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UCT astronomer part of international team that discovers a giant stellar void in the Milky Way Galaxy

A major revision is required in our understanding of the Milky Way Galaxy, according to an international team of astronomers led by Professor Noriyuki Matsunaga of the University of Tokyo. This is after a team of astronomers, which included Professor Michael Feast of the University of Cape Town, found that there was a huge region around the centre of our own Galaxy which was devoid of young stars.

The Milky Way is a spiral galaxy containing many billions of stars with our Sun about
26,000 light years from its centre. A knowledge of the distribution of these stars is crucial to an understanding of how the Milky Way formed and evolved. Pulsating stars called Cepheids are ideal for this. They are much younger (10 – 300 million years old) than the Sun (4.6 billion years) and their distances can be determined.

Cepheids are stars which vary in light due to pulsation. Pulsation changes the radius of the star and hence its brightness. These stars are much more massive than the Sun (masses of around twice to ten times that of the Sun). They are important for astronomers because their periods depend on their intrinsic luminosity and this allows astronomers to determine their distances rather accurately when they measure their periods and their brightness in the sky. The periods range from a few days to about 100 days, and their brightness from about 50 to 50,000 times that of our Sun. So they can be seen from very large distances.

Professor 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.”

Professor Feast from UCT’s Astronomy Department notes: "Our conclusions are contrary to other recent work, though they are consistent with observations by radio astronomers which show that no stars are currently being formed in this region known as the Extreme Inner Disk."

Another author, Professor Giuseppe Bono of the University of Rome Tor Vergata points out: "These results show that there has been no significant star formation in this large region over hundreds of millions of years and theories of galaxy evolution must explain why."

The results are published in the Monthly Notices of the Royal Astronomical Society
by N. Matsunaga (Tokyo), M Feast (Cape Town), G Bono (Rome), N Kobayashi (Tokyo, Kyoto), L Inno (Heidelberg),T Nagayama (Kagoshima), S. Nishiyama (Sendai), Y Matsuoka (Tokyo), T Nagata (Kyoto). The paper is titled: A lack of classical Cepheids in the inner part of the Galactic disc.

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