Astronomy Second Midterm - Chapt. 16

Front 1

What are the three major classes of galaxies?

Back 1

1) Spiral Galaxies
-disks, often with spiral pattern
-young and old stars in ISM
-evidence of ongoing star formation
-brightest galaxies

2) Elliptical Galaxies
-round or elongated with no disk
-old stars only
-little ISM so no recent star formation
-have largest range of masses (the least AND most massive galaxies are ellipticals)

3) Irregular Galaxies
-don't fit into above categories

Front 2

Besides the MW galaxy, what other large galaxies are near us?

Back 2

M33, and Andromeda (MW is 1/3 the size of Andromeda)

Front 3

Why did Van Maanen believe that M31 was within the MW?

Back 3

He claimed to have directly measured the spiral motion of the stars in M31, so it had to be close by

Front 4

Why can't we use trigonometric parallax to observe other galaxies?

Back 4

-parallax only works for nearby stars (limited baseline of the Earth's orbit)

* Need a standard candle (something that is easily recognizable and that has a known luminosity, so we can work out how far it needs to be to appear as faint as it does)

Front 5

How can we study extragallactic system (other galaxies)?

Back 5

Cepheid Variable Stars (massive, evolved/non main sequences stars) - stars that periodically change their brightness over timescales of a few to hundreds of days
-recognized by their characteristic light curves
-longer the period, the brighter the cepheid variable star was (since all stars in the large magellinic cloud are about the same distance from us, long period c-variables must be intrinsically brighter/more luminous)

-found in large magellanic clouds by Leavitt

Front 6

You take the spectrum of a distant spiral galaxy...what spectrum do you expect to see?

Back 6

An absorption line spectrum with some Emission line - have stars and gas (gas itself can emit light, so the hot gas gives off an emission line spectrum)
-but also have light source (stars) that will pass through cool gas in ISM, so we get absorption spectrum

Front 7

You take the spectrum of a distant elliptical galaxy...what spectrum do you expect to see?

Back 7

Old cool stars (so still have light source passing through cool gas --> so absorption spectra)

-no emission lines because no hot gas from new star formation

Front 8

Cepheid variable stars are stars that regularly change their brightness. What is the likely physical reason for this?

Back 8

They are changing their size (radius) and temperature

Front 9

How are cepheid variables represented on the H-R diagram?

Back 9

They are found on the instability strip - a region on the H-R diagram on its way to the Red Giant Branch (massive, evolved stars)
-on the instability strip, stars pulsate (expand and contract), changing their size and temperature

-larger cepheid variable, longer period light curve, more luminous

Front 10

Why do cepheid variable pulsate in luminosity?

Back 10

As they pass through the instability strip, their outward pressure and inward gravity compression are out of sync, so the star SIZE and TEMPERATURE puslate

Front 11

Explain the ladder of techniques for observing things in the universe:

1) Solar system
2) Nearby stars
3) MW
4) Nearby galaxies
5) Galaxy Clusters

Back 11

1) Radar ranging - can give you distance to things in the solar system
2) Parallax - tells you how many baselines away from something you are
3) Main-sequence fitting
4) Cepheids
5) Hubbles Law or White dwarf supernovae - distant standards

Front 12

What is unexpected about the universe expanding?

Back 12

It is not only expanding, but its expansion is at an accelerating rate

Front 13

What was Hubble's contribution to studying distant galaxies?

Back 13

-he measured distances to other galaxies using cepheid technique
-took spectra of other galaxies (absorption line spectra) - noticed that all the galactic spectra are shifted red (moving away from us) - Z shifts
-if we know a galaxies spectra, we can use its red shift to determine is recession velocity, and therefore (v = H0 x d), our distance to the galaxy

Front 14

If you get a red shift (Z = .05) of 5%, what does this tell you about how fast the galaxy is moving away from us?

Back 14

5% x speed of light (300,000 km/s) = 15,000 km/s

Front 15

What is Hubble's constant?

