Download PHYS3380_102615_bw

Homework Set #8
10/26/15
Due 11/2/15
Chapter 10
Review Questions
7, 9
Problems
3, 7
Chapter 11
Review Questions
3, 7
Problems
5, 9
Maximum Masses of Main-Sequence Stars
Mmax ~ 50 - 100 solar masses
a) More massive clouds fragment into
smaller pieces during star formation.
b) Very massive stars lose mass
in strong stellar winds
Eta Carinae
Example: Eta Carinae: Estimated to be over 100 Msun. Dramatic mass
loss; major eruption in 1843 created double lobes.
Minimum Mass of Main-Sequence Stars
Mmin = 0.08 Msun
Gliese 229B
At masses below 0.08
Msun, stellar progenitors
do not get hot enough to
ignite thermonuclear
fusion.
 Brown Dwarfs
The Life Cycle of Stars
Dense, dark
clouds,
possibly
forming stars
in the future
Aging
supergiant
Young stars, still
in their birth
nebulae
Stars are produced in dense nebulae in which much of
the hydrogen is in the molecular (H2) form, so these
nebulae are called molecular clouds. The largest such
formations are called giant molecular clouds.
Giant Molecular Clouds
Barnard 68
Infrared
Visible
Star formation collapse of the cores of giant molecular clouds:
Dark, cold, dense clouds obscuring the light of stars behind them.

(More transparent in infrared light.)
Parameters of Giant Molecular Clouds
Size: r ~ 50 pc
Mass: > 100,000 Msun
Temp.: a few 0K
Dense cores:
R ~ 0.1 pc
M ~ 1 Msun
Much too cold and too low density to
ignite thermonuclear processes
Clouds need to contract and heat up
in order to form stars.
Contraction of Giant Molecular Cloud Cores
Horse
Head
Nebula
• Thermal Energy (pressure)
• Magnetic Fields
• Rotation (angular momentum)
• Turbulence
 External trigger required to initiate
the collapse of clouds to form stars.
Three Kinds of Such Nebulae
1) Emission Nebulae
Hot star illuminates
a gas cloud;
excites and/or
ionizes the gas
(electrons kicked
into higher energy
states);
electrons
recombining, falling
back to ground
state produce
emission lines.
The Trifid
The Fox Fur Nebula NGC 2246
Nebula
Three Kinds of Nebulae
Star illuminates a gas and
dust cloud;
star light is reflected by the
dust;
reflection nebulae appear blue
because blue light is scattered
by larger angles than red light;
the same phenomenon makes
the day sky appear blue (if it’s
not cloudy).
2) Reflection Nebulae
Three Kinds of Nebulae
Dense clouds of gas and dust absorb the light from the stars
behind;
3) Dark Nebulae
appear dark
in front of the
brighter
background;
Barnard 86
Horsehead Nebula
Interstellar Extinction
The dimming of light from stars and other distant objects, especially
pronounced in the galactic plane, due the combined effects of interstellar
absorption and scattering of light by dust particles.
- About 2 magnitudes per 1000 pc in solar neighborhood. - increases at shorter (bluer) wavelengths, resulting in interstellar
reddening.
- least in the radio and infrared region - makes these wavelengths
suitable for seeing across large distances in the galactic plane and, in
particular, for
probing the nucleus of the Milky Way.
Extinction curve - broad 'bump' at about
2200 Å, well into the UV region of
electromagnetic spectrum.
- first observed in the 1960s - origin
still not well understood.
- thought to be caused by organic
carbon and amorphous silicates
present in interstellar
grains.
PHYS
3380 -
Observing Neutral Hydrogen: The 21-cm (radio) line
Electrons in the ground state of neutral hydrogen have slightly
different energies, depending on their spin orientation.
Opposite
magnetic fields
attract => Lower
energy
Magnetic field
due to proton
spin
21 cm
line
Magnetic field
due to electron
spin
Equal
magnetic
fields repel =>
Higher energy
The 21-cm Line of Neutral Hydrogen
Transitions from the higher-energy to the lower-energy spin state
produce a characteristic 21-cm radio emission line.
=> Neutral
hydrogen
(HI) can be
traced by
observing
this radio
emission.
Observations of the 21-cm Line
G a l a c t i c
p l a n e
All-sky map of emission in the 21-cm line
Observations of the 21-cm Line
HI clouds moving towards Earth
HI clouds moving
away from Earth
Individual HI clouds with
different radial velocities
resolved
Can be used to calculate the relative
of each armofofline)
our galaxy and the rotation
(fromspeed
redshift/blueshift
curve of our (and other) galaxy. It is then possible to use the plot of the rotation curve
and the velocity to determine the distance to a certain point within the galaxy.
Rotation curve of the typical spiral galaxy M 33 (yellow and blue points with
errorbars) and the predicted one from distribution of the visible matter (white
line). The discrepancy between the two curves is accounted for by adding a
dark matter halo surrounding the galaxy.
Gravitational Collapse
How do large, cold, high density clouds/nebulae become stars?
