Download In the beginning… Astronomical Observations of Star Formation

In the beginning…

Today’s Lecture:
 Modern version of the creation story: how stars are born,
and how our solar system formed
EAS 302
1
Astronomical Observations of Star
Formation




Just as with humans,
birth and death of stars
is an “everyday”
everyday”
occurrence.
Giant Nebulae (e.g.,
Tarantula Nebula in the
Large Magellanic Cloud)
Cloud)
are the stellar nurseries.
Stars form by collapse of
parts of these great
clouds of gas and dust.
(These are also regions
of nucleosynthesis since
many of the stars born in
them are massive and
end their short lives as
supernovae).
EAS 302
2
1
Great Nebula in Orion
EAS 302
3
Star Birth in the Orion Nebula
EAS 302
4
2
“Proplyds” in Orion Nebula
EAS 302
5
Edge on View of a Proplyd
EAS 302
6
3
Formation of Our Solar System


Widely accepted “nebular” or “protoplanetary” theory of
solar system evolution has evolved from ideas of
Immanuel Kant (1724-1804) and Pierre Laplace (17491827).
Local instability in nebula causes gravitational collapse
 Shockwave from supernova might provide a trigger.


Calculations predict collapse into central sphere
Because of conservation of angular momentum, sphere
(proto-star) surrounded by disk.
 To conserve angular momentum, gas must spin faster as it
collapses.

Observations, combined with physics-based calculations,
yield following scenario of 4 stages
EAS 302
7
EAS 302
8
4
Formation of the central star and disk
EAS 302
9
Stage 1: Collapse of cloud



~ 106 yrs
Temperatures reach 1500K in inner solar system
heat due to gravitational collapse (conversion of
kinetic to thermal energy), but is short-lived.
EAS 302
10
5
Nebular Temperatures & Condensation
Strong radial
temperature gradient in
inner solar system
produces variation in
the amount and
composition of the dust
in the disk
Because of temperature
most elements are in
gas in inner most solar
system
In outer solar system,
most elements are in
dust (but most mass in
gas).
A key factor in planet
formation is
condensation of water
(ice) , which makes
grains sticky.




EAS 302
11
Stage 2: Disk begins to dissipate







Bipolar winds eject mass
from system
magnetic fields, asymmetry
pull matter in, angular
momentum out.
Disk begins to cool, dust
condenses.
Some material heated
during cycling close to star
Material is accreted from
disk to star
Disk begins to clear from
center outward
Young star becomes visible
in center of disk
EAS 302
12
6
Stage 3: T-Tauri: Terminal
Accumulation of Sun

surface of star is
now 4000 K
 Shines due to
gravitational heating





final stages of
accumulation of star
Dust is accumulating
into planetesimals
disk continues to
dissipate
Giant planets form
planetesimal
formation begins
EAS 302
13
Stage 4: Residual Static Nebula




Final dissipation of
nebula (photoevaporation may
help).
Terrestrial planets
accumulate from
planetesimals
Fusion ignites in
stellar cores and star
becomes ‘main
sequence’
Process has taken
~10 million years for
Sun-sized star (much
less for giant stars).
EAS 302
14
7
Summary

Solar system formed 4.56
b.y. ago by collapse of a
cloud of gas and dust
 Kinetic heating of inner solar
system.


Planets formed by oligarchic
growth, starting with micron
and mm-sized grains that
coalesced to form larger and
larger objects.
Isotopic anomalies support
the possibility that a
Supernova may have
triggered collapse of an
interstellar cloud to form the
solar system
EAS 302
15
Let’s Compare the theory with
observations
EAS 302
16
8
Observations from Meteorites

Primitive meteorites (chondrites)
 Presence of chondrules and refractory inclusions tell of at
least locally high temperatures at the distance of the
asteroid belt

