Download Star Formation

Galaxy Formation
Universe was filled with gas initially composed only of H & He
Galaxy Formation
Gas flattens into a disk to form second generation of stars
Star Formation
Interstellar Medium (ISM): Gas & dust between stars
(100 – 200 atoms/cm3)
Composition
Gas: 90% H (neutral H, H+, H2)
6% He
4% “metals”
Dust: Silicate grains (rock/sand)
Graphite (Carbon)
Basic organic material
Star Formation
Nebula (Large cloud of ISM)
- low density (200 atoms/cm3)
- T ~ 20 K
- D ~ 150 L.Y.
- M ~ 104 – 106 MSuns
Great Orion Nebula
Within the nebula, tiny pockets of gas (globules) begin to form
Gravity causes:
- globule contraction
- material accumulation
- heating of central region
Rotation causes:
- Densest region to become spherical (proto-star)
- Outer gases cast off into proto-planetary disk
Proto-Sun & Proto-planetary Disk
Proto-planetary Disks in Orion
Condensation Sequence
Nebula material is uniformly spread through the proto-planetary disk
Different materials condense at different temperatures which leads to
rock/metal worlds close to Sun & gas giants further away
Solar Nebula Theory
Inner disk: metals/silicates (Terrestrial Planets)
Outer disk: ices and gases (Jovian Planets)
Habitable Zone: region around a star where H2O can exist as liquid
Accretion
Dust grains stick together through electricity to form planetesimals
Collisions between planetesimals melt surface which acts as a
“glue” causing them to grow larger
Largest planetesimals pull in material through gravity and
become proto-planets
Terrestrial Planet Formation
Terrestrial
- Interior heating causes differentiation; leads to layered interior
- Primitive H, He atmospheres heated away
- Out-gassing creates CO2 dominated atmosphere
- Water vapor washes out CO2; life forms oxygenate atmosphere
Jovian Planet Formation
Jovian
- Gases (H, He) and ices (NH3, CH4) accrete more quickly which
causes planets to grow more massive
- Once Sun ignites, solar wind sweeps gases and dust from Solar System
Formation of the Moon
Large Impact Theory
1) Mars-sized object strikes Earth early on in its history vaporizing crust
2) Debris settles into a disk that lies within the ecliptic plane
3) Material accretes to form the Moon
Explains Moon’s age, density, orbit, & lack of water
A STAR IS BORN!!!
(Once fusion reactions in the core begin & the star stabilizes)
Hydrostatic Equilibrium:
Equilibrium Inward Gravity = Outward Pressure
Revolutionized physics with publication of Principia Mathematica
• Principles of optics
• Development of calculus
• Laws of motion
• Law of gravity
• Derives Kepler’s Laws of Planetary Motion
Isaac Newton (Britain)
Laws of Motion
1st law of motion: object @ rest (in constant motion) will stay that way
unless an outside force acts on it (Law of Inertia)
2nd law of motion: Force on an object = mass x acceleration
F = ma
3rd law of motion: every force is countered by another that is equal in
magnitude and opposite in direction.
FA = - FB
Planetary Properties
Surface Gravity – strength of a planet’s gravitational pull at its surface
- depends on a planet’s size & mass
Earth’s surface gravity
9.8 m/s2 (1 g)
Moon’s surface gravity
1.6 m/s2 (1/6 g)
Weight = mass x surface gravity
W = mg
Planetary Properties
Escape Velocity – speed required to escape a planet’s gravitational pull
Earth’s escape velocity:
11 km/s
25,000 m.p.h.
