Galaxies - Cornell Universityhosting.astro.cornell.edu/.../lectures/23Galaxies.pdf · Lec. 23:...
Transcript of Galaxies - Cornell Universityhosting.astro.cornell.edu/.../lectures/23Galaxies.pdf · Lec. 23:...
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GalaxiesLecture 23
APOD: NGC 3628 (The Hamburger Galaxy)
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Lecture Topics
Discovering Galaxies History
Cepheid variable stars
Galaxy properties Classifying galaxies
Local Group
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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
distance
Edwin Hubble (1924) Discovered Cepheid variables in M31 (Andromeda Galaxy) –
also in NGC 6822 and M33 Using the Period-Luminosity Relation for Cepheids (more on
this in a bit) Determine that M31 is a galaxy, an “Island Universe”
M31 (Andromeda Galaxy)
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M100 Cepheid Variation
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Periodic Variable Stars A small fraction of stars have brightness
variations that are periodic. Due to “radial oscillations” (pulsation which
causes expansion and contraction) These are stars which have evolved off the
main-sequence (post main-sequence stars). Two types: RR Lyrae Variables Cepheid Variables
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Periodic Variable Stars
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
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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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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)
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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.
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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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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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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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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
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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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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.
Thumbnail Version of Hubble Tuning Fork
Source:M65, M77, M84, M101: IPAC Multiwavelength gallery; M49, M89, M91, M109: NOAO Image Gallery;M95: Nial R. Tanvir (through SEDS)
M101, ScM77, SbM65, Sa
M49, E4 M84, S0 (E1?)M89, E0
M109, SBcM91, SBbM95, SBa/b
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Other Types of Galaxies
Dwarfs 106 to 108 stars
Peculiar Exploding, Rings, Disrupted
Seyfert Very Bright Nucleus
Interacting Tidal Effects, Tails (pairs)
QSO Collapsed Nuclei?
Dwarf Galaxies
Leo I
NGC 1313
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Cen A: Two Different Wavelengths
The image of Cen A (NGC 5128), the nearest radio galaxy. Here it is at two different wavelengths. Images are from APOD (Astronomy Picture of the Day)
Left image: antwrp.gsfc.nasa.gov/apod/ap030806.html Right image: antwrp.gsfc.nasa.gov/apod/ap040624.html
visible infrared
Interacting Galaxies
The Antennae (NGC 4038 and NGC 4039)
Cartwheel Galaxy
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The Local Group
100 kpcLMC
SMC Milky Way
M33M31
SMC(Dwarf)
50kpc
Nearby galaxies – SMC
47 TucGlobularCluster
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LMC(Dwarf)
50kpc
Nearby galaxies – LMC
M31(Sb)
700kpc
Nearby galaxies – M31
Great galaxy in Andromeda
NGC205 (dE5) = M110
M32 (cE2)=NGC 221
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Galaxies statistics
~1/3 of all spirals are barred spirals.
There are “Field” and “Cluster” galaxies.
Ellipticals are most common in clusters.
Spirals are most common in the “field.”
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Clusters of Galaxies Poor clusters have
~ 10 galaxies
The Local Group is just outside the Virgo Cluster
Rich clusters have ~10,000 galaxies, with a size of ~ 10 Mpc
Millions of clusters in the universe
Hickson Compact Group 87
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Virgo Cluster: D ~ 20 Mpc
Virgo Cluster: A closer view
NGC 4435
NGC 4458
NGC 4473 NGC 4461
NGC 4438
NGC 4388
M86M84NGC 4402
NGC 4477
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Cluster MS1054-03
HSTImage
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Superclusters
Clusters of clusters
The Local Group is part of the Virgo Supercluster
Large scale structures containing many clusters have been found.
Does clustering keep going?
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Nearest Superclusterswithin 1 Glyr