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Galaxies
The Basic
Element of Cosmology
Warm Up
Please complete a K-W-L chart for
the topic of galaxies.
Warm Up-01/03/12
1.
2.
3.
What was the Hubble “Deep Field”
project?
What did it discover?
What type of galaxy is our Milky
Way galaxy?
Edwin Hubble Webquest
http://www.nisk.k12.ny.us/faculty/hug
hes/Hubble3/edwin_hubble_web_que
st.htm
Warm Up
1.
2.
3.
4.
5.
6.
7.
What is a galaxy?
What did Charles Messier do to link his name to
galaxies?
Who was Edwin Hubble and what did he do?
What are the three types of galaxies?
What is the difference between an E0 elliptical
galaxy and an E7?
What is the difference between an Sa spiral
galaxy and an Sc?
Draw and label the Hubble Galaxy Classification
scheme!
Deep Field The most important
picture ever taken?
Hubble Ultra-deep Field
Out beyond our galaxy…
Out beyond our galaxy, the depths of
space is filled with other star
systems. Some are like our own.
Galaxies
We call these star systems galaxies.
Galaxies come in many shapes and sizes.
Some galaxies are egg-shaped with clouds
of stars evenly spread throughout.
Galaxies
Some galaxies are completely irregular in
shape.
Galaxies
Galaxies not only differ in size and shape, but the
also differ in their content. Some galaxies are
young and some are old.
What Is a Galaxy?
A galaxy is an
immense cloud of
millions or billions of
stars. Each star while
following its own orbit
is locked in place by
the gravitational
influence of all the
other stars in its
galaxy. Each galaxy
is an independent and
isolated star system.
Early Observation
Because galaxies are
so distant, they are
difficult to see with
the unaided eye. The
closest galaxy to our
Milky Way is over
150,000 light years
away. Al-Sufi, a
Persian astronomer,
noted observing the
M31 (Andromeda)
galaxy in the 10th
century.
Early Observation
19th century
Frenchman Charles
Messier, studied
comets. While looking
for them he noticed a
number of faint,
diffuse patches of
light. To avoid
confusing them with
comets in the future
he began to assign
them numbers.
Galaxies are still
referred to by their
“M” or Messier
number.
Early Observation
Other astronomers came
along, such as William
Herschel and John Dryer
and created their own
catalog. It became
known as the New
General Catalog or “NGC”
for short. Some galaxies
appear in both catalogs
and therefore have two
names. M82 is the same
galaxy as NGC 3034
Edwin Hubble
While today we know
that galaxies are
immense star
systems, the idea was
not widely accepted
until the 1920’s. It
was a graduate
student at the
University of Chicago
that would define the
nature of galaxies.
That man was Edwin
Hubble.
Edwin Hubble
Edwin Hubble noticed that when he looked at
galaxies that they looked quite different.
However, many shared certain characteristics.
Hubble used these similarities the categorize all
galaxies.
Types of Galaxies
Hubble defines three basic types of
galaxies: the spiral galaxy, the
elliptical galaxy and the irregular
galaxy. These are often denoted by
the letters S, E and Irr, respectively.
Subtypes of Galaxies/Elliptical
Hubble subdivided the elliptical
galaxies (denoted as E). He ranked
them from E0 to E7. E0 galaxies had
zero eccentricity while E7 are highly
elliptical.
The Hubble Tuning Fork
Galaxy E0 and E1
Galaxy E2 and E3
Galaxy E4 and E5
Galaxy E6 and E7
Spiral Galaxies
Spiral galaxies (S) consist of central
bulges surrounded by sweeping
spiral arms emanating from their
centers. Hubble classified them from
Sa to Sd. Sa galaxies have large
central bulges and tightly wound
arms while Sd galaxies have small
central bulges with loosely wound
spiral arms.
The Hubble Tuning Fork
Spiral Galaxies
Subtypes of Galaxies/Barred
Hubble recognized subgroups within the
major types as well. The first of these is
the barred spiral galaxy (denoted SB).
Barred spiral galaxies have arms that
emerge from an elongated central region.
Barred spirals are ranked from a to d, with
SBa galaxies having large bulges and
tightly wound arms to SBd galaxies having
very small bulges with very loosely wound
arms.
Barred Spiral Galaxies: SBa
Barred Spiral Galaxies: SBb
Barred Spiral Galaxies: SBc
Irregular Galaxies
S0 Galaxies
S0 galaxies are those with nuclei surrounded by a
disk-like structure without arms. The S0 galaxy
shares properties of both spiral and elliptical
galaxies and seems to bridge the gap between
the two major types of galaxies.
Hubble introduced the S0 class long after his
original classification scheme had been
universally adopted largely because he noticed
many highly flattened objects that otherwise had
the properties of elliptical galaxies.
S0 Galaxies
Spiral Galaxies
Galaxies differ in ways other than
shape, too. They differ in content as
well. Spiral galaxies have a pretty
even mixture of both old (population
II) and new (population I) stars.
Usually, spiral galaxies contain about
15% of their mass as gas and dust
Elliptical Galaxies
Elliptical galaxies contain mostly
older, population II stars. However,
they generally retain about half their
mass as dust and gas suggesting
that they still have vast amounts of
stars to create.
Evolution?