Laws of Planetary Motion
Planet
Sun
Perihelion
Aphelion
Johannes Kepler (Germany)
1st: Planetary orbits are elliptical w/ the Sun @ one focus
Kepler’s First Law of Planetary Motion
Semi-major axis (a) – average distance between Sun and planet
c
a
Eccentricity (e = c/a) – describes the “flatness” of the ellipse
Kepler’s Second Law of Planetary Motion
A line connecting a planet to the Sun will “sweep out” equal areas in
equal time
1 month
1 month
1 month
Kepler’s Third Law of Planetary Motion
The orbital period (Porb) is related to the average distance (a)
according to the relation:
2
(Porb ) = (a)
Mercury : Porb = 0.241 years
(Porb ) 2 = (a) 3
(0.241) 2 = (a) 3
0.0581 = (a) 3
a =
3
0.0581 = 0.387 AU
3
Newton’s Law of Gravity
Force of gravity between two objects depends:
• Directly on the mass of each object
• Inversely on the square of the separation
GMm
F=
2
r
G ≡ Gravitational Constant
= 6.67 x 10-11
r = 1.5 x 1011 m
MSun = 2 x 1030 kg
MEarth = 6 x 1024 kg
(6.67 x 10 -11 ) (2 x 1030 ) (6 x 10 24 )
F =
(1.5 x 1011 ) 2
= 3.56 x 1022 N
Planetary Properties
Orbital Plane: “flat surface” that a planet’s orbit lies on
Earth’s orbital plane = “ecliptic plane”
Side View
Top View
Orbital Inclination (iorb) – angle a planet’s orbital plane makes with
respect to the ecliptic plane
Planetary Properties
Obliquity (ε) – angle a planet’s equator makes with respect to the
planet’s orbital plane
Earth: ε = 23.5º
Planetary Properties
Average density (ρ) – ratio of a planet’s mass to its Volume (m/Vol)
Material
Iron
Rock
Water
Oak
Air
Density (g/cm3)
7.86
2.0 – 5.5
1.0
0.6 – 0.9
0.0013
Albedo – the fraction (%) of sunlight reflected by a planet (0 – 1.0)
High albedo indicates: presence of an atmosphere
liquid/ice on surface of a planet
Planetary Properties
Equatorial Radius – a planet’s center-to-surface distance (size)
Oblateness – measure of the “flatness” of a planet
oblateness =
R equator - R pole
R equator
=
D equator - D pole
D equator
Planetary Comparison
Solar System planets are separated into two groups:
• Terrestrial (Earth-like) – Mercury, Venus, Earth, Mars
• Jovian (Jupiter-like) – Jupiter, Saturn, Uranus, Neptune
Property
Terrestrial
Jovian
Pluto
a (AU)
0.39 – 1.52
5.2 – 30.1
39.48
Porb (yrs)
0.24 – 1.88
11.9 – 163.7
248
e
0.007 – 0.21
0.009 – 0.054
0.25
iorb
0° - 7°
0.8° - 2.5°
17.1°
Req (Earth radii)
0.38 - 1
3.87 – 11.0
0.18
M (Earth masses)
0.055 – 1
14.5 - 318
0.0025
ρavg (g/cm3)
3.93 – 5.52
0.69 – 1.64
1.1
Prot (days)
1 – 243
0.41 – 0.73
6.39
Oblateness
0 – 0.005
0.03 – 0.11
~0
Albedo
0.11 – 0.65
0.41 – 0.52
0.3
Matter
Atoms (“indivisible”)
• 3 parts: protons, neutrons, electrons
(p+) (no) (e-)
nucleus
Atomic Number = # of protons in nucleus (defines element)
Each element can be found in different varieties (Isotopes)
Chemical Bonds
Two (or more) atoms can form new substances by bonding to each
other by sharing electrons (covalent) or exchanging electrons (ionic).
H
O
C
N
Molecule – two or more atoms chemically bonded
H2
O2
Compound – molecules made of two or more elements
H2O
NH3 (ammonia)
Organic Molecule – molecules that contain long carbon chains (life)
C4 H 8
Temperature
Measurement of the average energy of atoms within a substance
Low energy = Cold
High energy = Hot
Energy of the atoms or molecules is increased with the absorption
of light or by increasing pressure
States of Matter:
Matter will change states based on temperature (pressure)
The Nature of Light
Light can be described as a long continuous wave (classical) OR
as a packet of energy called a “photon” (quantum)
Continuous Wave
Photon
Wave Properties
Velocity (v) – speed at which the wave moves
Wavelength (λ) – distance over which a wave repeats
Photon Properties
Photon packet travels at c = 300,000 km/s
Photons are massless
Categorized according to wavelength or Energy
Reflection – the “bouncing” of light off of surfaces
Law of Reflection: Reflected angle = Incident angle
Refraction – the “bending” of light as it passes from one
medium to another