At Home in the Universe

Astronomers lay bare the Milky Way’s biggest secrets

When Lewis and Clark started exploring the West, they didn’t know much about what lay beyond St. Louis. Neither, at first, did astronomers know much about cosmic realms beyond Uranus.

If Earth-dwellers could peer through clouds of dust, they’d see this bustle of activity around Sagittarius A* at the Milky Way’s core (shown in white in lower right). X-ray: NASA, CXC, UMass, D. Wang et al; Optical: NASA, ESA, STScI, D. Wang et al; IR: NASA, JPL-Caltech, SSC, S. Stolovy

A galactic map created by William and Caroline Herschel in the late 18th century (top) shows the sun close to the center of a puddle of stars. Today maps of the Milky Way put the sun off to the side of a central bar (bottom). The symmetry of this map has recently been enhanced with the discovery of two new arms (outlined in yellow). today’s galaxy: NASA, JPL-Caltech

GASSY TENDRIL A cloud of gas is thought to be streaming toward the Milky Way’s central black hole, destined to one day break apart. The image above simulates what the remnants of the cloud (orange and yellow, orbit in red) and its surrounding stars (blue orbits) may look like in 2021. ESO, Marc Schartmann/MPE

GALACTIC EXTENSION | When viewed in wavelengths corresponding to atomic hydrogen gas, the newfound Outer Scutum-Centaurus arm of the Milky Way appears as a tilted band. Astronomers hadn’t spotted the arm until recently because this part of the galactic disk is warped with respect to the middle of the galaxy, like a phonograph record bent by the heat of the sun. T.M. Dame and P. Thaddeus/Harvard-Smithsonian Center for Astrophysics, Adapted by Stephen Egts

But just as 19th century explorers filled in huge blanks on the American map, so did 20th century skywatchers flesh out a much greater map — of frontiers far beyond the solar system, out across the entire Milky Way. Now, in the last few years, cosmic cartography has again redrawn modern science’s picture of the galaxy, from the inside out.

Surprising new findings from this endeavor begin at the Milky Way’s heart, where astronomers recently spotted a tendril of gas streaming toward the galaxy’s central black hole. Next year, scientists will have a ringside seat for the first time as the matter swings perilously close to its doom.

Farther out from this voracious maw, astronomers have looked at the Milky Way’s central clump of stars and found that much of it rotates not only with the pinwheel shape of the galaxy, but also in a different direction. And in the widest possible view, attained by peering at radio waves emanating from the far side of the galaxy, researchers have started to map out the full symmetry of the Milky Way — including the startling discovery of spiral arms that had long lain unseen.

Studying the galaxy illuminates more than just this corner of the cosmos. It also helps astronomers better understand the origin and fate of other galaxies, such as nearly two dozen small ones that dance alongside the Milky Way and billions of other spiral galaxies throughout the universe.

The newest galactic explorations may even reveal the solar system’s ultimate end. The Andromeda galaxy, which is zooming this way, is expected to smash into the Milky Way some 3 billion years from now. Just how massive the Milky Way is — a crucial statistic currently being updated — will determine how the cosmic collision plays out.

Milky Way metropolis

In many ways, it’s easier to study a galaxy millions of light-years away than to probe this one. Astronomers must infer what the Milky Way looks like from the outside while embedded deep within it, and huge dust clouds obscure much of the view. As a result, “we know galaxies across the universe much better than we know the Milky Way,” says Mark Reid, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Early scientists were convinced that the Earth lay near the center of the galaxy; the first map of the Milky Way, compiled by British astronomers William and Caroline Herschel in 1785, showed the solar system drifting in what looked like a starry puddle. But the Herschels generated that map by counting stars, a method that doesn’t include the all-important gas, dust and other free-floating stuff. By 1920, American astronomer Harlow Shapley had looked instead at star clusters orbiting the Milky Way and figured out that the solar system is perched off to the side.

Today astronomers know that the sun is 27,000 light-years away from the galactic center, requiring about 230 million years to complete an orbit around it.

At the center lurks the Milky Way’s central black hole, also known as Sagittarius A* (pronounced “A-star”) because it lies in the constellation Sagittarius. Most galaxies have such a central black hole around which stars, gas and dust swirl as if going down a drain. Sagittarius A* is a heavyweight among black holes, coming in at around 4 million times the mass of the sun (although it is only about 15 times as wide). The black hole’s massive gravitational pull makes it a sort of Grand Central Station, where stars, gas and dust assemble in an urban galactic buzz.

