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Astronomers find explanation for mysterious stars

Photo: NASA/JPL-Caltech
Photo: NASA/JPL-Caltech

Researchers from Lund University in Sweden have now drawn an exciting conclusion regarding massive stars that can be observed in the outskirts of many spiral galaxies. These objects appear to be “runaway” stars that have been ejected to areas where no stars can form at all.

Some ten years ago, the research community discovered that ultraviolet light radiates from the outer regions of many spiral galaxies; something that has puzzled astrophysicists ever since. Only very massive stars can emit this energy-rich UV light, but at the same time such stars can hardly have formed in the outer regions of galaxies because there is not enough cold and dense gas in those areas.

Furthermore, these massive stars only live for a relatively short time, so it has been thought that they cannot move very far. The research community therefore had to assume that these stars were indeed created at the outskirts of galaxies, thus posing a major problem for the models explaining how stars are formed. But now it seems there may be a solution on the horizon. In a recent study, three astronomy researchers at Lund University investigated these fascinating runaway stars.

“We can now show that these stars may have been created in the same way as other stars, but got a little kick along the way”, explains Eric Andersson, doctoral student in astronomy at Lund University.

The results of the study are based on computer simulations of the possible movements of the runaway stars and the speed of such movements. These stars are relatively common in spiral galaxies. Some of the massive runaway stars can live up to 150 million years, which is in fact a relatively short time in terms of the universe as a whole.

In the current study, the researchers note that these stars have been ejected at a young age from the places they were originally formed, due to gravitational interactions with other young stars. The speed of their involuntary parting is estimated to be about 100 kilometres per second.

“Even if they only live for a short time, they can move a long way out into the galaxy”, notes Andersson.

The current study moves researchers a step closer to a uniform theory of how stars are created and how galaxies evolve. The three Lund researchers were surprised at how well their computer models matched the data collected through observations made by other researchers.

“In a previous study where we looked at the energy coming from runaway stars, by chance we saw indications of what became the focus of this study. When we later did a more thorough investigation, the results became even clearer and we realised that we might have made an important discovery”, adds Andersson.

Publication: Runaway stars masquerading as star formation in galactic outskirts

Eric Andersson, doctoral student
Department of Astronomy and Theoretical Physics, Lund University
eric [at] astro [dot] lu [dot] se
+46 768 19 59 82