Mathematical Principles of Theoretical Physics

442 CHAPTER 7. ASTROPHYSICS AND COSMOLOGY

Table 7.1: Levels of Cosmic Structure

levels Sun

star clusters

(globular)

galaxies

(spiral)

clusters of

galaxies

Universe

radius(pc) 10 −^8101051061010

distance(pc) 1 103 106 108

mass(M⊙) 1 106 1011 1014 1021

density

(kg/m^3 )

103 10 −^1810 −^2010 −^2310 −^27

For the earth,M= 6 × 1024 kg,R= 6. 4 × 106 m. Thus we have

Meb= 1024 kg, T= 3. 3 × 104.

5.The radii of the solar and earth’s black hole cores.The radius of solar black hole is
given by
R⊙s=500 m,

and the radius of black hole of the earth is as

Res=

3

2

cm.

7.4 Galaxies

7.4.1 Introduction

Galaxy is an elementary unit in the large scale structure of our Universe, which is composed
of star clusters, stars, gases and dusts. In the following weintroduce this topic.

1.Levels of cosmical structure.There are mainly five levels of cosmic structure: stars,
star clusters, galaxies, clusters of galaxies, and the Universe, and their basic scales are given
inTable7.1.

2.Galaxy types.Galaxies consist of the following three types:

Elliptical (E), Spiral (S), irregular (Irr).

The spiral galaxies are divided into two different sequences:

normal spirals,denoted byS,

barred spirals,denoted bySB.

Elliptical galaxies have an oval appearance, which are classified into eight grades, denoted
byE 0 ,E 1 ,···,E 7 , according to their apparent ellipticitye= (a−b)/a, whereais the major