Gravity is the key.
Cloud given a “push” by some event.
perhaps the shock wave from a nearby supernova
As the cloud shrinks, gravity increases, causing collapse.
As the cloud “falls” inward, gravitational potential energy is converted to heat.
Conservation of Energy
As the nebula’s radius decreases, it rotates faster
Conservation of Angular Momentum
Star forms in the very center of the nebula.
temperature & density high enough for nuclear fusion reactions to begin
Shocks Triggering Star Formation
Trifid
Nebula
Globules = sites where stars
are being born right now!
Sources of Shock Waves Triggering Star Formation
Previous star formation can trigger further star
formation through:
a) Shocks from
supernovae:
Massive stars die
young =>
Supernovae tend to
happen near sites of
recent star formation
The Crab Nebula
Sources of Shock Waves Triggering Star Formation
Previous star formation can trigger further star
formation through:
b) Ionization fronts of
hot, massive O or B
stars which produce a
lot of UV radiation:
Massive stars die
young => O and B
stars only exist near
sites of recent star
formation
Sources of Shock Waves Triggering Star Formation
Giant molecular clouds are very large
and may occasionally collide with each
other
c) Collisions of
giant molecular
clouds.
Sources of Shock Waves Triggering Star Formation
d) Spiral arms in
galaxies like our
Milky Way:
Spirals’ arms are
probably rotating
shock wave
patterns.
Original cloud large and diffuse - begins to
collapse. Final density, shape, size, and
temperature the result of three processes:
• Heating - cloud heats up due to conservation
of energy - as cloud shrank, gravitational energy
converted to kinetic energy - collisions convert
KE into random motions of thermal energy density and temperature greatest at center
• Spinning - conservation of angular momentum
causes rotation to increase as cloud collapses all material doesn’t collapse to middle because
the greater the angular momentum of a cloud
the more spread out it will be.
• Flattening - cloud flattens to a disk - different
clumps of gas collide and merged - random
motion of clumps becomes average motion becomes more orderly flattening original cloud’s
lumpy shape - orbits also become more circular
Nebula Flattening
As a nebula collapses, clumps of gas collided and merged.
Their random velocities averaged out into the nebula’s direction of
rotation.
The spinning nebula assumed the shape of a disk.
Collapse of Solar Nebula Animation
Formation of Protoplanetary Disk Animation
Protostars
Protostars =
pre-birth
state of
stars:
Hydrogen to
Helium fusion
not yet
ignited
Still enshrouded in opaque “cocoons” of dust => barely visible in the
optical, but bright in the infrared
- dust cocoon absorbs almost all of the visible radiation - grows
warm and reemits energy as IR radiation
Heating By Contraction
As a protostar contracts, it heats up:
Life tracks from protostar to the main sequence for stars of different masses.
Star emerges
from the
enshrouding
dust cocoon
(birth line) solar wind
blows dust out
and away
More massive
stars have
higher gravity
and contract
faster
Ignition of H
 He fusion
processes
Formation of the Solar Protoplanetary Disk
By the time solar nebula had shrunk to 200 AU, became flattened, spinning
disk - called a protoplanetary disk
The Sun formed in the very center of the nebula.
– temperature & density were high enough for nuclear fusion reactions
to begin
The planets formed in the rest of the disk.
Three processes - heating, spinning, flattening - produced orderly motions.
Explains:
–
–
–
–
–
–
all planets lie along one plane (in the disk)
all planets orbit in one direction (the spin direction of the disk)
the Sun rotates in the same direction
the planets would tend to rotate in this same direction
most moons orbit in this direction
most planetary orbits are near circular (collisions in the disk)
Strong Support for the Nebular Theory
Computer simulations can reproduce most of the observed motions
We have observed disks around other stars.
These could be new planetary systems in formation.
 Pictoris
AB Auriga
Proplyds
Proplyds - disks of dust and gas surrounding newly formed stars.
- of the five stars - all pre main sequence - in this field which spans
about 0.14 light years, four appear to have associated proplyds - three
bright ones and one dark one seen in silhouette against the bright
nebula.
- more complete survey of 110 stars in the region found 56 with
proplyds.
Disks seen only in silhouette,
- the absence of emission lines at an edge indicates that they are not
being illuminated by ionizing photons or flux is so low that the
emission is less than that of the background nebula.
- may be located within the foreground
Some bright proplyds have dark disks silhouetted against both the background
nebula as well as the ionization fronts of the proplyd.
- bright cusp, and extended comet-like tails.
- well defined axes tended to be pointed toward an ionizing star.
- form envelopes of dust as protoplanetary disks overtaken by the
ionization front.
HST10
- a protostar in the Orion Nebula
surrounded by a cocoon of dust and
gas distorted into a teardrop shape by
interstellar winds and radiation from
nearby hot stars. Inside the teardrop,
a disk of dark protoplanetary material
roughly the size of our solar system
orbits the star. The other images
depict a model of HST10 from
viewpoints left of, beside, and right of
the proplyd.
PHYS 3380 -