26 ASTRONOMY • MARCH 2020
figure out how to bridge the
gap. (See “Tension at the
heart of cosmology” in the
June 2019 issue.)
When the space telescope
looks at galaxies in full, it
usually finds a neighbor or
two. In fact, Hubble has
shown that collisions between
galaxies are more the rule
than the exception. Some of
these collisions are in their
early stages. For example, the
Whirlpool Galaxy and its
companion, NGC 5195, show
only subtle traces of an initial
interaction. Others, such as
the Mice, display the dis-
torted shapes and long tidal
tails that develop as gravity
acts over longer periods of
time. And still others, exem-
plified by the Antennaegalaxies, are experiencing a
full-blown impact. Here, the
galaxies have lost much of
their individual identities
as gas clouds ram into one
another and trigger a mael-
strom of star birth.
Speaking of galaxy
collisions, Hubble has veri-
fied that the Milky Way
is not immune. Detailedobservations of our large
neighbor, the Andromeda
Galaxy, show that it is on a
collision course with our
galaxy and will not merely
pass closely. Over the next
several billion years, these
two behemoths will act out
the same scenes now being
performed by the Mice
and Antennae.WATER ON EXOPLANET WASP-39b
The Crab Nebula (M1)
represents the glowing tendrils
of a massive star that earthbound
observers saw explode nearly
1,000 years ago, in the year 1054.
Astronomers have been able to
deduce the nebula’s expansion
rate thanks in part to Hubble
observations. NASA/ESA/ALLISON LOLL
AND JEFF HESTER (ARIZONA STATE UNIVERSITY)Galaxies like the beautiful
Whirlpool (M51) played a key role
in many of Hubble’s cosmological
discoveries. Such island universes
are held together by dark matter,
harbor supermassive black holes
at their centers, help define the
expansion rate of the cosmos, and
fly away from one another at
increasing rates thanks to dark
energy. NASA/ESA/S. BECKWITH (STSCI)/THE
HUBBLE HERITAGE TEAM (STSCI/AURA)Spectra made by Hubble and
its sister Great Observatory, the
Spitzer Space Telescope, helped
nail down the atmospheric
composition of this Saturn-mass
exoplanet. The observations
show the presence of water
vapor and several other important
compounds. ASTRONOMY: ROEN KELLY, AFTER
NASA/ESA/G. BACON AND A. FEILD (STSCI)/H. WAKEFORD
(STSCI/UNIV. OF EXETER)