Formation of Galaxies
Types of Galaxies
Learning Goals
 Students will:
1) Learn to classify types of galaxies
2) Explain the formation of galaxies
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)
 http://www.youtube.com/watch?v=ENiqpA2yCfc
(Introduction to galaxies)
 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.
Hubble’s 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.
Hubble’s Tuning Fork
 Hubble’s 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 subclassifications such as Sab, which is an intermediate of Sa and
Sb).
 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
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.
Types of Galaxies
2. Spiral (Sa-Sc or SBa-SBc)
 Spiral galaxies have three main components: a
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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.
Types of Galaxies
2. Spiral Galaxies
 This visual 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.
Types of Galaxies
Spiral Galaxies with no bars (S)
the Sombrero galaxy, M104
Whirlpool Galaxy (M51)
Types of Galaxies
Barred Spiral (SB) have a straight bar of gas extending
from either side of the nucleus.
NGC 1365
Barred Spiral Galaxy NGC 1300
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
NGC 2363
Cigar Galaxy (M82)
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.
Some peculiar Galaxies
interact
NGC3808A & NGC3808
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.
Other Types of Classification
 DDO (or van den Bergh) classification of galaxies
This system is based on shaped and luminosity.
Relating Age to the Structure
of Galaxies
 The shape of galaxies evolve as they get older. They start off as
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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.
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.
The Milky Way
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
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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.
The Structure of the Milky
Way
The nuclear bulge
2.
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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.
The Structure of the Milky
Way
The galactic disk
3.
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
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Thin disk – this contains 95% of the stars of our galaxy
Thick disk – Contain older stars and is 3 times as tall.
The Structure of the Milky
Way
The Halo
4.
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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.
Homework Questions
1.
2.
P. 838#1,2 & 3
p. 846 #1,2,4, & 5