Mathematical Principles of Theoretical Physics

7.4. GALAXIES 443

Table 7.2: Galaxy distribution

Types E S 0 S+SB Irr Undetermined

% 13.0 21.5 61.1 3.1 0.9

radius andbis the minor radius. TheE 0 -type of elliptical galaxies are spherical in shape with
e 0 =0, and theE 7 -types are of the most ablate appearance, i.e. the apparent ellipticityekof
Ekis arranged in order
e 0 <e 1 <···<e 7.

TheS-spirals form a sequence of three types:Sa,Sb,Sc. TheSahave large galaxy nuclei
and tightly wound arms, theSbhave moderate galaxy nuclei and less tightly wound arms,
and theSchave the smallest nuclear bulges and the least wound arms. Our galaxy (the Milky
Way) is typeSb.
About one third of all spiral galaxies are theSB-type. They also consist of three types:
SBa,SBb,SBc, according to the size of the galaxy nuclei and tightness of the spiral arms,
exactly as inS-spiral sequence.
The galaxy classification given above was introduced by E. Hubble, who arranged the
galaxies in an orderly diagram as shown in Figure7.5.

E 0 E^4 E^7 SO

Sa Sb Sc

SBa SBb SBc

Figure 7.5: Hubble classification of galaxies, the ellipticals arranged on the left, and the
spirals arranged on the right in two parallel sequences.

TheS0 (S-zero) galaxies have the characteristicsof ellipticals and spirals. They are disk
shaped, like spirals, but lack spiral structure, and therefore like flat ellipticals.
The amount of interstellar gas, also called nebula, in galaxies increases from left to right
in the diagram given by Figure7.2. In ellipticals the amount of gas is very small, and it is
also small inSOgalaxies. However, the nebula increases progressively in the spirals from
SaandSBatoScandSBc. in addition, another important distinction between ellipticals and
spirals is that clusters of galaxies contain 80% elliptics,and outside clusters, 80% of galaxies
are spirals.

3.Galaxy properties.Various properties of galaxies are listed in Tables7.2-7.4.

4.Active galaxies.According to their active extent, galaxies are divided intotwo types:
normal galaxies and active galaxies. Active galaxies mainly include

Starbursts, Seyferts, Quasars, Radio-galaxies