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Astronomy 110: SURVEY OF ASTRONOMY 

13. THE REALM OF THE NEBULAE 

1. Distances and Types of Galaxies 

2. Hubble’s Law and Galaxy Evolution 

3. Peculiar and Active Galaxies

Galaxy History Revealed in This Colorful Hubble View

1. DISTANCES AND TYPES OF GALAXIES 

a. An Extragalactic Distance Scale 

b. Galaxy Morphology 

c. Groups and Clusters

Parallax Distances 

Nearby stars appear 

to shift back and forth 

as we orbit the Sun. 

Dec 

2p 

June 

The parallax angle p is 

inversely proportional to 

D 

the distance D: 

D 

1 

2π 

360° AU 

p 

p 

AU 

Using parallax, we can measure stellar distances out to 

a few hundred light-years.

Luminosity Distances 

The same luminosity L must 

pass through each sphere. 

A sphere of radius D has area 

A = 4πD 2 

So brightness is inversely 

proportional to (distance) 2 : 

B = L A 

L 

= 4πD 2 

D = 

√ 

L 

4πB 

An star of known luminosity L is a standard candle.

Main Sequence Fitting 

All stars in a cluster 

have the same distance, 

so plot HR diagrams 

for clusters using 

apparent brightness. 

MS in Hyades appears 

7.5 times brighter than 

MS in Pleiades; why? 

Pleiades are √7.5 2.7 times further away than Hyades!

Distance Scale: Summary 

1. Parallax measurements within the solar system gave 

an accurate value for the astronomical unit: 

1 AU = 1.496×10 8 km 

2. Using the Earth’s orbit as a baseline, stellar parallax 

provides a distance to the Hyades cluster: 

DHyades = 9.56×10 6 AU = 151 ly 

3. Main sequence fitting yields the distance to other 

clusters in the galaxy in terms of DHyades. 

At each step, known distances are 

used to find unknown distances.

Cepheid Variable Stars 

Brightness (mag) 

3.5 

4.0 

4.5 

Period 

2 4 6 8 10 12 

Time (days) 

delta Cephei 

Massive stars become Cepheid Variables at one 

phase of their lives after leaving the main sequence. 

During this phase, they vary in size, temperature, and 

brightness in regular ways with well-defined periods.

Period-Luminosity Relationship. I 

Cepheids in the Large Magellanic Cloud 

were found to obey a relationship 

between period and apparent brightness. 

Large Magellanic Cloud 

100 

These stars are all at the 

same distance, so their 

apparent brightnesses are 

proportional to their 

absolute luminosities. 

relative apparent brightness 

absolute luminosity 

10 

1.0 

0.1 

0.01 

So Cepheids must obey a period-luminosity relationship!

Period-Luminosity Relationship. II 

To be useful for distance measurements, the P-L 

relationship must be calibrated in units of L by 

measuring absolute luminosities of some Cepheids. 

Cepheids in star clusters 

are handy for this, since 

distances are available via 

main-sequence fitting. 

Once this is done, a 

Cepheid’s luminosity can 

be found from its period.

The Distance to Andromeda (M31) 

Is M31 another galaxy, or part of the Milky Way? 

The luminosities of several Cepheids 

in M31 were determined from their 

periods via the P-L relationship. 

Andromeda Nebula: Var! 

Given their luminosities and brightnesses, distances to 

these Cepheids could be computed: DM31 2.4×10 6 ly. 

M31 is far beyond the Milky Way! 

M31: The Andromeda Galaxy

White Dwarf Supernovae: Standard Bombs 

These supernovae have a very narrow range of peak 

luminosities since they always occur in the same way. 

To calibrate this peak, we must observe supernovae in 

galaxies with distances known from Cepheid variables.

An Extragalactic Distance Scale 

10 -3 ly 1 ly 10 3 ly 10 6 ly 10 9 ly 

Interlocking methods allow distances up to ~10 10 ly to 

be measured fairly reliably.

