Homework Set #810/26/15

Due 11/2/15Chapter 10

Review Questions7, 9Problems3, 7

Chapter 11Review Questions 3, 7Problems5, 9

Maximum Masses of Main-Sequence Stars

Eta Carinae

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

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

At masses below 0.08 Msun, stellar progenitors do not get hot enough to ignite thermonuclear fusion.

Brown Dwarfs

Gliese 229B

The Life Cycle of Stars

Dense, dark clouds, possibly forming stars in the future

Young stars, still in their birth

nebulae

Aging supergiant

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

VisibleInfrared

Barnard 68

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

Dense cores:

Temp.: a few 0K

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

• Thermal Energy (pressure)

• Magnetic Fields

• Rotation (angular momentum)

External trigger required to initiate the collapse of clouds to form stars.

Horse Head Nebula

• Turbulence

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 Fox Fur Nebula NGC 2246The Trifid

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;

Barnard 86

Horsehead Nebula

appear dark in front of the

brighter background;

3) Dark Nebulae

PHYS 3380 - Astronomy

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.

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.

Magnetic field due to proton

spin

Magnetic field due to electron

spin

Opposite magnetic fields attract => Lower

energy

Equal magnetic

fields repel => Higher energy

21 cm line

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

All-sky map of emission in the 21-cm line

G a l a c t i c p l a n e

Observations of the 21-cm Line

HI clouds moving towards Earth

(from redshift/blueshift of line)

HI clouds moving away from Earth

Individual HI clouds with different radial velocities

resolved

Can be used to calculate the relative speed of each arm of our galaxy and the rotation 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

Globules = sites where stars are being born right now!

Trifid Nebula

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:

Free-fall contraction→ Heating

Life tracks from protostar to the main sequence for stars of different masses.

Ignition of H He fusion processes

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

Formation of the Solar Protoplanetary DiskBy 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 TheoryComputer simulations can reproduce most of the observed motions

We have observed disks around other stars.

These could be new planetary systems in formation.

AB Auriga

Pictoris

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.

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.

PHYS 3380 - Astronomy

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.