Sagittarius A (Sgr A)
A strong source of radio
waves at the heart of the Milky Way Galaxy
made up of several components. Sagittarius A West is a
thermal radio source associated with three small arms of ionized gas and
dust that are spinning around the center, and resembles a miniature spiral
galaxy. Sgr A East is a nonthermal source, about 39 light-years
across, that appears to be a supernova remnant.
The very center of the Milky Way is marked by an intense radio source, called
Sagittarius A*, which is probably a supermassive
| Chandra X-ray Observatory image of
Sagittarius A and A*
Astronomers have been able to obtain detailed information about the nature
of Sag A* from observing a star called S2, which is in a small elliptical
orbit around the object at the center. S2 is an elliptical orbit with a
period of 15.2 years and a pericenter (point of closest approach) of just
17 light-hours (18 billion km) from the center of the central mass. This
gives an estimate for the central mass of about 4.1 million solar masses.
All of this material must be confined with a region no more than a few light-hours
across (since otherwise the star S2 would have fallen into it by now), leading
to the conclusion that it must be a black hole.
In 2012, astronomers reported seeing an ionized cloud of gas falling into
Sag A*. A team at the Harvard-Smithsonian Center for Astrophysics subsequently
explained the observations in terms of a young, low-mass star and its protoplanetary
disk plummeting toward the central black hole and being torn apart in the
process. The fact that the doomed star has a system of planets-in-the-making
is interesting in itself because scientists had doubted whether worlds could
form in such an inhospitable place as the vicinity of a supermassive black
hole. There are other stars in more stable orbits in a ring around Sag A*,
and seems some of these may have planets despite their extreme circumstances.
Surely no life could exist of them, given the intense radiation barrage
they'd be continually subjected to. However, it's interesting to speculate
on the view to be had of the central black hole and its environs from such
a vantage point.
|Artist's impression of a protoplanetary
disk being pulled apart as it falls toward Sgr A*
- Murray-Clay, R. A. and Loeb, A. "Disruption of a proto-planetary disc
by the black hole at the milky way centre". Nature Communications
3, Article number 1049 (11 September 2012) doi:10.1038/ncomms2044
AND INTERPLANETARY MATTER