28 ASTRONOMY • MARCH 2020
When astronomers look at
large galaxy clusters, they
often see wispy arcs and,
sometimes, multiple images
of background galaxies gravi-
tationally lensed by material
in the foreground cluster.
These lenses magnify and
distort light from more dis-
tant objects. Astronomers
dissect Hubble’s images and
calculate where dark matterhas to be to produce the
observed distortions.
Although astronomers
had seen hints that presaged
many of Hubble’s notable
findings, the telescope’s
greatest discovery came out
of the blue. Two research
groups — one led by Saul
Perlmutter of the University
of California, Berkeley, and
the other by Brian Schmidt
at the Australian National
University — were observing
distant type Ia supernovae.
These blasts occur in binary
star systems in which a white
dwarf pulls material from
a red giant companion.
When the white dwarfThe Antennae galaxies, NGC
4038 and NGC 4039, are merging.
This cosmic collision is giving birth
to billions of new stars, most of
which belong to bright blue star
clusters. The large yellowish globes
near the top and bottom of the image
are the cores of the original galaxies.
ESA/HUBBLE AND NASATidal forces have pulled material
from this pair of interacting spiral
galaxies. The Mice sideswiped each
other 160 million years ago and are
now heading back for round two.
Eventually, they will merge into a
single elliptical galaxy. NASA/ESA/H. FORD
(JHU)/G. ILLINGWORTH (UCSC/LO)/M. CLAMPIN (STSCI)/
G. HARTIG (STSCI)/THE ACS SCIENCE TEAMThe Sunburst Arc Galaxy has
been gravitationally lensed into at
least 12 separate images that reside
in four separate arcs around a
foreground galaxy cluster chock
full of dark matter. The lensed galaxy
lies some 11 billion light-years from
Earth, while the intervening cluster
is 4.6 billion light-years away.
NASA/ESA/E. RIVERA-THORSEN ET AL.