Mathematical Principles of Theoretical Physics

444 CHAPTER 7. ASTROPHYSICS AND COSMOLOGY

Table 7.3: Average physical quantities

Types E Sa Sb Sc Irr

mass (M⊙) 2. 0 × 1011 1. 6 × 1011 1. 3 × 1011 1. 6 × 1010 1 × 109

mass/luminosity

(m⊙/L⊙) 20-70 6.6 3.6 1.4 0.9

density(M⊙/pc^3 ) 0.16 0.08 0.025 0.013 0.003

gas mass(%) ≤ 0. 2 1.3 3 20 40

Table 7.4: Properties of ellipticals and spirals

Properties ellipticals spirals

interstellarmatter non plentiful

young stars non yes

appearance elliptical shape disk shape

stellar motion random rotation

color red blue

Active galaxies are usually characterized by their extraordinary energy emision. The
starburst galaxies emit a large amount of infrared radiation, which is caused by a great number
of newly formed stars. But the three types of galaxies: Seyferts, Quasars and Radio-galaxies
are known as active galaxies because they possess a compact region at their centers that has
a much higher radiation than normal luminosity, which is called the active galactic nucleus
(AGN).
AGN generates and emits immense quantities of energy over a wide range of electromag-
netic wavelengths. Today, it is widely believed that AGN is an accretion disk generated by
a supermassive black hole. Though we have not completely confirmed evidence to show the
existence of black holes, many astronomical observations strongly suggest their existence.
The main features of active galaxies are listed in Table7.5.

5.The Milky Way.Our own galaxy is known as the Milky Way, which is ofSbtype and
has 10^11 stars and an enormous quantities of clouds of gases and dusts. Its radius is about 10^5
ly.
The Milky Way consists of two parts: the disk and the halo. Thedisk has the radius of
about 5× 105 ly and the thick of about 5× 104 ly. The disk is composed of stars and nebulae,

Table 7.5: Features of active galaxies

Galaxy Type Active nuclei strong radiation Jets

Normal No no no

Starburst No yes no

Seyfert yes yes yes

Quasar yes yes yes

Radio galaxy yes yes yes