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


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


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


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


© 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


Superclusters


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


Galaxy Interactions15-28


Gravitational Lensing15-29


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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


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!


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