Spiral Galaxy, Inclined 

M63: The Sunflower Galaxy

Spiral Galaxy, Edge-On 

NGC 4565: Needle Galaxy

Barred Spiral Galaxy 

NGC 1365: A Nearby Barred Spiral Galaxy

‘Grand Design’ Spiral Galaxy 

M51 Hubble Remix

Disk Galaxy With Large Bulge 

The Sombrero Galaxy from VLT

Lenticular Galaxy With Dust 

NGC 2787: A Barred Lenticular Galaxy

Giant Elliptical Galaxy With Companions 

Galaxies Away

Dwarf Elliptical Galaxy 

Companions to M31 

M32: Blue Stars in an Elliptical Galaxy 

M31: The Andromeda Galaxy

Irregular Galaxy (Large Magellanic Cloud) 

The Large Cloud of Magellan

Peculiar Galaxy 

The Colliding Galaxies of NGC 520

Hubble’s Galaxy 

Classification 

Irregular and peculiar 

galaxies not included. 

The Hubble Tuning Fork — Classification of Galaxies

The Local Group: Over 30 Galaxies 

two large spirals 

with satellites 

one smaller 

spiral 

many dwarf elliptical and irregular galaxies 

Local Group

The Virgo Cluster: : Over 1000 Galaxies! 

Distance: ~6 × 10 7 ly 

three massive elliptical galaxies 

many MW-sized galaxies 

M86 in the Virgo Cluster

A Rich Regular Galaxy Cluster 

Distance: ~2.5 × 10 8 ly 

mostly elliptical galaxies 

Galaxies of the Perseus Cluster

A Rich Irregular Galaxy Cluster 


many disk galaxies 

some are colliding 

The Hercules Cluster of Galaxies

A Compact Group 


one elliptical galaxy 

three spiral galaxies 

Galaxy Group Hickson 44

2. HUBBLE’S LAW AND GALAXY EVOLUTION 

a. The Expanding Universe 

b. Looking Back in Time 

c. Class Survey

The Doppler Shift 

Doppler Effect 

Doppler Effect 

A stationary source sends 

out waves of the same 

wavelength in all 

directions. 

If the source is moving, 

the waves bunch up 

ahead of its motion, and 

spread out behind.

The Doppler Shift: Light 

We get a similar effect 

with light. The change in 

wavelength λ depends on 

the source’s velocity v 

toward or away from us: 

red-shift 

blue-shift 

λshift - λrest 

λrest 

= v c 

Note: valid for v c 

where λshift is the observed (shifted) wavelength, λrest is 

the wavelength with the source at rest, and c is the 

speed of light.

The Redshift 

Text 

Most galaxies have spectra systematically shifted toward 

the red, implying that they’re moving away from us.

The Redshift: An Example 

Define the redshift: 

λshift - λrest 

λrest 

= z 

line 

λrest 

(nm) 

λshift 

(nm) 

z 

v = c z 

(km/s) 

Hβ 486.1 500.9 0.0304 9120 

Hγ 434.1 447.3 0.0304 9120 

Hδ 410.2 422.7 0.0304 9120 

Arp 188 and the Tadpole's Tidal Tail

The Expansion of the Universe 

Plotting galaxy velocities, 

v, against their distances, 

d, revealed a relationship: 

v H0 d, 

where H0 is Hubble’s 

“constant”:

The Expansion of the Universe 

Plotting galaxy velocities, 

v, against their distances, 

d, revealed a relationship: 

v H0 d, 

where H0 is Hubble’s 

“constant”: 

H0 22 km/s/Mly. 

Two consequences: 

(1) galaxy redshifts can be used to estimate distances; 

(2) the universe is expanding.

The Cartoon History of the Universe

The Universe Has No Center! 

Observed from Galaxy A 

Observed from MW 

Observed from Galaxy B 

MW 

MW 

MW 

A 

B 

A 

B 

A 

B 

All observers see other galaxies moving away from their 

position with speeds proportional to distances. 

The expansion does not define a center!

The Universe Has No Center! 

The universe shows no sign 

of edges — it seems to be 

infinite in all directions. 

Cosmological Principle: 

The universe looks 

roughly the same 

everywhere. 

• Matter is evenly distributed on very large scales. 

• There is no center and no edges. 

• Not proved but consistent with observations.

The Universe Has An Age! 

Assume that galaxies move apart at constant speeds; 

how long ago were they all ‘on top of each other’? 

A galaxy at distance d = 1000 Mly moves away at speed 

v = H0 d = (22 km/s/Mly) × 1000 Mly = 22000 km/s 

The time required to travel this distance is 

d 

v = 

1000 Mly 

22000 km/s = 9.5×1021 km 

22000 km/s = 4.32×1017 s = 13.7 Gyr 

(Note: d cancels out; you get the same time for any d!) 

