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42 Cards in this Set

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  • Back
is a collection of stellar and interstellar matter – stars, gas, dust, neutron stars, black holes – held together by gravity.
When looking into the sky what galaxy do we see?
MOSTLY OUR OWN! the milky way galaxy
Give 2 examples of spiral galaxies?
Andromeda and the Milky way
what are the three main parts of a spiral galaxy?
Galactic Disk
Galactic Bulge
Galactic Halo
Who was the first to predict the size and shape of our galaxy? How?
William Herschel. By counting the stars he saw in all directions.
1.) He assumed all stars had the same luminosity

2.) Dust which blocks out stars further than a few Kpc.
What are variable stars?
Stars whose luminosity is changing because of its pulsating.
What are the two classes of variable stars? what are they used for?
RR lyraes and Cepheids.

Distance markers to meausre how far other structures are
Which of the two classes has the highest period and therefore is brightest?
the Cepheids (bright)
Where are variable stars on the H-R diagram?
Variable stars are in the instability strip of the H-R diagram
Which of the two branches of variable stars are more massive (heavier)?
What relationship exists that allows us to use Variable stars to determine distances?
Relationship between pulsation period and luminosity.
Who discovered Cepheids and where? impact?
Edwin hubble;

on the andromeda galaxy and showed it was external to our own galaxy

this had a profound impact on the field (like the modern Coppernican revolution
Where do we see RR lyres?
They are too faint to be seen in distant galaxies. But they can be seen in globular clusters
what 2 conclusions were drawn about global clusters?
1.) most are great distances from the sun

2.) their 3D distribution is roughly spherical with a diameter of 30kpc.
What other major finding did Shapely find?
Not only is our planet not in the center of our solar system, our sun is not at the center of the galaxy.
Where do Globular clusters reside in the galaxy?
In the halo
Old stars in the galaxy are in the...
halo and bulge
Where is most of the gas and dust at?
-Stars= young
-Gas= Hydrogen and singly ionized hydrogen (HI)
-Dust= alot
-Metals-solar metal abundance

-Stars- mostly old stars
-Gas- small amounts of H and HI
-Dust- little or none
-Metals- poor

-Stars=older stars
-Gas= little or none
-Dust= little or none
-Metals- metal rich
How are things moving in the galaxy?
Doppler shifts reveal orbiting motions around the GALACTIC CENTER. with an orbit taking 225 million years.
The formation of the galaxy:
First young stars form (keep their random motion)

Next there gravitational attraction to eachother causes rotation and flattening of the neighboring dust and gas into a flat plane.

Take home message--> disk formed later
How do we know that the Milky way is a spiral galaxy?
The presence of O and B stars (young stars) on the spiral arms suggest that star formation begins at the spiral arms.
What is the other line of defense that the galaxy is in fact a spiral one?
giant molecular clouds trace out the locations of spiral arms.
What is the last line of defense for a spiral galaxy?
Tracing the HI in the galaxy shows the spiral arms.
what are the three explanations of how we know that the milky way is a spiral galaxy?
1.) O and B stars

2.) Giant molecular clouds

3.) Neutral hydrogen
What is the winding dilemna?
the fact that since the galaxy has rotated several times, tzhe spiral arms should be wound so tightly they should NOT be seen.
What is the theory that explains why we can see the spiral arms of the galaxy?
Spiral density waves-

Spiral arms are caused by the compression of gas as it orbits the Galactic center and encounters density waves that are essentially stationary. Compression of gas causes stars to form which we see as spiral arms.
What is the alternative theory for the formation of the spiral arms?
Self-propogating star formation:

The formation of stars drives the waves – shock waves from the later evolution of stars creates denser regions where new stars are created.
How do we measure the mass of the galaxy? What is it?
From Kepler's third law.

100 billion SUNS
What is the equation for orbital speed?
sqrt (G M(interior)/radius)
What surprising observation do we see with respect to the radius and orbital velocity?
Stars further from the center of the galaxy are NOT MOVING SLOWER they are either moving at the same speed as closer stars or FASTER!
This is evidence that there is something else at play... what is that something else?
Dark matter.
What portion of the galaxies mass is "invisible/dark matter"?
2/3 (66%)
What are the leading dark matter candidates?
What is gravitational lensing?
The effect that allows us to see MACHOs.

The fainter foreground object bends the light of the background star (gravity) making the fainter star appear brighter.
Where is the solar system in the galaxy?
Far from the center of the galaxy (near the edge)... about 8kpsc from the center.
Why cant we see the galactic center?
it is covered by dust and since dust preferentially scatters light in the blue, the galactic center is best studied in the infared and radio wavelengths.
The center of the galaxy is a strong source of...
radio waves
All of this means that the center of the galaxy is a...
supermassive black hole
Recall: by observations of the movement around the center of the galaxy tells us...
the mass of the galaxy.
If there is a black hole in the center of the galaxy, what is its effect on the Sun?
No effect. the sun feels the effect of hundreds of millions of solar masses and the black hole is merely 2-3 million solar masses.