Astronomy Test 3

Do we always see the same part of the milky way?

no, we see different parts of the milky way at different points in the year

How do we view our galaxy, and what are its features?

edge on. Primary features: disk, bulge, halo, globular clusters

Bulge

 brighter, spherical region concentrated towards the center of the galaxy

Halo

areas above and below the plane of the galaxy. Globular clusters like to hang out there

Disk

the flat plane the milky way is on. Open clusters lie here

What kind of galaxy is the milky way?

barred spiral galaxy

How do we know what our galaxy looks like from the outside when we’re in it?

21-cm Flip method

21-cm Flip

In (neutral) hydrogen atoms, electrons that flip their spin to lower energy emit 21-cm radio photons. By mapping large clouds of atomic hydrogen (HI regions, which emit these waves), it can show the 5 spiral arms of the milky way galaxy

What is HI?

neutral hydrogen

How do HI detections measure galaxy rotations?

By measuring red shift and blue shift on different parts of the galaxy, you can tell how galaxies are rotating

Where do halo stars travel?

high above and far below the disk on orbits with random orientations

How do bulge stars orbit?

with random orientations

How do disk stars orbit?

in circles with the same orientation, except for a little up and down motion, producing a bending wave (a “warp”)

Why do different parts of the galaxy have different motions?

It’s not fully known why these parts have different motions 

A theory is that major individual clouds of gas and dust came together in some random larger scale motion, which eventually collapsed on itself and started rotating

Why are stars not likely to collide?

the stars are very small compared to the distances between them

How is gas in our galaxy recycled?

 through the star-gas-star cycle; Gas from old stars is used to form new star systems

Star-Gas Cycle

• Interstellar gas clouds fill the galactic disk 

• Gas in the disk gradually cools and forms molecules

• Gravity makes stars from molecular hydrogen gas 

• Fusion in the cores of stars makes new elements from hydrogen 

• Supernovae and stellar winds return gas and new elements to interstellar space

• Returning gas cools and then blends into atomic hydrogen clouds

How do high mass stars contribute to the star-gas cycle?

have strong stellar winds that blow bubbles of hot gas

How do lower mass stars contribute to the star-gas cycle?

return gas to interstellar space through stellar winds and planetary nebulae

What do X-rays from hot gas in supernova remnants reveal?

newly made heavy elements

Galactic fountain model 

Basically, gas is being transported in between galaxies through cosmic web filaments

What is the galactic fountain model?

• Multiple supernovae create hot bubbles that can blow out of the disk 

• Gas clouds cooling in the halo can rain back down on the disk 

• Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons

• Molecular clouds result next, after gas cools enough to allow atoms to combine into molecules