Astronomy

r-shaped

Ellipticals Lenticulars Spirals Irregulars

s-shaped

Sa Sb Sc Sd

A Normal

B Barred

ES

s-shaped r-shaped

A

Normal

B

Barred

Sm I

WWW.ASTRONOMY.COM 15

THE DE VAUCOULEURS SYSTEM. Galaxies come in a variety

of shapes and sizes, but if you look at enough of them, you start

to pick out visual consistencies. Galaxy classifications are largely

based on their morphologies, or shapes, and galaxies are still

typically classified by eye (rather than with a computer algorithm)

because of the human brain’s exceptional ability to pick out

shapes and patterns. Following in the footsteps of Edwin Hubble,

Gerard Henri de Vaucouleurs developed a “lemon-shaped” galaxy

classification system in 1959 that is still used today. Each slice of

the lemon represents a different morphological class of galaxy.

The lemon’s width is related to the number of galaxies with that

classification, with the widest part of the lemon reflecting the

most common types of galaxies. ILLUSTRATION BY KELLIE JAEGER

THE MILKY WAY

is a barred spiral,

classified SAB(rs)bc.

ASTRONOMY

: ROEN KELLY

NGC 6822

is an irregular

IBm-type dwarf

galaxy with no

clear structure.

NOAO/LGGST

M101 is classified

SAB(rs)cd for its well-

organized spiral arms.

CFHT/J.-C. CUILLANDRE/COELUM

M33, a face-on

spiral classified

as SA(s)cd.

CFHT/J.-C. CUILLANDRE/COELUM

M87 is a round

E0-1 elliptical

galaxy, also

classified as

peculiar (“pec”)

because of a jet

from its super-

massive black

hole.

M66 is an s-shaped

SAB(s)b spiral galaxy.

CFHT/J.-C. CUILLANDRE/COELUM

CFHT/J.-C. CUILLANDRE/COELUM

NGC 4622 is

an SA(r)ab spiral.

NGC 3115, an S0-type

lenticular galaxy with a

flattened, featureless

disk of old stars.

NASA/HUBBLE HERITAGE TEAM

NOAO/AURA/NSF