the universe looking at galaxies. the universe early in the history of the universe, hydrogen and...
TRANSCRIPT
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.
© 2011 Pearson Education, Inc.
Hubble’s Galaxy Classification
This pair of images shows the Coma cluster of galaxies. Almost every object visible is a galaxy.
Galaxies
• Galaxies are systems of stars, gas and dust
• They exist in a wide variety of shapes and sizes
• 4 basic types
Galactic InquiryPurpose:
•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
© 2011 Pearson Education, Inc.
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.
Hubble’s Galaxy Classification
© 2011 Pearson Education, Inc.
Ellipticals are classified according to their shape from E0 (almost spherical) to E7 (the most elongated)
Hubble’s Galaxy Classification
© 2011 Pearson Education, Inc.
What would you classify this galaxy? Tell your elbow partner
why.
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
Spiral galaxies are classified according to the size of their central bulge
Hubble’s Galaxy Classification
© 2011 Pearson Education, Inc.
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.
Hubble’s Galaxy Classification
© 2011 Pearson Education, Inc.
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.
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?
© 2011 Pearson Education, Inc.
Similar to the spiral galaxies are the barred spirals
Hubble’s Galaxy Classification
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 structureAbout 10% of the galaxies
© 2011 Pearson Education, Inc.
The irregular galaxies have a wide variety of shapes. The small and large Magellanic Clouds are close neighbors to our own Milky Way.
Hubble’s Galaxy Classification
© 2011 Pearson Education, Inc.
Here are several other irregular galaxies: AM 0644-741 and its neighbors on the left, and NGC 1569 on the right.
Hubble’s Galaxy Classification
© 2011 Pearson Education, Inc.
Hubble’s “tuning fork” is a convenient way to remember the galaxy classifications.
Hubble’s Galaxy Classification
© 2011 Pearson Education, Inc.
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