HUBBLE RESOLVES A BLAZE OF STARS IN A GALAXY'S CORE
The central region of the small galaxy NGC 1705
blazes with the light of thousands of young and
old stars in this image, taken by NASA's Hubble
At 17 million light-years away, the individual
stars of the dwarf irregular galaxy NGC 1705 are
out of range of all but the sharp eyes of Hubble.
NGC 1705 is an ideal laboratory to conduct investigations
on star formation history. Young, blue, hot stars
are strongly concentrated toward the galaxy's center,
while older, red, cooler stars are more spread out.
This galaxy has been forming new stars throughout
its lifetime, but a burst of star-formation activity
occurred as recently as 26 to 31 million years ago.
This "starburst" is responsible for many of the
young stars on the outskirts of the galaxy's core,
as well as the central giant star cluster.
NGC 1705 is classified as a dwarf irregular because
it is small and lacks any regular structure. Many
astronomers now believe that dwarf galaxies, like
NGC 1705, were the first systems to collapse and
start forming stars in the early universe. They
represent the building blocks from which more massive
objects (spiral and elliptical galaxies) were later
formed through mergers and accretion. Nearby small
galaxies are thought to be the leftovers of the
Dwarf irregulars are similar in many ways to very
young galaxies, but they are much nearer and easier
to study. These galaxies seem to have consumed only
a tiny percentage of their reservoir of gas. Their
stars have a much lower fraction of heavy elements
than does the Sun. These are all indications that
only a few generations of stars have formed there
over time. Current star formation is taking place
at a fairly high rate in starburst episodes. All
these characteristics make dwarf irregular galaxies
the ideal local analogues to young galaxies from
the early universe. Understanding their evolution
is extremely useful and sheds light on the many
processes related to galaxy formation and evolution.
Dwarf irregulars play a key role in astronomers'
attempts to unravel the history of galaxies in the
early universe. These galaxies are probably best
described as fairly old stellar systems whose chemical
and physical properties can be ascribed to a process
of slow evolution. The Hubble observations of the
stars in NGC 1705 and other close irregulars have
demonstrated that these galaxies are several billion
years old. NGC 1705 could be as old as 13.5 billion
This image was taken in March 1999 and November
2000 by an international science team led by Monica
Tosi at Italy's National Institute of Astrophysics
(INAF) at the Osservatorio Astronomico di Bologna.
Other team members include Alessandra Aloisi (JHU),
Mark Clampin (STScI), Laura Greggio (INAF, Osservatorio
Astronomico di Padova), Claus Leitherer and Antonella
Nota (STScI). Hubble's Wide Field Planetary Camera
2 observed the galaxy in ultraviolet, blue, visible,
and infrared light. Although not included in this
image, NICMOS (Near Infrared Camera and Multi-Object
Spectrometer) observations were also made of the
galaxy's central core.
Credit: NASA, ESA, and The Hubble Heritage
Acknowledgment: M. Tosi (INAF, Osservatorio Astronomico