General GeoAstro II: Astronomy

General GeoAstro II: Astronomy

The name of the gameThe name of the game::

slides slides willwill NOT NOT be putbe put on the web on the web attend the lectures, take notes !attend the lectures, take notes !

suggested suggested reading: “Universe”reading: “Universe” (Kaufmann & Freedman) (Kaufmann & Freedman)

no laptops, no mobiles no laptops, no mobiles during classduring class

classes are not complicated, but please classes are not complicated, but please repeatrepeat them them regularly regularly

onlyonly few formulae few formulae, but you have to know them, but you have to know them

General GeoAstro II: Astronomy

StarsStars

- Nature of starsNature of stars- Birth of starsBirth of stars- Stellar evolutionStellar evolution- Endpoints:Endpoints: White Dwarfs White Dwarfs

Neutron Stars Neutron Stars Black HolesBlack Holes

GalaxiesGalaxies

- Milky Way- Milky Way

- Other galaxies- Other galaxies

- Supermassive - Supermassive

black holesblack holes

CosmologyCosmology

- Cosm. Expansion- Cosm. Expansion

- Big Bang- Big Bang- Tests of our Tests of our

theoriestheories

- New - New

developmentsdevelopments

Distance to the stars

From brightness? No!From brightness? No! Parallax-experiment …Parallax-experiment … Stellar Stellar parallax …parallax …

d= 1/pd= 1/p


DefineDefine: “star has a distance of 1 parsec (pc) if its: “star has a distance of 1 parsec (pc) if its parallax is one arcsecond”parallax is one arcsecond”

1 pc = 3.26 light years1 pc = 3.26 light years

Brightest stars on the night sky: too far to measure parallaxBrightest stars on the night sky: too far to measure parallax

Blurring of atmosphereBlurring of atmosphere: parallaxes < 0.01 arcsec extremely : parallaxes < 0.01 arcsec extremely hard to measure hard to measure

reliable out toreliable out to d= 1/p = d= 1/p = 100 pc100 pc


Hipparcos: Hipparcos: High Precision Parallax High Precision Parallax Collecting Satellite Collecting Satellite (Hipparchus: greek astronomer)(Hipparchus: greek astronomer)

Parallaxes still importantParallaxes still important to gauge other to gauge other distance indicatorsdistance indicators

Stellar motionsStellar motions …. ….

Brightness and Distance (“Inverse square law”)

Distance and brightness Distance and brightness luminosityluminosity

Stars have different masses Stars have different masses

different different luminositiesluminosities

““luminosityluminosity= energy/time” [J/s]= energy/time” [J/s]

““brightnessbrightness= energy/(time surface area)” [J/s m= energy/(time surface area)” [J/s m22]]

Brightness and distanceBrightness and distance

•bb= L/(4 = L/(4 dd22))

•““double the distance double the distance brightness reduced by a factor 4”brightness reduced by a factor 4”

• brightness brightness ….….

luminosities

huge variety huge variety of stellar luminosities:of stellar luminosities:

LLmaxmax =10 =101010 L Lmin min

(10(1010 10 = number of all people that = number of all people that everever lived on earth) lived on earth)

The Magnitude system System toSystem to classify stellar brightness classify stellar brightness Very old: Hipparchus (200 B.C.):Very old: Hipparchus (200 B.C.):

“ “ brightest stars: first magnitudebrightest stars: first magnitude

half as bright: second magnitudehalf as bright: second magnitude

…… …… sixth magnitude”sixth magnitude”

“ “apparent magnitudes”apparent magnitudes”

Attention: Attention: “scale backwards”“scale backwards”

Magnitude system

1919thth century astronomers century astronomers: : “first magnitude stars “first magnitude stars shall be 100 times brighter than sixth magnitude shall be 100 times brighter than sixth magnitude stars”stars”

differencedifference of 5 mag corresponds to a of 5 mag corresponds to a factorfactor of 100 in brightness, of 100 in brightness, i.e. xi.e. x5 5 = 100 x= 2.52= 100 x= 2.52

“ “half as bright half as bright 11//2.522.52 as bright” as bright”

Magnitude system

Scales backwards:Scales backwards: “the brighter the more “the brighter the more

negative”negative” Examples:Examples:

Venus: m= - 4Venus: m= - 4 Full moon: m= - 13Full moon: m= - 13 Our sun: m= - 26.8Our sun: m= - 26.8

