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Galaxies
The Universe
• Early in the history of the universe,
hydrogen and helium (and other forms of
matter) clumped together by gravitational
attraction to form countless trillions of
stars. Billions of galaxies, each a cluster
of billions of stars, now form most of the
visible mass in the universe.
When stars are viewed
through a telescope, they
appear as points of
light. However some objects
in the sky look like "fuzzy"
clouds. Some are nebulae
(star-forming regions) Others
are actually islands of stars
that are much farther from us
than the individual stars we
see in the night sky. These
islands are galaxies.
Hubble’s Galaxy Classification
This pair of images shows the Coma cluster of galaxies.
Almost every object visible is a galaxy.
© 2011 Pearson Education, Inc.
Galaxies
• Galaxies are systems of stars, gas and
dust
• They exist in a wide variety of shapes and
sizes
Galactic Inquiry
Purpose:
• Recognize that galaxies are a collection of
stars
• Understand that galaxies take a variety of
forms
• Learn to develop your own classification
scheme to understand how scientists
classify
• Comprehend the four major categories of
galaxies
Galaxy Classification Intro
Activity
• Follow the instructions carefully, in the first
part you will be developing your own
classification of galaxies, then you will look
at what Hubble thought.
Galaxies
• Galaxies are systems of stars, gas and
dust
• They exist in a wide variety of shapes and
sizes
• 4 basic types
Let’s Review Four basic types
elliptical, spiral,
barred spiral, and
irregular
Hubble Tuning Fork Scheme
The Milky Way is a spiral galaxy.
Most galaxies that we can see
are spiral galaxies. Looking at
other spiral galaxies, we can
begin to learn more about how
the spirals work.
Spirals
Hubble’s Galaxy Classification
Spiral galaxies are classified according to the size of
their central bulge
Hubble’s Galaxy Classification
Type Sa has the largest central bulge,
Type Sb is smaller, and Type Sc is the
smallest.
Type Sa tends to have the most tightly
bound spiral arms with Types Sb and
Sc progressively less tight, although
the correlation is not perfect.
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Hubble’s Galaxy Classification
The Sombrero galaxy, with its large central bulge, is a type
Sa. We cannot see the spiral arms, as they are edge-on.
© 2011 Pearson Education, Inc.
Turn to your elbow partner
 One of you hold up a text book or
notebook for your partner with the edge
facing your partner.
 Now hold the book with the title facing
your partner. How does the book look
different. Which is more recognizable as a
book?
 Try this with a pen. Look down the top of
the pen. Now look at it from the side.
How is the image different?
Variety of Spiral Arms
Flocculent spirals
(fleecy)
Grand-design spirals
(highly organized)
What do you
notice about
the colors of
the stars in the
spiral arms of
this galaxy?
The arms of
spiral galaxies
contain most of
the bright, blue
O and B type
stars.
Stellar Lifetimes
• Which live longer,
Red or Blue stars?
Stellar Lifetimes
• So the stars in the spiral arms are
mostly bright, hot, and young stars!
Since O stars
have such a
short lifetime, it
must be that:
the arms of
spiral galaxies
are where star
formation
happens!
If stars form in the spiral
arms, how do they end
up getting spread out
over the whole disk?
Spiral arms are not
permanent structures,
rather they are locations
where stars are
forming.
If spiral arms are not
permanent structures, then
what is making these locations
become active star forming
regions?
Spiral arms maybe caused by
spiral density waves – a spiral
shaped disturbances traveling
through the disk of the galaxy
that compressed gas and
.
triggers star formation.
Elliptical galaxies
Barred Spirals
Elliptical Galaxies
• About 60% of the galaxies
are round to oval in shape
Hubble’s Galaxy Classification
Elliptical galaxies have no spiral
arms and no disk. They come in
many sizes, from giant ellipticals
of trillions of stars, down to dwarf
ellipticals of less than a million
stars.
Ellipticals also contain very little,
if any, cool gas and dust, and
they show no evidence of
ongoing star formation.
Elipticals have large clouds of hot
gas, extending far beyond the
visible boundaries of the galaxy.
© 2011 Pearson Education, Inc.
Hubble’s Galaxy Classification
Ellipticals are classified according to their shape from E0
(almost spherical) to E7 (the most elongated)
© 2011 Pearson Education, Inc.
What would you classify this
galaxy? Tell your elbow partner
why.
© 2011 Pearson Education, Inc.
Galaxies that don’t seem to look
like spirals or ellipticals are
called….
Irregular
Irregular Galaxy
These galaxies have little symmetry in their structure
About 10% of the galaxies
Hubble’s Galaxy Classification
The irregular galaxies have a wide variety of shapes. The small
and large Magellanic Clouds are close neighbors to our own
Milky Way.
