster of many stars. Once again a weakness in Aristotelian Cosmology was found - the Milky Way wasn't the result of interactions between the terrestrial and celestial spheres. Galileo's observations demonstrated the Milky Way was a massive grouping of individual stars, planets and other nebulous elements.
Island Universes and External Creations
In 1750, English astronomer Thomas Wright, published An original theory or new hypothesis of the Universe. In this book, Wright speculated that the Milky Way was a flat layer of stars, a part of which which was our solar system.
Beyond this he suggested that many of the very faint nebulae "in all likelihood may be external creation, bordering upon the known one, too remote for even our telescopes to reach." The idea that the faint nebulae could be their own "external creations" suggested the universe was much large than previously imagined. In 1755, philosopher Immanuel Kant elaborated on Wright's ideas and referred to these faint nebulae as "island universes." Both the notions of external creations and island universes struggled to capture the implications of this new larger scale of the universe. Beyond the fact that our sun was a star, could nebulae be their own universes or completely separate creations?
Surveying the Milky Way
In the 1780s William Herschel surveyed the stars in a range of different directions. He found that the stars were much denser on one side of the sky than those of the other side.
His son John Herschel conducted a similar study of the sky in the southern hemisphere and found the same pattern. What they were seeing was the core of the Milky Way galaxy, where there is a much greater density of stars.
Herschel had placed our sun nearly at the center of the Milky Way; it wouldn't be until the 1920's when Harlow Shapley's demonstrated that our sun was far from the center of the Milky Way.
Andromeda and Other Nebulae
Nebulous stars have been observed for thousands of years. In 964 Islamic astronomer Al-Sufi had observed and recorded what he called "a small cloud" in an illustration of the constellation Andromeda. We now understand this description as the Andromeda galaxy. Only with the advent and refinement of the telescope was it possible to start to document different kinds of nebulous stars.
As already mentioned, Thomas Wright and Immanuel Kant had published their speculations that faint nebulous stars like this were in fact independent entities like the Milky Way. In the late 18th century Charles Messier compiled a catalog of the 109 brightest nebulae, which was followed by a William Herschel's much larger catalog of over 5,000. Even while documenting all of these nebulae it remained unclear as to exactly what they were.
Finding and Interpreting Red Shift
Studying the light spectrum of nebulae like Andromeda would ultimately provide the information about what exactly these objects were. A range of astronomers worked on this issue in the early 20th century. In 1912 astronomer Vesto Slipher studied the light spectra of some of the brightest nebulae. He was interested in determining if they were made of the kinds of chemicals one would expect to find in a planetary system.
Slipher found something very interesting - it is possible to calculate the relative speed and distance of a star or nebulae is moving by examining the light spectrum it gives off and seeing how much the indicators for elements have shifted into the blue or red color spectrum. Objects shifted blue are moving closer to us and red shifted objects are moving away from us. In Slipher's analysis, the spectrums for the nebula were shifted so far into the red that these nebulae must be moving away from the earth at speeds beyond the escape velocity of the Milky Way. Along with this evidence, in 1917 Herber Curtis observed a nova, the brightening of an exploding star, inside the Andromeda Nebula. Looking back over photographs of the Nebula he was able to document 11 more novae that were on average 10 times fainter than those of the Milky Way. The evidence was mounting to suggest that these nebulae were well outside the Milky Way.
In 1920, Harlow Shapley and Heber Curtis debated the nature of the Milky Way, nebulae and the scale of the universe. Using the 100 inch telescope at Mt. Wilson, Edwin Hubble was able to resolve the edges of some spiral nebulae to identify they were in fact collections of stars, some of which matched standard patterns that enable astronomers to calculate that the stars were too distant to be part of the Milky Way. Thus, the idea of the Milky Way as just one of many galaxies came to be the dominant scientific perspective.
Where the Earth was once understood to be the center of a relatively small universe we have come to understand it as one world orbiting one of the 300 billion stars in our galaxy which is itself just one of more than a hundred billion of galaxies in the observable universe. Even today it remains difficult to grasp just how tiny and small our planet is in the vastness of the observable universe.
Four billion years from now, our galaxy, the Milky Way, will collide with our large spiraled neighbor, Andromeda.
The galaxies as we know them will not survive.
In fact, our solar system is going to outlive our galaxy. At that point, the sun will not yet be a red giant star – but it will have grown bright enough to roast Earth’s surface. Any life forms still there, though, will be treated to some pretty spectacular cosmic choreography.
Currently, Andromeda and the Milky Way are about 2.5 million light-years apart. Fueled by gravity, the two galaxies are hurtling toward one another at 402,000 kilometers per hour. But even at that speed, they won’t meet for another four billion years. Then, the two galaxies will collide head-on and fly through one another, leaving gassy, starry tendrils in their wakes. For eons, the pair will continue to come together and fly apart, scrambling stars and redrawing constellations until eventually, after a billion or so years have passed, the two galaxies merge.
Then, the solar system will have a new cosmic address: A giant elliptical galaxy, formed by the collision and merger of the Milky Way and Andromeda.
