spiral pattern. The classification
system is as follows: an “S” for
“spiral,” is followed by an “a,” “b,”
“c,” or “d,” to indicate the tightness
of the spiral, with “a” being tightly
wound and “d” being loose. Varying
degrees of compactness are indicated
by additional letters (e.g., an Scd
galaxy is between an Sc and an Sd). A
barred spiral is indicated with an
“SB” followed by the corresponding a,
b, c, or d. 
 S0 galaxies are spirals that closely
resemble ellipticals; the spiral arms 
are so tightly wound around the
bulge, which itself is very large, that