© 2011 Pearson Education, Inc.
Hubble’s Galaxy Classification
Here are several other irregular galaxies: AM 0644-741 and
its neighbors on the left, and NGC 1569 on the right.
© 2011 Pearson Education, Inc.
Hubble’s Galaxy Classification
Hubble’s “tuning fork” is a convenient way to remember the
galaxy classifications.
© 2011 Pearson Education, Inc.
Tutorial: Galaxy Classification
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
another. Take time to understand it now!!!!
• Come to a consensus answer you both agree
on.
• If you get stuck or are not sure of your answer,
ask another group.
Galaxy Classification and Properties:
Lecture Tutorial (handout)
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5
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3
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A galaxy that appears to be populated by mostly
red stars, likely:
1. never had blue stars in the galaxy
2. had blue stars that are not present
anymore but were at one time long
ago
3. has been around long enough for
the blue stars to have already
evolved into red stars
4. never contained enough gas to
have blue stars develop
5. has blue stars that are being
blocked by dust
0/0
Cross-Tab Label
Why are the arms of spiral
galaxies typically blue in color?
1. They are usually moving toward us and
are Doppler shifted to blue wavelengths.
2. The gas and dust in the arms filter out all
but the blue light from stars in the arms
3. Stars are forming in the spiral arms so
there are high mass, hot blue stars in the
arms.
4. Almost all the stars are in the arms of the
disk of the galaxy and their light makes
the arms appear blue.
See if you can locate a few
galaxies on Stellarium
M31 in Andromeda a spiral galaxy
M81 in Ursa Major a spiral galaxy
M95 in Leo a barred spiral
M87 in Virgo (near Mars) an elliptical
© 2011 Pearson Education, Inc.
The Local Group
The Andromeda Galaxy
• The Andromeda Galaxy is
the nearest large galaxy to
the Milky Way.
• It is 2.5 million light years
away.
• It takes the light from the
stars and gas in
Andromeda 2.5 million
years to get to us.
• That means, we see
Andromeda as it was 2.5
million years ago.
Tutorial: Looking At Distant
Objects
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
another. Take time to understand it now!!!!
• Come to a consensus answer you both agree
on.
• If you get stuck or are not sure of your answer,
ask another group.
Imagine that you simultaneously receive the satellite
transmission of two pictures of two people that live on
planets orbiting two different stars. Each image shows the
people at their 21st birthday parties. Consider the following
possible interpretations that could be made from your
observations.
Which do you think is the most plausible interpretation?
1. Both people are the same age but at different distances
from you.
2. The people are actually different ages but at the same
distance from you.
3. The person that is closer to you is actually the older of the
two people.
4. The person that is farther from you is actually the older of
the two people
Fifteen years ago, a quasar was observed
that was found to be located 8 billion light
years away. If our universe is approximately
15 billion years old, when did the quasar
emit the light that we observe?
1.
2.
3.
4.
5.
0/0
15 years ago
7 billion years ago
8 billion years ago
15 billion years ago
23 billion years ago
Cross-Tab Label
Colliding galaxies
• Take notes on the
following
• https://www.youtub
e.com/watch?v=U
QNo1hkcrYU
24.4 Active Galactic Nuclei
About 20–25% of galaxies don’t fit well into the Hubble
scheme—they are far too luminous.
Such galaxies are called
active galaxies. They differ
from normal galaxies in
both the luminosity and
type of radiation they emit.
Active galactic nuclei have
some or all of the following
properties:
high luminosity
• nonstellar energy emission
• variable energy output, indicating small nucleus
• jets and other signs of explosive activity
• broad emission lines, indicating rapid rotation
24.4 Active Galactic Nuclei
This active galaxy has star-formation rings surrounding a
very luminous core
Many active galaxies
have jets, and most
show signs of
interactions with
other galaxies.
Quasars—quasistellar objects—are
starlike in
appearance, but have
very unusual spectral
lines
Quasars (QSO’s)
• A quasi-stellar radio source is a very
energetic and distant active galactic
nucleus.
• Quasars are extremely luminous.
• A quasar is a compact region in the
center of a massive galaxy
surrounding a supermassive black
hole.
• 10–10,000 times the Schwarzschild
radius of the black hole.
Quasars
The Central Engine of an Active
Galaxy
This is the leading theory
for the energy source in an
active galactic nucleus: a
black hole, surrounded by
an accretion disk. The
strong magnetic field lines
around the black hole
channel particles into jets
perpendicular to the
magnetic axis.
This pair of images shows evidence for a black hole at the
center of NGC 4261.
Make your own Active Galaxy
Read through the introduction and background information
to Active Galaxies. Highlight anything that might make a
good test question or is an important point. With our
partners follow the instructions on the student worksheet to
make your own active galaxy.