slide 1 m31: the andromeda galaxy 2 million light years from us
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Slide 1
M31: The Andromeda Galaxy 2 million light years from us
http://antwrp.gsfc.nasa.gov/apod/
Slide 2
200 billion stars
Milky Way Galaxy
25,000 light years,Or ~ 8 kpc 1 pc = 3.26 ly
Galactic year = 225 million yrOur sun is 4.6 billion yr old
Slide 3
“Milky Way” – a milky patch of stars that rings the Earth
Galactos = milk in Greek
Slide 4
Galileo found that the Milky Way is made up of stars
Slide 5
The Structure of the Milky Way (1)
Disk
Nuclear Bulge
HaloSun
Globular Clusters
Slide 6
Local group: ~ 30 galaxies
Slide 7
Galaxy M31 in Andromeda – similar to the Milky Way Galaxy
1 Mpc from us
Slide 8
The Structure of the Milky Way (2)
Galactic Plane
Galactic Center
The structure is hard to determine because:1) We are inside2) Distance measurements are difficult3) Our view towards the center is obscured by gas and dust
Slide 9
William and Caroline Herschel, 1785
Herschel could not see very far because of the interstellar gas and dust. He concluded that we live near the center of a relatively small disk of stars.
Slide 10
Strategies to Explore the Structure of Our Milky Way
I. Select bright objects that you can see throughout the Milky Way and trace their directions and distances
II. Observe objects at wavelengths other than visible (to circumvent the problem of optical obscuration), and catalogue their directions and distances
III. Trace the orbital velocities of objects in different directions relative to our position
Slide 11
The key to determining the size and the shape of the Galaxy: measure the distances to the most distant stars and star clusters
Parallax works only for the closest stars within 500 pc
To probe the distance at larger scales, we must find standard candles – very bright objects of known luminosity (or absolute magnitude)
Then we can measure their intensities or apparent magnitudes and find the distance using the inverse square law:
R
d
)mJ/(s4
22 d
LI
5/)5(10pc)( Mmd
Slide 12
Henrietta Leavitt (1868-1921)
The key to the distance scale in the Universe: variable stars cepheids
Discovered and catalogued over 2000 variable stars in Small Magellanic Cloud
Slide 13
Delta Cephei Discovered in 1784 by John Goodricke (deaf-mute)
Slide 14
John Goodricke 1764-1786
Also explained a puzzle of Algol!
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Leavitt noticed that the brightest variable stars had longest periods
Slide 17
Period-magnitude dependence for cepheids in SMC
Slide 18
Period-luminosity relation
Ave
rag
e m
agn
itu
de
M<
V>
Period (log P)
0.4 2.01.40.80.6 1.0 1.2 1.6 1.8
-2-7
-5-6
-4-3
Since all cepheids in SMC are at the same distance from us, the same relationship should be between their periods and absolute magnitudes!
Leavitt did not know the distance to SMC and could not calibrate this relation.It was done by Shapley
Slide 19
Harlow Shapley(1885-1972)
Proposed to use globular clusters as tracers of the mass distribution in our Galaxy
Slide 20
Exploring the Galaxy Using Clusters of Stars
Two types of star clusters:
1) Open clusters: young clusters of recently formed stars; within the disk of the Galaxy
Open clusters h and Persei
2) Globular clusters: old, centrally concentrated clusters of stars; mostly in a halo around the GalaxyGlobular Cluster M 19
Slide 21
Globular Clusters• Dense clusters of 50,000 – 1 million stars
• Old (~ 11 billion years), lower-main-sequence stars
• Approx. 200 globular clusters in our Milky Way
Globular Cluster M80
Slide 22
Locating the Center of the Milky Way
Distribution of globular clusters is not centered on the sun…
…but on a location which is heavily obscured from direct (visual) observation
Slide 23
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Infrared View of the Milky Way
Interstellar dust (absorbing optical light) emits mostly infrared
Near infrared image
Infrared emission is not strongly absorbed and provides a clear view throughout the Milky Way
Nuclear bulge
Galactic Plane
Slide 25
Cepheid Distance Measurements
Comparing absolute and apparent magnitudes of Cepheids, we can measure their distances (using the 1/d2 law)!
