The Caldwell Objects

36
NGC 4559
Type: Mixed Spiral Galaxy (SABcd)
Con: Coma Berenices
RA: 12 h 36.0 m
Dec: +27° 58'
Mag: 10.0
Dim: 11.3' x 5.0'
SB: 14.3
Dist: 32 million light-years
Disc: William Herschel, 1785
W. H ERSCHEL: [Observed 11 April
1785] Very bright, very large, much
extended from north preceding to
south following [from northwest to
southeast] 10' or 12' long. 4 stars in it.
(Η I-92)
GC/ NGC: Very bright, very large, much extended toward
position angle 150°, gradually brighter in the middle, 3 stars
following [to the east of the object].
IN COMA BERENICES MORE THAN A DOZEN GAL-axies
shine at 10th magnitude or brighter. Among
these are the famous Black Eye Galaxy, M64, and
the constellation's flagship spiral, NGC 4565
(Caldwell 38), a textbook edge-on system. Just 2°
due north of NGC 4565 (and perhaps overlooked
because of its proximity to it) lies NGC 4559, a
remarkable example of a spiral system in an
intermediate morphological class.
The galaxy classification scheme developed
by Edwin P Hubble . in 1925 is simple enough.
First it divides galaxies into three major types:
ellipticals (E); unbarred spirals (S); and barred spirals
(SB). Hubble's famous "tuning fork" diagram,
which illustrated his galaxy types, also included
a then-hypothetical transitional form of galaxy,
which he dubbed simply "S0." S0, or lenticular,
galaxies (since discov-
The Caldwell Objects
36
ered) are intermediate in form between ellipticals
and spirals. They are lens-shaped spheroids
embedded in flattened disks that lack spiral
structure. The unbarred and barred spiral families
each branch off of the lenticular SO form on
Hubble's diagram. Each branch is then subdivided
largely according to the size of the
galaxy's central bulge and the tightness of its
spiral arms; both parameters are described by
lowercase letters that range from "a" to "d". An
"Sa" or "SBa" spiral has the largest nuclear region
and most tightly wound arms, while an "Sd" or
"SBd" spiral has the smallest nucleus and most
loosely wound spiral arms.
So where does NGC 4559 fit in? According
to the so-called Revised Morphological Galaxy
Classification System, an elaboration of Hubble's
original scheme, the galaxy is a mixed SAB (rs)
cd.
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