This isn’t a chapter ripped from science fiction – it’s a real, scientific prediction. That science can forecast such events was the focus of the third episode of Cosmos: A Spacetime Odyssey. That Newton could describe the orbits of planets, and Halley the return of his eponymous comet, and contemporary astronomers, the end of the Milky Way – this gift of foresight is really a mathematical understanding of the physical laws that govern the movements of celestial bodies.
“Using nothing more than Newton’s laws of gravitation, we astronomers can confidently predict that several billion years from now, our home galaxy, the Milky Way, will merge with our neighboring galaxy, Andromeda,” host Neil DeGrasse Tyson says. “Because the distances between the stars are so great compared to their sizes, few if any stars in either galaxy will actually collide. Any life on the worlds of that far-off future should be safe, but they will be treated to an amazing, billion-year long light show.”
The galactic collision that closes out the third Cosmos episode follows the sequence in the animation below, which is based on a 2006 simulation by astrophysicist Brant Robertson*.
Now, how on Earth do we know this is going to happen?
The story starts in the early 1900s, when astronomer Vesto Slipher measured the radial velocity of Andromeda — in other words, he calculated the speed at which the galaxy was moving toward or away from Earth. Slipher did this by looking for a telltale stretching or compression in the light from Andromeda arriving at Earth: Light from objects that are moving away from us is slightly stretched, or red-shifted. Light from objects moving toward us is blue-shifted, or compressed.
The result was a little bit surprising.
“We may conclude that the Andromeda Nebula is approaching the solar system with a velocity of about 300 kilometers per second,” Slipher wrote in the Lowell Observatory Bulletin in 1913 (Andromeda was called a nebula back then because astronomers didn’t realize it wasn’t part of the Milky Way; Slipher’s calculation strongly suggested that idea needed rejiggering).
So Andromeda was zooming toward us – that much at least seemed clear. Whether its arrival would mean the end of the Milky Way was still uncertain. For decades, scientists had no way of knowing whether Andromeda and the Milky Way would collide head-on, or if they would slip past one another like star-filled vessels in the cosmic night.
Turns out, it’s relatively easy to measure the velocity of faraway objects moving toward or away from us, but much more difficult to determine their sideways motion (something astronomers call “proper motion”). The farther away something is, the harder it is to measure its sideways motion, which doesn’t produce those telltale stretched or compressed wavelengths that astronomers can work with. Instead, astronomers rely on detailed observations of an object’s position relative to background stars – a small and subtle shift that without superior telescopes can take centuries to become apparent.Around 2007, Harvard University astrophysicist Avi Loeb decided to revisit the question of Andromeda’s impending arrival. “Most theorists are interested in reproducing systems from our past that are observed now, and are reluctant to make predictions that will be tested only billions of years from now,” Loeb says. “The rationale was unclear to me; I am curious about the future as much as I am about the past.”
Loeb and then post-doc T.J. Cox simulated the impending collision and merger of Andromeda and the Milky Way using estimates of Andromeda’s proper motion. Their results showed a better than decent chance of the two galaxies smashing into one another, and a pretty good possibility of the solar system being punted to the outskirts of the resulting elliptical galaxy, which Loeb named “Milkomeda.”
In 2012, a team of astronomers based at the Space Telescope Science Institute re-did the collision calculations, this time using direct measurements of Andromeda’s proper motion. After all those years, the team was able to get those measurements with the Hubble space telescope – and an observing campaign that used years of data, beginning with images snapped in 2002.
“We compared images taken at different times with the Hubble Space Telescope, and measured how much the Andromeda stars have moved relative to the fuzzy galaxies in the distant background,” says astronomer Sangmo Tony Sohn. “This gives us a sense of how fast the Andromeda stars moved across the sky.”
The team concluded that Andromeda’s proper motion was tiny – and that a head-on collision was pretty much inevitable. That might sound a little bit traumatic, but it’s not all that unusual for galaxies to merge. The Hubble space telescope has captured some glorious images of faraway mergers and collisions, and astronomer Halton Arp included a number of galactic interactions in his “Atlas of Peculiar Galaxies,” published in 1966. They’re all really pretty.
The good news is that, as Tyson says, stars are so far apart that even though galaxies are colliding, the probabilities of stellar collisions are small. So the sun and its planets will likely survive the birth of Milkomeda, though Earth will no longer be able to call the Milky Way home. And we’ll no longer live in a spiral galaxy: Milkomeda will be elliptical in shape, and it’ll probably look pretty red, which you can see toward the end of the 2012 team’s animation, and in the animation above.
So there’s no doubt this merger is going to be a spectacle – and there’s a good chance that the Triangulum, a smaller, nearby galaxy, will get sucked into the fray. I, for one, am disappointed that I won’t be able to watch this great cosmic light show. For now, the best I can do is enjoy the sequence of illustrations below.
*9:45pm PDT, 3/24: This post has been updated to attribute the embedded animation to astrophysicist Brant Robertson, now at the University of Arizona, and his colleagues. NASA recently redid the animation.