Because the black hole itself is, well, black, astronomers had to deduce its existence by carefully measuring the paths of stars circling it. Two teams — one in California, the other in Germany — have been doing this for nearly two decades and still have plenty to learn.

In 2018, for instance, a star known as S02 will make its closest approach to Sagittarius A* in about 16 years, zooming by at just three times the average distance between Pluto and the sun. By tracking how the star swings past the black hole like a passenger train, scientists can test Einstein’s theory of general relativity, which makes specific predictions about how matter should behave so close to a gravitational sink. “This would be a test on the largest mass scale that’s ever been done,” says Andrea Ghez of UCLA, leader of the California group.

New upgrades to existing telescopes should give astronomers a far better view than the last time S02 swung past, in 2002. Some of the world’s biggest facilities, such as the Very Large Telescope and Gemini telescopes in Chile, are installing a new generation of adaptive optics, shining multiple lasers into the sky as guides so that technicians can measure and correct for the distorting effects of Earth’s atmosphere. This summer, Ghez will help lead a worldwide push to observe the galactic center. Eventually she aims to expand her study of stars from the current maximum of around 0.3 light-years away from Sagittarius A* to those that sit around three light-years away.

She also wants to track stars that weigh less than the current observing limit, around eight times the sun’s mass, to see if lighter stars behave differently. “There’s so much excitement, which is why we keep going,” Ghez says. “You can make predictions about what kind of stars exist near a black hole, which tells you about how black holes and galaxies form and evolve over time.”

Stars aren’t the only things careening around the hub of Sagittarius A*. So too is a gas cloud on its death march, described in Nature in January by the German-led research team. Using the Very Large Telescope, the scientists took pictures of this blob — only about three times the mass of the Earth — speeding up from 1,200 kilometers per second in 2004 to 2,350 km/s in 2011. Already it is stretching out, like a strand of spaghetti headed directly for Sagittarius A*.

By the middle of next year, the cloud should make its closest sweep by the black hole, at a distance of around 40 billion kilometers, says Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. Part of the cloud may even fall into the black hole, which would burp extra X-rays as it digests its meal. “It’s absolutely wonderful that we can watch this in real time,” Gillessen says. “We really don’t know what may happen.”

Not all astronomers are convinced the blob is really a gas cloud or that it is destined to fall into Sagittarius A*. Ghez sees the object in her data but thinks it might be a fast-moving star on an everyday path around the black hole. And others have proposed the cloud could instead be chunks of debris, such as the shattered remains of a planet-forming disk that could continually feed Sagittarius A*.

Bar of the stars

If the urban core of the Milky Way is Sagittarius A*, then suburbia is the galaxy’s bulge — the sphere-shaped blob of stars in its middle. If you could look at the Milky Way edge-on, like a phonograph record from the side, you’d see that the bulge extends both above and below the galactic plane like an orange at the record’s center — an orange about 8,000 light-years across.

How and why the bulge formed is still something of a mystery. Most theories suggest that it came together soon after the Milky Way was born 12 billion to 13 billion years ago. In this scenario, no more than a billion years passed between the huge starry disk coalescing and its center building up and bulging out from the main galactic plane.

But new studies of some bulge stars suggest they are much younger than expected — on the order of only 2 billion to 5 billion years, says Michael Rich, a UCLA astronomer who led a survey of 10,000 stars. So astronomers need to figure out whether those youthful stars are simply a few newcomers on the block, or whether they indicate a bigger problem for the standard ideas about when the entire subdivision was built.

Suburbia wouldn’t be complete without a neighborhood bar, and the galactic bulge is no exception. Running right through the bulge is a dense elongated concentration of stars, as if someone had rammed a thick straw into the orange along the plane of the phonograph record. This tubelike clump of stars is known as the bar. From either end of it, great streams of stars pour off to form the iconic pinwheel shape of the Milky Way.

Rich’s survey uncovered a curious feature of how the bar rotates: cylindrically, like a toilet roll holder, even as it spins with the pinwheel of the rest of the galaxy. This cylindrical rotation has been seen in other galaxies and could be common throughout the universe, the team wrote in March in the Astronomical Journal.

But because the bulge and its bar are buried in dust, astronomers have a tough time seeing what’s going on. Many can’t even agree on where the bar leaves off and the bulge begins. “It’s just a mess right now,” says Robert Benjamin, an astronomer at the University of Wisconsin–Whitewater.

