Stars born in galactic centre

BY KEITH COOPER

ASTRONOMY NOW

Posted: 10 June, 2009

Stars have been seen being born in the inner sanctum of our Milky Way Galaxy, answering the question of whether stars can form there or instead have to migrate there.

The reason that doubt existed was because the conditions in the galactic centre are extremely harsh – awash with high energy radiation, powerful stellar winds, supernova shockwaves and of course the mighty 4.3 billion solar mass black hole at our Galaxy’s heart, it’s not for the faint hearted. Indeed, astronomers have been bemused by the existence of stars within the central 600 light-year wide region of the Galaxy, and puzzled by how they could form in such conditions. The new observations of three young stellar objects on their way to becoming stars should begin to answer some of these questions.

The galactic centre, imaged by the Spitzer Space Telescope. Image: NASA/JPL–Caltech/ S Stolovy (SSC/Caltech).

A team led by Solange Ramirez, from NASA’s Exoplanet Science Institute at the California Institute of Technology, began the process for hunting for the young stars by inspecting a million different objects in high-resolution mosaics of the galactic centre taken by NASA’s Spitzer Space Telescope. Viewing the Universe in infrared light, Spitzer is able to peer through the thick veils of dust that otherwise hide the galactic centre. Picking 100 candidates, Ramirez’s team then followed up on them with Spitzer’s spectrograph instrument, which was able to identify the signature of certain warm gases found in nebulae where stars are being born. Each young stellar object, or protostar, is less than a million years old and is still encased in its nebulous womb.

“It is amazing that we have found these stars,” says Ramirez. “The galactic centre is a very interesting place: it has young stars, old stars, black holes, everything.”

It is still uncertain whether migration also takes place to transport stars into the galactic centre. Stars have been seen within 100 light years of the black hole, but these are all O- and B-type stars, which are the most luminous and easily seen. A wider range of star types would be expected if stars were randomly migrating into the galactic centre rather than being born there. But clusters such as the Arches Cluster (see our story here) embedded deep in the galactic centre, appear quite normal, while clouds of charged gas are an indication that young stars are beginning to ionise the gas with their ultraviolet radiation. The centre of the galaxy is also plentiful in the material required to build stars; ten percent of all the gas in the Galaxy is found within the galactic centre, funnelled there by the Milky Way’s central bar.

Ramirez’s team’s next step is to try and identify more protostars, in order to better understand the range of stars that form there, and how the conditions allow them to do so.