Astronomy 114 - University of Massachusetts Amherstastro.umass.edu/~weinberg/a114/lectures/lec30.pdfMessier Catalog 1783, 1784, 1787 First catalog of deep sky objects Messier’s published

Astronomy 114

Lecture 30: Types and Classification of Galaxies

Martin D. Weinberg

[email protected]

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)


Announcements

Exam #2 redux: due Friday (27 Apr)

PS#7: due next Monday (30 Apr)

Galaxies beyond the Milky Way. . .

Galaxies, Chap. 26


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


Edwin Hubble: 1923

DiscoveredCepheid variablesin M31(AndromedaGalaxy)

Used thePeriod-LuminosityRelation forCepheids

Determined thatM31 is a galaxy, an"Island Universe"


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)


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


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


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


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⊙


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


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


Galaxy catalogs


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


Galaxy catalogs




IC 1 1895 & IC2 1907 – additions and corrections toNGC


Galaxy catalogs




IC 1 1895 & IC2 1907 – additions and corrections toNGC

. . .


Elliptical Galaxies


Elliptical Galaxies

Center of Virgo Cluster


Elliptical Galaxies

M87, giant elliptical galaxy


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



Spiral Galaxies

Typical type outside of clusters

Average spectral type: A, F, G, K

109 to 1011 M⊙


Spiral Galaxies

Typical type outside of clusters

Average spectral type: A, F, G, K

109 to 1011 M⊙


Irregulars

By definition, irregular inshape

Mostly young stars (PopI)

Lots of gas and dust


Found in the “field”outside clusters

Average spectral type:A, F

108 to 1010 M⊙

Leo I


Irregulars

LMC


Irregulars

SMC


Early classification attempt

Wolf’s system

1906

Classification isnot unique

Many systemspossible


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


Hubble Tuning Fork Diagram


Classification depends on waveband

Spiral galaxy: M81

X-ray UV Visible

Near IR Far IR


Documents

Astronomy 114 - University of Massachusetts Amherstastro.umass.edu/~weinberg/a114/lectures/lec30.pdfMessier Catalog 1783, 1784, 1787 First catalog of deep sky objects Messier’s published