AN INTRODUCTION TO ASTRONOMY Dr. Uri Griv Department of Physics, Ben-Gurion University

BEN-GURION UNIVERSITY
AN INTRODUCTION TO ASTRONOMY
Dr. Uri Griv
Department of Physics, Ben-Gurion University
Tel.: 08-6428226 Email: [email protected]
Accretion Disks
• Accretion disks: mass transfer from a
lobe-filling star toward a compact
companion (white dwarf, neutron star, or
black hole)
2
Accretion Disks
• Schematic diagram of accretion disk
3
Star Clusters
• Open clusters: ∼ 103 stars; total number of
open clusters in the Galaxy N ∼ 104
• Globular clusters: ∼ 106 stars; total number
of globular clusters in the Galaxy N ∼ 103
4
Star Clusters
• Open cluster M 67 = NGC 2682
5
Globular Clusters in Our Galaxy
6
Dynamics of Star Clusters
• The stars move freely past one another
without direct collisions
• A cluster is made of a discrete number of
stars → random velocities → no rotation
• The thermodynamic Maxwell–Boltzmann
(MB) distribution of velocities
7
Dynamics of Star Clusters
Number of stars
1
0.8
0.6
0.4
0.2
0
−1
−0.5
0
Velocity
0.5
1
• The velocity dispersion V ≈ (3kT /m)1/2
or in q
a stellar system
PN
V = (1/N ) i=1 (vi − v̄)2 where
PN
v̄ = (1/N ) i=1 vi is the mean velocity
and N is the total number of stars
• A true MB distribution has a “tail”
• Stars will be lost from the cluster
• The escape speed ve = 2V
8
Relaxation Time
• The stars move freely past one another
without direct collisions
• The relaxation time trelax is the time to
establish the MB distribution
• Long-range ∝ 1/r2 gravitational force
• Elementary Theory: trelax ≈
9
V3
4πG2 m2 n
Open Clusters – Stellar Orbits
• No regular cluster rotation !
10
Galaxies
• Hubble “fork” diagram
• Elliptical galaxies
• S0 galaxies
• Spiral galaxies (normal and barred ones)
• Irregular galaxies
11
Spiral Galaxies
12
Spiral Galaxies
• The spiral galaxy M 81
13
Elliptical Galaxies
14