Sky and Telescope - March 2018 - 33

of Beta (β) Cassiopeiae, it's visible as a soft 10th-magnitude
glow. Moderately large-sized imaging setups will not only
resolve the H II regions but will also allow for imaging of the
brightest stars in the galaxy.

SUPERWINDS: N ASA / ESA / HUBBLE HERITAG E TE A M (STSCI / AUR A) / A . E VA NS (UNIV. OF VIRGINIA ,
CH A RLOT TESVILLE / NR AO / STON Y BROOK UNIV ERSIT Y )

Cosmic Menagerie: BCDs, LIRGs & ULIRGs
So far, the galaxies in this survey have been rather typical
starburst galaxies. But there are far more exotic examples in
the cosmos . . . if you know where to look! Let's start with
the blue compact dwarf (BCD) class: These are typically small
irregular or lenticular galaxies that contain large clusters of
young, massive stars, often responsible for shifting the galaxies' appearance toward a bluish color. Somewhat similar are
the Wolf-Rayet galaxies, which instead have unusually large
populations of Wolf-Rayet stars. These rare superluminous
blue stars are burning through a short-lived evolutionary
phase characterized by a distinct spectral signature that's
often useful in determining the rate of star formation.
The BCD best known to amateurs is NGC 2537, the "Bear
Paw" galaxy in Lynx. This is a small, unusual-looking galaxy
with numerous bright knots and H II regions that give rise
to the "bear paw" nickname. Though it does share its ﬁeld
of view with a super-thin edge-on galaxy (IC 2233), the two
aren't physically interacting, so the origin of the star-forming
activity isn't exactly clear. Like many of these galaxies, the
"Bear Paw" has an Arp Atlas of Peculiar Galaxies designation,
Arp 6, due to its unusual shape. Visually, this object lives up
to its name, as it actually does look like an animal paw in
the eyepiece. My notes using a 17.5-inch telescope describe it
as a "round glow, about 1.5′ across with an outer horseshoeshaped ring of knots and a faint knotty inner bar."
As galaxies go, Hen 2-10 (ESO 495-21) has a curious
history. Located deep in the dense star ﬁelds of Pyxis, even
at 12th magnitude it was missed by many observers. In the
1960s, Karl Henize ﬁnally cataloged it as a planetary nebula
due to its strong emission lines. The nature of this source was
readdressed in the 1970s, and today it's classiﬁed as a WolfRayet galaxy. Though situated in a dense Milky Way star ﬁeld,
it's not too hard to ﬁnd: You can start by locating Gamma (γ)
Pyxidis and star-hopping to Eta (η) Pyxidis. From there it's a
short 24′ hop to the southwest. You'll see a small, roundish,
12th-magnitude glow around 1.5′ across that hardly dims
with a nebula ﬁlter - try it out!
Most of the galaxies in this survey have been small- to
medium-sized systems, in direct contrast to the last two
classes we will visit here: luminous infrared galaxies (LIRGs)
and ultra-luminous infrared galaxies (ULIRGs). Both are
extreme examples of starburst galaxies, with the former being
some one hundred billion times and the latter one trillion
times brighter than our Sun in the infrared. One of the clos-

p A STARBURST GALAXY FAR, FAR AWAY If you want to witness
galactic cannibalism firsthand, look in the direction of Ophiuchus toward
NGC 6240, a ULIRG around 350 million light-years away. Not only are
two galaxies merging in this system, triggering starburst activity, but their
central supermassive black holes are also spiraling toward each other.
The black holes are a mere 3,000 light-years apart and will soon (on
cosmic timescales) coalesce into one gargantuan black hole.

est LIRGs is the lenticular galaxy NGC 5010 in Virgo. It's in
the process of evolving from a spiral into an elliptical, with
most of its luminosity derived from old, red stars. Located
around 25′ north of 53 Virginis, it's a small, 13th-magnitude,
spindle-shaped glow with a dusty/mottled core.
Much more extreme is NGC 6240 in Ophiuchus, a massive ULIRG. Here, the process of galactic cannibalism is well
underway, as two large galaxies merge into one immense system. Huge tidal tails, dusty gas plumes, and vast star-forming
regions are evident in images of this system. Its unusual
shape earned it the nickname "Rumpled Starﬁsh," though to
me it resembles a lobster. Indeed, when viewed with a 20-inch
scope at 175×, some of the diffuse plume structure visible is
suggestive of the appendages of some description of cosmic
crustacean or starﬁsh. I'll let readers be the judge of this.

¢ RICHARD JAKIEL, a dedicated deep-sky observer, has contributed to Sky & Telescope since 1999.

FURTHER READING: To learn more about interacting galaxies, specifically the Antennae and NGC 6946, see Steve Gottlieb's articles
"Galaxies in Collision" (S&T: May 2017, p. 28) and "Unraveling NGC 6946" (S&T: July 2013, p. 60). See Howard Banich's article "Two
Cool Galaxies" (S&T: May 2016, p. 18) to read more about his sketches of M82.
sk yandtelescope.com * M A RCH 2 018

http://www.skyandtelescope.com

Sky and Telescope - March 2018

Table of Contents for the Digital Edition of Sky and Telescope - March 2018