Mystery At The Center Of The Milky Way Turns Out To Be An Optical Illusion

Composite color infrared image of the center of our Milky Way galaxy. Hubble: NASA, ESA, and Q.D. Wang (University of Massachusetts, Amherst); Spitzer: NASA, Jet Propulsion Laboratory, and S. Stolovy (Spitzer Science Center/Caltech)

A recent study looked at the composition of some of the closest stars to the supermassive black hole at the center of the Milky Way and discovered something very bizarre. These stars appear to have a lot of rare elements – scandium, in particular – something unexpected in models. But new research now claims that this is not the case. It’s an optical illusion.

The composition of stars is estimated by looking at the absorption lines in the spectrum of light. Each element absorbs light at several specific wavelengths creating a “fingerprint” on the light coming from the star. But researchers argue that for cooler stars, some of these distinct lines become muddled up and that confuses where the “scandium signal” comes from. The findings are reported in the Astrophysical Journal.

The team used the advanced camera on the world’s largest telescope at the W. M. Keck Observatory. They looked at both these cool red giant stars within 3 light-years of the center of the Milky Way and red giants near the Solar System. Both show an overabundance of scandium although it isn’t clear why this would be the case. The team suggests that the scandium lines shouldn’t be used for stars colder than 3,500°C (6,380°F).

“These giant red stars have used up most of their hydrogen fuel and their temperatures are therefore only half of the Sun’s,” lead author Brian Thorsbro, a doctoral student in astronomy at Lund University, said in a statement.

The work is an international collaboration between astronomers at UCLA and atomic physicists studying spectral lines. The team is trying to map the detailed composition of stars at the center of the Milky Way, in particular, the bulk of stars that formed at a short distance from the supermassive black hole.

These stars are roughly 25,000 light-years away from us and the technology to study them in detail is only now becoming good enough. In particular, researchers are looking at starlight emitted in near-infrared. Near-infrared light doesn’t get absorbed by interstellar gas, so the information gained from this analysis is more accurate. The project is still in its infancy but it is already giving us some interesting information.  

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