The Cepheid distance measurements were the first distance determinations that worked out to distances beyond our Milky Way!
Cepheids are up to ~ 40,000 times more luminous than our sun
=> can be identified in other galaxies.
Slide 26
Cepheids help to determine distances to other galaxies
Slide 27
Cepheid Variables: The Period-Luminosity Relation
The variability period of a Cepheid variable is correlated with its luminosity.
=> Measuring a Cepheid’s period, we can determine its absolute magnitude!
The more luminous it is, the more slowly it pulsates.
Slide 28
What are cepheids??
Slide 29
Cepheids: what happens when the balance between the thermal pressure and gravity becomes unstable
Slide 30
Pulsating Variables: The Instability Strip
For specific combinations of radius and temperature, stars can maintain periodic oscillations.
Those combinations correspond to locations in the Instability Strip
Cepheids pulsate with radius changes of ~ 5 – 10 %.
Slide 31
-2
0
2
4
6
5.0 4.5 4.0 3.5
log Tefflog Teff
log
(L
/L)
log
(L
/L)
Main Sequence
Solar-type stars
Mira LPVs
Cepheids
Irregular LPVs
DBVs
DDVs
PNNVs
Instability Strip
Classical Cepheids
RR Lyrae Scutis
VW Virginis
ZZ Ceti (DAVs)
There are several different types of variable stars
Slide 32
Mechanism of pulsations: the battle between opacity, the temperature and gravity
Explained by Sergei Zhevakin in 1950s
Slide 33
A simple pulsation cycle
•At one point in the pulsation cycle, a layer of stellar material loses support against the star’s gravity and falls inwards.
•This inward motion tends to compress the layer, which heats up and becomes more opaque to radiation.
•Since radiation diffuses more slowly through the layer (as a consequence of its increased opacity), heat builds up beneath it.
N.B. These diagrams are definitely not to scale!!
N.B. These diagrams are definitely not to scale!!
Slide 34
•The pressure rises below the layer, pushing it outwards.
•As it moves outwards, the layer expands, cools, and becomes more transparent to radiation.
•Energy can now escape from below the layer, and pressure beneath the layer drops.
•The layer falls inwards and the cycle repeats.
Slide 35
•This animation illustrates two stellar pulsation cycles.
General idea is OK, but it does not work – pulsations will be damped
Slide 36
•In most stars there are two main partial ionisation zones.
Partial ionisation zones
•The hydrogen partial ionisation zone is a broad region with a characteristic temperature of 1 to 1.5 104 K, in which the following cyclical ionisations occur:
•The helium II partial ionisation zone is a region deeper in the stellar interior with a characteristic temperature of 4 104 K, where further ionisation of helium takes place:
Slide 37
Pulsating Variables: The Valve Mechanism
Partial He ionization zone is opaque and absorbs more energy than necessary to balance the weight from higher layers.
=> Expansion
Upon expansion, partial He ionization zone becomes more transparent, absorbs less energy => weight from higher layers pushes it back inward. => Contraction.
Upon compression, partial He ionization zone becomes more opaque again, absorbs more energy than needed for equilibrium => Expansion
Slide 38
•The pulsation properties of a star depend primarily on where its partial ionisation zones are found within the stellar interior.
•For stars hotter than Teff ~
7500K, the partial ionisation zones are located too close to the star’s surface, where there is not enough mass to drive the oscillations effectively.
•The location of the partial ionisation zones is determined by the star’s temperature.
-5
-7
-9
-10
-3
-6
-8
-4lo
g (
1-M
r/Mst
ar)
log
(1-
Mr/M
star)
He
H Su
rfac
eC
entr
e
Teff ~ 7500K
Slide 39
•However at low temperatures, energy transport via convection becomes quite efficient in the stellar interior, preventing the build-up of heat and pressure beneath the driving pulsation layer.
-5
-7
-9
-10
-3
-6
-8
-4
log
(1-
Mr/M
star)
log
(1-
Mr/M
star)
He
H
Su
rfac
eC
entr
e
Teff ~ 5500K
•For stars cooler than Teff ~ 5500K, on
the other hand, the partial ionisation zones are deep in the stellar interior.
Slide 40
•Computer modelling of stellar pulsation suggests that it is primarily the helium II ionisation zone which is responsible for the observed oscillations of stars on the instability strip.
