­­­A team of astronomers has found signs of small stars forming within a few light-years of the Milky Way’s central black hole.

galactic center, outflows marked
Marked on this ALMA image are the locations and orientations of 11 gas outflows, which look like the bipolar lobes made by young protostars. These outflows are all within about 3 light-years of our galaxy's supermassive black hole, marked with a star. Outflow #1 has the most obvious structure; the rest don't show up well at this scale.

The galactic center churns out a surprising number of stars. They even cluster close to the supermassive black hole, called Sagittarius A* (pronounced “A-star”). Young, beefy O­- and B­­­-type stars encircle Sgr A* about a light-year out. A little beyond this — from about 6 to 15 or 20 light-years from the black hole — is a big ring of molecular gas. New ALMA observations suggest that here, too, stars are coming to life.

It’s a wee odd to have so much star formation near Sgr A*. Theoretically, the black hole’s violent gravitational effects, coupled with the high-powered radiation pumped out by nearby massive stars, should make star formation about as easy as cooking a perfect omelet on a storm-tossed boat. Clouds here need to be at least 10 times denser than normal in order to coalesce into stars.

Yet clearly that’s not stopping them. Astronomers have seen hints of current starbirth here before, such as lopsided gas concentrations that resemble the disks around forming suns. These observations have not been definitive, however.

Farhad Yusef-Zadeh (Northwestern University) and colleagues report in the December 1st Astrophysical Journal Letters that they’ve now uncovered strong evidence of low-mass stars forming near the black hole. Using ALMA to peer through the galactic center’s dust, the team found 11 gas dumbbells, all embedded in the inner edge of the molecular gas ring or sitting just within it. These look a lot like the two-lobed outflows created by protostars as they pull in gas from their natal clouds.

Making some assumptions about the lobes’ masses and how big they are, the team estimates that the 11 objects are between 3,000 and 15,000 years old. That’s roughly comparable to previous protostar suspects, and much younger than the O and B stars, which are several million years old. The O and B stars might have formed as part of a major event: several astronomers suspect that they were forged in a massive but short-lived disk of gas accreting onto the black hole, which might have also fueled the creation of two gargantuan outflows called the Fermi bubbles, each some 25,000 light-years tall.

The ALMA observations only include the innermost few light-years, so it remains unclear if there are more of these outflows peppering the gas ring, Yusef-Zadeh says. The team will need to create a larger map in order to know how common these features are.

Assuming these lobes really are from forming stars, the question of how stars assemble with such apparent ease in the galactic center becomes more pressing. Perhaps the very factors that astronomers once thought would stymie starbirth in these regions actually encourage it. Only dense clouds may be able to survive here, and dense clouds make for great stellar nurseries. Jet outbursts from the black hole might plow into the gas and pummel it into dense pockets, too.


You can read more about the result in NRAO’s press release.



F. Yusef-Zadeh et al. “ALMA Detection of Bipolar Outflows: Evidence for Low-mass Star Formation within 1 pc of Sgr A*.” Astrophysical Journal Letters. December 1, 2017. Full text here.


Sgr A*


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