Relation brightness – magnitudes...Relation brightness – magnitudes...

mm22-m-m11= 2.5 log(b= 2.5 log(b11/b/b22))

Absolute magnitudes

Definition: Definition: ”absolute mag.= relative mag. as seen ”absolute mag.= relative mag. as seen from a distance of 10 pc”from a distance of 10 pc”

Distance modulus Distance modulus (m-M)…(m-M)…

m - M= 5 log(dm - M= 5 log(dpcpc) – 5) – 5

ddpcpc: distance in pc: distance in pc

m : apparent magnitudem : apparent magnitude

M : absolute M : absolute magnitudemagnitude

Stellar colours

Stellar colours Stellar colours depend depend

on theon the surface temperature ! surface temperature !

Wien’s law: Wien’s law: maxmax T = const T = const … …

Spectra of stars How do we know the same laws of physics hold in How do we know the same laws of physics hold in

the observable universe?the observable universe?

Sun:Sun: absorption line spectrum absorption line spectrum (=continuum + dark lines)(=continuum + dark lines)

Spectral classificationSpectral classification: O B A F G K M : O B A F G K M

““Oh be a fine girl/guy kiss me…”Oh be a fine girl/guy kiss me…”

““hot”hot” T Tsurf surf ~ 25 000 K~ 25 000 K

SunSun

““cool”cool” T Tsurf surf ~ 3000 K~ 3000 K

Spectra of Stars

Advent of quantum mechanics:Advent of quantum mechanics: Interpretation of absorption lines in terms of atomic energy levelsInterpretation of absorption lines in terms of atomic energy levels

Stellar sizes impossible impossible to measureto measure with telescopes with telescopes measure measure i) brightnessi) brightness

ii) distance ii) distance (parallax)(parallax)

iii) surface iii) surface temperaturetemperature (spectral (spectral type)type)

..

..

luminosityluminosity

RadiusRadius

Stefan Boltzmann lawStefan Boltzmann law

Hertzsprung-Russel diagram IdeaIdea: : plot luminosity vs. temperature plot luminosity vs. temperature (spectral type)(spectral type)

information about radiusinformation about radiusclassification of starsclassification of stars

Hertzpsrung-Russel diagram

not random, just anot random, just a few classes few classes most stars onmost stars on “Main Sequence” “Main Sequence” (hydrogen burning)(hydrogen burning)

White dwarfs: White dwarfs: same temperature, but lowersame temperature, but lower luminosity luminosity small radiussmall radius RRWDWD ~ 10 000 km ~ R ~ 10 000 km ~ Rearthearth

Giants: Giants: same temperature, but higher same temperature, but higher luminosity luminosity large radiuslarge radius RRgiantgiant = 10 - 100 R = 10 - 100 Rsun sun

TTsurf surf = 3000 – 6000 K= 3000 – 6000 K

Supergiants: Supergiants: up to 1000up to 1000 RRsun sun

Stellar Masses needneed binary stars ! binary stars ! (~50% of all stars in binaries)(~50% of all stars in binaries)

““double stars”double stars” either either i) “optical double stars”i) “optical double stars”

ii) true binary starii) true binary star How to get masses???How to get masses???

Kepler IIIKepler III: : GGMM11+M+M22)/a)/a33

M1: mass star 1M1: mass star 1 M2: mass star 2M2: mass star 2

a : separation between starsa : separation between stars G : gravitational constantG : gravitational constant = 2 = 2 /T, T: orbital period/T, T: orbital period

measure a andmeasure a and T T total system masstotal system mass

Stellar masses

individualindividual masses?masses?

i) find i) find center of masscenter of mass (CM) (CM)

ii) distances from CM to stars, ii) distances from CM to stars, aa11 & a & a22

aa11= (M= (M22/M/Mtottot) a) a

aa22= (M= (M11/M/Mtottot) a) a

Mass-luminosity relation Observation: Observation:

L ML M3.53.5 ….. …..

