The five stars, known as Cepheid variables, were situated on the far side of the galaxy, 80,000 light years from the Earth and beyond the galactic centre, said SA Astronomical Observatory (SAAO) spokeswoman, Thembela Mantungwa.
They changed their brightness every few days and had characteristics that allowed their distance to be measured accurately.
"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," she said.
"In particular these stars... will help astronomers trace the distribution of the very mysterious dark matter."
Dark matter was known to be an important component of all galaxies but its nature and distribution remained elusive.
Mantungwa said most of the stars in Earth's galaxy, including the sun, were distributed in a flat disk.
Radio astronomers in the 21st century discovered that hydrogen gas flared away from the disk at large distances from the centre of the galaxy. Until now, no one knew that stars did the same thing.
The stars were discovered by Prof Michael Feast, Dr John Menzies, and Prof Patricia Whitelock from South Africa, and Dr Noriyuki Matsunaga from Japan.
Observations were made with the Southern African Large Telescope and the Infrared Survey Facility, both at the SAAO site at Sutherland in the Northern Cape.
* Watch eNCA reporter, Bibi-Aisha Wadvalla's video report in the gallery above.
Additional reporting Sapa