A 12-year study of massive stars has reaffirmed that our Galaxy has
four spiral arms, following years of debate sparked by images taken by
NASA’s Spitzer Space Telescope that only showed two arms.
The new research, which is published online in the Monthly Notices of
the Royal Astronomical Society, is part of the RMS Survey, which was
launched by academics at the University of Leeds.
Astronomers cannot see what our Galaxy, which is called the Milky
Way, looks like because we are on the inside looking out. But they can
deduce its shape by careful observation of its stars and their distances
from us.
"The Milky Way is our galactic home and studying its structure gives
us a unique opportunity to understand how a very typical spiral galaxy
works in terms of where stars are born and why," said Professor Melvin
Hoare, a member of the RMS Survey Team in the School of Physics &
Astronomy at the University of Leeds and a co-author of the research
paper.
In the 1950s astronomers used radio telescopes to map our Galaxy.
Their observations focussed on clouds of gas in the Milky Way in which
new stars are born, revealing four major arms. NASA’s Spitzer Space
Telescope, on the other hand, scoured the Galaxy for infrared light
emitted by stars. It was announced in 2008 that Spitzer had detected
about 110 million stars, but only found evidence of two spiral arms.
The astronomers behind the new study used several radio telescopes in
Australia, USA and China to individually observe about 1650 massive
stars that had been identified by the RMS Survey. From their
observations, the distances and luminosities of the massive stars were
calculated, revealing a distribution across four spiral arms.
“It isn’t a case of our results being right and those from Spitzer’s
data being wrong – both surveys were looking for different things,” said
Professor Hoare. “Spitzer only sees much cooler, lower mass stars –
stars like our Sun – which are much more numerous than the massive stars
that we were targeting.”
Massive stars are much less common than their lower mass counterparts
because they only live for a short time – about 10 million years. The
shorter lifetimes of massive stars means that they are only found in the
arms in which they formed, which could explain the discrepancy in the
number of galactic arms that different research teams have claimed.
“Lower mass stars live much longer than massive stars and rotate
around our Galaxy many times, spreading out in the disc. The
gravitational pull in the two stellar arms that Spitzer revealed is
enough to pile up the majority of stars in those arms, but not in the
other two,” explains Professor Hoare. “However, the gas is compressed
enough in all four arms to lead to massive star formation.”
Dr James Urquhart from the Max Planck Institute for Radio Astronomy
in Bonn, Germany, and lead author of the paper, said: “It's exciting
that we are able to use the distribution of young massive stars to probe
the structure of the Milky Way and match the most intense region of
star formation with a model with four spiral arms.”
Professor Hoare concludes, “Star formation researchers, like me, grew
up with the idea that our Galaxy has four spiral arms. It’s great that
we have been able to reaffirm that picture.”
Media contacts
(To arrange interviews with Professor Melvin Hoare)
Sarah Reed
Press Officer
University of Leeds
Tel: +44 (0)113 343 4196
s.j.reed@leeds.ac.uk
Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
rm@ras.org.uk
Further information
The new work appears in the paper “The RMS survey: galactic
distribution of massive star formation”, J. S. Urquhart, C. C. Figura,
T. J. T. Moore, M. G. Hoare, S. L. Lumsden, J. C. Mottram, M. A.
Thompson and R. D. Oudmaijer, Monthly Notices of the Royal Astronomical
Society, published by Oxford University Press. The paper is available
from http://mnras.oxfordjournals.org/content/early/2013/11/13/mnras.stt2006
Notes for editors
University of Leeds
The University of Leeds is one of the largest higher education
institutions in the UK and a member of the Russell Group of
research-intensive universities.
The 2008 Research Assessment Exercise showed the University of Leeds to be the UK's eighth biggest research powerhouse and the University's vision is to secure a place among the world's leading universities by 2015. www.leeds.ac.uk
Royal Astronomical Society
The Royal Astronomical Society (RAS, www.ras.org.uk
), founded in 1820, encourages and promotes the study of astronomy,
solar-system science, geophysics and closely related branches of
science. The RAS organizes scientific meetings, publishes international
research and review journals, recognizes outstanding achievements by the
award of medals and prizes, maintains an extensive library, supports
education through grants and outreach activities and represents UK
astronomy nationally and internationally. Its more than 3500 members
(Fellows), a third based overseas, include scientific researchers in
universities, observatories and laboratories as well as historians of
astronomy and others.
