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Click to see selection as Astronomy Picture of the Day (APOD) - October 5, 2000
NASA's Hubble Space Telescope has peered deep into a neighboring
galaxy to reveal details of the formation of new stars. Hubble's
target was a newborn star cluster within the Small Magellanic Cloud,
a small galaxy that is a satellite of our own Milky Way. The new
images show young, brilliant stars cradled within a nebula, or
glowing cloud of gas, cataloged as N 81.
These massive, recently formed stars inside N 81 are losing material
at a high rate, sending out strong stellar winds and shock waves and
hollowing out a cocoon within the surrounding nebula. The two most
luminous stars, seen in the Hubble image as a very close pair near
the center of N 81, emit copious ultraviolet radiation, causing the
nebula to glow through fluorescence.
Outside the hot, glowing gas is cooler material consisting of
hydrogen molecules and dust. Normally this material is invisible,
but some of it can be seen in silhouette against the nebular
background, as long dust lanes and a small, dark, elliptical-shaped
knot. It is believed that the young stars have formed from this cold
matter through gravitational contraction.
Few features can be seen in N 81 from ground-based telescopes,
earning it the informal nick-name "The Blob." Astronomers were not
sure if just one or a few hot stars were embedded in the cloud, or
if it was a stellar nursery containing a large number of less massive
stars. Hubble's high-resolution imaging shows the latter to be the
case, revealing that numerous young, white-hot stars---easily visible
in the color picture---are contained within N 81.
This crucial information bears strongly on theories of star
formation, and N 81 offers a singular opportunity for a close-up
look at the turbulent conditions accompanying the birth of massive
stars. The brightest stars in the cluster have a luminosity equal
to 300,000 stars like our own Sun. Astronomers are especially keen
to study star formation in the Small Magellanic Cloud, because its
chemical composition is different from that of the Milky Way. All
of the chemical elements, other than hydrogen and helium, have only
about one-tenth the abundances seen in our own galaxy.
The study of N81 thus provides an excellent template for studying
the star formation that occurred long ago in very distant galaxies,
before nuclear reactions inside stars had synthesized the elements
heavier than helium.
The Small Magellanic Cloud, named after the explorer Ferdinand
Magellan, lies 200,000 light-years away, and is visible only from
the Earth's southern hemisphere. N 81 is the 81st nebula cataloged
in a survey of the SMC carried out in the 1950's by astronomer Karl
Henize, who later became an astronomer-astronaut who flew into space
aboard NASA's space shuttle.
The Hubble Heritage image of N 81 is a color representation of data
taken in September, 1997, with Hubble's Wide Field Planetary Camera 2.
Color filters were used to sample light emitted by oxygen ([O III])
and hydrogen (H-alpha, H-beta).
N 81 is the target of investigations by European astronomers Mohammad
Heydari-Malayeri from the Paris Observatory in France; Michael Rosa
from the Space Telescope-European Coordinating Facility in Munich,
Germany; Hans Zinnecker of the Astrophysical Institute in Potsdam,
Germany; Lise Deharveng of Marseille Observatory, France; and Vassilis
Charmadaris of Cornell University, USA (formerly at Paris Observatory).
Members of this team are interested in understanding the formation of
hot, massive stars, especially under conditions different from those
in the Milky Way.
October 5, 2000
Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)