Download Ch. 14 Formation of Stars

The Formation of Stars
Chapter 14
Where Do the Stars Form?
• Most stars form within giant clouds of
molecular gas. These clouds typically
contain enough mass to form 100,000 suns
spread out over a diameter of 50 pc. Their
temperatures begin just a few degrees Kelvin
(very cold!).
• But with mass comes gravity. However,
there are 4 factors that gravity must
overcome in order to compress the gas into a
star.
The Four Factors
1. Thermal energy: molecules are still moving fast
enough (~800 mph) to drift apart even in the cold.
2. Interstellar magnetic field: though weak, this
magnetic field works like a spring trying to prevent
compression.
3. Cloud rotation: as the cloud begins to contract,
its rotational speed increases due to conservation
of angular momentum. Faster rotation makes it
harder to collapse.
4. Turbulence: “winds” in the ISM distort the clouds
and make them more difficult to contract.
Gravity Finally Wins!
• One way that gravity might win out is with the help
of a shock wave, such as from a supernova
explosion. This has the effect of destabilizing
regions of the cloud forming dense pockets
• Once the cloud begins collapsing, the core
temperature increases. Gravitational potential
energy is transformed into thermal energy.
• Some of these pockets become hot enough so that
they emit IR. This core, that now emits IR but has
not yet begun nuclear fusion, is called a protostar
(proto = “before”).
Stellar Nurseries
M 8 (left) and M 42 (right)
[note how the stars are clearer
in the right image in IR].
Stellar Nurseries
• A cocoon is the cloud of gas and dust
surrounding a contracting protostar and
blocks our view in visible light.
Note: this is an
artist’s concept.
But note the
rapidly-spinning
disk of dust and
gas around the hot
core!
Eagle Nebula Wide View
http://soccerdad.baltiblogs.com/archives/2007/01/25/nasas_recent_pictures_of_the_day.html
Stellar
nurseries are
located in
these pillars,
called the
“Pillars of
Creation.”
New stars are
most likely
located at the
tops of these
pillars, where
dust is most
dense.
Close ups of the
stellar “cocoons.”
More Examples of Cocoons
Birth Timelines
• Massive protostars can collapse more rapidly and
begin nuclear fusion sooner than less massive
ones. As protostars collapse their position on the
HR Diagram moves toward the main sequence line
(MS runs diagonal from upper left to lower right).
The exact path depends on mass.
• The sun took about 30 million years to reach main
sequence, but one 30 times more massive only
takes about 30 thousand years!
• Star formation
• http://ircamera.as.arizona.edu/NatSci102/NatSci
102/lectures/starform.htm -
Star Formation Paths on the HR
Diagram
Different
paths take
different
amounts of
time and
depend on the
starting mass.
Fusion Review
• Stars like the sun, having core temperatures of at
least 10 million K, undergo nuclear fusion via the
proton-proton chain. Essentially, this process
fuses individual protons (H) into helium (He) nuclei
and releases much energy (E = mc2).
• Heavier stars, with higher core temperatures (16
million K or above), often use another process. The
CNO cycle uses carbon (C), nitrogen (N), and
oxygen (O) as steppingstones in the process. The
outcome (production of He) is the same for both
processes, except in the amount of energy
released.
The Crossover Point between PP
and CNO
Structures of Stars
• Many stars have a structure similar to the sun,
which we have already studied in chapter 12.
• Core: At the center, where nuclear fusion takes
place. Must be at least 10 million K.
• Radiative Zone: Energy flow in the form of photons
that are absorbed and reemitted in random
directions. Each super high energy photon get
broken down into 2500 photons of visible light.
• Convective Zone: Begins where photons can no
longer penetrate the gas (cooler, more opaque) so
energy builds up, begins churning. Hotter gas rises
while cooler gas sinks.
Dance of the Forces
• Two forces must be balanced in order for a star to remain
stable: the inward pull of gravity and the outward push of
thermal pressure (because gases expand when heated).
• The gravity-pressure balance that supports the sun is a
fundamental part of stellar structure known as the law of
hydrostatic equilibrium (hydro = fluid, static = stable).
• A star can be thought of as having multiple layers, like an
onion, each of which must be in hydrostatic equilibrium.
• The interior of the sun must be very hot, in order to support
the great weight of the above layers.
What’s Mass Got to Do With It??
• It turns out that the model we used for the sun
applies only to stars between 0.4 and 1.1 times the
sun’s mass.
• Why? Mass largely determines core temperature,
which determines which nuclear process dominates
at the core!
• Main Sequence “Tweener” Stars: These stars
generate most of their energy via the proton-proton
chain, so the core is not as concentrated in dead
center.
• Ex. Sun: 50% of its energy comes from 11% of its
volume
• So, core  radiative zone  convective zone (little core
mixing)
Sun-like stars: radiative
zone inside of
convective zone.
More Massive Stars—Role
Reversal?
• Stars that are heavier than 1.1 solar masses have
core temperatures that are hot enough to fuse
much of their hydrogen using the CNO cycle,
which is extremely sensitive to temperature.
• Ex. 10-Msun stars generate 50% of their energy
from only 2% of their volume!
• Therefore, a “log jam” occurs as energy tries to
leave the center. Therefore, convection occurs
around the core instead of radiation!!
• So, core  convective zone  radiative zone
Comparing 3 Types of Stellar
Interiors
Mini Me Stars—Missing
Something?
• Stars that are lighter than 0.4 suns have relatively
cool interiors, which makes them more opaque to
photons.
• As a result, radiation cannot easily flow through any
part of them, so they only have convective zones.
No radiative zones.
• Core  convective zone
• “Stars” between 0.08 and 0.012 solar masses are
not really stars at all; called “brown dwarfs”
(failed stars). No nuclear fusion is ever sustained.
For comparison, Jupiter is ~0.001 solar masses.
Brown Dwarf Candidate Gliese
229b
Star Regulation
• How does the star maintain the right amount of
energy so that it neither expands or contracts too
much?
• If the star begins producing more energy, then it
expands. This results in a lower central
temperature and density and slows nuclear fusion
until the star regained stability.
• The same process works in reverse: if less energy
were generated, then the star would contract
slightly and cause central temperature and density
(and energy production) to increase. A stellar
thermostat!
Star Families
• Most emission nebulae are massive enough
to form clusters of stars that tend to
disperse over a few hundred million years.
• These dispersed stars become isolated
individuals or small groups (such as binaries)
after enough time.
• Our sun must have been a member of a
cluster at one time, but has since drifted
away. A good thing!