Galaxies

• How many galaxies do you think are
there in the universe?
• Is it possible for us to circumnavigate
our own galaxy?
• Do you think that there is also another
“Earth” somewhere out there?

• Types of Galaxies
• Common Galaxies
• Location of our Solar System in the
Galaxy
• Great Nebula of Andromeda
• Quasars
• Clouds of Magellan (Large and Small
Magellanic Cloud)

• Elliptical galaxies are smooth and elliptical in
appearance.
• Most common type of galaxy
• There are four distinguishing characteristics of
the ellipticals:
(a)they have much more random star motion than
orderly rotational motion (star orbits are aligned
in a wide range of angles and have a wide
range of eccentricities)
(b) they have very little dust and gas left between
the stars;

(a) this means that they have no new star
formation occuring now and no hot, bright,
massive stars in them (those stars are too
short-lived); and
(b) they have no spiral structure.

• Spiral galaxies have flattened disks
with a spiral pattern in the disk.
• The spiral arms can go all of the way
into the bulge or be attached to the
ends of a long bar of gas and dust
that bisects the bulge.

• The four distinguishing characteristics of the spirals
are:
(a) they have more orderly, rotational motion than
random motion (the rotation refers to the disk as a
whole and means that the star orbits are closely
confined to a narrow range of angles and are fairly
circular);
(b) they have some or a lot of gas and dust between the
stars;
(c) this means they can have new star formation
occuring in the disk, particularly in the spiral arms; and
(d) they have a spiral structure.

• Spiral galaxies are sub-classified into ``a'',
``b'', ``c'', and ``d'' groups according to how
loose their spiral arms are and how big the
nucleus is.
• The ``a'' group spirals have large bulges
and very tightly wound spiral arms and the
``d'' group spirals have almost no bulge
and very loose arms.

• The Milky Way is between the ``b'' and ``c''
groups with a bar, so it is an SBbc-type
spiral galaxy. Most spirals are luminous.
Some other examples of spiral galaxies
are M31 (the Andromeda Galaxy) and
M33 (a small spiral in the Local Group).

• Irregular galaxies have no definite structure.
• The stars are bunched up but the patches are
randomly distributed throughout the galaxy.
• Some irregulars have a lot of dust and gas so
star formation is possible. Some are
undergoing a burst of star formation now, so
many H II regions are seen in them.
• Others have very little star formation going on
in them (even some of those with a lot of gas
and dust still in them).

• Most irregulars are small and faint.
• The dwarf irregulars may be the most
common type of galaxy in the universe (or
maybe the dwarf ellipticals are).
• The estimates of the number of dwarf
irregulars and dwarf ellipticals are based
on the proportions of these types of
galaxies in nearby groups.

• The dwarf galaxies far away are too faint
to be seen and are, therefore, overlooked
in surveys of the sky. Perhaps if the dwarf
galaxies were brighter, Hubble would have
arranged the galaxies in a different
sequence instead of the two-pronged
sequence.
• Examples of irregular galaxies are the
Large and Small Magellanic Clouds (two
small irregulars that orbit the Milky Way).

• What are the types of galaxies?
• What is the most common type of
galaxy?
• What type of galaxy is the Milky
Way?
• Describe an Elliptical Galaxy.
• Describe a Spiral Galaxy.
• Describe an Irregular Galaxy.

GALAXY DESCRIPTION
Milky Way
Galaxy
This is the galaxy that
contains Earth, it is named
after the nebulosity in the
night sky that marks the
densest concentration of
stars of our galaxy in the sky,
which appears to blur
together into a faint glow,
called the Milky Way.

GALAXY DESCRIPTION
Andromeda Commonly just Andromeda,
this, called the Andromeda
Galaxy, Andromeda
Nebula, Great Andromeda
Nebula,Andromeda Spiral
Nebula, and such, has been
traditionally called
Andromeda, after the
constellation in which it lies.

GALAXY DESCRIPTION
Large
Magellanic
Cloud
This is the fourth largest
galaxy in the Local Group,
and forms a pair with
the SMC, and from recent
research, may not be part of
the Milky Way system of
satellites at all.
It is visible with the naked
eye.

GALAXY DESCRIPTION
Small
Magellanic
Cloud
This forms a pair with
the LMC, and from
recent research, may
not be part of the Milky
Way system of satellites
at all.
One of the four irregular
galaxies

• Our home lies within the disk of the Milky
Way. It's the disk where most of the
galaxy's gas and dust is located.
• As a result this is where most new stars
are being born. Just as the Earth has an
equator, so too does the galaxy and we
are about 14 light years above what's
called the equatorial symmetry plane.

• As to the thickness of the disk, most
current estimates put it at around 1,000
light years thick. Obviously our solar
system lies very close to the galaxy's
equator.
• As to our distance from the center of the
galaxy, the best guess is that we are
26,000 to 28,000 light years from the
center.

• The estimates vary due to
uncertainty in the exact size of the
galaxy and the time it takes the
solar system to complete one
orbit of our galaxy.

• The yellow circle in the picture shows
the orbital path of our solar system as
it travels around the center of the
Milky Way. The red dot, located just
above the 'r' in the 'arm' associated
with the Orion Arm shows our current
location on this path.

• Which part of the Milky
Way Galaxy is our Solar
System located?

• A quasi-stellar radio source ("quasar") is a
very energetic and distant active galactic
nucleus.
• Quasars are extremely luminous and were first
identified as being high red shift sources
of electromagnetic energy, including radio
waves and visible light, that were point-like,
similar to stars, rather than extended sources
similar to galaxies.

• Quasars have all the same properties as active
galaxies, but are more powerful: the irradiation is
partially 'nonthermal' (i.e., not due to a black
body), and approximately 10 percent are
observed to also have jets and lobes like those
of radio galaxies that also carry significant (but
poorly known) amounts of energy in the form of
high energy (i.e., rapidly moving, close to the
speed of light) particles (either electrons and
protons or electrons and positrons).

• The energy radiated by the quasar is
about a thousand times more than that of
a normal galaxy.
• The brightness of a quasar varies over
periods of days or years.
• Astronomers believe that quasars are
eruptions at the center of large galaxies.
• They get their energy from a black hole.

• A black hole is created when an old
star collapses.
• It is a region in space with
tremendous gravity.
• A body pulled by this gravity gets
large amount of energy.

• What are the different types of
galaxies?
• What type of galaxy is the Milky
Way?
• Which part of the Milky Way
Galaxy is our Solar System
located?

Answer page 283 of your
Science book

Galaxies

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