““proportional to”proportional to”

Stellar Stellar

lifetimelifetime

“ “fat blokes die young”fat blokes die young”

The Birth of Stars

““We see a region of space extending from We see a region of space extending from the centre of the sun to unknown distances the centre of the sun to unknown distances contained between two planes not far from contained between two planes not far from each other…” each other…” ((Immanuel KantImmanuel Kant: “Allgemeine Naturgeschichte und : “Allgemeine Naturgeschichte und

Theorie des Himmels”) Theorie des Himmels”)

Nuclear burning in the sunNuclear burning in the sun (“hydrogen to helium”): (“hydrogen to helium”):

consumes 6 10consumes 6 1011 11 kg/s of hydrogenkg/s of hydrogen

no infinite fuel resources: finite life timeno infinite fuel resources: finite life time

stellar evolution stellar evolution (“birth, evolution, death”)(“birth, evolution, death”)

Birth of Stars

““snapshot problematic”snapshot problematic”

stellar >> human lifetimestellar >> human lifetime

Derive evolutionary sequence from a set of Derive evolutionary sequence from a set of “snapshots”“snapshots”

Stellar Birth Stars are born in the Stars are born in the

gravitational collapse of gravitational collapse of

giant molecular cloudsgiant molecular clouds

Stellar Birth

computer-simulationcomputer-simulation of of

the collapse of a giant the collapse of a giant

molecular cloud by molecular cloud by

Mathew BateMathew Bate

very dynamic processvery dynamic process stars form in groupsstars form in groups many binary/multiplemany binary/multiple

star systems formstar systems form observation:observation:

~ 50% of stars are in~ 50% of stars are in

binary systemsbinary systems

Stellar birth

Where does star formation take place?Where does star formation take place?

……in thein the spiral arms spiral arms of galaxies…of galaxies…

Interstellar Medium (ISM) ISM providesISM provides matter of which stars are matter of which stars are

mademade

ISM consists of aISM consists of a combination of gas and combination of gas and dustdust

Interstellar clouds Interstellar clouds are (for historical are (for historical reasons)reasons) called nebulae called nebulae

Interstellar medium

Three kinds of nebulae:Three kinds of nebulae:

Emission N. Reflection N. Dark N.Emission N. Reflection N. Dark N.

Interstellar medium

Emission nebulaeEmission nebulae:: temperaturestemperatures: ~ 10 000 K: ~ 10 000 K massesmasses: ~ 10 – 10 000 M: ~ 10 – 10 000 Msolarsolar densitydensity: n ~ few 1000 atoms/cm: n ~ few 1000 atoms/cm33

((compare with: “air” ~ 10compare with: “air” ~ 101919 atoms/cm atoms/cm33

ISM ~ 1 atom/cmISM ~ 1 atom/cm33))

found found near hot, young starsnear hot, young stars (O and B (O and B stars with Tstars with Tsurfsurf > 10 000K) > 10 000K)

Interstellar medium: emission nebulae

InterstellarInterstellar hydrogen hydrogen found infound in two forms” two forms” ““HI-region”: HI-region”: neutral hydrogenneutral hydrogen ““HII-region”: HII-region”: ionized hydrogen ionized hydrogen (i.e. protons(i.e. protons

and electrons)and electrons)

Interstellar medium: emission nebulae

Emission mechanism Emission mechanism HII-region:HII-region:

recombinationrecombination (proton captures electron, emits light (proton captures electron, emits light

as it cascades down)as it cascades down)

most important transition most important transition

from n=3 to n=2 from n=3 to n=2

(“Ha-photons”)(“Ha-photons”)

reddish colourreddish colour

•Reflection nebulae

Lots of fine-grainedLots of fine-grained dust dust, low density, low density

reflects short-wavelengths morereflects short-wavelengths more

efficiently than long onesefficiently than long ones

blue colourblue colour

•Dark Nebulae

High density of dust grainsHigh density of dust grains

block view to the starsblock view to the stars TemperatureTemperature: 10 – 100 K hydrogen : 10 – 100 K hydrogen

moleculesmolecules DensityDensity: n ~: n ~ 10 1044 – 10 – 109 9 atoms/cmatoms/cm33

Stellar Evolution Protostars:Protostars:

Gravity has to overcome gas pressureGravity has to overcome gas pressure dense & cold regions preferreddense & cold regions preferred dark nebulaedark nebulae (“stellar nurseries”) (“stellar nurseries”)

“ “standard cosmic standard cosmic abundancesabundances”:”:75 % 75 % HHydrogenydrogen24 % 24 % HeHeliumlium 1 % 1 % heavierheavier elements elements

Protostars

youngyoung protostars more luminous protostars more luminous than later on the than later on the main sequence (gravitational energy)main sequence (gravitational energy)

