The Milky Way is “S-shaped”, according to a new 3D map of our home in the cosmos. And the further out you go, the more distorted it becomes.
- We have long thought the Milky Way was warped, but it is also twisted, according to a new study of pulsating stars
- The new map is the most accurate representation of the Milky Way so far, say astronomers
- The Gaia telescope will give us a much better picture in the future
The map provides an “unprecedented view of the Milky Way”, said study co-author Richard de Grijs of Macquarie University.
The paper, which was published today in the journal Nature Astronomy, maps the location of more than 1,000 bright young stars across our galaxy.
The Milky Way is a spiral galaxy that contains 250 billion or so stars, most of which are found in its inner region.
Astronomers have long thought the galaxy we live in was warped, based on observations of gas clouds that sit between and extend out about twice as far as the stars.
“But what we’ve seen here is that the young stars [in the stellar disk] are also warped,” Professor de Grijs said.
Even more importantly, the pattern of stars indicated the galaxy was not just warped but twisted into an S-shape — a phenomenon known as precession.
“It’s likely this was caused because the main disk, where most of the stars are located, is rotating and it’s dragging behind the outer layer so you get a bit of a twisted spiral pattern in that warped layer,” Professor de Grijs said.
Professor de Grijs said the new map could help in the hunt for dark matter, adding that it could act as a “benchmark” for other work, such as the European Space Agency’s Gaia telescope’s mission to map a billion stars in the Milky Way.
Cosmic beacons shed light on our galaxy
To create the map, Professor de Grijs and colleagues from the Chinese Academy of Sciences focused on 1,339 pulsating beacons known as Cepheid stars.
These young, blue stars, which are 20 times as massive and 100,000 times brighter than our sun, are used by astronomers to calculate distances across the cosmos.
RS Puppis is one of the brightest known Cepheid stars in the Milky Way.
Supplied: NASA/ESA/Hubble Heritage Team (STScl/AURA)-Hubble/Europe Collaboration
To do this they calculate the time it takes the star to brighten and estimate how luminous the star is — the intrinsic brightness — and compare this to the actual brightness observed — the apparent brightness.
“The difference between the intrinsic brightness you expect and the apparent brightness you observe is a direct measure of the distance to the star, provided you know how much dust there is between you and the star,” Professor de Grijs explained.
To cut through the dust, Professor de Grijs and colleagues based their calculations on observations of Cepheid stars made using the Wide-Field Infrared Survey Explorer, which detects longer wavelengths than visible light.
The data shows that while most stars are confined to a thin disk held together by gravity, the proportion of gravity becomes weaker towards the outer regions where there are fewer stars.
“At great distances, the gravity from the inner parts of the Milky Way is not so strong any more, and so any deviation from a flat disk is magnified, so that’s what we are seeing out there.”
Professor De Grijs said understanding the dynamics and distribution of mass could help in the hunt for dark matter — a mysterious substance that interacts with our universe only through its gravitational pull.
“Most of the [Milky Way] is composed of dark matter — we don’t know what it is, we just know there’s a lot of mass,” he explained.
“The fact that the outer disk of the Milky Way is not confined to the thin plane may tell us something about where the dark matter is located and how important it is at that radii.”
Gaia is the Holy Grail of Milky Way science
Astronomer Naomi McClure-Griffiths of the Australian National University said the new study confirmed what we already knew about our galaxy, but in a “different and perhaps more reliable way”.
“The paper is saying the stars match the hydrogen gas,” said Professor McClure-Griffiths, who led a team two years ago that mapped the galaxy’s hydrogen gas clouds.
But in the future, she said, the Gaia telescope will transform the way we see the Milky Way.
The Gaia telescope will map a billion stars in the Milky Way.
Supplied: ESA/ATG medialab ESO/S.Brunier
“In a study like this, not only could they map the 3D structure of the galaxy, but then they could look at how the galaxy is moving, which gives the dynamical information about where things were and how things will go and that becomes much more interesting,” she said.
Joss Bland-Hawthorn of the University of Sydney agreed the new study was a first step, which will be followed up in more detail by observatories such as the Gaia space telescope.
“This is probably the cleanest demonstration of the warp in stars and it looks different from the warp in gas,” said Professor Bland-Hawthorn, who was also not involved in the new study.
But we don’t really know what causes these warps, he added.
“It is not a simple warp, and as the authors say, has a strong precession associated with it,” Professor Bland-Hawthorn said.
Last year the Gaia telescope detected ripples in the Milky Way, possibly created by a close encounter with a massive dwarf galaxy some 300-900 million years ago.
The question is whether the warp is “internal to the Galaxy or induced by infalling dwarf galaxies, or something dark and unseen? Time will tell,” he said.