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40 Cards in this Set

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When interstellar space appears dark this is usually a result of...
a dust cloud that gets in between the stars and the observer.
How large are dust grains found in space?
On the order of visible light waves. (100-1000 nm).
the process of Dust absorbing light and scattering it is known as...
EXTINCTION (IMPORTANT)
Dust in the ISM and in Earth's atmosphere has a tendency to extinct (Absorb and scatter) what color light? What does this cause?
Blue light (higher energy light); thus objects appear to be reddened in the sky.
Why is the sky blue?
Atmospheric particles are about the size of the blue wavelength of light. The Sun’s blue light is scattered in all directions – we are seeing this scattered light in the sky.
Although dust and gas absorb alot of light, spectral lines can still be produced to figure out...
the star's spectral class

and then its luminosity, temperature and shape.
Warm dust grains emit like blackbodies, with spectra peaking in
the infared (lower energy than visible. In the visible recall they block out light when in reality they too are emiting radiation at lower energy.
What does the peak wavelength given off by the dust indicate? According to what law?
its temperature. Wien's law.
How do we study gas in the ISM?
We observe the spectral output and we can determine not only the temperature of the gas but what element it is.
Emission lines tell us about the star's...
composition, temperature, density and motion of the interstellar gas.
Emission line nebulae
We are searching for the ionization of H to H+ at 8000K. PHOTOIONIZATION!
ISM conditions:

Gas temperature
Gas density
Dust density
Gas temp= very low (below water's freezing point)

Gas density= very dense (no man-made vacuum is as dense)

Dust density= not dense at all, very wide open spaces (1 particle/ 300 ft)
Summary: How do we study dust (2 ways)
1.) Dust Extinction- dims and reddens radiationfrom space.

2.) Dust emission- warm dust emits lika a blackbody (infared)
Summary: Studying GAS
(2 ways)
1.) Emission nebulae: Atoms in hot gas emit spectral lines

2.) absorption line spectra- atoms in cool gas absorb in lines.
Since gas only emits when it is hot or is cool and has a hot background (recall Kirchoff's laws). What do we do when neither is the case?
21 cm radiation of atomic hydrogen
21 cm radiation comes from the idea that...
when a hydrogen's proton and electron are excited and change energy levels, they emit 21 cm radiation. Thus when we look out to space for these emissions we see plenty of stars presumably changing their energy state.
Other cooler MOLECULES (other than H2) NOT ATOMS exist in the ISM including
CO, HCN, NH3, H20, CH3OH, FORMALDAHYDE
How are these "other gases" observed?
through vibrational and rotational energy they give off.
Why do stars form?
Gravity
Theoretically, every particle in the world is attracted to eachother by gravity, however each particle is most attracted to...
its nearest neighbor (recall gravity is an inverse square law)
Interstellar clouds are pulled inward by...

While they are forced apart by...
their own weight (gravitational attraction)

The movement of particles due to increasing temperature.
which force is stronger?
the force outward of particle motion (a result of incereasing temperature).
How many atoms (how much total mass) is required for the collapse of an ISM cloud?
That depends on the temperature (which constantly pushes out)
Where is the birthplace of stars?
Dense dark clouds are the birthplace of stars.
Star formation:

Stage 1
Cloud collapse and Fragmentation (2 million years)
which clouds collapse first?
those which are DENSEST!
As a result of the collapse and fragmentation...
stars are formed in groups not indvidually.
Star formation:

Step 2:
Continuing Collapse of each Fragment (3000 years)
Continued collapse (gravity) causes more heat and motion to be generated which pushes outward. This process is called.
radiative cooling.
Star formation:

Stage 3
Fragmentation Ceases, a Protostar is Born (10000 years)
During stage 3 what happens to the temperature of the star?
it soars. the protostar has a photosphere surface like regular stars have.
Formation of a star:

Stage 4
Protostar I (1 million years)

protostar further collapses, heat keeps increasing, pressure increases, and luminosity increases.
Formation of a star:

Stage 5
Protostar II (10 million years)

Contraction proceeds slowly as the internal temperature and pressure increase…
Formation of a star:

Stage 6
A Star is Born! (30 million years)
What is the benchmark to be considered a star and not a protostar?
the temperature of the star must exceed 10^7K and nuclear fusion must begin at the core.
What happens once the protostar becomes a star?
it continues to compact (originally stars are larger than the sun) over many years they continue to compact.
Star formation:

Stage 7:
Joining the main sequence

Once a star becomes a main sequence star it remains one for 10 billion years.
More massive stars form...
in much the same way and populate a different "neighborhood" of the H-R diagram.

Recall more massive stars live LESS!
Star clusters are...
They are the result of...
Contain stars of different masses, that land on different parts of the main sequence.

FRAGMENTATION during their birth.
What are brown dwarfs?
fragments which do not have enough mass to become stars.
They reach equilibrium (pressure vs. gravity) but their central temperatures remain too low for nuclear burning.