Modelling pulsations
•The hydrogen ionisation zone, however, still plays an important role, producing an observable phase lag between the star’s maximum brightness and its minimum radius.
Time (days)Time (days)
rR
/Rm
inT
eff
V
Observed properties of a classical Cepheid
Note the phase lag between the star’s maximum brightness and
its minimum radius.
Note the phase lag between the star’s maximum brightness and
its minimum radius.
Slide 41
The structure of the Galaxy
Two components:• Disk • Spherical (halo and bulge)
Slide 42
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Spiral arms
Slide 44
Methods to map out the spiral arms: • galactic (or open) clusters
(found in spiral arms)• O & B stars: bright, short lived• HI 21 cm line
(HI is the astronomer's name for hydrogen atoms) when the electron "flips its spin", it emits a 21 cm radio wave (radio frequency passes through dust)
• HII regions HII ionized Hydrogen (found preferentially in spiral arms)
• Molecular clouds ( CO)
Slide 45
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The puzzle of spiral arms
M 51
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Density wave theory of the spiral arms
Slide 52
Star Formation in Spiral Arms
Shock waves from supernovae, ionization fronts initiated by O and B stars, and the shock fronts forming spiral arms trigger star formation
Spiral arms are stationary shock waves, initiating star formation
Slide 53
Star Formation in Spiral Arms (2)
Spiral arms are basically stationary shock waves
Stars and gas clouds orbit around the Galactic center and cross spiral arms
Shocks initiate star formation
Star formation self-sustaining through O and B ionization fronts and supernova shock waves
Slide 54
Winds from other stars drive new star formation cycles
Slide 55
Measuring the mass of the Galaxy
Slide 56
Key idea: to measure rotation velocities
Slide 57
Orbital Motion in the Milky Way
Differential Rotation• Sun orbits around Galactic center with 220 km/s
• 1 orbit takes approx. 240 million years
• Stars closer to the galactic center orbit faster
• Stars farther out orbit more slowly
Slide 58
Slide 59
r
GMv
r
rGMv
)(
3
3
4~)( rrM
)(~ rrv
Slide 60
Matter extends beyond the visible disk!
Slide 61
There is much more matter than we see!
Slide 62
Dark matter halo
Slide 63
Fritz Zwicky 1898-1974
"spherical bastards”
Walter Baade 1893-1960
Slide 64
What is dark matter???
• White dwarfs
• Brown dwarfs
• Black holes
• Exotic particles
• ???
• Revision of the Standard Model of elementary particles may be necessary
Slide 65
The age and origin of our Galaxy
Slide 66
Age of the star clusters from turnoff points
The age of our Galaxy
12-13 billion years for oldest globular clusters in the halo
9-10 billion years for oldest open clusters in the disk
Disk seems to be younger than halo (?!)
Slide 67
Two populations of stars
Walter Baade1893-1960
Ages of the stars
Their main difference is in chemical composition
Population I – metal-richPopulation II – metal-poor
Metals: all elements heavier than helium
Slide 68
The Abundance of Elements in the Universe
Logarithmic Scale
All elements heavier than He
are very rare.
Linear Scale
Slide 69
Metals in StarsAbsorption lines almost exclusively from hydrogen: Population II
Many absorption lines also from heavier elements (metals): Population I
At the time of formation, the gases forming the Milky Way consisted exclusively of hydrogen and helium. Heavier elements (“metals”) were later only produced in stars.
=> Young stars contain more metals than older stars
Slide 70
Slide 71
Continuing cycle of stellar evolution
Slide 72
Metal abundance in stars is related to their age!
More metals Younger star
Slide 73
Stellar Populations
Population I: Young stars: metal rich; located in spiral
arms and disk
Population II: Old stars: metal poor; located in the halo (globular clusters) and
nuclear bulge
Slide 74
There were no metals in the early universe, before the first stars were born
All heavy elements were synthesized in stars
That is why metals are so rare in the universe
The lack of metals in Population II stars also suggests that the disk is younger than the halo
Slide 75
History of the Milky Way
The traditional theory:
Quasi-spherical gas cloud fragments into smaller pieces, forming the first, metal-poor stars (pop. II);
Rotating cloud collapses into a disk-like structure
Later populations of stars (pop. I) are restricted to the disk of the Galaxy
Slide 76
Problems with the traditional model
• Some of the oldest stars are in the bulge, and some younger clusters are in the halo
• Many Population II stars in the bulge are metal-rich• Even the oldest stars have some metals• Maybe the bulge formed first and the halo later?• Our Galaxy may have absorbed many small galaxies
in the past (galactic cannibalism)• Many globular clusters have been destroyed and gave
their stars to the halo
Slide 77
Slide 78
What is in the Galactic Center??