Differentiated meteorites
 Pieces of planetesimals that melted and differentiated to
form iron cores and silicate mantles
 Ages of differentiated meteorites are within a few 10’s of
millions of years of primitive meteorites
 Therefore: planetesimal-sized bodies formed and
differentiated within a few 10’s of millions of years at most
EAS 302
17
Data on the Planets
Mass
(kg)
Sun
1.99!1030
Terrestrial Planets
Mercury
3.35 !1023
Venus
4.87 !1024
Earth
5.98 !1024
Moon
7.35 !1022
Mars
6.42 !1023
asteroids
4 !1021
Giant Planets
Jupiter
1.90 !1027
Saturn
5.69 !1026
Icy Planets
Uranus
8.73 !1025
Neptune
1.03 !1026
Pluto
2.88!1022
Anhydrous
Chondrit e s
Radial
distance
(AU)
Radius
(km)
6.96!105
Density 1 atm
(g/cc) density
(g/cc)
1.4
Atmospheric
components
1.6
2.44!103
6.05!103
6.38!103
1.74!103
3.39!103
2.8
!103
5.2
9.6
6.99!104
5.95!104
1.31
0.69
H, He
H, He
19.1
30.8
39.4
2.54!104
2.13!104
1.15!103
1.30
1.76
~2.0
3.4-3.9
H, He, CH4
H, He, CH4
—
0.39
0.72
1.0
5.42
5.24
5.52
3.3
3.93
5.3
3.95
4.03
3.4
3.7
—
CO2, N2, Ar
N2, O2, Ar
—
CO2, N2, Ar
EAS 302
18
9
Terrestrial Planets: Mercury, Venus,
Earth, and Mars



Rocky (silicate) outer parts
(crust and mantle) and inner
cores of Fe-Ni metal.
Silicates are compounds of
silica (SiO2) and the oxides of
other metals.
Common silicates:




Olivine: (Mg,Fe)SiO4
Pyroxene: (Mg,Ca,Fe)2 Si2O6
feldspar (e.g., (Na,K)AlSi3O8)
mica (e.g., biotite
K(Mg,Fe)3 AlSi3 O 10(OH)).
EAS 302
19
The Giant Planets: Jupiter and Saturn


Like the Sun, they consist primarily of H and He.
Thus they are approximately “solar” in
composition (though not exactly).
Small rocky core overlain by metallic H layer,
overlain by molecular H, He layer (gradual
transitions from liquid to gas).
EAS 302
20
10
The Icy Planets: Uranus and Neptune


These too consist of largely of H and He, but
they are enriched in C and N compared to
Jupiter and Saturn.
Their solid parts consist of ices of methane and
nitrogen.
EAS 302
21
So What?

Pattern consistent with nebular model of
solar system formation:
 the inner planets are extremely volatile-depleted
compared to the solar composition.
 The giant planets are only slightly depleted in the
most volatile components (H and He) compared
to the Sun
 The outer planets, while distinctly depleted in H
and He relative to the Sun, are nevertheless very
rich in volatiles (C and N) compared to the Earth.
EAS 302
22
11
Conclusions from this:

The giant planets must have formed early, before the gas
of the nebula dissipated.
 Position of Jupiter at the “snow line critical”:

Condensation of water ice makes particles sticky, which greatly
speeds accretion into larger bodies.

Icy planets may form more slowly because of lower
densities in outer part of nebula

Process of oligarchic growth in inner solar system begins
while inner solar system is still hot.
 Partial nebular dissipation results in H and He depletion
 The terrestrial planets are depleted not only in gaseous elements
such as H, He, C, and N, but in “moderately volatile” elements as
well. These include the alkalis (Na, K, Rb, Cs) and elements
such as sulfur, lead, and indium. Planetesimal accretion took
place before these elements could condense.
 Final assembly of the terrestrial planets took longer and was not
complete until gas had cleared from inner solar system.
EAS 302
23
Formation of the Terrestrial Planets

Idea is that planets
formed by “Oligarchic
Growth”, i.e.,
progressive accretion.
 Collisions between
bodies forms
progressively larger
bodies.


Process is initially fast,
but eventually slows
when there are only a
few bodies left in each
orbital zone.
Final stage of planetary
accretion would involve
collisions between large
bodies.
EAS 302
24
12
Lunar constraints on planet formation


Moon is the only other planetary body we
have directly sampled and explored
Moon and Earth and closely related, both
physically and chemically
 Therefore, they probably have a shared history

‘Hadean’ record preserved on the Moon
 Oldest rocks from the Moon are 4.45 Ga
 Record of the ‘early Earth’ (Hadean period) is
missing, destroyed by subsequent events

Oldest terrestrial mineral grain is 4.4 Ga
EAS 302
25
The Moon: some key observations






No other planet has such a large moon relative to its size (except
Pluto).
Moon has only a very small iron core
Moon has a bulk density about the same as the Earth’s mantle
(suggests compositional similarity).
Highly depleted in highly volatile elements (gaseous elements);
depleted in moderately volatile elements.
Has identical oxygen isotope composition to the Earth
Bottom line: Earth and Moon are both similar and different.
EAS 302
26
13