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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 eye 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 galaxy-formation process.
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 years.
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
March 6, 2003
Image Credit: NASA, ESA,and The Hubble Heritage Team