The Hubble “tuning fork” was not
created to imply an evolutionary
path for galaxies. However,
astronomers have seen evidence of
galaxies changing types. The
collision between spiral galaxies of
similar mass is believed to create
elliptical galaxies.
Warm Up
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6.
What is galactic cannibalism?
When one galaxy absorbs another, what
evidence do you see?
If two spiral galaxies combine, what is
the end product?
What is the closest galaxy to our own?
What is the period-luminosity
relationship?
How does the period-luminosity help to
determine how far another galaxy is
away from us?
Creation of Galaxies
Merger seems to play an important role in
the formation of galaxies. Primarily
amount star clusters. This may be one
way to explain the vast difference in the
ages of stars contained in spiral galaxies.
As a smaller galaxy is absorbed by
another, its stars are attracted to the
central portion of the larger bulge.
Galactic collision. It’s called galactic
cannibalism, the big eating the small.
Galactic Merger
The Causes of Galactic Types
Astronomers are still uncertain as to
the exact causes of galaxy types.
Spiral disks rotate rapidly when
compared to elliptical halos.
However, halos and bulges in spiral
galaxies rotate much more slowly.
Thus astronomers believe that more
than a galaxy’s rotation determines
its type.
Many astronomers believe that
galaxies form from merger of like
galaxies followed my galactic
cannibalism. Streams of stars can
be seen trailing stretching from our
galactic halo. Astronomers think
theses stars are the remnants of
smaller galaxies absorbed by our
Milky Way.
Galactic Cannibalism
This beautiful, eerie silhouette of dark dust clouds against
the glowing nucleus of the elliptical galaxy NGC 1316 may
represent the aftermath of a 100-million-year-old cosmic
collision between the elliptical and a smaller companion
galaxy
Can Merger Explain the Differences
between spiral and Elliptical Galaxies?
Many astronomers say yes. Both
observation and computer modeling
are compelling astronomers to
propose a new hypotheses for the
origin of spiral and elliptical galaxies.
The theory should say that new
galaxies are born as disk-like
systems lacking a central bulge.
Subsequent collisions and mergers
with other star systems create
elliptical galaxies. This theory has
added attractions as well.
Many astronomers once believed that
galaxies were formed from one
creation event. But, looking at the
bulge of our galaxy, you see stars of
various ages throughout.
A massive black hole hidden at the center of
nearby galaxy, Centaursus A, feeds on a
smaller galaxy in a spectacular collision.
Merger Model
With each merger, the left over gas
from the cannibalized galaxy would
gravitate to the center and begin to
form new stars. This accounts for
the stars of various ages existing
there.
Merger Model
Another feature of the merger model
is that it also explains large spiral
galaxies. When a large spiral galaxy
merges with an elliptical galaxy, the
elliptical galaxy becomes the bulge of
the new spiral galaxy.
Measuring the Properties of
Galaxies
Galaxies are
separated by
enormous distances.
The closest galaxy to
our own is the Large
Magellanic Cloud
about 150,000 light
years away. The
distances are so great
that using parallax will
not work to determine
distance.
Candle Power
One of the techniques used to
measure the size of the Milky Way
was “standard candles” of
luminosity, discussed in Chapter 15
(The Milky Way). The most reliable
sources of known standard candles is
the “Cepheid variable” (Chapter 13).
Techniques for Measuring Distances
Cepheids
Cepheids are convenient to use for several
reasons. One, the are very bright,
averaging about 1 million times the Sun’s
luminosity. They can be seen a very long
way away.
Cepheids
Because Cepheids
are variable,
astronomers can
use the periodluminosity
relationship to
determine exactly
how bright the
variables are.
Red-shift and the Hubble Constant
We know that
when we look at
other galaxies, the
spectrum from
those galaxies is
red-shifted. This is
due primarily to
the Universe
expanding. This
was first noticed by
Vesto Slipher.
Red-shift and the Hubble Constant
Others, like Hubble, also noticed that the dimmer
the star (i.e. the farther away the star), the
larger the red-shift. The speed that a galaxy
moves away from us (“V”), its recessional
velocity, increases with distance (“D”).
The Hubble law
Hubble defined the relationship with the Hubble
law which says:
V=HD, where
V = Recessional Velocity, in kilometers/second
D = Distance, in parsecs (3.26 million light-years)
and
H = Hubble Constant (70 km/sec/Mpc)
The Hubble Constant
The Hubble Constant
created lots on
controversy initially.
To calibrate the
formula, you have to
know both the
distance and velocity
of at least a few
galaxies. The solution
was to simply assign
the number 70
km/sec/Mpc.
Measuring the Diameter of a
Galaxy
To determine the diameter of a
galaxy, use the formula:
d = 2pAD
360
where:
d=diameter
A=angular size
D=distance
Measuring the Mass of a Galaxy
We can use Kepler’s third law. The
orbital velocity tells you the mass.
There is one problem though. When
you add up all the mass that we see,
it does not equal the mass indicated
by our galactic motion. It is always
less.
Why Less?
How is it that our galaxy is more
massive than the sum of all that we
see? Easy, there is stuff out there
we can’t see. It’s called dark matter!
Dark Matter
Dark matter is what scientists
believe accounts for the discrepancy
between the mass we see and the
mass needed to make our galaxy
move as it does. How much are we
talking here? Maybe 10 times more
massive. We are only seeing on
average about 10% of a galaxies
mass.