Astronomy Second Midterm - Chapt. 15

Front 1

A band of light is apparent in the MW galaxy (spiral galaxy) - does this have any relation to the ecliptic?

Back 1

No

The constellations of the MW are not the same as the zodiac
-The MW does not run along the ecliptic

Front 2

What are the components of the MW galaxy?

Back 2

1) Disk
-spiral arms, ISM, and young stars are located in the disk

2) Nuclear Bulge - central bulge (black hole at centre)

3) Stellar Halo
-old stars, globular clusters

Front 3

Where is the Sun located in the MW galaxy?

Back 3

-The Sun is located 27,000 Lyrs from the centre of disk

-disk is 100,000 lyrs long, 1,000 lyrs wide

-mass enclosed by the Sun's orbit around the center is about 100 billion solar masses

Front 4

Where does most of the mass in the MW come from?

Back 4

Dark Matter!

-does not interact with light, its origin is unknown
-its presence is only known by it gravitational force it exerts on matter (explains the rotation of stars around the center of the MW galaxy)

Front 5

The MW is not made of solid components, so why does it have a distinct shape?

Back 5

-there are two distinct patterns of stellar orbits that give the MW its shape

Front 6

How much of the MW's mass is accounted for by ISM? What types of telescopes must we use to examine ISM?

Back 6

10%

-for cool molecular clouds (For star formation) - radio telescopes

-for hot gas (when gas is returned to ISM) - x-ray telescopes

Front 7

What are the three states of gas found in the ISM?

Back 7

1) Hot Bubbles - ionized H
-1 million K temp
-pockets of gas heated by stellar winds (created by young, massive, O-type stars from their massive light pressure pushing some material away) or supernovae

2) Atomic H Clouds - atomic H
-100-10,000 K
-Most common form of gas filling much of galactic disk in MW

3) Molecular Clouds - molecular H
-30 K
-Regions of star formation

Front 8

You take a spectrum of the interstellar gas comprising the "bubble" on the previous slide...you can expect what time of spectrum? What can you determine from the spectra?

Back 8

-Emission spectrum (hot gas that emits light)

-Temperature of gas, composition of gas, motion of gas

Front 9

How are hot bubbles in the ISM formed and returned to the ISM of of the galactic disk?

Back 9

-formed from multiple supernova from young star clusters, which blow a giant bubble in the galactic disk
-bubble becomes thicker and blows hot gas into halo
-bubble cools, forms clouds
-clouds rain back onto galactic disk

Front 10

Why must cool gas in the ISM be very dense for star formation to occur?

Back 10

Very massive stars my erode molecular clouds in ISM by radiation

Only densest molecular clouds an survive, and stay cool enough in their centers for stars to form

Front 11

Why do we see the prominent spiral pattern in the MW?

Back 11

-we see the pattern because of light of young stars and HII regions
-spiral pattern is less apparent in distribution of older stars (such as the Sun)

Front 12

What is the Winding Problem of the MW spiral pattern?

Back 12

-The Sun has circled the centre of the MW galaxy nearly 40 times...but we don't see incredibly tight spirals that resemble this?

-WHY? The spiral pattern is a wave-like pattern that travels through the disk (stars and gas continuously enter and leave the pattern)
-the density wave compresses the ISM gas and triggers new star formation
-massive stars don't travel far from where they formed, older stars travel much farther

Front 13

Is the number of stars in the universe infinite? What is the size/shape of the MW? Where is the center of the MW?

Back 13

-Herschel built a giant telescope to look at faint stars and counted stars in different directions
-this model put the Sun near the center
-Kapteyn built on these findings with a better telescope, but still had Sun at center

Front 14

How did the globular clusters inform us that the Sun isn't at the center of the MW?

Back 14

Shapley - discovered that the Sun is part of a globular cluster, and the globular cluster is very condensed to one side of the Sun
-the centre of the globular cluster distribution is where the center of the MW must be
-So the Sun gets displaced from the center about 27 lys away

Front 15

How had Kapteyn been fooled in his map of the MW galaxy?

Back 15

-tons of dust in the MW would have obscured stars, all but the ones close to the Sun
-most of the galaxy is invisible to optical light

Front 16

How long does it take the Sun to orbit the MW?

Back 16

250 million years (220km/s)

Front 17

The rotation speed of the planets in our solar system as you move farther away from the Sun do what?

Back 17

Decrease as you move further from the Sun (your orbital speed is determined by how much mass is enclosed in your orbit) - Keplerian rotation
-move farther away from sun, have more mass enclosed (but insiginificant), but proportionally gravitational pull is much weaker as you move farther away

Front 18

What does the rotation curve (the rotation of stars) as you move further away from the center look like? Why does it look this way?

Back 18

-There is no drop with distance
-each larger orbit must be enclosing more mass then!
-Once we move past the distance of the sun, the number of stars begins to drop...so where does the mass come from? DARK MATTER

Front 19

You are searching for an example of a "zero-metal" star, that is a star made on only H and He. Where should you look?

Back 19

Galactic Halo and Globular Clusters (old stars) - low mass stars which burn H--> He)

Front 20

What are stars like in the galactic disk?

Back 20

-Regular, circular orbits in common plane
-mix of young and old stars
-metalicity similar to Sun (2% metals)
-contains raw material for new stars

Front 21

What are stars like in the galactic halo?

Back 21

-Random, elipitical orbits with no common plane
-all old stars (12 billion years old) globular clusters
-very low metalicity (0.02%)
-no raw material for star formation

Front 22

Why did star formation begin in the Halo (now where it is not seen) and move to the disk (where star formation currently happens)?

Back 22

-At the Big Bang, He and H were made (not enough time for other nuclear reactions to occur)
-First generation of stars formed from galactic cloud that collapsed in Halo with low metal content (oldest stars now)
-b/c of conservation of angular momentum, remaining gas flattened into spinning disk (so raw material for star formation moved from halo into disk)

Front 23

What is at the exact dynamical centre of the MW?

Back 23

A radio source known as Sagittarius A-
-strong evidence that there is a black hole at the center - 4 million solar masses) - almost all spiral galaxies have a black hole at their centers
-around the center there is hot gas (millions of degrees), massive star clusters, and a strong magnetic field

Front 24

In radio images of the inner 200 pc of the galactic center, there seems to be few stars. Why is this?

Back 24

Stars are very faint at radio wavelengths

Front 25

Is the central black hole at the center of the MW spiral galaxy accreting material? What do we call a central black hole that accrets material?

Back 25

No - if it were accreting material, it woulb be brighter than it appears to be

Quasar

Front 26

Do the stars orbit the center of the MW b/c of the gravitational pull of the central 10^6 solar mass black hole?

Back 26

NO
-the central black hole is a negligible component to the overall mass of the MW (which is about 10^12 solar masses)