Transcript Galaxy notes

The Universe
Looking at 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.
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Galaxies
• Galaxies are systems of stars, gas and
dust
• They exist in a wide variety of shapes and
sizes
• 4 basic types
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
Galactic Inquiry
1. Looking at the 15 pictures of galaxies
develop your own classification system
2. Get a Hubble classification system from
me and use the Hubble tuning fork to
reclassify the galaxies
3. Answer the reflection questions.
Review Four basic types
elliptical, spiral,
barred spiral, and
irregular
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.
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Hubble’s Galaxy Classification
Ellipticals are classified according to their shape from E0
(almost spherical) to E7 (the most elongated)
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What would you classify this
galaxy? Tell your elbow partner
why.
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Spiral galaxies
• Have a central nucleus surrounded by a
flattened disc with the stars, gas, and dust
organized into a pattern of spiral arms.
• Greater concentration of stars near their centers
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.
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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?
Hubble’s Galaxy Classification
Similar to the spiral galaxies are the barred spirals
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Barred Spiral
Has a bar through the center with arms extending
through the bar
The variation from SBa to SBc is similar to that for
the spirals
What is this classified as?
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.
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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.
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Hubble’s Galaxy Classification
Hubble’s “tuning fork” is a convenient way to remember the
galaxy classifications.
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Let’s 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
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