Decrease of luminosity at almost Decrease of luminosity at almost constant surface constant surface temperaturetemperature,,

but central temperaturebut central temperature

risesrises

Evolutionary Evolutionary path inpath in

HR-diagramHR-diagram……

Protostars

At TAt Tcentralcentral ~ 10 ~ 1066 K: thermonuclear reactions K: thermonuclear reactions

(H He)(H He) set in set in produce energy/produce energy/pressurepressure stop contractionstop contraction

hydrostatic equilibrium+nuclear burninghydrostatic equilibrium+nuclear burning = Main sequence (MS) = Main sequence (MS) reachedreached

Exact Exact position on MS determined byposition on MS determined by stellar stellar mass…mass…

Main sequence masses

Extreme cases:Extreme cases: Mass too small Mass too small (<0.08 M(<0.08 Msolsol))

no ignition of hydrogen, no main sequence stageno ignition of hydrogen, no main sequence stage

Brown DwarfBrown Dwarf Mass too big Mass too big (>100 M(>100 Msolsol))

violent winds violent winds

disruption of the stardisruption of the star

Main sequence: 0.08 < MMain sequence: 0.08 < MMS MS < 100 M< 100 Msolsol

Young stellar objects (YSOs): …youngsters in revolution…

Accretion disks: Accretion disks:

Jets:Jets:

Young stellar objects

examples of examples of accretion disk – jet connectionaccretion disk – jet connection

interaction of these outflows with surrounding interaction of these outflows with surrounding matter:matter: Herbig-Haro objects Herbig-Haro objects

Jets Jets are usuallyare usually short-lived: 10 short-lived: 1044 years, years,

but canbut can eject large masses eject large masses (~1 M(~1 Msolsol) during this ) during this timetime

many young stars lose mass viamany young stars lose mass via strong winds: strong winds: mass lossmass loss 10 10-7-7 M Msolsol/year/year

(our sun: (our sun: 1010-14 -14 MMsolsol/year)/year)

Young stellar objects

Young stars like to hang aroundYoung stars like to hang around in groups in groups

(see previous movie)(see previous movie)

““open clusters”open clusters”

fastest stars may leavefastest stars may leave

““evaporation” of open clustersevaporation” of open clusters

Stellar evolution: overview

once formed,once formed, evolution evolution of starsof stars depends on depends on theirtheir masses: masses:

M < 0.08 MM < 0.08 Msolsol:: no no nuclear nuclear fusionfusion

“ “Brown dwarfs”Brown dwarfs”

0.08 < M < 8 M0.08 < M < 8 Msolsol: i) : i) Main sequenceMain sequence

ii) ii) GiantGiant phase phase iii) iii) White DwarfWhite Dwarf + + planetary nebula planetary nebula

Stellar evolution: overview

8 < M < 25 M8 < M < 25 Msolsol: : i) Main Sequencei) Main Sequence

ii) Giantii) Giant phasephase iii) iii) supernovasupernova explosionexplosion

neutron starneutron star

M > M > 25 M25 Msolsol: : i) Main Sequencei) Main Sequence

ii) Giantii) Giant phasephase iii) iii) supernovasupernova explosionexplosion

black holeblack hole

Evolution of a M < 8 Msol star ““our sun”:our sun”: -- MS-starMS-star, H-burning in core, H-burning in core

- - Red GiantRed Giant: H in core : H in core exhausted, H-burning in exhausted, H-burning in shellshell

- - Red GiantRed Giant:He ignites in:He ignites in stellar core, radius ~ 1 AUstellar core, radius ~ 1 AU earth swallowedearth swallowed (~ 5 10(~ 5 109 9 years from now)years from now)

- - final stagesfinal stages: hot, cooling : hot, cooling

Carbon-Oxygen core,Carbon-Oxygen core, eject envelopeeject envelope

White dwarf + “planetary nebula”White dwarf + “planetary nebula”

8 Msol -star

Planetary nebulaePlanetary nebulae::

8 Msol -star

Evolution in the HR-diagram:Evolution in the HR-diagram:

