New Stars in the Shadow of a Black Hole

New observations suggest that several dozen low-mass stars, and eventually perhaps even planets, are forming just 2 light-years from our galaxy’s supermassive black hole.

Radio-wave image of galactic center

Farhad Yusef-Zadeh and colleagues used the Very Large Array to image the galactic center's radio emissions. They discovered 44 protoplanetary disks around low-mass stars, grouped within radio cloud 1 (labeled RC1) and radio cloud 2 (RC2). These star-forming clouds lie 2 and 2.6 light-years from the supermassive black hole (Sgr A*), respectively.
Yusef-Zadeh & others, 2015

Despite the gravitational pull of a black hole with the mass of 4.3 million Suns, massive stars apparently have no trouble forming in our galaxy’s stormy center. Young, massive stars circle the behemoth black hole, and even younger stars-to-be form nearby.

But theorists still don’t understand how stars manage to coalesce around the Milky Way’s deceptively quiet supermassive black hole, known to astronomers as Sgr A*. The fierce gravitational field ought to rip clouds apart long before stars have a chance to form. And that’s if the intense radiation from nearby stars in the galaxy’s busy downtown doesn’t blow the clouds apart.

Now Farhad Yusef-Zadeh (Northwestern University) and colleagues have added another layer to the mystery. While past evidence for star formation in the galactic center has relied on high-mass stars, Yusef-Zadeh’s team reports in an upcoming Astrophysical Journal Letters new observations showing low-mass stars forming just 2 light-years from the black hole.

Proplyds in the galactic center

Two examples of the 44 protoplanetary disks that Yusef-Zadeh's team imaged are shown here. Nearby stars emit intense radiation and winds that shape the gas cocooning low-mass stars into comet-like objects, with bright heads and swept-back tails.
Yusef-Zadeh & others, 2015

The astronomers used the Very Large Array to image what appear to be 44 protoplanetary disks, reservoirs of dust and gas that feed low-mass newborn stars. Intense ultraviolet radiation and stellar winds have shaped these objects into comet-like fuzzies, with bright heads and blown-back tails. (Protoplanetary disks in the Orion Nebula show similar shapes.)

The disks sit in two clusters (labeled RC1 and RC2 in the first image), lying 2 and 2.6 light-years away from Sgr A*, respectively. These clusters are both between 10,000 and 100,000 years old. Even though nearby stars' winds and intense radiation will steal mass from these disks, Yusef-Zadeh estimates they could have enough material left to form planets around the protostars. But any planets in the galactic center would face a harsh environment that could strip their atmospheres before life had a chance to begin.

These results confirm previous observations that showed stars have formed very close to the black hole as recently as 10,000 years ago. Astronomers using adaptive optics have also long sat in vigilant watch of massive stars less than 10 million years old hurtling about the galactic center—in fact, these stars’ orbits enable astronomers to measure the black hole’s mass. And radio observations have revealed massive stars just 10,000 years old, still embedded in their natal cocoons, forming a few light-years away from Sgr A*.

The formation of low-mass stars suggests that, rather than a one-time formation event, stars are probably churned out continuously in our galaxy’s center, says Andreas Eckart (University of Cologne, Germany). The galactic center has one of the highest star-formation rates in the Milky Way, so if this activity is long-standing, that makes theorists’ conundrum more than a temporary curiosity.

How To Make Stars Near a Black Hole

Observed and predicted positions of 13 stars close to the galactic center around Sgr A*. Keck / UCLA Galactic Center Group / Cosmus

Observed and predicted positions of 13 young, massive stars orbiting Sgr A*. The galactic center's long-known "paradox of youth" has inspired simulations to explain how stars might form in the presence of a black hole, but much work remains to be done.
Keck / UCLA Galactic Center Group / Cosmus

Clever minds have managed to come up with two scenarios, both of which use the black hole’s pull to their advantage. In 2005, Sergei Nayakshin (University of Leicester, UK) and colleagues suggested that a cloud might break apart in the strong gravitational field, reassembling into a disk around the black hole.

The disk could then form stars (albeit only massive ones) the same way that disks around stars ultimately form planets. That might explain why many young, massive stars around Sgr A* encircle the black hole in two rings.

Last year, another method suggested by Behrang Jalali (University of Cologne, Germany) and colleagues suggested that molecular clouds on highly elongated orbits that pass very close to the black hole would spaghettify, compressing even as they stretch out along the orbit. That compression would in turn trigger star formation inside the clouds. A spaghettified, star-bearing cloud might explain the mystery object G2, which hurtled past Sgr A* last spring, the team proposed.

Now the theorists will have their turn again, to see whether the same simulations that were able to form massive stars can form their smaller cousins as well. “The subject needs a lot more work,” says Yusef-Zadeh.


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