Learning Goals Students will: 1) Learn to classify types of
galaxies 2) Explain the formation of galaxies
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Success Criteria Students will show their understanding of
learning goals by: 1) Explaining how galaxies are classified based
upon shape (and structure) 2) Explaining how galaxies were formed
3) How the classification of galaxies strongly indicates the age
(stage of development) of the galaxy)
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http://www.youtube.com/watch?v=ENiqpA2yCfc (Introduction to
galaxies) http://www.youtube.com/watch?v=ENiqpA2yCfc This is the
Andromeda Galaxy (M31), our closest neighbouring large galaxy. It
is one of the farthest things you can see with the naked eye in the
night sky. It is 2.4 million light years away.
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Hubbles Tuning Fork Created in 1925 by Edwin Hubble. A
classification used to classify galaxies that are elliptical (E) in
shape or are spiral with (SB) or without bars (S). Classified based
on bulge strength and arm characteristics for spirals. This system
is only based on photographed evidence. Astronomers in the past few
decades have found that many galaxies do not fit into this system
very well. One problem is that galaxies often interact with each
other with the result that gravity pulls the two galaxies into
irregular shapes.
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Hubbles Tuning Fork Hubbles Tuning Fork is the method of
classifying galaxies note that the Spiral galaxies are broken into
two groups unbarred and barred. Note the sub-classifications
(Astronomers will use sub- classifications such as Sab, which is an
intermediate of Sa and Sb).
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Elliptical galaxies are broken into seven division based on
shape, E0 galaxies are circular while E7 galaxies are elliptical.
Spiral galaxies are broken into two classes, the Spirals and Barred
Spirals
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Types of Galaxies 1. Elliptical Galaxy (E0 - E7) Elliptical
galaxies are shaped like a spheriod, or elongated sphere. In the
sky, where we can only see two of their three dimensions, these
galaxies look like elliptical, or oval, shaped disks. The
sub-classifications (E0 to E7) are based on the axial ratio of the
elliptical shape. E0 galaxies are circular in shape and E7 are very
flat. One third of the galaxies are elliptical. Range in size form
6000 ly across to 300 000 ly.
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Types of Galaxies 2. Spiral (Sa-Sc or SBa-SBc) Spiral galaxies
have three main components : a bulge, disk, and halo (see right).
The bulge is a spherical structure found in the center of the
galaxy. This feature mostly contains older stars. The disk is made
up of dust, gas, and younger stars. The arm structures extend
directly from the central bulge. Our Sun is located in an arm of
our galaxy, the Milky Way. Sa - Sc : Unbarred Spirals - these
galaxies have a large nuclear bulge and tightly wound, smooth arms.
SBa - SBc: Barred Spirals - this group has a small bulge, loosely
wrapped, textured arms.
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Types of Galaxies 2. Spiral Galaxies This v isual
classification chart shows examples of each type of galaxy. Note
that the S0 galaxies are an intermediate between elliptical and
spiral galaxies called lenticular galaxies. Lenticular galaxies
contain a central bulge like a spiral galaxy, but there are no
well-defined arms.
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Types of Galaxies Spiral Galaxies with no bars (S) Whirlpool
Galaxy (M51) the Sombrero galaxy, M104
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Types of Galaxies Barred Spiral (SB) have a straight bar of gas
extending from either side of the nucleus. Barred Spiral Galaxy NGC
1300 NGC 1365
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Types of Galaxies 3. Irregular Galaxies These have no regular
shape and are composed of mostly gas and no dust. They do not fit
the Hubble classification system. They are really faint, so they
are hard to see. Barnard's Galaxy or NGC 6822
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NGC 2363 Cigar Galaxy (M82)
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Types of Galaxies 4. Peculiar Galaxies These are galaxies that
look regular, but have one major difference. Many peculiar galaxies
are the result of the interaction of two galaxies the gravitational
effects disturb the spiral shape. Oddly, as two galaxies pass
through each other, there are very few star collisions. It is
thought that our own galaxy will eventually collide with its larger
neighbour, the Andromeda galaxy. Centaurus A shows evidence of a
large explosion in the middle.
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Some peculiar Galaxies interact NGC3808A & NGC3808
NASA/ESA
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Newer Classification Systems 1. Morgan classification - This
system is based on the shape of the galaxy as well as its spectrum,
rotational symmetry, and central concentration. 2. de
Vaucouleurs-Sandage classification This system is a more detailed
version of the Hubble system and includes irregular galaxies.
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Other Types of Classification DDO (or van den Bergh)
classification of galaxies This system is based on shaped and
luminosity.
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Relating Age to the Structure of Galaxies The shape of galaxies
evolve as they get older. They start off as a disk containing a
number of bright clumps in which stars are forming but with little
star formation elsewhere in the disk. As a galaxy ages, the central
bulge of star formation emerges. Clumps of stars at the edges of
the galaxy begin to spread out, forming indistinct arms. Only
later, do the arms become more distinct and take on the familiar
spiral shape. At first, spiral galaxies have only two arms, but
later they evolve to have more arms. Hence, the position a galaxy
has on the Hubble Tuning fork is suggestive of the age of
galaxies.
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Local Groups Once thought to be isolated, galaxies actually
exist in groups and they can interact and/or collide. The Milky Way
is one of the largest galaxies in our local group of about 40 known
galaxies. Our group also includes the Andromeda galaxy (the largest
in the Local Group and the closest) as well as the Large Magellanic
Cloud (LMC) a dwarf galaxy.
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The Milky Way
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The Structure of the Milky Way In general our galaxy looks like
a pancake with a bulge in the middle. There are four parts to our
galaxy: 1. The nucleus Located 25,000 light-years from us. It is
hidden from view by dust, but it can be observed by infrared and
radio waves. At the centre is a large black hole that has a mass of
millions of times that of our sun.
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The Structure of the Milky Way 2. The nuclear bulge Looks like
a flattened sphere that is twice as long as it is wide. Contains a
lot of dust and densely packed old stars. 10, 000 light-years in
radius.
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The Structure of the Milky Way 3. The galactic disk The part of
the pancake that is outside the bulge. It extends for 70, 000
light-years from the middle. Our sun is located 28, 000 light-years
from the middle. The galactic disk is broken down into two parts
Thin disk this contains 95% of the stars of our galaxy Thick disk
Contain older stars and is 3 times as tall.
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The Structure of the Milky Way 4. The Halo Composed of very old
stars, dust, gases, and Globular clusters. Globular clusters are
compact stellar systems that contain lots of stars. These object do
not rotated around the nucleus.
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Homework Questions 1. P. 838#1,2 & 3 2. p. 846 #1,2,4,
& 5
