Milky Way has four spiral arms, says new study

The research looked at massive stars in the Milky Way and found them to be distributed across four arms, whereas NASA's Spitzer Space Telescope looking at low-mass stars in 2008 found two arms.

By Ananth Baliga

Massive stars helped researchers confirm that the Milky Way has four spiral arms, contrary to earlier data from NASA’s Spitzer Space Telescope, which indicated two arms.

The 12-year study used radio telescopes in Australia and China to observe 1650 massive stars and used readings of their distances and luminosities to confirm that the galaxy has four arms.


In the 1950s astronomers used observations from clouds of gas in the Milky Way, in which stars are born, to reveal four arms. But this was challenged by data from NASA's Spitzer Space Telescope, which looked at the galaxy for infrared light emitted by low mass stars. In 2008, they announced finding 110 million stars and only two 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 co-author of the paper Professor Melvin Hoare of the University of Leeds. “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.”

The astronomers looked at massive stars which only live for a short time, about 10 million years. Because they live for such a short period they are found in the arms in which they are formed and were used to calculate the number of arms in our galaxy.


“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,” said Hoare.

[University of Leeds]

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