Slide 79
Slide 80
The Galactic Center (1)
Wide-angle optical view of the GC region
Galactic center
Our view (in visible light) towards the galactic center (GC) is heavily obscured by gas and dust
Extinction by 30 magnitudes
Only 1 out of 1012 optical photons makes its way from the GC towards Earth!
Slide 81
Need to observe the GC in the radio, infrared, or X-ray range
Infrared imagesCentral 2 pc
Slide 82
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Slide 84
Synchrotron radiation of relativistic particles
Slide 85
The central radio emission consists of three parts: Sagittarius A East (blue): a supernova remnant, which was produced by a violent explosion only several tens of thousands of years ago. The origin is unknown. Explanations range from a star disrupted by a black hole to a chain reaction of ordinary supernovae or even a gamma-ray burst. Sagittarius A West or Minispiral (red): Gas and dust streamers ionized by stars and spiraling around the very center, possibly feeding the nucleus. Sagittarius A *: A bright and very compact radio point at the intersection of the arms of the Minispiral (difficult to see in this image)
Slide 86
Slide 87
Fast rotation of spiral filaments around Sgr A*
Slide 88
If one looks at this region with big telescopes and near-infrared cameras one can see lots of stars. If one takes pictures every year it seems that some stars are moving very fast (up to 1500 kilometers per second). The fastest stars are in the very center - the position marked by the radio nucleus Sagittarius A* (cross).
Distance between stars is less that 0.01 pc
Slide 89
Explanation: the dark mass ~ 2.6 million solar masses
Rotation curve for the Galactic Center
Is this a black hole?!
Slide 90
A Black Hole at the Center of Our Galaxy
By following the orbits of individual stars near the center of the Milky Way, the mass of the central black hole could be determined to ~ 2.6 million solar masses
Slide 91
Radio observations with Very Long Baseline Interferometry (VLBI) which are thousands of times more precise than optical observations (good enough to easily pin-point a source the size of a pea in New York when sitting in Paris)
Slide 92
Slide 93
Resolution limit:
D
~rad)(
Determined by diffraction
For visible light ~ 0.5 m and Dmax ~ 10 m
D is the telescope mirror
For radio waves ~ 1 cm = 104 m but Dmax ~ 107 m !
Slide 94The size of Sagittarius A* as measured with VLBI at different wavelengths of the observed radiation
Race to measure the size of Sgr A*0.1 mas = 100x106 km2 Rs = 8x106 km
Slide 95
Size ~ 1 AU (12 Schwarzschild Radii)
Density ~ 7x1021 Msun/pc3
Recent VLBI observations (Nature 2005)
No other explanation but a black hole!
Indicate heavier black hole ~ 4 million solar masses?
Slide 96
“Shadow” of a black hole
Slide 97
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Black hole vicinity is probably very messy …
Slide 99
Cores of other galaxies show an accretion disk with a possible black hole
Slide 100
What about X-ray emission due to accretion?
Rather weak X-ray source
Chandra X-ray image of the Sgr A West region
Slide 101
Another Chandra X-ray image
Rapid X-ray variability and minute-long flares!
Slide 102
X-ray View of the Galactic Center
Chandra X-ray image of Sgr A*
Supermassive black hole in the galactic center is unusually faint in X-rays, compared to those in other galaxies
Galactic center region contains many black-hole and neutron-star X-ray binaries
Slide 103
Evidence for a black hole of ~ 3-4 million solar masses:
• Rotation curve indicating an ultra-compact object• No motion of the central object• Rapid variability• Dense stellar population• Radio jets
Radio jets but rather weak X-ray emission
Other galaxies contain much heavier black holes and stronger activity