Astronomy 113 Dr. Joseph E. Pesce, Ph.D. Galaxies and...
Transcript of Astronomy 113 Dr. Joseph E. Pesce, Ph.D. Galaxies and...
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© Dr. Joseph E. Pesce, Ph.D.
Astronomy 113Dr. Joseph E. Pesce, Ph.D. Galaxies and Clusters
© Dr. Joseph E. Pesce, Ph.D.
Types of GalaxiesThe Hubble Sequence (1920s)
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Spiral Galaxies³Nuclear bulge³Spiral arms³Knots of glowing Interstellar clouds (star
formation)³Gas & dust³Predominantly blue
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Spiral Classification³Classification based on size of bulge &
tightness of spirals:
²Sa = tight arms, large bulge²Sb = moderate spiral, moderate bulge²Sc = loose spiral, tiny bulge
*** Don’t always see face-on (arms), so bulge size is used ***
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blue
red
© Dr. Joseph E. Pesce, Ph.D.
Spiral-Galaxy Properties³100s of billions of stars³100,000 ly in diameter³On-going star formation³Lots of gas and dust
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Barred Spirals³Spiral with a bar of stars
crossing bulge³Most spirals unbarred³Milky Way is barred³Probably caused by
galaxy interactions
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S0 Galaxies³Spiral galaxies, but with enormous bulges and
no arms³No ongoing star formation³A transition between spirals and ellipticals?
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Elliptical Galaxies³Do NOT have spiral arms³Classification based on how round/oval
³Roundest = E0
³Most Elongated = E7
³But this is 2-D!
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E0
E7
Probably look different from every angle
© Dr. Joseph E. Pesce, Ph.D.
Elliptical-Galaxy Properties - I³Large range in size and number of stars³No current star formation³Little gas & dust (at least cool gas)³Are reddish, made up of low-mass, long-lived
stars
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Elliptical-Galaxy Properties - II³Dwarf Elliptical Galaxies
²Millions of stars²100,000 ly in diameter²The most common galaxy in the universe
³Giant Elliptical Galaxies²Trillions of stars²Millions of ly in diameter²The largest galaxies in the universe
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Irregular Galaxies³No spiral structure or disk³Not elliptical in shape³Doesn’t fit other categories³Unusual shapes³Can have very high rates of star formation³Satellite galaxies to Milky Way are Irregular
galaxies
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Galaxy Sizes15-12
Dwarf Elliptical(106 stars)
Milky Way(1011 stars)
Giant Elliptical (1013 stars)
1 Mpc© Dr. Joseph E. Pesce, Ph.D.
Star-Formation Rates15-13
NowTime
Elliptical
Spiral
Irregular
© Dr. Joseph E. Pesce, Ph.D.
© 2007-2014 Joseph E. Pesce, Ph.D.© Dr. Joseph E. Pesce, Ph.D.
© 2007-2014 Joseph E. Pesce, Ph.D.© Dr. Joseph E. Pesce, Ph.D.
© 2007-2014 Joseph E. Pesce, Ph.D.© Dr. Joseph E. Pesce, Ph.D.
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Clusters of Galaxies³Galaxies grouped into CLUSTERS
²Members are gravitationally bound³Clusters grouped into SUPERCLUSTERS
²Contain dozens of clusters over 100 million ly across
²Largest gravitationally bound structures in universe
³Voids between superclusters: vast empty regions
³Galaxy clusters concentrated in sheets around voids – like soap bubbles
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Cluster Types
³Clusters classified as �poor� or �rich� – number of galaxies
³LOCAL GROUP: poor cluster with ~30 galaxies
³Nearest rich cluster: VIRGO, ~50 million lyaway² Has ~1,000 galaxies over large area (~7 million ly diameter)
² Dominated by several giant ellipticals (each about the size of the local group!) – this is typical of rich clusters
³Rich clusters have mostly Elliptical galaxies
³Poor clusters have more Spiral than Ellipticals
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Local Group15-23
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A Big Cluster
© Dr. Joseph E. Pesce, Ph.D.
Superclusters
© Dr. Joseph E. Pesce, Ph.D.
Collisions in Clusters³Density high in clusters: Galaxies collide
²Stars don’t interact, but gas/dust clouds do²Gas stripped from individual galaxies, heated and
added to cluster – Intracluster medium (HOT)³Collisions can cause star formation³Galaxy Mergers: similar sized galaxies³Galactic Cannibalism: big galaxy absorbs
smaller one²Giant Elliptical at center of cluster
³Collisions turn spirals into ellipticals (?)
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Collisions in Clusters15-27
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Galaxy Interactions15-28
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Gravitational Lensing15-29
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© 2007-2014 Joseph E. Pesce, Ph.D.© Dr. Joseph E. Pesce, Ph.D.
Questions³How do galaxies form?
²Spiralsq Top down or bottom up
²Ellipticalsq Spiral-spiral collisions?
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Redshift³Hubble observed galaxy spectral lines shifted
to red (what does this mean?): Redshift²All galaxies observed expect a few in the Local
Group³Redshift linked to recessional velocity (z)³Measure distance (D) with Cepheids or
Supernovae³Hubble’s Law: recessional velocity
proportional to distance
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Redshift
© Dr. Joseph E. Pesce, Ph.D.
Hubble’s Lawvr = Ho * D
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Hubble’s Law
vr = Ho * D
³Ho = Hubble Constant = ?
²Need to measure z (easy) and D (hard)
²HST Key Project (observe Cepheids to 200Mly)
³More distant galaxies moving away faster
³Why?
³Universe is expanding (from an �explosion�)
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Thank You!
© Dr. Joseph E. Pesce, Ph.D.