13.7 Gyr is a good estimate for the Universe’s age!

High Redshift 

Redshifts z bigger than 

one can’t be interpreted 

dtoday 

in terms of galaxy velocity: 

v = c z 

A correct interpretation: 

dtoday 

1 + z = d then 

then 

dthen 

Example: from redshift 

z = 2 to today, galaxy 

distances have tripled. 

galaxy 

location 

then

Looking Back in Time 

Light travels at finite speed, so when we look out into 

space we are also looking back in time! 

Many of these galaxies are billions of lightyears away, so 

we’re seeing them as they were billions of years ago. 

Galaxy History Revealed in This Colorful Hubble View

Lookback Time 

Lookback time is related 

to redshift: longer times 

correspond to higher 

redshifts. 

z 

tback (Gyr) 

then 

1 7.73 

2 10.3 

3 11.5 

∞ 13.7 

galaxy 

location 

then

z ≈ 2

High-Redshift Galaxies 

200-million-year-old baby galaxies 


These galaxies have redshifts z ≈ 7 to 7.5, implying 

lookback times of ~ 13 Gyr; back then, the age of the 

universe was only 700 Myr.

High-Redshift Galaxies 



• Irregular shapes; no apparent symmetry 

• Very high rates of star formation 

• Powerful outflows of gas

1. What do we need to know about a star before we 

can use it as a standard candle? 

A. Mass 

B. Diameter 

C. Age 

D. Luminosity 

E. Temperature

2. We compute the peak luminosity of a white-dwarf 

supernovae in another galaxy by determining its 

distance using __________ and measuring its 

__________. 

A. parallax; temperature from spectra 

B. Cepheids in the same galaxy; apparent brightness 

C. main-sequence fitting; apparent brightness 

D. Cepheids in the same galaxy; makeup from spectra 

E. parallax; mass using orbital motion

3. What kind of galaxy is this? 

A. Irregular 

B. Elliptical 

C. Barred spiral 

D. Regular spiral 

E. High-Redshift 

NGC 1365

4. Where do we see evidence of recent star formation? 

D 

A 

B 

C 

E 

NGC 1365

5. Which of these is an elliptical galaxy? 

A 

D 

E 

B 

C 

M86 in the Virgo Cluster

6. How can we tell another galaxy is moving away? 

A. It appears smaller from year to year. 

B. It appears fainter from year to year. 

C. Its spectral lines are shifted toward the blue. 

D. Its spectral lines are shifted toward the red. 

E. Its parallax angle gets smaller over time.

7. If galaxy A has redshift zA = 0.05 and galaxy B has 

redshift zB = 0.1, 

A. galaxy A is twice as far as galaxy B. 

B. both galaxies have the same distance. 

C. galaxy B is twice as far as galaxy A. 

D. galaxy B is four times as far as galaxy A. 

E. we cannot tell which galaxy is further.

8. Which statement is most likely to be correct? 

A. Other galaxies are moving away from the Milky 

Way, but not from each other. 

B. Every galaxy in the universe is surrounded by other 

galaxies which are moving away from it. 

C. The universe is a finite sphere of galaxies expanding 

into empty space. 

D. All galaxies are orbiting the center of the universe. 

E. Galaxies are moving away from the Milky Way with 

speeds which do not depend on their distances.

3. PECULIAR AND ACTIVE GALAXIES 

a. Galaxy Collisions 

b. Starburst Galaxies 

c. Active Galaxies

e 1.4 Galaxy sample in this study. Top and middle row from left to right: Arp 

7469, NGC 4676 and Arp 299. Bottom row from left to right: IC 883, NGC 2623 

Interacting and Merging Galaxies 

Figure 1.4 Galaxy sample in this study. Top and middle row fro 

NGC 7469, NGC 4676 and Arp 299. Bottom row from left to right 

NGC 7252. North is up, and east is to the left. Most colored image 

ACS/WFC images (courtesy of NASA, the Hubble Heritage, A. Ev 

http://hubblesite.org/newscenter/archive/releases/galaxy/interac 

and courtesy of NASA, H. Ford, G. Illingworth, M. Clampin, G. Ha 

Team, taken from http://hubblesite.org/newscenter/archive/relea 

Image of NGC 7252 is restored from B- and R-band images taken 

from Hibbard et al. (1994). 

Some galaxies don’t fit the elliptical/spiral/irregular classification. 

Figure 1.4 Galaxy sample in this st 

12

How Can Galaxies Collide? 

If galaxies move away from each other as the universe 

expands, how can they ever collide? 