Testing stellar evolution:globular clusters Globular clustersGlobular clusters

~~101055 stars stars

in in halohalo of galaxy of galaxy

OldOld: about same age as: about same age as galaxygalaxy

Globular clusters and HR-diagrams

Basic ideaBasic idea: : - - heaviest starsheaviest stars have already have already evolvedevolved away from away from

main sequencemain sequence

- - lightest stars stilllightest stars still on on main sequencemain sequence

age of age of clustercluster

Evolution for M > 8 Msol

Stages:Stages:

• Main sequenceMain sequence

• Giant stageGiant stage

• Final stage:Final stage:


No more No more nuclear nuclear fuel fuel (beyond iron)(beyond iron)

“ “core”-collapsecore”-collapse

supernova explosionsupernova explosion (type II) (type II)


Supernova explosionSupernova explosion results in results in

eithereither

i) a i) a neutron starneutron star (M < 25 M (M < 25 Msolsol))

oror

ii) a ii) a black holeblack hole (M > 25 M (M > 25 Msolsol))

End stages of stellar evolution: White dwarfs: White dwarfs:

Left behind in Left behind in center of planetary nebulacenter of planetary nebula

NoNo more nuclear more nuclear burningburning just coolsjust cools until it fades until it fades awayaway

MassesMasses: 0.2 – : 0.2 – 1.4 M1.4 Msolsol

above 1.4 Mabove 1.4 Msol sol collapse to collapse to neutron neutron starstar

DensitiesDensities: ~ 10: ~ 1066 – 10 – 108 8 g/cmg/cm3 3 (earth: ~ 5 g/cm(earth: ~ 5 g/cm33))

EquilibriumEquilibrium between between gravitygravity and and degeneracy pressuredegeneracy pressure

White dwarfs

Degeneracy pressure:Degeneracy pressure: purely purely quantum mechanical quantum mechanical effect:effect:

Electrons are “Fermions” (spin= ½)Electrons are “Fermions” (spin= ½)

don’t want to be in the same statedon’t want to be in the same state

(Pauli-exclusion principle)(Pauli-exclusion principle)

resist compression even at zero temperatureresist compression even at zero temperature

• all all mass from neutrons and protonsmass from neutrons and protons

•all all pressure from electronspressure from electrons

white dwarfs

Mass-Radius relationship: Mass-Radius relationship: R MR M -1/3-1/3

““More massive More massive WDs areWDs are smaller” smaller”

End Stages of stellar evolution: Neutron Stars

Masses: Masses: ~1.4 M~1.4 Msolsol

Radius: Radius: ~10 - 15 km~10 - 15 km

Density: Density: fewfew 101014 14 g/cmg/cm33

observed neutron star mass distributionobserved neutron star mass distribution

Magnetic field: Magnetic field: 101012 12 - 10- 101515 G G (earth: ~ 0.5 G)(earth: ~ 0.5 G)

Neutron stars

hard to detect:hard to detect:

new-born neutronnew-born neutron

star in Supernova star in Supernova

remnantremnant

Neutron Stars

Internal structure:Internal structure:

mostly neutronsmostly neutrons (~90% neutrons, ~10% protons)(~90% neutrons, ~10% protons)

crust: iron-likecrust: iron-like

nucleinuclei

center: “exotic”center: “exotic”

particles?particles?

End Stages of stellar evolution:

Black holes

neutron star hasneutron star has limiting mass, limiting mass, above that above that mass: mass: collapse to a black holecollapse to a black hole

not even light can escape not even light can escape from a black from a black hole…hole…

HowHow can a black hole be can a black hole be detected?detected?

Black holes

Black hole “accretes”Black hole “accretes” mass from mass from

companion starcompanion star

x-ray binaryx-ray binary

Galaxies:Our Galaxy: the Milky Way

.

The Structure of the Milky Way

Galactic Plane

Galactic Center

The actual structure of our Milky Way is very hard to determine because:

1) We are inside2) Distance measurements are difficult3) Our view towards the center is

obscured by gas and dust

Structure of the Milky Way (MW) So what can we do toSo what can we do to explore the MW ?? explore the MW ??

a) space craft? a) space craft? No No

b)b) select select bright objects bright objects that can be seen that can be seen throughout the MWthroughout the MW

c)c) observe in observe in different wavelengthsdifferent wavelengths

d) trace velocities d) trace velocities of all visible objectsof all visible objects

Structure of Milky Way: a) space craft

a)a) How long would it take to get How long would it take to get good good “outside view” “outside view” of our Galaxyof our Galaxy

(travel at speed of light)(travel at speed of light)??

i) 2 monthsi) 2 months

ii) 1 yearii) 1 year

iii) 500 yearsiii) 500 years

iv) 30 000 yearsiv) 30 000 years

v) 5 million years ??v) 5 million years ??

