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Astronomers map 219 million stars in the Milky Way

Paul Sutherland, Feature writer
Sep 18, 2014, 4:01 UTC

Sen—UK astronomers have built the most detailed catalogue of stars in our Milky Way ever assembled charting the positions of 219 million stars.

The team, led by Geert Barentsen of the University of Hertfordshire, spent ten years working with the powerful Isaac Newton Telescope (INT) on La Palma in the Canary Islands to produce their epic study.

It plots details of stars visible in the northern part of the glowing band in the sky called the Milky Way which is formed due to our edge-on view through our 100,000 light-years wide home galaxy.

With its 2.5-metre wide mirror, the INT was able to capture the faint light and separate, or resolve, the millions of distant stars in the Milky Way. The catalogue charts all star brighter than 20th magnitude, which includes stars one million times fainter than the human eye alone could detect.

By plotting this vast catalogue, the astronomers were able to demonstrate how the density of stars varies across the disk of the Galaxy. Dark lanes or pools which resemble empty space are actually caused by clouds of dust blocking out the light of more distant stars.


The Milky Way stretched over the Isaac Newton Telescope, on La Palma. Image credit: Nik Szymanek

Now the team is offering to freely share its catalogue with astronomers around the world, so that it might produce other interesting findings. It is known as IPHAS DR2 because it is the second release from the survey programme the INT Photometric H-alpha Survey of the Northern Galactic Plane, or IPHAS. A paper about the catalogue is published this week in the journal Monthly Notices of the Royal Astronomical Society.

Sen asked Dr Barentsen how much of a challenge it had been to map so many stars.

He told us: “To achieve our goals, we used the Isaac Newton Telescope for more than 350 nights over a period of ten years. Across these nights, the sensitive detectors of the telescope’s camera provided us with more than 250,000 detailed images of the Milky Way, at a resolution of 8 megapixels each.  

“Every image typically contained several thousands of stars, each of which had to be painstakingly measured in order to construct an accurate catalogue. Needless to say, we required a high-performance supercomputer along with purpose-built software to assemble this new atlas.”

We also asked how the astronomers imagined this catalogue being used, and how will it help astronomers to learn more about our Milky Way galaxy.

Barentsen told us: “A unique feature of our survey is the use of a narrow-band filter which captures the brightest hydrogen emission line, H-alpha. If we detect a strong signature of H-alpha emission towards a star, it tells us that the object is likely to be very young or very old, because that is when stars tend to be surrounded by diffuse ionized gas.  

“Such pre- or post-main-sequence stars represent relatively short-lived phases of evolution and are hence hard to find amongst the vast majority of “normal” main sequence stars that populate our galaxy. It is necessary to discover more of these young and old objects to develop and test models for these crucial stages in the life of a star. We have already identified several hundreds of previously unknown objects in the catalogue which are likely to be very young or old, and have started studying them in more detail.”

He added: “It is worth noting that IPHAS complements the work of ESA’s recently launched Gaia mission, focusing on the astrometric problem—how far away stars are, and their space motions—that will start releasing data from 2016.”

Barentsen told us that he looks forward to seeing the ways in which scientists will work with the data his team has assembled. He said: “Whenever large data sets have been released in the past, astronomers have always come up with new and creative methods to use the data in ways which were not originally envisaged by the authors. For this reason, we are excited to share our data with the rest of the world, because it opens the door to new and serendipitous discoveries!”

Barentsen was part of another team, led by Professor Janet Drew, of the University of Hertfordshire, that announced the first detailed 3D map of dust in the Milky Way in June.