Overview: NGC 772 is a
unbarred spiral galaxy that lies in the direction of the constellation
Aries, Latin for the ram or male sheep. Constellations
are merely made-up concepts based on the random patterns of
stars visually organized and grouped by ancient humans,
therefore galaxies and all objects in the universe have no
physical association with a particular constellation. By
coincidence, however, this galaxy in Aries the Ram has some
resemblance to a ram’s horn, and even more interesting, it is
very close to where the imaginary ram’s horn would be in the
constellation of Aries. NGC 772 is also known as Arp 78 in the
Atlas of Peculiar Galaxies (Arp) as it is a disturbed,
interacting galaxy with multiple companion galaxies described in
more detail below.
Unbarred Spiral Galaxy.
NGC 772 is about 130 million light-years away and about 200,000
light years in diameter making it about twice as large as our own
Milky Way
galaxy. It is unbarred in that it has no obvious central bar
connecting any of its arms, however observations reveal there
may be some central bar structure. It’s designation is SA
(normal spiral) as opposed to SB (barred spiral) in the
galaxy morphological classification. Most normal spiral and
barred spiral galaxies are flat disks of stars rotating around a
nucleus or bulge. The disks have various organizations of stars
in spiral arms that often extend from the center to the edges of
the galaxy. The arms are believed to be formed by both
density waves of matter and gravity differential as well as
shock waves by
interstellar winds and
supernovae.
Approximatey one-third of galaxies are normal spirals and
two-thirds are barred spirals which brings up the questions as
to why barred are not considered normal. See the recently
processed image of
NGC 1097 for an example of a barred spiral galaxy.
Interacting Galaxy. Imagine a billiard table with zero
resistance where a billiard ball moves around, bouncing off the
table sides forever, without slowing down. Then imagine each
billiard ball is swarm of billions of stars gravitationally
bound. As these balls of stars move around the frictionless
surface they inevitably come close to, or even collide, with
another ball of stars. These close encounters and collisions are
how galaxies interact with and disturb each other, occasionally
merging with one another to form even more massive galaxies.
Most galaxies seem to be the more boring
elliptical type without arms or spiral structure. The spiral
arms form when galactic density waves occur to align the galaxy
into arms. One contributor to the density waves affect may be
when galaxies interact with each other. NGC 772 multiple
companions including elliptical galaxies NGC 770 and PGC 212884
as well as irregular galaxy PGC 1577957 visible here.
Annotations. The above image shows annotations when hovering
over the image. Starting with the lower left image at about 7
o’clock and progressing clockwise, PGC 1577957 is an irregular
elliptical galaxy with an unrelated foreground star near its
center bulge. Visible at 9 o’clock is the enlargement of the
galactic nucleus containing a supermassive black hole of
millions of suns compressed into an infinitesimally small point.
Next at about 11 o’clock is a galaxy with no major catalog lable
and is likely in the far distant at hundreds of millions of
light years away. Following that at 1 o’clock and 3 o’clock are
galaxies PGC 212884 and NGC 770 responsible for some of the
disturbances to NGC 772. Visually near PGC 212884 is a much more
distant spiral galaxy without catalog designation. Finally at
about 5 o’clock, looking through the billions of disturbed stars
of NGC 772 are two very distant galaxies including PGC 1577099
(upper right in inset). Between these two galaxies even further
away (a billion light years distant) are 4 points of light that
are actually 4 additional entire galaxies each with 100 billion
or more stars. The universe is incomprehensibly vast!
