billion light-years
10.25 10.53C 9 (quasar)
10.44 billion light-years54 ASTRONOMY • DECEMBER 2015
million light-years from home, this group’s
largest and most famous member is M106.
Water vapor in this galaxy pulses out
microwave radiation, creating a giant
microwave equivalent of a laser called a
megamaser. Astronomers used the water
megamasers orbiting M106’s supermassive
black hole to directly measure its distance.
The galaxy also contains visible
Cepheid variable stars, “standard candles”
whose predictable brightness changes
astronomers also can use to tabulate dis-
tance. These stars dim and brighten peri-
odically like seizure-inducing Christmas
lights, and the period gives away the star’s
intrinsic luminosity. By comparing this to
the star’s observed brightness, astronomers
can calculate how far away it is. Thus, the
water megamasers and Cepheid variables
in M106 help scientists calibrate the cos-
mic distance scale.
One ring rules them all
The M96 Group in Leo lies some 36 mil-
lion light-years away and has a number of
big, bright galaxies — 12 with diameters
above 30,000 light-years. In 2010, it also
helped astronomers investigate how gal-
axies form thanks to a ring of chilly gas
650,000 light-years wide that encircles the
group. For 30 years, no one understood
where it came from or what exactly it was.
Then a team led by astronomers at Lyon
Observatory decided to tease out the ring’s
nature. The researchers believed that it
might reveal itself to be “primordial gas,”
gas that has never lived inside another gal-
axy and can’t, in its current form, transform
into stars. For galaxies to form, astrono-
mers think cold primordial gas has to fall
into the system, like high-nutrition food to
help fuel the growth of early years. But no
telescope had seen such old-school atoms
around a growing galaxy. The Leo Ring,
these scientists thought, might be just that.
But when they turned their telescope on
it, they found bright optical light, the kind
that massive young stars emit. Primordial
gas, by definition, cannot create such stars.
They had a new mystery.
Using computer simulations, the team
discovered that the ring represents a scar
left over from a colossal collision: NGC
3384, a central elliptical galaxy, and M96, a
peripheral spiral, smashed into each other
more than a billion years ago. The gas from
one blew away, eventually forming the ring.The biggest and baddest
The Virgo Cluster, the most significant col-
lection of galaxies with the home address“Local Supercluster,” has a center some 55
million light-years away. As its name sug-
gests, you can find it in the direction of
the constellation Virgo. Compared to the
meager dozens of galaxies in the previ-
ous groups and clusters, Virgo has 1,300
— and maybe even 2,000 — constituents.
They add up to 1.2 quadrillion solar mass-
es spread across 7.2 million light-years.
In between these galaxies roams a
rogue set of intergalactic stars — up to
10 percent of the cluster’s total. Globular
clusters, dwarf galaxies ripped from par-
ents, and at least one star-forming region
also populate the intergalactic region. Hot
X-ray-emitting gas permeates the space
where these orphans live.
The Virgo Cluster is so big that it actu-
ally has subclumps: Virgo A, Virgo B, and
Virgo C. Virgo A dominates, encompass-
ing 10 times as much mass as the other
two. The three eventually will merge into
a single giant cluster. Because these sub-
groups have yet to coalesce, astronomers
suspect Virgo is young and still figuring
out its identity.
At the cluster’s center — and the center
of subgroup Virgo A — lies the giant ellip-
tical galaxy M87. Bright galaxies like M87
fill the Virgo Cluster. They are, in fact,
the concentration that Messier
and the Herschels noticed
— the first evidence that
superclusters exist.
Fifteen of Messier’s
objects are found in
this congregation.
And you can seeGalaxy M82 resides in a large group named for its neighbor, M81. An ongoing interaction between
these two has sparked M82 to produce vast numbers of new stars. NASA/ESA/THE HUBBLE HERITAGE TEAM (STSCI/AURA)
Microwaves from water vapor in M106 helped pin
down its 30-million-light-year distance. NASA/ESA/THE
HUBBLE HERITAGE TEAM (STSCI/AURA)/R. GENDLER (FOR HUBBLE HERITAGE TEAM)Spiral galaxy M101 is the dominant member of
its galaxy group, which lies 18 million light-years
from Earth. The 170,000-light-year-wide island
universe appears face-on from our perspective.
NASA/ESA/K. KUNTZ (JHU), ET AL./STSCIDSS2 (3C 9); NASA/ESA/R. GOBAT, ET AL. (CL J1449+0856)