How old is this galaxy?
The galaxy on the left,
I Zwicky 18,
was once thought to be one of the youngest galaxies on record
since its bright stars indicated an age of only 500 million years.
The galaxy was also intriguing because it resembled
galaxies forming in the very early universe,
but mysterious since it is so nearby -- only 59 million
years away -- and surrounded by galaxies that are significantly older.
images of I Zwicky 18 by the
Hubble Space Telescope
have helped resolve this mystery, discovering a population of
old faint stars intermixed with the
bright star population.
I Zwicky 18
is now thought to be just as old as its neighbors,
roughly 10 billion years old,
but with an intense episode of relative
new star formation.
Possibly the trigger for this recent episode of bright star formation
is the changing gravitational influence of
18's smaller companion galaxy, visible at the upper right.