FIREWORKS OF STAR FORMATION LIGHT
UP A GALAXY
Newly released images obtained with
NASA'S Hubble Space Telescope in July 1997 reveal
episodes of star formation that are occurring across
the face of the nearby galaxy NGC 4214.
Located some 13 million light-years
from Earth, NGC 4214 is currently forming clusters
of new stars from its interstellar gas and dust.
In the Hubble image, we can see a sequence of steps
in the formation and evolution of stars and star
clusters. The picture was created from exposures
taken in several color filters with Hubble's Wide
Field Planetary Camera 2.
NGC 4214 contains a multitude of faint
stars covering most of the frame, but the picture
is dominated by filigreed clouds of glowing gas
surrounding bright stellar clusters.
The youngest of these star clusters
are located at the lower right of the picture, where
they appear as about half a dozen bright clumps
of glowing gas. Each cloud fluoresces because of
the strong ultraviolet light emitted from the embedded
young stars, which have formed within them due to
gravitational collapse of the gas.
Young, hot stars have a whitish to
bluish color in the Hubble image, because of their
high surface temperatures, ranging from 10,000 up
to about 50,000 degrees Celsius. In addition to
pouring out ultraviolet light, these hot stars eject
fast "stellar winds," moving at thousands of kilometers
per second, which plow out into the surrounding
gas. The radiation and wind forces from the young
stars literally blow bubbles in the gas. Over millions
of years, the bubbles increase in size as the stars
inside them grow older.
Moving to the lower left from the
youngest clusters, we find an older star cluster,
around which a gas bubble has inflated to the point
that there is an obvious cavity around the central
The most spectacular feature in the
Hubble picture lies near the center of NGC 4214.
This object is a cluster of hundreds of massive
blue stars, each of them more than 10,000 times
brighter than our own Sun. A vast heart-shaped bubble,
inflated by the combined stellar winds and radiation
pressure, surrounds the cluster. The expansion of
the bubble is augmented as the most massive stars
in the center reach the ends of their lives and
explode as supernovae.
Deprived of gas, the cluster at the
center of NGC 4214 will be unable to form further
new stars, and its luminous stars will continue
to go supernova and disappear. Elsewhere in the
galaxy, however, gas will start to collapse and
form yet another new generation of stars, even as
the clusters visible today gradually fade away.
The faint stars covering most of the
picture are much older than the bright blue supergiants,
and show us that episodes of star birth have been
occurring in NGC 4214 for billions of years.
The principal astronomers are: John
MacKenty, Jesús Maíz-Apellániz
(Space Telescope Science Institute), Colin Norman
(Johns Hopkins University), Nolan Walborn (Space
Telescope Science Institute), Richard Burg (Johns
Hopkins University), Richard Griffiths (Carnegie
Mellon University), and Rosemary Wyse (Johns Hopkins
Credit: NASA and The Hubble
Heritage Team (STScI)
Acknowledgment: J. MacKenty and J. Maíz-Apellániz