Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a...
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Transcript of Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a...
![Page 1: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/1.jpg)
Chapter 15
Galaxies
![Page 2: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/2.jpg)
What do you think?
• Do galaxies all have spiral arms?
• Are most of the stars in a spiral galaxy in its arms?
• Are galaxies isolated objects?
• Are all galaxies moving away from the Milky Way?
![Page 3: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/3.jpg)
Galaxies seem to take one of four different appearances
SPIRALS
![Page 4: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/4.jpg)
Galaxies seem to take one of four different appearances
SPIRALS
![Page 5: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/5.jpg)
Galaxies seem to take one of four different appearances
SPIRALS
![Page 6: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/6.jpg)
The tightness of a spiral galaxy’s arms is correlated to the size of
its nuclear bulge
Type Sa Type Sb Type Sc
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Variety of Spiral Arms
Flocculent spirals (fleecy)
Grand-design spirals (highly organized)
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We easily see these spiral arms because they contain numerous bright O and B stars which illuminate dust in the arms.
However, stars in total seem to be evenly distributed throughout the disk.
![Page 9: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/9.jpg)
Self-propagating star formation and spiral density waves produce spiral arms
![Page 10: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/10.jpg)
Galaxies seem to take one of four different appearances
BARRED SPIRALS
![Page 11: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/11.jpg)
Galaxies seem to take one of four different appearances
BARRED SPIRALS
![Page 12: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/12.jpg)
Galaxies seem to take one of four different appearances
BARRED SPIRALS
![Page 13: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/13.jpg)
Bars of stars run through the nuclear bulges of barred spiral galaxies
Type SBa Type SBb Type SBc
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Galaxies seem to take one of four different appearances
ELLIPTICALS
Type E0 Type E3 Type E7
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Galaxies seem to take one of four different appearances
ELLIPTICALS
![Page 16: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/16.jpg)
Elliptical galaxies display a variety of sizes and masses
• Giant elliptical galaxies can be 20 times larger than the Milky Way
• Dwarf elliptical galaxies are extremely common and can contain as few as a million stars
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Galaxies seem to take one of four different appearances
IRREGULAR
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Galaxies seem to take one of four different appearances
• Spirals
• Barred Spirals
• Ellipticals
• Irregulars
![Page 19: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/19.jpg)
This classification scheme is known as the Hubble Tuning Fork Scheme
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Galaxies are gravitationally bound into clusters and superclusters
• Galaxies are not scattered randomly throughout the galaxy
• Galaxies are found in clusters– The Milky Way is part of the Local Group of about 35 galaxies
• Clusters of galaxies are clustered as well in groups called superclusters– Our Local Group is part of the Local Supercluster
• The majority of space is empty - called voids.
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The Local Group
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Virgo Cluster
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Fornax Cluster
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Hercules Cluster
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Each dot represents a
single galaxy
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Galaxies in a cluster can collide and combine
NOTE: There is so much space between stars in a galaxy that the probability of two stars crashing into each other is extremely small.
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Colliding galaxies ….
• Do not involve colliding stars
• Might form hot intergalactic gas• Could initiate rapid star formation - called
STARBURST GALAXIES
• Cause galaxy mergers called “galactic cannibalism”
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Three galaxies, M81 (big), M82 (medium), and
NGC 3077 (small).
Are they related to one
another?
![Page 29: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/29.jpg)
![Page 30: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/30.jpg)
![Page 31: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/31.jpg)
Galactic Cannibalism Computer Simulation
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NOTE: No cluster or supercluster of galaxies contains enough visible matter to provide enough gravitational “glue” to keep the spinning clusters bound together.
Non-luminous material in galactic halos may account for some dark
matter in the universe
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The redshifts of remote superclusters indicate that the universe is expanding
• During the 1920s, Edwin Hubble and Milton Humanson were studying galaxy spectra
• They found that galaxy spectral lines were redshifted
• They measured the distance to galaxies by finding Cepheid variables and using the period-luminosity relationship
• They found that the most distant galaxies had the largest redshifts
• This is called the HUBBLE FLOW.
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Hubble’s Law
slope = 75 km/s/Mpc called Hubble’s Constant
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Astronomers now measure distances to galaxies using supernovae and the Tully-Fisher relation
• Standard candle method– If you know how bright something really is and compare that
to how bright it looks, then the distance can be determined
• Supernovae– All Type II a supernovae have the same luminosity
• Tully-Fischer Relation– The broader the spectral line, the faster it is rotating and the
more luminous the galaxy.
![Page 38: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/38.jpg)
What did you think?• Do galaxies all have spiral arms?
No, galaxies may be either spiral, barred spiral, elliptical, or irregular. Only spirals and barred spirals have arms.
• Are most of the stars in a spiral galaxy in its arms?No, the spiral arms contain only 5% more stars than the regions between the
arms.
• Are galaxies isolated objects?No, galaxies are grouped in clusters, and clusters are grouped in superclusters.
• Are all galaxies moving away from the Milky Way?No, only the galaxies in other superclusters are necessarily receding from us.
![Page 39: Chapter 15 Galaxies What do you think? Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated.](https://reader036.fdocuments.net/reader036/viewer/2022062308/56649e485503460f94b3ba6d/html5/thumbnails/39.jpg)
Self-Check1: Draw and label the tuning fork diagram, showing the principal Hubble classes for
galaxies, and describe the criteria for the assignment of subclasses.
2: Discuss the differences between flocculent spirals and grand-design spirals.
3: Explain what spiral arms are and how they are sustained.
4: Compare and contrast the contents of the various Hubble classes of galaxies.
5: Describe the distribution of galaxies in space, and distinguish between regular and irregular clusters of galaxies.
6: Explain the observational basis for the dark matter problem in spiral galaxies and in clusters of galaxies.
7: Discuss the role of collisions and mergers in our understanding of galaxies.
8: State the Hubble law and describe how the Hubble constant is evaluated and how it is used to determine the distances of galaxies.