Interacting Galaxy UGC 9618 

The gravitational attraction of two massive galaxy halos 

can locally reverse the expansion and cause a collision. 

Most interacting pairs probably fell together “recently”.

Tides between disk galaxies create filaments of stars. 

SPIN 

–0.5 

0 

0.5 

1 

1.5 

2 

2.5 

3 

Galactic Bridges and Tails

A Simulated Interaction

A Simulated Interaction 

Tidal Interaction

The Whirlpool Nebula 

M51 Hubble Remix

The Mice: Two Colliding Spirals 

"The Mice": Colliding Galaxies 

NGC 4676: True-Color RGB Image

Simulation of the Mice 

The Mice at Play

Why do Galaxies Merge? 

Tidal forces transform the organized orbital motion of 

galaxies into random motions of stars and dark matter. 

• This is a form of friction — it slows galaxies down. 

• Dark matter plays critical role — absorbs momentum.

What Kind of Galaxy is Produced? 

Random stellar orbits can naturally account for the oval 

shapes and slow rotation of elliptical galaxies. 

• Merger hypothesis: spiral galaxies merge to form 

elliptical galaxies. 

• Estimated merger rates can produce right number of 

elliptical galaxies. 

• Need additional star formation in mergers to form 

cores of elliptical galaxies.

The Antennae 

Super Star Clusters in the Antennae Galaxies 

Rapid star formation is common 

NGC 4038/4039 

in merging spiral galaxies!

Antennae Simulation With Star Formation

Starburst Galaxies 

Galaxy Wars: M81 versus M82

Starburst Galaxies 

Starburst Galaxy M82 

Star formation rate: 

~10 × Milky Way’s. 

Gas outflow driven 

by supernovae 

Galaxy Wars: M81 versus M82

Arp 299: Supernova Factory 

First encounter 

~700 Mry ago 

Ultra-Luminous 

Infrared Galaxy 

(L > 10 12 L ) 

Interacting Galaxy NGC 3690

Arp 220: Merger Remnant 

Core contains as much 

gas as entire Milky Way! 

A Collision In The Heart Of A Galaxy 

Star formation rate: 

~100 × Milky Way’s! 

Active nucleus 

as well as stars. 

Interacting Galaxy Arp 220

If the center of a 

galaxy is 

unusually bright, 

we call it an 

active galactic 

nucleus. 

Quasars are the 

most luminous 

examples. 

Active Nucleus in M87 

Copyright © 2009 Pearson Education, Inc.

Galaxies 

around 

quasars 

sometimes 

appear 

disturbed by 

collisions. 


Radio galaxies contain active nuclei shooting out vast jets of 

plasma that emit radio waves coming from electrons moving at 

near light speed. 


Characteristics of Active Galaxies 

• Luminosity can be enormous (>10 12 L Sun ). 

• Luminosity can rapidly vary (comes from a space 

smaller than solar system). 

• They emit energy over a wide range of wavelengths 

(contain matter with wide temperature range). 

• Some drive jets of plasma at near light speed. 


What is the power source for quasars 

and other active galactic nuclei? 


The accretion of gas onto a supermassive black hole appears to 

be the only way to explain all the properties of quasars. 


Energy from a Black Hole 

• The gravitational potential energy of matter falling 

into a black hole turns into kinetic energy. 

• Friction in the accretion disk turns kinetic energy 

into thermal energy (heat). 

• Heat produces thermal radiation (photons). 

• This process can convert 10–40% of E = mc 2 into 

radiation. 


Jets are thought to come from the twisting of a magnetic field 

in the inner part of the accretion disk. 


Copyright © 2009 Pearson Education, Inc. 

Do supermassive black holes 

really exist?

Orbital speed and distance of gas orbiting center of M87 

indicate a black hole with mass of 3 billion M Sun . 


Orbits of stars 

at center of 

Milky Way 

indicate a black 

hole with mass 

of 

4 million M Sun . 


Galaxies and Black Holes 

• The mass of a 

galaxy’s 

central black 

hole is closely 

related to the 

mass of its 

bulge. 


Galaxies and Black Holes 

• The 

development 

of a central 

black hole 

must 

somehow be 

related to 

galaxy 

evolution. 


Galaxy Mergers With Gas 

Transformations of Galaxies II: Gas Only 

Transformations of Galaxies II: Final Encounter

Mergers With Gas and Black Holes 

Galaxy Collisions Awaken Dormant Black Holes

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