Astronomy 114
Lecture 30: Types and Classification of Galaxies
Martin D. Weinberg
UMass/Astronomy Department
A114: Lecture 30—25 Apr 2007 Read: Ch. 26 Astronomy 114—1/21
Announcements
Exam #2 redux: due Friday (27 Apr)
PS#7: due next Monday (30 Apr)
A114: Lecture 30—25 Apr 2007 Read: Ch. 26 Astronomy 114—2/21
Announcements
Exam #2 redux: due Friday (27 Apr)
PS#7: due next Monday (30 Apr)
Galaxies beyond the Milky Way. . .
Galaxies, Chap. 26
A114: Lecture 30—25 Apr 2007 Read: Ch. 26 Astronomy 114—2/21
The Discovery of Galaxies
Debate on The nature of spiral nebulae & The size ofour galaxy
Heber Curtis vs. Harlow Shapley
April 26, 1920
Spiral nebulae: Messierobjects
Shapley argued spiral nebulaewere in the Galaxy
Curtis argued that they weredistant, “Island Universes” M51 drawn by Her-
schel
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Edwin Hubble: 1923
DiscoveredCepheid variablesin M31(AndromedaGalaxy)
Used thePeriod-LuminosityRelation forCepheids
Determined thatM31 is a galaxy, an"Island Universe"
A114: Lecture 30—25 Apr 2007 Read: Ch. 26 Astronomy 114—4/21
Cepheid Variables: review
Periods: ∼ 1 to 100 days
Luminosity is a function of period
Period-Luminosity relation discovered byHenrietta Leavitt in 1908
Two types, determine from spectrum (see Ch. 21):
Type I“Classical” CepheidsLuminosity: 400 to 20,000 L⊙
Location: Galactic disk (Pop I stars)
Type IILuminosity: 100 to 5,000 L⊙
Location: Globular clusters (Pop II stars)
A114: Lecture 30—25 Apr 2007 Read: Ch. 26 Astronomy 114—5/21
Period-Luminosity Relation
Distances with P-L Relation:
Recall magnitude: m1 − m2 = −2.5 log10(b1/b2)
If the two stars are the same but at differentdistances,
b1 =L
4πd21
sob1
b2=
d22
d21
Put the two relations together:
m − M = −2.5 log10(102/d2) = 5 log10 d − 5
d = 100.2(m−M)+1
m − M is called distance modulus
A114: Lecture 30—25 Apr 2007 Read: Ch. 26 Astronomy 114—6/21
Distance modulus
Measured Period gives:
Luminosity
Mv (absolute magnitude)
Measure mv (apparent magnitude) to get distance!
Example:
m − M = 25
d = 100.2×25+1 = 106 pc = 1 Mpc
A Hubble "key project" is to determine the distancesto galaxies w/ Cepheids
Standard candle analysis
A114: Lecture 30—25 Apr 2007 Read: Ch. 26 Astronomy 114—7/21
Galaxies
A galaxy is a collection of stars, gas, dust, magneticfields, dark matter . . . all gravitationally bound
Four basic categories used today:
E: elliptical
S: spiral (normal & barred)
S0: lenticular (no arms)
I: irregular
Historical
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Elliptical Galaxies
Range from spherical tohighly flattened
E0 to E7
Contain old stars (Pop II)
Very little gas and dust
1-200 kpc in diameter
Typical in clusters of galaxies
Average spectral type: K
106 to 1013 M⊙
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Galaxy catalogs
Messier Catalog 1783, 1784, 1787First catalog of deep sky objects
Messier’s published list in French contained 103objects
Can be seen in 3" telescopes, binoculars
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Galaxy catalogs
Messier Catalog 1783, 1784, 1787
GC (General Catalog) 1863Compiled by John Herschel (son of William)
5079 deep sky objects found from 1758 to 1860
Includes all of Messier’s catalog
A114: Lecture 30—25 Apr 2007 Read: Ch. 26 Astronomy 114—10/21
Galaxy catalogs
Messier Catalog 1783, 1784, 1787
GC (General Catalog) 1863
THE NGC/IC (New General Catalogue of Nebulaeand Star Clusters and Index Catalogue) 1888, 1895,1908
J.L.E. Dreyer (Danish Astronomer)
Over 7,840 nebulae known at the end of the year 1887
A114: Lecture 30—25 Apr 2007 Read: Ch. 26 Astronomy 114—10/21
Galaxy catalogs
Messier Catalog 1783, 1784, 1787
GC (General Catalog) 1863
THE NGC/IC (New General Catalogue of Nebulaeand Star Clusters and Index Catalogue) 1888, 1895,1908
IC 1 1895 & IC2 1907 – additions and corrections toNGC
A114: Lecture 30—25 Apr 2007 Read: Ch. 26 Astronomy 114—10/21
Galaxy catalogs
Messier Catalog 1783, 1784, 1787
GC (General Catalog) 1863
THE NGC/IC (New General Catalogue of Nebulaeand Star Clusters and Index Catalogue) 1888, 1895,1908
IC 1 1895 & IC2 1907 – additions and corrections toNGC
. . .
A114: Lecture 30—25 Apr 2007 Read: Ch. 26 Astronomy 114—10/21
Elliptical Galaxies
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Elliptical Galaxies
Center of Virgo Cluster
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Elliptical Galaxies
M87, giant elliptical galaxy
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Spiral Galaxies
Thin disk, like Milky Way
Spiral structure
Divided into barred (SB) and unbarred (S) spirals
Further subdivided into classes a, b, and c; e.g. SBb,Sc, ... where:
a: large nuclear bulge & tightly wound spiral arms
c: small nuclear bulge & loosely wound spiralarms
Consist of young (Pop I) and old (Pop II) stars
Gas and dust, forming stars
5-50 kpc in diameter
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Spiral Galaxies
Typical type outside of clusters
Average spectral type: A, F, G, K
109 to 1011 M⊙
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Spiral Galaxies
Typical type outside of clusters
Average spectral type: A, F, G, K
109 to 1011 M⊙
A114: Lecture 30—25 Apr 2007 Read: Ch. 26 Astronomy 114—15/21
Irregulars
By definition, irregular inshape
Mostly young stars (PopI)
Lots of gas and dust
1-10 kpc in diameter
Found in the “field”outside clusters
Average spectral type:A, F
108 to 1010 M⊙
Leo I
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Irregulars
LMC
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Irregulars
SMC
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Early classification attempt
Wolf’s system
1906
Classification isnot unique
Many systemspossible
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Hubble Tuning Fork Diagram
The classification scheme is morphological (basedupon physical appearance)
Does not imply an evolutionary sequence.
A galaxy may remain the same type over lifetime
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Hubble Tuning Fork Diagram
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Classification depends on waveband
Spiral galaxy: M81
X-ray UV Visible
Near IR Far IR
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