Growth of a Protostar
• Matter from the
cloud continues to
fall onto the
protostar until
either the
protostar or a
neighboring star
blows the
surrounding gas
away
How did the Solar System come to
be ?
The hot, inner region
At about 5 AU
from the Sun,
water freezes
The colder, outer region
ASTEROIDS
collisions
fragmentation
METEORITES
Irons
Stony-Irons
Stones
Primitive
perturbations
MERCURY
Condensation of
nebular material
Rock chunks
only
TERRESTRIAL
TERRESTRIAL
PLANETS
PLANETS
accretion
(100 M-yrs)
surface evolution
outgassing
EARTH / MOON
MARS
inner disk
(hot)
k
VENUS
outer disk
(cold)
Rock & Ice
chunks
accretion
(20 M-yrs)
JUPITER
JOVIAN
JOVIAN
cooling
PLANETS
PLANETS
SATURN
URANUS
NEPTUNE
perturbations
in central
region
orbits unaffected
KBO’s
KUIPER BELT
KUIPER
BELT
OORT
CLOUD
OORT CLOUD
COMETS
orbits perturbed, come
within 3 AU of Sun
20 M-yrs
Slow brightening
Solar wind starts,
gases removed
PLUTO
METEORS
The SUN
Star Formation in our Galaxy
• Throughout the Milky Way Galaxy we find
Molecular Clouds where new stars are
forming.
• Many of these stars may have planets.
• Giant Molecular Clouds are also seen in
many other galaxies.
Molecular clouds
in which star
formation is in
progress.
“Pillars of
Creation”
or
“The Fingers of
God”
The Orion Nebula a stellar nursery
in the
Milky Way Galaxy,
1500 light years
from the
Solar System
This is a giant
molecular cloud
Evidence from the
Solar System
•
The nebular
theory of solar
system formation
illustrates the
importance of
rotation
A newly forming star in its dusty cloud
Jets are
observed
coming from
the centers of
disks around
protostars
The disk of dust around a Sun-like star – Beta Pictoris
The disk of dust and gas around a young star
(The star has been blocked out to show the faint disk)
This is the star HD 141569A,
about 320 light years away.
There is a gap between the
star and the inner edge of
the disk. This gap may have
been swept out by planets,
although the planets cannot
be seen in this image.
The presence of an orbiting planet causes
the star to move (“wobble”) as
seen from Earth.
The motion is detected by the Doppler shift
of the spectral lines.
Fig 20-16, p.454
The periodic shifting of the spectral lines reveals the
presence of an orbiting planet, and tells us its orbital period
Away
Toward
Another case where a periodic wobble in the
star’s motion reveals the presence of a planet.
Table 20-2, p.457
Limitations to the Doppler technique
• The technique is not sufficiently accurate to
detect Earth-size planets
• Planets detected so far are large -- at least as
large as Neptune, and many are larger than
Jupiter
• It is hard to detect multiple planet systems
• It is hard to detect planets with long orbital
periods (e.g., Jupiter’s period is 12 years)
As seen from a great distance, the Sun wobbles
because of the gravity effects of the planets.
Which planets have the greatest effect on the
Sun’s wobble ?
A map of the Sun’s
changing position,
as seen from outside
the Solar System
Another method by which planets can be
detected -- transits
As seen from Earth, a planet
may cross in front of the star.
This event is called a transit.
The planet’s orbit must line
up exactly with our line of
sight. This is fairly rare.
If a transit occurs, the light from the star is slightly
dimmed for a short time, as the planet covers
a part of the star.
Although the planet cannot be seen directly, it is
revealed by the slight dimming of the starlight.
The Habitable Zone around the Sun.
What is meant by “habitable zone” ?
As the Sun evolves and its temperature changes ,
the habitable zone of the Solar System will change
How does nuclear fusion
begin in a newborn star?
From Protostar to Main
Sequence
• Contraction must continue until the core
becomes hot enough for nuclear fusion —
the star then becomes a main-sequence
star
Assembly of a Protostar
•
Luminosity and temperature grow as
matter collects into a protostar
Self-Sustaining Fusion
•
Core temperature continues to rise
until star arrives on the main sequence
Life Tracks for Different
Masses
• Models show that
Sun required about
30 million years to go
from protostar to
main sequence
• Higher-mass stars
form faster
• Lower-mass stars
form more slowly
What is the smallest mass a
newborn star can have?
Fusion and Contraction
• Fusion will not begin if contraction stops
before the core temperature rises above
107 K.
• Is there another form of pressure that can
stop contraction?
Thermal Pressure:
Depends on heat content
The main form of pressure
in most stars
Degeneracy Pressure:
Particles can’t be in same
state in same place
Doesn’t depend on heat
content
Brown Dwarfs
• Degeneracy pressure
halts the contraction
of objects with
<0.08MSun before
core temperature
become hot enough
for fusion
• Starlike objects not
massive enough to
start fusion are
brown dwarfs
Upper Limit on a Star’s Mass
• Models of stars
suggest that radiation
pressure limits how
massive a star can be
without blowing itself
apart
• Observations have
not found stars more
massive than about
150MSun
Luminosity
Stars more
massive
than
150MSun
would blow
apart
Temperature
Stars less
massive
than
0.08MSun
can’t
sustain
fusion
Demographics of Stars
• Observations of star clusters show that star formation
makes many more low-mass stars than high-mass stars