WWW.ASTRONOMY.COM 27
It’s not surprising that
galaxies collide frequently.
After all, Hubble observa-
tions show that the universe
holds at least 100 billion gal-
axies. The number comes
from the study of several
“deep fields” the telescope
has taken by turning its cam-
eras on small areas of appar-
ently empty sky and exposing
for days at a time. Each deep
field captures thousands of
galaxies, from which astron-
omers can estimate the uni-
verse’s total number.
To t h e d a rk s i d e
Hubble’s sensitive detectors
only pick up the light coming
from celestial objects. But
clever scientists have used
its observations to map out
the darkness that dominates
our universe. Dark matter
is a mysterious material that
radiates no light but whose
gravity serves as the glue that
holds individual galaxies and
galaxy clusters together. It
makes up 27 percent of the
mass-energy content of theuniverse, more than five
times what the normal mat-
ter that forms stars, planets,
and people contributes.The space telescope has
been able to map the distri-
bution of cosmic dark mat-
ter despite its invisibility.The Tarantula Nebula (NGC
2070) in the Large Magellanic
Cloud is the largest star-forming
region in the known universe.
Hubble, which resolved the central
cluster (top center) into myriad
stars, enables scientists to see
objects in other galaxies with the
clarity once possible only in our
own. NASA/ESA/ESO/DANNY LACRUE