galaxies astronomy 115. first, which of the following is a galaxy? open cluster globular cluster...
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Galaxies
Astronomy 115
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First, which of the following is a galaxy?
• Open cluster• Globular cluster• Nebula• Interstellar medium (gas and dust)• Supernova remnant• None of these
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First, which of the following is a galaxy?
• Open cluster• Globular cluster• Nebula• Interstellar medium (gas and dust)• Supernova remnant• None of these
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What is a “star cluster”?What is a “star cluster”?
• stars formed together at same time
• stars are at least weakly gravitationally bound together
• two types: open (galactic) and globular (shown to right)
Image at http://hubblesite.org/newscenter/archive/releases/star%20cluster/globular/2007/18/image/a/format/web/results/50/
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Open ClustersOpen Clusters• dozens to
thousands of stars• young stars! only
a few million years old
• may still be surrounded by nebula from which they formed
• located in the spiral arms of a galaxy
• example: Pleiades• Fate: generally,
the stars drift apart (not enough gravity)
Image at http://hubblesite.org/newscenter/archive/releases/star%20cluster/open/2004/20/image/a/results/50/
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More open star clusters
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Globular ClustersGlobular Clusters• millions to hundreds of millions of
stars• old! 6 to 13 billion years• mostly red giants and dwarfs• stars are clumped closely together,
especially near the center of the cluster (densely); stars don’t drift apart
• surround our disk as a halo
Image at http://hubblesite.org/newscenter/archive/releases/star%20cluster/globular/1999/26/image/a/results/50/
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What is a “nebula”?What is a “nebula”?
• A cloud in space• Made of gas and dust
– Can have stars inside
• Most of the ones we see are inside our Milky Way Galaxy
• Different types
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Large, massive, bright nebulae
Emission NebulaEmission Nebula•The hot gas is emitting lightThe hot gas is emitting light
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Colder, darker nebulaeDark dust blocking Dark dust blocking the hot gas behind the hot gas behind itit
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Leftovers from an Explosion
Supernova remnantSupernova remnant (smaller, less gas) (smaller, less gas)
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So, what is a “galaxy”?So, what is a “galaxy”?
• A large group of stars outside of our own Milky Way• Made of billions to trillions of stars, held together by
its own gravity, with all different ages of stars– Also may have gas and dust
• Spiral, or elliptical, or irregular shaped
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Galaxy ClassificationSa
Sb
Sc
E0 = Spherical
Small nucleus; loosely
wound arms
E1
E6
E0, …, E7 Large nucleus;
tightly wound arms
E7 = Highly elliptical
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Spiral galaxy--Andromeda
NOAO/AURA/NSF Images at http://www.noao.edu/image_gallery/html/im0606.html and http://www.noao.edu/image_gallery/html/im0685.html
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Elliptical Galaxies
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Irregular Galaxies
NASA and NOAO/AURA/NSF Images at http://hubblesite.org/newscenter/archive/releases/galaxy/irregular/2005/09/results/50/ , http://www.noao.edu/image_gallery/html/im0560.html , and http://www.noao.edu/image_gallery/html/im0993.html
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Irregular GalaxiesOften: result of galaxy collisions / mergers
Often: Very active star formation (“Starburst galaxies”)
Some: Small (“dwarf galaxies”) satellites of larger galaxies
(e.g., Magellanic Clouds)
Large Magellanic
Cloud
NGC 4038/4039
The Cocoon Galaxy
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Galaxy Diversity
The Hubble Deep Field:
10-day exposure on an apparently empty field in the sky
Even seemingly empty regions
of the sky contain
thousands of very faint, very distant galaxies
Large variety of galaxy
morphologies:
Spirals
Ellipticals
Irregular
(some interacting)
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Gas and Dust in GalaxiesSpirals are rich in
gas and dustEllipticals are almost
devoid of gas and dust
Galaxies with disk and bulge, but no dust are termed S0
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Barred Spirals
• Some spirals show a pronounced bar structure in the center
• They are termed barred spiral galaxies
• Sequence:
SBa, …, SBc,
analogous to regular spirals
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Our Galaxy: the Milky WayOur Galaxy: the Milky Way
• has about 200 billion stars, and lots of gas and dust
• is a barred-spiral (we think)• about 100,000 light-years wide• our Sun is halfway to the edge, revolving at
half a million miles per hour around the center of the Galaxy
• takes our Solar System about 200 million years to revolve once around our galaxy
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The Milky WayThe Milky Way
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Mapping the Milky Way
We can see: • Stars and star clusters – microwaves generated by water from H II
regions (called the MASER technique) traces the Milky Way’s spiral arms
• Nebulae – infrared light (detected by the Spitzer Space Telescope) shows the outline of the heat generated by the bar
• Other galaxies (analogous structure as our galaxy)
How do we know what our Galaxy looks like?How do we know what our Galaxy looks like?
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Do galaxies evolve over time?
Edwin Hubble (whom we’ll hear more about next lecture) in 1926 classified known galaxies according to shape, and suggested an “evolution” of galaxies from elliptical to spiral as they aged. The diagram was called a “tuning fork” due to its shape
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No one evolutionary path for galaxies
As detection methods grew more sophisticated, using the infrared (Spitzer telescope), radio (Very Long Baseline Array) and gamma ray (Compton telescope) portions of the EM spectrum, the tuning fork is no longer regarded as containing an evolutionary sequence – it’s simply a way of classifying galaxies.
It is true that irregular galaxies seem to form from galactic collisions, and that some spiral galaxies lose their arms to become elliptical (Milky Way + Andromeda fate), there is no good model to describe galactic evolution.