South African astronomers have discovered the very first known stars in the flared disk of our Milky Way Galaxy. These stars are situated on the far side of our Galaxy, 80 thousand light years from the Earth and beyond the Galactic Centre.
The discovery is important because stars like these will allow astronomers to test theoretical ideas about how galaxies, like the Milky Way in which we live, formed. In particular these stars, which are close to the effective edge of the Milky Way, will help astronomers trace the distribution of the very mysterious dark matter. Dark matter is known to be an important component of all galaxies, but its nature and distribution remain elusive.
The five stars involved in this discovery are very special ones, known as Cepheid variables, whose brightness changes regularly on a cycle time of a few days. These Cepheid variables have characteristics that allow their distances to be measured accurately. A team of astronomers led by Prof. Michael Feast used observations made with the Southern African Large Telescope (SALT) and the Infrared Survey Facility (IRSF), both at the South African Astronomical Observatory’s (SAAO) site at Sutherland in the Northern Cape, to determine the distances of these stars and hence their locations within our Galaxy.
The majority of stars in our Galaxy, including our own sun, are distributed in a flat disk (see illustration). Early in the 21st century radio astronomers discovered that hydrogen gas, of which the Galaxy contains a great deal, flared away from the disk at large distances from the Galactic centre, but until now no one knew that stars did the same thing.
The team who made the discovery are from South Africa and Japan: Prof Michael Feast (University of Cape Town – UCT, SAAO), Dr John Menzies (SAAO), Dr Noriyuki Matsunaga (the University of Tokyo, Japan) and Prof Patricia Whitelock (SAAO, UCT).
These results will be published in detail on 15 May, in the international journal Nature.
(1) In this artist’s impression the Milky Way galaxy is seen edge on. The sun is shown as a large yellow circle and the approximate position of the known Cepheid variables are shown as pale blue circles. The hydrogen gas is shown in pink and the five Cepheid variables in the flare are shown as dark blue circles (credit Robin Catchpole, University of Cambridge, UK).
(2) The authors: from left to right: Feast, Menzies, Whitelock and Matsunaga (credit Matsunaga, University of Tokyo, Japan)
(3) Left: an infrared image of one of the Cepheid variables (named OGLE-BLG-CEP-32) and nearby stars taken with the IRSF. Right: a SALT spectrum of the same Cepheid variable.