Back 15

Graphing the recession velocities of other galaxies, and their apparent distances, found that their was a linear relationship (more distant galaxies moving away from us faster)
- velocity of recession is proportional to distance of galaxy (v = H0 x d)

-found that Hubble's constant (H0) = 22km/s/Mly - this gives us the current rate of expansion of the universe

Front 16

What was Einstein's cosmological constant? (his biggest blunder)

Back 16

-he did not like the expanding/contracting solutions to the universe
-added a cosmological constant to balance the force of gravity at large distances (explains why our universe is expanding at an accelerating rate)

Front 17

How did general relativity solve the problem of us thinking Earth was at the centre of the Universe?

Back 17

-expansion is not away from a single point, but uniform expansion in every direction
-every observer sees everyone else moving away from them

Ex. blowing up a balloon

Front 18

You travel back in time in the distant past when the Universe was expanding more slowly and you measure the Hubble constant. How will the value you obtain compare to the present value of 22km/s/Mly?

Back 18

It would be smaller (V = H0 x d) - v would be smaller (so smaller numerator, same denominator, smaller H0)

Front 19

What is wrong with interpreting red shifts as velocities at which galaxies are moving away from us?

Back 19

- you can find red shifts greater than 1 (does this mean moving faster than the speed of light through space?)
-velocity interpretation of red shifts is incorrect
-distant galaxies are not moving through space away from us, its space itself that is expanding and carrying the galaxies along with it

Front 20

Will the expansion of the universe make me taller? Will it make the solar system bigger? Why only distant galaxies being swept up in cosmic expansion?

Back 20

-NO, the electromagnetic forces holding us together are much stronger

-NO, gravitational forces in solar system too strong

-gravitational force between distant galaxies is very weak so they get swept up in expansion

Front 21

What happens to light in the expanding universe?

Back 21

-Photons are stretched out
-so photons are being shifted to lower energies and longer wavelengths (hence red shift)

Front 22

So what does a redshift for a galaxy actually mean?

Back 22

(1 + z) = the ratio of the average distance between when the light was emitted and received

-so if an object has a redshift of z=2, it is not receding at twice the speed of light
-the universe has trippled in size (1 + 2) between the emission and reception of this light

Front 23

What is a quasar?

Back 23

quasi-stellar radio sources - luminous centers of distant galaxies

-intense radio sources that have no optical counterpart on the sky
-see hydrogen lines in absorption spectra, but LARGELY shifted to redshift (z = .16) - Schmidt

-large red shift implies large distance (with an insane luminosity - 100 million MW's)

Front 24

Are quasars stars?

Back 24

-They are points of light
-we have emissions and luminosities, but we don't know what they are from this information
-quasars seem to be surrounded by jets of material causing emission

Front 25

Why are quasars and radio galaxies classified as ACTIVE GALAXIES?

Back 25

-these galaxies have physical processes beyond that in normal galaxies
-these galaxies need an additional component beyond the stars and ISM to account for their properties

Front 26

What is the key missing feature in active galaxies?

Back 26

- A supermassive black hole (10^6 - 10 ^10 solar masses) in the center of these galaxies
-accounts for:
1) Large luminosities - through accretion of material onto black hole (accretion energy is much more efficient for converting mass into energy through nuclear reactions)
2) Jets - magnetic field funnels material into a jet
3) Star like appearance of quasars (nucleus of galaxy outshines rest of galaxy)

Front 27

Energy to power luminosity in these active galaxies also comes from gas falling into black hole under gravitational contraction...but how can falling into a black hole create light? Isn't light trapped inside the event horizon along with the mass?

Back 27

It is very difficult to fall directly into a black hole
-when you drop gas into a black hole it wants to orbit, so it spins, collides, and heats up (creating energy for luminosity)

Front 28

Active galaxies emit strongly at almost every wavelength of light...why?

Back 28

there is a wide range of temperatures in the accretion disk of the black hole in the center of these active galaxies

Front 29

What are the two methods of Hubble Black hole Measurements?

Back 29

1) look at light from individual stars around black hole

2) look at light combined from many stars around black hole

Front 30

What is the relationship between the mass of the central bulge in a galaxy and the size of a black hole?

Back 30

Bigger the bulge, the bigger the black hole mass