Answer:iv) 30,000 years

The Sun is about

8.5 kpc = 8,500 pc

≈ 30,000 light years

from the Galactic center.

Sun

Galactic Center

=> No spacecraft will ever travel a significant distance through or even out of the Milky Way

Structure of Milky Way: b) bright objects

b) b) What areWhat are bright objects? bright objects?

A A typetype stars ? stars ?

Brown dwarfs ?Brown dwarfs ?

White dwarfs ?White dwarfs ?

O O typetype stars ? stars ?


Answer: O- and B-stars ! Answer: O- and B-stars !

Remember: O and B stars are the most massive, most luminous stars

Look for very young

clusters or associations:

O/B- Associations !

Structure of Milky Way: b) bright objects optically bright objects

O/B Associations

Distances to O/B Associations determined using Cepheid Variables

O/B Associations trace out 3 spiral arms near the Sun.

Sagittarius arm

Orion-Cygnus arm

Perseus arm

Sun


Globular Clusters

• Dense clusters of 50,000 – a million stars

• Approx. 200 globular clusters in our Milky Way

• Old (11 billion years), lower-main-sequence stars

Globular Cluster M80


Globular cluster distribution:Globular cluster distribution:

we are not in the centre of our Galaxywe are not in the centre of our Galaxy

Structure of Milky Way: c) different wavelengths

Galaxy (optical):Galaxy (optical):

absorption by gas and dustabsorption by gas and dust

Galaxy (near-infrared):Galaxy (near-infrared):

emission from warm dustemission from warm dust

Structure of Milky Way: c) different wavelengths

Galaxy moreGalaxy more transparent at longer transparent at longer (than optical)(than optical) wavelengths….wavelengths….

most transitions in hydrogen atom at most transitions in hydrogen atom at “short” wavelengths,“short” wavelengths, but … but …

coupling magnetic moments electron and coupling magnetic moments electron and proton in neutral hydrogen hydrogen:proton in neutral hydrogen hydrogen:

21cm radio emission21cm radio emission

Structure of the Milky Way

Neutral hydrogen createsNeutral hydrogen creates

radio emission radio emission ((= 21cm):= 21cm):

coupling between magnetic coupling between magnetic

moments of proton and electron…moments of proton and electron…

“ “21-cm radiation” can be 21-cm radiation” can be

used to trace the used to trace the

distribution of neutral distribution of neutral

hydrogen in the Galaxyhydrogen in the Galaxy

Structure of the Milky Way

Animation

75,000 light years

Disk

Nuclear Bulge

HaloSun

Globular Clusters

Open Clusters, O/B Associations

Structure of Milky Way

Stellar Populations:Stellar Populations:

……heavier elements are formed in variousheavier elements are formed in various

burning stages of stars…burning stages of stars…

QuestionQuestion::

1) Old stars should be more metal-rich…

2) Young stars should be more metal-rich…

3) They should be the same…

how does the metal content of young and old stars differ?

Structure of Milky Way

Of course:Of course: young stars are “metal”-rich young stars are “metal”-rich

Stellar PopulationsStellar 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

Dynamics in the Milky Way (I)

Population I (disk stars)

Population II (halo stars)

Dynamics in the Milky Way (II)

Differential Rotation

Sun orbits around Galactic center with 220 km/s

1 orbit takes approx. 240 million years.

Dynamics in the Milky Way

QuestionQuestion: : What determines the velocity with which the What determines the velocity with which the sun is moving around the Galactic centre?sun is moving around the Galactic centre?

MassMass of the of the sunsun??

Rotational period of the Rotational period of the spiral arm patternspiral arm pattern??

Mass insideMass inside the orbit of the sun ? the orbit of the sun ?

Angular momentumAngular momentum of the of the Milky WayMilky Way ? ?


