This artist's concept shows young, blue stars encircling a supermassive black hole at the core of a spiral galaxy like the Milky Way. The background stars are the typical older, redder population of stars that inhabit the cores of most galaxies (including our own). CfA astronomers caught two stars in the act of forming within a few light-years of the Milky Way's center. Their find demonstrates that stars can form at our galaxy's core despite the powerful gravitational tides generated by the black hole. Credit: NASA, ESA, and A. Schaller (for STScI)Two embryonic stars discovered just a few light years away from the Milky Way's center show that stars can form in the potentially destructive reach of the powerful black hole at our galaxy's center.
Astronomers have long known that young stars could be found near the center of the galaxy, but they had no idea how the stars got there.
The region wasn't thought to be conducive to star formation because of the powerful gravitational tides stirred up by the 4 million solar-mass black hole at the galaxy's center. Scientists had figured that the tides would rip apart any gas clouds that could act as stellar nurseries.
An alternative explanation, that the stars fell in toward the galaxy's center after forming elsewhere, was thought to be a rare event.
But the new discovery, presented here today at the 213th annual meeting of the American Astronomical Society, shows that the stars did form in place.
"We literally caught these stars in the act of forming," said Elizabeth Humphreys of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Because the gas and dust between Earth and the galactic center blocks visible light from getting out, astronomers use infrared and radio wavelengths to peer into the region.
Humphreys and her colleagues (at the Harvard-Smithsonian Center and the Max Planck Institute for Radio Astronomy in Germany) used the Very Large Array of radio telescopes to search for water masers — radio signals that signal proto-stars still embedded in their birth cocoons.
The team found the proto-stars at seven light-years and 10 light-years from the galactic center (a light-year is the distance light will travel in a year, about 6 trillion miles, or 10 trillion kilometers). Combined with one previously identified proto-star, the objects show that star formation is taking place near the Milky Way's center.
The finding suggests that the molecular gas at the center of the Milky Way from which the stars form is denser than previously thought. The higher density gas makes it easier for the self-gravity of the condensing cloud to overcome the strong pull of the black hole and to collapse to form new stars.
The discovery also supports recent supercomputer simulations that showed star formation within a few light years of the Milky Way's central black hole.
This 0.6 by 0.7-degree infrared photograph of the galactic center shows a large population of old, red stars. However, the discovery of two young protostars within a few light-years of the center of the Milky Way shows that stars can form there despite powerful gravitational tides due to the supermassive black hole. Credit: 2MASS/E. Kopan (IPAC/Caltech)
"We don't understand the environment at the galactic center very well yet," Humphreys said. "By combining observational studies like ours with theoretical work, we hope to get a better handle on what's happening at our galaxy's core. Then, we can extrapolate to more distant galaxies."