Lec. 23: Normal and Spiral Galaxies 23 - 1 Lec 23: Norman & Active Galaxies 1 Galaxies Lecture 23 APOD: NGC 3628 (The Hamburger Galaxy) Lec 23: Norman & Active Galaxies 2 Lecture Topics Discovering Galaxies History Cepheid variable stars Galaxy properties Classifying galaxies Local Group
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Lec. 23: Normal and Spiral Galaxies
23 - 1
Lec 23: Norman & Active Galaxies 1
GalaxiesLecture 23
APOD: NGC 3628 (The Hamburger Galaxy)
Lec 23: Norman & Active Galaxies 2
Lecture Topics
Discovering Galaxies History
Cepheid variable stars
Galaxy properties Classifying galaxies
Local Group
Lec. 23: Normal and Spiral Galaxies
23 - 2
Lec 23: Norman & Active Galaxies 3
Discovering Galaxies From the late 1700’s to the 1920 astronomers had
noticed may spiral nebulae. It was not known whether these nebula were far
away or nearby Various arguments were put forward in support of each
Although the periods from 0.5 to 100 days. Any given star has a constant period.
Period
Brig
htne
ss
Time (Days)1 4 632 5
1.5
.75
7
Lec 23: Norman & Active Galaxies 8
Cepheids Variables
Named after Cephei (first discovered) Red Giants and Supergiants
Periods: ~ 1 to 100 days
Luminosity is a function of period Period-Luminosity relation discovered by
Henrietta Leavitt in 1908.
There are two types (labeled Type I and II Cepheids)
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Lec 23: Norman & Active Galaxies 9
Types of Cepheids
Type I Cepheids (Classical Cepheids) Luminosity: 400 to 20,000 Lsun
Found in Open clusters and the galactic disk (Pop I stars)
Type II Cepheids (W Virginis Stars) Luminosity: 100 to 5,000 Lsun
Found in Globular clusters (Pop II stars)
Lec 23: Norman & Active Galaxies 10
Type ICepheids
Period-Luminosity Relations
102
103
104
Lum
inos
ity (
L su
n)
0.3 1 3 10 30 100Period (days)
Type IICepheids
Cephei
RR Lyrae Stars
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Distances with P-L relation
Measure Period
Luminosity
Mv (absolute magnitude)
Measure mv (apparent magnitude)
Mv and mv distance from distance modulus
mv - Mv = - 5 + 5 log10( d ) A Hubble “key project” was to determine the
distances to galaxies w/ Cepheids.
Lec 23: Norman & Active Galaxies 12
RR Lyrae Variables
Horizontal branch stars (because of where they appear in the H-R diagram).
Periods: ~ 12 to 24 hours
Luminosity: ~ 50 Lsun
Found in Globular clusters (Pop II stars)
Luminosity is independent of period
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Lec 23: Norman & Active Galaxies 13
H-R Instability Strip
Cepheids
RR LyraeVariables
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Galaxies
A galaxy is a collection of stars, gas and dust along w/ associated starlight, magnetic fields and cosmic rays.
Four broad categories: E elliptical S spiral (normal & barred) S0 lenticular I irregular
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Lec 23: Norman & Active Galaxies 15
Ellipticals Galaxies Range from spherical to highly flattened
with designations E0 to E7 Contain old stars (Pop II) Very little gas and dust 1-200 kpc in diameter Mostly found in clusters of galaxies Average spectral type: K 106 to 1013 Msun
NGC 4636 (E0/S01)
NGC 3377 (E5-6) NGC 3115 (E7/S01)NGC 4406 (E3)NGC 4278 (E1)
M87
Giant Elliptical Galaxy
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Lec 23: Norman & Active Galaxies 17
Spiral Galaxies Flattened systems which have a thin disk Display 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 spiral arms Young (Pop I) and old (Pop II) stars Copious amounts of gas and dust 5-50 kpc in diameter Found mostly in the “field” (outside clusters of galaxies) Average spectral type: A, F, G, K 109 to 1011 Msun
M33 - Sc
Sc: Whirlpool (M51) SABc: Southern Pinwheel (M83)
Spiral Galaxy Images
SBc: M109Sa: M65
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HI + cont.
8.0 um3.6 um
B Color visual
M81 at different wavelengths
0.23 um (yellow), 0.16 um (blue)
24 um
70 um 160 um
Lec 23: Norman & Active Galaxies 20
Irregulars By definition, irregular
in shape Mostly young stars
(Pop I)
Lots of gas and dust 1-10 kpc in diameter Found in the field (outside
clusters) Average spectral type: A, F 108 to 1010 Msun
M82
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Lec 23: Norman & Active Galaxies 21
Hubble Tuning Fork – For classifying galaxies
Ellipticals
E0 E7 S0
Normal Spirals
Barred Spirals
Sa
SBa
Sb
SBb
Sc
SBc
The classification scheme is strictly morphological and does not necessarily imply an evolutionary sequence.