An artist's impression of the implied distribution of young stars, represented here by Cepheids shown as blue stars plotted on the background of a drawing of the Milky Way. With the exception of a small clump in the Galactic centre, the central 8000 light years appear to have very few Cepheids, and hence very few young stars. Credit: The University of Tokyo. Click for a full size image
A major revision is required in our understanding of our Milky Way Galaxy according to an international team led by Prof Noriyuki Matsunaga of the University of Tokyo. The Japanese, South African and Italian astronomers find that there is a huge region around the centre of our own Galaxy, which is devoid of young stars. The team publish their work in a paper in Monthly Notices of the Royal Astronomical Society.
The Milky Way is a spiral galaxy containing many billions of stars,
with our Sun about 26,000 light years from its centre. Measuring the
distribution of these stars is crucial to our understanding of how our
Galaxy formed and evolved. Pulsating stars called Cepheids
are ideal for this. They are much younger (between 10 and 300 million
years old) than our Sun (4.6 billion years old) and they pulsate in
brightness in a regular cycle. The length of this cycle is related to
the luminosity of the Cepheid, so if astronomers monitor them they can
establish how bright the star really is, compare it with what we see
from Earth, and work out its distance.
Despite this, finding Cepheids in the inner Milky Way is difficult, as the Galaxy is full of interstellar dust
which blocks out light and hides many stars from view. Matsunaga's team
compensated for this, with an analysis of near-infrared observations
made with a Japanese-South African telescope
located at Sutherland, South Africa. To their surprise they found
hardly any Cepheids in a huge region stretching for thousands of light
years from the core of the Galaxy.
Noriyuki Matsunaga explains: "We already found some while ago that
there are Cepheids in the central heart of our Milky Way (in a region
about 150 light years in radius). Now we find that outside this there is
a huge Cepheid desert extending out to 8000 light years from the
centre."
This suggests that a large part of our Galaxy, called the Extreme Inner Disk, has no young stars. Co-author Michael Feast
notes: "Our conclusions are contrary to other recent work, but in line
with the work of radio astronomers who see no new stars being born in
this desert."
Another author, Giuseppe Bono,
points out: "The current results indicate that there has been no
significant star formation in this large region over hundreds of
millions years. The movement and the chemical composition of the new
Cepheids are helping us to better understand the formation and evolution
of the Milky Way."
Cepheids have more typically been used to measure the distances of
objects in the distant Universe, and the new work is an example instead
of the same technique revealing the structure of our own Milky Way.
Media contacts
Ms Kanako Takeda
The University of Tokyo
Japan
kouhou.s@gs.mail.u-tokyo.ac.jp
Thami Nkwanyane
University of Cape Town
Tel: +27 21 650 5672
thami.nkwanyane@uct.ac.za
Thembela Mantungwa
South African Astronomical Observatory
Tel: +27 21 460 9319
tm@saao.ac.za
Science contacts
Prof Noriyuki Matsunaga
The University of Tokyo
Japan
matsunaga@astron.s.u-tokyo.ac.jp
Prof Michael Feast
University of Cape Town and
South African Astronomical Observatory
Tel: +27 21 650 2396
mwf@ast.uct.ac.za
Prof Giuseppe Bono
University of Rome Tor Vergata
Italy
bono@roma2.infn.it
Prof Marco Galliani
INAF
Italy
marco.galliani@inaf.it
Prof Marco Malaspina
INAF
Italy
marco.malaspina@inaf.it
Further information
The new work appears in "A lack of classical Cepheids in the inner part of the Galactic disc",
N. Matsunaga, M. Feast, G. Bono, N. Kobayashi, L. Inno, T. Nagayama, S.
Nishiyama, Y. Matsuoka and T. Nagata (Kyoto), Monthly Notices of the
Royal Astronomical Society, Oxford University Press, in press.
A preprint of the paper is available on the arXiv
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Source: Royal Astronomical Society (RAS)
