The Milky Way is all around us, so to determine the shape, we would have to map the distribution of stars in all directions
From a great distance our Milky Way would look like a thin disk with stars that rotates around its central region once every few hundred million years. Hundreds of billions of stars supply the gravity glue to keep everything together.
But the attraction of gravity is much weaker in the far outer disk of the galaxy. Outside, the hydrogen clouds that form the largest part of the gas disk of the Milky Way are no longer limited to a thin surface. Instead, they give the disc an S-like, contorted appearance.
Although the distorted hydrogen gas layer of the Milky Way has been known for decades, research has been conducted today Nature astronomy, we discovered that a disc with young, massive stars there is also crooked, and in a progressively distorted spiral pattern.
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We were able to determine this distorted appearance after we had developed the first accurate three-dimensional image of the Milky Way's stellar disk to its distant outer areas.
Map the Milky Way
Trying to determine the true shape of our galaxy is like standing in a garden in Sydney and trying to determine the shape of Australia. The Milky Way is all around us, so to determine the shape, we would have to map the distribution of stars in all directions.
Although this is not particularly difficult in directions above and below the stellar disk plane, it becomes much more difficult along the plane of the Milky Way.
Unlike stars and hydrogen gas clouds in the plane of the Milky Way, our view is obscured by enormous amounts of dust. The material astronomers who call dust consist of carbon particles. It is not too different from the soot that accumulates in your home if you have an open fire, for example.
Large quantities of dust obscure our view of what lies behind, but dust also makes light appear redder. This is because the size of those carbon particles is close to the wavelength of blue light. Therefore, blue light can be absorbed easily by the dust, while red light passes without too many problems.
But it is not just the presence of dust that makes it difficult to chart our Galaxy. It is notoriously difficult to determine distances from the sun to parts of the outer disk of the galaxy without having a clear idea what that disk actually looks like.
One of the researchers in my international team – Xiaodian Chen of the National Astronomical Observatories (Chinese Academy of Sciences) in Beijing – compiled a new catalog of well-behaved changeable stars known as the classic Cepheids. These stars vary in brightness over a certain period.
These stars are among the best milestones in astronomy: they can be used to determine very accurate distances with uncertainties of only 3% to 5%. This is as good as possible in astronomy, allowing us to obtain the most accurate map of the outer galaxy so far.
Our new catalog was based on observations with NASA's Wide-field Infrared Survey Explorer (WISE), a space telescope equipped with long-wavelength (infrared) glasses, ideal for looking at all the dust on the galaxy's disk.
The charted Cepens reach from the center of the Milky Way to its outer areas with most on the near side of the center of our galaxy because of observation restrictions.
Classic Cepheids are young stars that are about 4 to 20 times as massive as our sun and up to 100,000 times brighter. Such high star masses imply that these stars live quickly and die young, and very quickly, sometimes in just a few million years, burn the hydrogen fuel in their star interior.
Cepheid exhibits day- to month-long pulsations, which can be perceived quite easily as changes in their brightness. Combined with the perceived average brightness of a Cepheid, the period of its pulsation cycle can be used to obtain an accurate distance.
We all fall together
Somewhat to our surprise, we discovered that our collection of 1339 Cepheus stars and the gas disk of the Milky Way follow us closely. Until our recent study, it was not possible to connect the distribution of young stars with the outer disc of the Milky Way to the flickering and warped disc consisting of hydrogen gas clouds.
But perhaps more importantly, we discovered that the stellar disk is contorted in a progressively distorted spiral pattern.
Many spiral galaxies are warped to varying degrees, such as the galaxy ESO 510-G13 (depicted top) in the southern constellation Hydra, about 150 million light years from Earth. However, only a dozen other galaxies were known to show distorted patterns in their outer discs.
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By combining our results with these previous observations, we concluded that the warped and distorted spiral pattern of the Milky Way is probably caused by forced moments of the massive inner disk of the galaxy. The rotating inner disc essentially drags the outer disc with it, but because the rotation of the outer disc remains, the resulting structure is a spiral pattern.
This new map provides a crucial update for research into the stellar movements of our galaxy and the origin of the galaxy's disc. This is particularly interesting given the wealth of information we expect to receive from the Gaia satellite mission of the European Space Agency.
Gaia wants to map our Milky Way in unprecedented detail, based on the most accurate distance measurements for the brightest stars ever of the galaxy.
– The conversation Rappler.com
Richard de Grijs is Associate Dean (Global Engagement) and professor of astrophysics at Macquarie University.
This article has been republished in The Conversation under a Creative Commons license. Read the original article.