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]
Astronomy Picture of the Day
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.
2004 January 17
Saturn: Lord of the Rings
Credit: Hubble Heritage Team, (AURA / STScI), R.G. French (Wellesley College), J. Cuzzi
(NASA/Ames), L. Dones (SwRI), J. Lissauer (NASA/Ames)
Explanation: Born in 1564, Galileo used a telescope to explore the Solar System. In 1610, he became
the first to be amazed by Saturn’s rings, After nearly 400 years, Saturn’s magnificent rings still offer one
of the most stunning astronomical sights. Uniquely bright compared to the rings of the other gas giants,
Saturn’s ring system is around 250,000 kilometers wide but in places only a few tens of meters thick.
Modern astronomers believe the rings are perhaps only a hundred million years young. Accumulating
dust and dynamically interacting with Saturn’s moons, the rings may eventually darken and sag toward
the gas giant, losing their lustre over the next few hundred million years. Since Galileo, astronomers
have subjected the entrancing rings to intense scrutiny to unlock their secrets. On December 31, 2003,
Saturn made its closest approach to Earth for the next 29 years, a mere 1,200,000,000 kilometers. It will
remain a tantalizing target for earthbound telescopes in the coming months.
Tomorrow’s picture: Closest Planet Yet
Astronomy
• “astron” ≡ star
“nomos” ≡ law
• Total number of galaxies ∼ 1010
• Number of stars in each one ∼ 1011
• Space is almost empty!
• Gas and dust ∼ 10%
• Sun - ordinary star. Temperature ∼ 6 000◦ K
• Solar system: 8 (eight!) planets,
∼ 100 000 000 asteroids, meteoroids
• Jupiter is the largest planet ∼ 0.1%M⊙
• ∼ 300 extrasolar planetary systems
• Origin of stars and planets →
Gravitation Attraction
2
Astronomy
• Earth
3
Astronomy
• Mars
4
Astronomy
Astronomy Picture of the Day
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.
2004 January 17
Saturn: Lord of the Rings
Credit: Hubble Heritage Team, (AURA / STScI), R.G. French (Wellesley College), J. Cuzzi
(NASA/Ames), L. Dones (SwRI), J. Lissauer (NASA/Ames)
Explanation: Born in 1564, Galileo used a telescope to explore the Solar System. In 1610, he became
the first to be amazed by Saturn’s rings, After nearly 400 years, Saturn’s magnificent rings still offer one
of the most stunning astronomical sights. Uniquely bright compared to the rings of the other gas giants,
Saturn’s ring system is around 250,000 kilometers wide but in places only a few tens of meters thick.
Modern astronomers believe the rings are perhaps only a hundred million years young. Accumulating
dust and dynamically interacting with Saturn’s moons, the rings may eventually darken and sag toward
the gas giant, losing their lustre over the next few hundred million years. Since Galileo, astronomers
have subjected the entrancing rings to intense scrutiny to unlock their secrets. On December 31, 2003,
Saturn made its closest approach to Earth for the next 29 years, a mere 1,200,000,000 kilometers. It will
remain a tantalizing target for earthbound telescopes in the coming months.
Tomorrow’s picture: Closest Planet Yet
• Saturn, its rings and moons
5
Astronomy
• Uranus, its rings and moons
6
Astronomy
• Solar system
7
Astronomy
• Extrasolar planetary system 55Cancri
8
Astronomy
• Spiral galaxy M83
9
Basic Law
• Newton’s law of gravity
mA · mB
d2
where G = 6.7 × 10−8 cm3 g−1 s−2
F =G
• Orbital velocity
10
(1)
Gravitation Attraction
• |F1 | = |F2 | even though m1 6= m2
• But F1 = −F2
11
Electric and Gravitational Forces – Similarities
• Coulomb’s force:
F12
•
q1 q2
= 2
r
Felectric
ee/r 2
e2
42
=
=
4.17
×
10
=
Fgravity
Gme me /r 2
Gm2e
• Both forces are long-range ∝ 1/r 2 forces
• We will use this similarity in our study
12
(2)
(3)
Exponent Notation
• 10n = 1000 · · · 000 → n zeroes
• 10−n = 1/10n = .000 · · · 000 → n − 1
• multiplication: 10n × 10m = 10n+m
10n /10m = 10n × 10−m = 10n−m
• nano = 10−9 (one-billionth)
• micro = 10−6 (one-millionth)
• mili = 10−3 (one-thousandth)
• centi = 10−2 (one-hundredth)
• kilo = 103 (one thousand)
• mega = 106 (one million)
• giga = 109 (one billion)
• Example: 1 kilometer = 103 m = 105 cm
13
The CGS Units and Rough Scales
• unit of length = centimeter = cm
• unit of mass = gram = g
• unit of time = second = s
• unit of force = dyne = g cm s−2
• unit of energy = erg = g cm2 s−2
• unit of power (luminosity) = erg s−1
• child → S ≈ 102 cm M ≈ 104 g A ≈ 108 s
• Sun → S ≈ 1011 cm M ≈ 1033 g
A ≈ 1017 s
• Galaxy → S ∼ 1023 cm M ∼ 1045 g
A & 1017 s
14
The CGS Units and Rough Scales
• Astronomical unit (AU) ≈ 1.5 × 1013 cm
• Year ≈ 3.2 × 107 ≈ 3 × 107 s
• Mass of electron me ≈ 9 × 10−28 g
• Solar mass M⊙ ≈ 2 × 1033 g
• Solar radius R⊙ ≈ 7 × 1010 cm
• Solar luminosity L⊙ ≈ 4 × 1033 erg s−1
• Mass of Earth M⊕ ≈ 6 × 1027 g
• Radius of Earth R⊕ ≈ 6.4 × 108 cm
• Speed of light c ≈ 3 × 1010 cm/s
• Gravitational constant
G ≈ 6.67 × 10−8 g−1 cm3 s−2
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Classical Mechanics, Light, Telescopes
• Mass m
• Inertial frame
• Newton’s laws: 1. When F = 0,
p = mv = const
• Newton’s laws: 2. When F 6= 0,
usually, F = m dp
dt = ma
dp
dt
= F,
• Newton’s laws: 3. F1 = −F2
• a 6= 0 even if v = |v| = const but changes
direction
2
• Gravity force Fattractive = −G m1r·m
r
3
16
Circle and Sphere
• Circle
= 360◦ = (360 × 60)′ = (360 × 60 × 60)′′
• Moon → 30′
• Circle l = 2πr cm, where π = 3.1416
• Sphere V = 43 πr3 cm3
• Sphere S = 4πr2 cm2
• Transfer Length → Angle: Circle
≡ 360 × 60 × 60 = 1 296 000 ≈ (1.3 × 106 )′′
• Volume density ρ =
m
V
g cm−3
• Surface density Σ =
m
S
g cm−2
• Number density n =
N
V
cm−3
17
Temperature
• T ◦ K = T ◦ C + 273◦
• Temperature minimum −273◦ C = 0◦ K
18