Astro 10-Lecture 13:Quiz

1. T/F We are near the center of our Galaxy

2. Cepheid variable stars can be used as distance indicators because

a) They all have the same parallax

b) They all have the same instrinsic brightness

c) The is a relationship between the period of their variability and their luminosity.

d) The are all the same size

3. Draw an edge on picture of the galaxy. Label the disk, bulge, globular clusters, and halo. Give the approximate diameter in light-years or parsecs.

4. T/F The brightest stars in our galaxy are found near spiral arms.

Astro 10-Lecture 13:Other Galaxies and the Expanding Universe

The “mystery” of the spiral nebulae:

• How far away are they?

• How big is the universe?

1920: The Shapley-Curtis Debate

Shapley: Our Galaxy is the entire universe. Spiral nebulae are clouds of gas. The sun is not near the center of the Galaxy/Universe, but is in the center of a cluster of stars 50,000 light years from the center. The galaxy is 300,000 light years across.

Curtis: Spiral nebulae are galaxies like our own. The sun is near the center of our Galaxy. The galaxy is less than 30,000 light years across.

1920: The Shapley-Curtis DebateThe Evidence

Shapley:– If spiral nebulae were galaxies the size of our own, then

the novae we see associated with them must be 15,000 times brighter than nearby novae. It’s more logical that they are nearby objects (less than 20,000 light years away) with normal novae.

– Globular clusters form a sphere 300,000 light years in diameter centered 50,000 light years away from here. So the sun isn’t in the center.

1920: The Shapley-Curtis DebateThe Evidence

Curtis– In the plane of the galaxy, stars seem to become less

common as you get farther away, so the sun is at the center of the galaxy. The stars appear to end about 30,000 light years away.

– Stars in globular clusters are faint, so the clusters are closer than Shapley estimates.

– The spectrum of a spiral nebula looks like the spectrum of a cluster of stars rather than light emitting gas, so it is logical to assume they are great clusters of stars.

– All other nebula types are concentrated near the plane of the Galaxy

Late 1920s: The answer at last

• Edwin Hubble finds Cepheid Variables in the Andromeda Galaxy (M31)– Distance: 1,000,000

light-years!

• Both Shapley and Curtis were right on some points and Wrong on others.

Types of Galaxies

Spiral Galaxies

BarredNormal

Elliptical Galaxies

Irregular and Peculiar Galaxies

Classification of Galaxies

No Star Formation

More loosely wound armsMore loosely wound armsSmaller bulgeSmaller bulgeMore star formationMore star formation

ConcepTest

T/F: In a galaxy with very little gas and dust you would expect to find many bright blue stars.

T/F: A galaxy that contains lots of emission nebulae is likely to have a high rate of star formation.

Properties of Galaxies

Spiral Elliptical Irregular

Mass 109-1012 M 106-1012 M

1013 M in cluster center

106-1011 M

Color Blue disk Red Bulge

Red Some blue stars

Gas & Dust Yes Very Little Variable

Star Formation Yes None Some

Fraction 20% (lower in clusters)

30% (higher in clusters)

50%

Rotation Yes No Some

Another look at the Virgo Cluster

Interacting Galaxies

• Click here for simulation

Interacting Galaxies

Click here for simulation

Interacting GalaxiesIs this how elliptical galaxies form?

cD Galaxies: Cluster Cannibals

How did spiral galaxies form?

Active Galactic Nuclei (AGN)

• Seyferts and BL Lacs and Quasars, Oh My!– Some are bright in the radio,

some in the X-rays, some in the optical.

– Some have jets, some are bright points.

– Don’t worry too much about the differences, deep down they are very similar...

Active Galactic Nuclei (AGN)Jets

Active Galactic Nuclei (AGN)Radio Lobes

Active Galactic Nuclei (AGN)

Bright Blue Nuclei &

Emission Line Regions

Active Galactic Nuclei (AGN)Quasars & QSOs (Quasi-Stellar Objects)

Before HST

Active Galactic Nuclei (AGN)Quasars & QSOs

After HST

So what is an AGN?

How do we figure out properties of galaxies?

Size:

• Measure angular size and use geometry:– R=D sin() (need to know the distance)

Luminosity:• Measure apparent brightness:

– L=4D2 B (need to know the distance)

Mass:• Measure rotational velocity (using Doppler shift)

and use Kepler’s Laws– M= v2 R/G = v2 D sin()/G (need to know the distance)

Rule 1: If we don’t know the distance we don’t know anything.

Distance:

• Cepheid variables: Measure the period to determine luminosity. From luminosity we can calculate distance– Only works for nearby galaxies.– Cepheids are calibrated to parallax of nearest

Cepheids.

Concept Test

Suppose the nearest Cepheids were farther than we had previously thought. What effect would that have on the luminosity we calculate for galaxies.

– If they are farther, they must be more luminous than we thought

– If Cepheids are more luminous that we thought, then the ones we see in other galaxies must be farther away than we thought.

– If other galaxies are farther away, they must be more luminous than we thought.

How do we get the distances to farther galaxies?

The distance ladder:

1. Measure parallax to nearby stars.

2. Use measured parallax to calculate the luminosity of Cepheid variables

3. Use luminosity of Cepheid variables to calculate other “standard candles”

“Standard Candles”

A type of object for which we think all objects of that type have the same luminosity

• Cepheid Variables (works out to 30 Mpc)

• Luminosity of brightest globular cluster around a galaxy.

• Luminosity of brightest planetary nebula

• Luminosity of Type I supernovae. (Brighter than some galaxies)

All of these methods are based upon the Cepheid calibration!

Type Ia Supernovae

Different from the Type II (death of a massive star)...

In Type Ia, a white dwarf is pushed over the 1.4 Solar Mass limit for a white dwarf.

Every one is a nearly identical explosion of a 1.4 solar mass white dwarf.

Now that we know the distance we can determine size,luminosity, mass,

Biggest surprise comes from the mass calculation:

There’s more mass than you would expect given

the amount of light.

DARK MATTER

Dark Matter: Clusters of Galaxies

“Dark Matter” also shows up in clusters of galaxies.– Velocities of galaxies in

clusters are higher than the escape velocity you would calculate from the visible matter.

– Hot X-ray emitting gas would escape from the clusters without extra mass to hold it in

Dark Matter: Gravitational LensingThe lensing we see

requires far more mass than is visible in the clusters.

We’ll talk more about this next

week.

The Expanding Universe

In the 20s and 30s, Edwin Hubble noticed that emission lines in the spectra of spiral nebulae were consistently red shifted.– The smaller the angular size of the nebula, the

greater the red shift.

– Once he measured the distance to Andromeda, he was able to express a relationship between the measured velocity and the distance.

• V=H0d

• Added benefit, v is much easier to measure than d

Cosmological Red Shift

The Expanding Universe

The Expanding Universe

What’s the deal? Are we in a special place in the universe? Why else would everything be moving away from us?

The Expanding Universe

What does it mean?

• In the distant past, every galaxy we can see was in one place. All matter and energy was essentially in the same place.

THE BIG BANG

(more about this next week, too)

Galaxy Evolution

• Remember that when we look into the distance we’re also looking into the past, one year ago per light year distant.

We see the Andromeda Galaxy as it was more than million years ago.

More distant galaxies we see longer ago.

Hubble Deep Field

1/4,000,000th of the entire sky

What do we see?

• In the past, AGN were more common.– It’s likely that most have run out of fuel

• Spiral were somewhat more common.– Less had been cannibalized

• Peculiarly shaped blue galaxies were more common.– The collisions that form ellipticals in progress?