One new theory might help with the cleanup. A team led by Juntai Shen of the Shanghai Astronomical Observatory in China has been running computer simulations of how the bulge might have formed, based on data from Rich’s Bulge Radial Velocity Assay survey. The findings go against a leading theory holding that the proto–Milky Way must have collided with other disks of stars and that the bulge was created when all those pieces merged together. Rather, Shen’s team suggests that the spinning disk of the protogalaxy could have naturally generated a handlelike bar that then thickened on its own.

The model provides a straightforward explanation for how the bulge and bar could have come to be, Rich says.

Outstretched arms

Galaxies with bars are more likely to have spiral arms tracing a beautiful symmetry into the outer reaches, as the Milky Way does. New explorations of these rural landscapes map out how these stars form a far-flung pinwheel about 100,000 light-years across.

Putting together the full picture isn’t easy. “Galactic astronomy is like a giant jigsaw puzzle where the pieces are coming in in the wrong order,” Benjamin says.

The first jigsaw piece fell into place in the 1950s, when astronomers discovered that they could study a particular spectral line in light coming from distant stars to trace how neutral hydrogen gas is distributed through the spiral arms. This particular wavelength, at 21 centimeters, is in the radio part of the electromagnetic spectrum, which means it can pass unscathed through the dust that blocks telescopes’ view in visible wavelengths. Suddenly, scientists could look for signals coming from gas clouds in other parts of the galaxy and begin to map out its large-scale structure.

Yet after those initial maps, understanding galactic structure essentially stalled as scientists faced problem after problem, such as distinguishing whether a particular cloud was on the Earth’s side or the other side of the galactic center. Only in the last decade have astronomers been able to use other data, such as how other kinds of gas clouds are distributed, to get around such issues.

In 2008, for instance, Thomas Dame and Patrick Thaddeus of the Harvard-Smithsonian Center reported finding a curving spiral arm on the far side of the galactic center. The arm, now called Far 3-kpc, is a counterpoint to a similar arm on Earth’s side. And last July, the team reported discovering one of the most distant spiral arm features yet known — the continuation of an arm called Scutum-Centaurus that wraps around this side of the galaxy, disappears on the other side and finally pops out again where it was spotted (SN: 6/18/11, p. 14).

Nobody had seen the arm segment before because the galactic disk turns out to be slightly warped in the outer reaches, like a Frisbee left too long in the sun. The team stumbled upon it by looking for carbon monoxide as well as atomic hydrogen gas. “It occurred to me when I first saw it that it could be the outer arm,” Dame says. “Then I said: ‘This can’t be.’ ”

Dame and other scientists are now trying to link the outer part of the arm to the inner part, like a game of celestial connect-the-dots. Dame readily admits the arm may not trace back to where he and his colleagues think it should; the galaxy may still hold surprises. To date, the best scientific summary of the Milky Way’s structure is not a scholarly publication but a picture: an artist’s conception of the galaxy as seen from above, in a beautiful mirror-image spiral. “The chances are very significant that the Milky Way is not as orderly as that model shows it,” Dame says.

Answers may soon come from a massive survey to map out galactic structure using cosmic masers, which amplify astronomical emissions like a laser. The Bessel Survey, co-led by Reid, is in the middle of five years of precisely measuring distances to roughly 400 masers throughout the galaxy, including looking through the galactic center and out on the other side into the distant reaches.

Already, the survey has turned up some shockers, such as revising official estimates for how far the sun is from the galactic center and how fast the galaxy is rotating. Bessel results have nudged the solar system closer to Sagittarius A*, from the old estimate of 27,700 light-years to 27,400 light-years, and sped up the galaxy’s rotation speed from 220 km/s to about 250 km/s.

That faster spin means that the Milky Way must be about 50 percent more massive than previously thought, Reid and colleagues reported in 2009 in the Astrophysical Journal. If so, then the Milky Way is no longer just the Andromeda galaxy’s little sibling, as scientists had long thought, but each galaxy weighs about the same amount. The mutual gravitational pull of the Milky Way and Andromeda are ushering the pair closer together, and the added Milky Way heft may speed up their collision a little (SN: 1/31/09, p. 8).

Even better maps of the entire Milky Way may come starting next year, when the European Space Agency plans to launch Gaia, a spacecraft designed to measure the locations of 1 billion (yes, billion) stars above and below the galactic plane. That’s about 1 percent of all the stars in the galaxy, charted with extreme precision. If it succeeds, Gaia will take galactic exploration far from its Lewis and Clark days and well into its space-odyssey future.

“We now have all of these different ways to map the galaxy,” Benjamin says. “It’s a question people sort of forgot was interesting, partly because the field had been so problematic for so many years. Now people are looking at data afresh and finding new things. We may run aground again. But we’ll see how the rest of it fills in.”

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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