Download 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]
plays, the distant ringed world Uranus was last visited in 1986 by the Voyager 2 spacecraft.
Astronomy Picture of the Day
Tomorrow’s picture: x-ray galaxy
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured,
along with a brief explanation written by a professional astronomer.
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2003 January 15
Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
NASA Technical Rep.: Jay Norris. Specific rights apply.
A service of: LHEA at NASA / GSFC
& NASA SEU Edu. Forum
& Michigan Tech. U.
Ringed Planet Uranus
Credit: E. Lellouch, T. Encrenaz (Obs. Paris), J. Cuby , A. Jaunsen (ESO-Chile), VLT Antu, ESO
Explanation: Yes it does look like Saturn, but Saturn is only one of four giant ringed planets in our
Solar System. And while Saturn has the brightest rings, this system of rings and moons actually belongs
to planet Uranus, imaged here in near-infrared light by the Antu telescope at the ESO Paranal
Observatory in Chile. Since gas giant Uranus’ methane-laced atmosphere absorbs sunlight at
near-infrared wavelengths the planet appears substantially darkened, improving the contrast between the
otherwise relatively bright planet and the normally faint rings. In fact, the narrow Uranian rings are all
but impossible to see in visible light with earthbound telescopes and were discovered only in 1977 as
careful astronomers noticed the then unknown rings blocking light from background stars. The rings are
thought to be younger than 100 million years and may be formed of debris from the collision of a small
moon with a passing comet or asteroid-like object. With moons named for characters in Shakespeare’s
The Sun as a Star
• The matter in the interior is a plasma
• The sizes of the particles (nuclei ∼ 10−13 )
• The plasma in the Sun behaves as a perfect
gas: the total pressure P = nkT , where
n = ρ/m is the number density
• The central mass density ρs is about 110
times its average value 1.44 g cm−3
• The central number density
ρs /mproton = 1 × 1026 cm−3
• The Sun’s temperature
T = 6 × 103 − 1.5 × 107 K
• The source of energy of the Sun →
thermonuclear reactions
2
The Sun as a Star
3
• Earth orbits the Sun approximately in a
circle with radius r = 1.5 × 1013 cm (1 AU)
and period P = 1 year = 3.6 × 107 s. Hence,
the speed v of the Earth’s orbital motion
2πr
v=
= 2.98 × 106 cm/s ≈ 30 km/s
P
• We may set F = ma in the form where a is
the centripetal acceleration (magnitude
v 2 /r) and F is the gravitational force
(magnitude = GM⊙ m/r) to obtain
rv 2
= 2 × 1033 g
(1)
M⊙ =
G
• The total energy
E = mv 2 /2 + (−GM⊙ m/r) of the Earth’s
orbital motion is equal to one-half of its
potential energy (−GM⊙ m/r)
4
Gravitational (Jeans) Instability
• The two-phase model of interstellar medium
5
Gravitational Instability
2
2
2
0
0
0
−2
−2
−2
−2
0
t=0.0
2
−2
0
t=0.5
2
2
2
2
0
0
0
−2
−2
−2
−2
0
t=1.5
2
−2
0
t=2.0
2
−2
0
t=1.0
2
−2
0
t=2.5
2
• A star is held together by gravitation
1 ∂P
∂Φ
=
∂r
ρ ∂r
• Suggestion: Stars are formed by
gravitational instability
• Galaxies have also been formed by
gravitational condensation from a
pre-galactic medium
∂Φ
2
=
rΩ
∂r
6
Gravitational Instability: Example
• Cartwheel ring-type galaxy
7
Gravitational Instability: Example
• M83 spiral galaxy
8
Gravitational Instability: Example
• Local group of galaxies
9
Gravitational Instability: Example
• Solar system
10
Gravitational Instability: Example
• Extrasolar planetary system 55Cancri
11