The most detailed three-dimensional map yet of the Milky Way has been revealed, showing that our galaxy is not a flat disc but has a “warped” shape like a fascinator hat or a vinyl record that has been left in the sun.
“The stars 60,000 light years away from the Milky Way’s centre are as far as 4,500 [light years] above or below the galactic plane – this is a big percentage,” said Dr Dorota Skowron of the University of Warsaw, first author of the latest research.
Both the new study and an earlier one published in February, which found a similar shape, are based on the distribution across the galaxy of stars known as Cepheids – bodies whose brightness varies in a regular cycle over time. This phenomenon of dimming and brightening is the key to creating the maps.
While a star might be fainter because it is further away, it could also be because it is less luminous. For Cepheids, the maximum brightness of the star is related to the length of time the cycle of brightening and dimming takes, with brighter Cepheids having longer cycles. By comparing this intrinsic brightness with how bright the star appears to be, researchers can work out how far away a Cepheid is.
“Cepheids are relatively young – younger than 200m years – whereas the Milky Way is believed to be about 10bn years old. This means that we can use Cepheids to study the relatively recent history of our galaxy,” said Skowron.
The new research shows our galaxy, which Skowron notes has a radius of about 70,000 light years and four spiral arms, is “warped”, with the outer regions of one side bent “up” from the galactic plane towards the north galactic pole, and the other bent “down”. The thickness of the disc is not the same all the way across – it flares towards the edges, like a pair of 1970s jeans.
These findings also tally with other work showing a warp and flare, including studies of the distribution of hydrogen atoms across the Milky Way, although Skowron notes that uncertain distances mean the shape of the warp could not be unpicked from those studies.
Prof Richard de Grijs of Macquarie University, who co-authored the earlier Cepheid study, said there were a number of explanations for our galaxy’s warp. These include mergers with smaller galaxies, or the gravitational pull of the Milky Way being weaker in its outer regions meaning bodies there might be deflected out of the galactic plane by the tug of other stars. Alternatively, he said, “gravitational interactions with nearby galaxies … could distort the gravitationally weakly bound outer regions into a warp-like structure”.
Skowron said warping was not seen in the central regions of the Milky Way because gravitational forces there were so strong that it was hard to deflect stars and gas out of the galactic plane.
The new study suggests the bending away from the plane of the galaxy begins about 26,000 light years from the centre of the galaxy – roughly where our solar system lies – but becomes steeper from about 32,000 light years.
A warped galaxy was not unusual, said Skowron. “In fact, it is estimated that about half of the galaxies could have some detectable warping. However the warp of our galaxy is quite substantial in comparison with others.”
As with the previous work, the new study shows the Cepheids disproportionately lie on one side of the warped galaxy, forming an arc-shaped spread.
The Polish team also found younger Cepheids lie nearer the centre of the Milky Way, while older Cepheids are further out. A computer simulation revealed there would need to have been star-forming events 64m, 113m and 175m years ago to produce the distribution of Cepheids seen today.
Dr Vivienne Wild, an expert on galaxies from the University of St Andrews said it was much more difficult to explore the structure of our own galaxy than that of others.
“This is because we are inside it, and determining the distances to stars is really difficult,” she said.
Wild welcomed the exploration of how and why the Cepheids are bunched up on one side of the galaxy. “By measuring the ages of the stars and plotting their 3D positions in the Milky Way’s disc,” she said, “we can see how they initially formed in the Milky Way’s spiral arms up to 175m years ago.”