Sky and Telescope - July 2017 - 24
Suspicion has therefore fallen on other suspects, such as the
stars within galaxies. In 2014 Geach and colleagues observed
outﬂows from intense star formation in an extremely compact
galaxy. Measuring less than 600 light-years across, this tiny
starburst probably arose during a gas-rich galactic merger.
Now it's spewing away a third of its gas reservoir.
p GALACTIC FOUNTAIN A bubble of hot gas rises from the core of a
spiral galaxy - perhaps driven by star formation.
J U L Y 2 0 1 7 * SK Y & TELESCOPE
Following up with the Institute of Millimeter Radio
Astronomy's Plateau de Bure Interferometer in the French
Alps, the astronomers observed a Doppler shift in emission
lines from ionized magnesium and doubly ionized oxygen,
indicating that powerful outﬂows are blowing gas out of this
galaxy at up to 1,000 km/s (2 million mph). Astronomers
ordinarily attribute such velocities to ﬁerce winds of radiation
from supermassive black holes, but there's no evidence of an
active black hole in this galaxy. It must be the stars that are
driving the gas out.
In the same way that sunlight can impart momentum to
push a solar sail, photons emitted from stars can also drive
away gas molecules. The more massive a star, the hotter it is
and the greater its radiation pressure.
"There's enough energy in stellar radiation pressure to
drive quite a lot of gas out of a galaxy," says Geach. He paints
a picture where a frenzied burst of star formation converts
gas into stars, which in turn produce photon winds that are
powerful enough to sweep out the remaining gas and bring
star formation to a halt.
So how many stars can a galaxy form before stellar feedback kicks in? It depends how crowded the stars are. Compact
galaxies concentrate their star formation rather than spreading it across a large disk, so even a few stars can kick out large
amounts of gas. That's not the case for the Milky Way, where
the bubbles that individual star-forming regions blow are tiny
compared to the scale of our galaxy.
Newborn stars aren't the only ones to create bubbles;
dying stars do, too. Intense bursts of star formation produce
massive stars that inevitably go supernova. A star cluster
might see rapid-ﬁre stellar destruction over the course of
just a few million years. While supernova shocks can trig-
CO M A CLUSTER: N ASA / ESA / HUBBLE HERITAG E TE A M (STSCI / AUR A);
G A L ACTIC FOUN TAIN: N ASA / CXC / STSCI / U NIV. NORTH CA ROLIN A / G. CECIL (2)
p COMA CLUSTER The hot gas between galaxies in a cluster such as Coma (pictured here) will strip away the gas inside the galaxies that might
have formed stars, leaving them aging and sterile.