Rather than being flat as a Frisbee, the Milky Way’s star-studded disk is twisted and warped, according to a new three-dimensional map of our home galaxy. If viewed from the side, the spiral arms girdling our galaxy’s bulging core would resemble a record bent into an S shape, or a softly poached egg sliding off a slotted spoon.
“The warp of the galaxy is so pronounced … if we would be able to see our galaxy from the side, we would clearly see it,” says Dorota Skowron of the University of Warsaw, whose team reports the results today in the journal Science.
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Made using thousands of rhythmically pulsating stars, the new map is among the most highly detailed representations of our home galaxy to date, and it supports previous work that similarly suggested the Milky Way got bent. Along the way, Skowron and her colleagues also uncovered evidence for recent bursts of star formation in our galactic neighborhood.
“We can see with our own eyes, and inside our own galaxy, that the star formation is not a constant process, but indeed is happening in bursts,” she says.
Stretching some 120,000 light-years from tip to tip, the Milky Way is what’s known as a spiral galaxy. Four large arms wind around its core, with our sun parked along a minor arm some 26,000 light-years from the centre.
The galaxy’s disk of stars and gas is mostly thin and flat toward the middle. But at roughly the sun’s distance from the core, the galaxy begins to bend, flexing upward in one direction and flopping down in the other.
Near the edges, it gets kind of sloppy: The disk flares, expanding in width from 500 light-years to more than 3,000 light-years, and the warp is even more prominent, with stars living as many as 5,000 light-years above or below the galactic plane.
“We think the warp may have been caused by interactions with satellite galaxies,” Skowron says, noting that the Milky Way today is surrounded by a swarm of dwarf galaxies. “Other ideas point to interactions with intergalactic gas or dark matter,” she says, referencing the invisible stuff thought to make up 85 per cent of the matter in the universe.
Finding a warped spiral galaxy is not at all unusual; astronomers have observed numerous warped galaxies among the spirals that we can see edge-on, and the stellar conglomerate next door, a giant spiral galaxy called Andromeda, is a similarly twisted sister. But since we sit parked inside the Milky Way, it’s much more challenging to see the large-scale structure of our own galaxy.
Skowron and her colleagues mapped the Milky Way in three dimensions using 2,431 classical Cepheid variable stars.
Sometimes shining thousands of times brighter than our sun, Cepheid variables rhythmically brighten and dim with a period that’s tightly linked to their intrinsic brightness. By monitoring those pulses, astronomers can figure out exactly how bright these stars should be, and then use that intrinsic brightness to calculate precise distances—a relationship that Harvard College astronomer Henrietta Swann Leavitt first determined in 1912.
For years, Skowron and her team watched these big, young stars periodically pulse—even monitoring the ones on the very fringes of our galaxy—using the OGLE instrument, which is mounted on a telescope in Chile, along with several other telescopes. And when Skowron and her colleagues plotted the precise distances to their 2,431 stars, the resulting 3-D galactic map was intriguingly kinked.
“The structures are well mapped, with accurate distances,” says Annie Robin of the Observatoire de Besancon in France, who has made similar maps using information about how gas is distributed and moves through the galactic disk.
“This is clearly compatible with the gas map,” she says.
Skowron and her colleagues also identified groups of Cepheids that had recently burst into existence, cosmically speaking. The team identified three discrete patches of stars, with ages spanning 20 million to 260 million years, that are narrowly distributed along several of the Milky Way’s spiral arms.
By contrast, the oldest known stars in the Milky Way are a whopping 10 to 13 billion years old.
Coloured dots represent older (red) and younger (blue) stars in the Milky Way belonging to three stellar groups that seem to have formed in relatively recent bursts.
IMAGE BY J. SKOWRON/OGLE/ASTRONOMICAL OBSERVATORY, UNIVERSITY OF WARSAW
It’s not exactly clear what might have triggered such recent bursts of star formation in a galaxy that now births only a handful of stars each year. Robin notes that slowly moving regions of higher density can compress the gas and dust in interstellar clouds, triggering star formation, while Skowron suggests that a recent interaction with a passing dwarf galaxy could be the culprit.
“We have to remember that Cepheids [as a group] are relatively young,” Skowron says. “This means that we can use the Cepheids to study only the relatively recent history of our galaxy … so there is still a lot to be discovered about the earlier history of the Milky Way.”
Lead Image: The Milky Way arcs over a mountain lake in California. From our vantage point inside one of the galaxy's spiral arms, we can see the dense inner region of the disk as a distinct band of stars, but it's harder for us to perceive the galaxy's large-scale shape.
PHOTOGRAPH BY BABAK TAFRESHI, NAT GEO IMAGE COLLECTION