Answer:Answer: Newton’s Laws tell us that it is the Newton’s Laws tell us that it is the mass inside the radius mass inside the radius

of the sunof the sun that determines its velocity that determines its velocity

The more mass there is inside the orbit, the faster the sun has to orbit around the Galactic center (argument similar to Kepler’s III. law)…

V= 220 km/s Minside ~ 1011Msol

R= 8.5 kpc


Forms of rotationForms of rotation

rigid rotation

differential

rotation…

Dynamics of the Milky Way

Use theUse the 21-cm-radiation of neutral hydrogen 21-cm-radiation of neutral hydrogen to to

determine thedetermine the rotation curve rotation curve (“velocity as a function of radius”)(“velocity as a function of radius”) of our Galaxyof our Galaxy

observedobserved

expected expected (if(if

mass concentratedmass concentrated

in centre)in centre)


explanation explanation for the observed rotation curve:for the observed rotation curve:

There must be more mass than is visible !!!There must be more mass than is visible !!!

“ “ DARK MATTER “ (DM)DARK MATTER “ (DM)

- 90 % - 90 % of the matter in the of the matter in the

Galaxy isGalaxy is “invisible” “invisible”

- - only only 10 % 10 % inin stars stars

Dynamics in the Galaxy

What could dark matter be made of?What could dark matter be made of?

i) dim stars, massive planets, black holes?i) dim stars, massive planets, black holes? (= (= mamassive ssive ccompact ompact hhalo alo oobjects= bjects= MACHOSMACHOS))

experiments: experiments: only small fractiononly small fraction of of DM are MACHOSDM are MACHOS

ii) A new kind of particle ?ii) A new kind of particle ? (=(=wweakly eakly iinteracting nteracting mmassive assive pparticle= article= WIMPWIMP))

maybe, but maybe, but none none such particle has such particle has been been detecteddetected yet… yet…

The centre of our Galaxy

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:

Only 1 out of 1012 optical photons makes its way from the GC towards Earth!

Radio View of the Galactic Center Many supernova remnants;

shells and filaments

Sgr A

Arc

Sgr A*: The Center of our Galaxy

Galactic Center contains a supermassive supermassive black holeblack hole of approx. 2.6 million solar masses2.6 million solar masses.

Sgr A

Centre of our Galaxy

motion of stars motion of stars close to Galactic centreclose to Galactic centre

(observed !)(observed !)

everything consistent witheverything consistent with a black a black

hole of 2.6 million solar masseshole of 2.6 million solar masses

Other Galaxies types galaxiestypes galaxies

spiral galaxies

barred spiral

galaxies

Other Galaxies

elliptical elliptical

galaxiesgalaxies

irregular irregular

galaxiesgalaxies

Other galaxies

...some more beautiful galaxies…...some more beautiful galaxies…

Sombrero galaxy Blackeye galaxy Andromeda galaxySombrero galaxy Blackeye galaxy Andromeda galaxy

Other galaxies

Stars do not collide !Stars do not collide !

But galaxies do…But galaxies do…

(observations !)(observations !)

Other Galaxies

galaxy mergersgalaxy mergers

(computer simulation !)(computer simulation !)

Other galaxies

Do other galaxies also contain supermassive Do other galaxies also contain supermassive black holes ? black holes ? YES !YES !

Similar to accretion Similar to accretion

disk-jet connection in disk-jet connection in

young stellar objectsyoung stellar objects

Cosmology

Ancient Mythology and Modern Ancient Mythology and Modern Cosmology:Cosmology:

Is there a Difference ?Is there a Difference ?

Creation Stories I:The Christian/Jewish View

Genesis: Genesis: In the beginning God created the heavens In the beginning God created the heavens and the earth. And the earth was waste and void; and the earth. And the earth was waste and void; and darkness was upon the face of the deep: …and darkness was upon the face of the deep: …

Creation Stories II:Greco-Roman MythologyHesiod: Hesiod: In the beginning there was only “chaos” [the infinite In the beginning there was only “chaos” [the infinite

emptiness]. Then out of the void appeared Erebus, the emptiness]. Then out of the void appeared Erebus, the unknowable place where death dwells, and Night. All else unknowable place where death dwells, and Night. All else was empty, silent, endless, darkness. Then somehow Love was empty, silent, endless, darkness. Then somehow Love was born bringing a start of order. From Love came Light was born bringing a start of order. From Love came Light and Day. Once there was Light and Day, Gaea, the earth and Day. Once there was Light and Day, Gaea, the earth appeared. appeared.

Then Erebus slept with Night, who gave birth to Ether, the Then Erebus slept with Night, who gave birth to Ether, the heavenly light, ...heavenly light, ...

Common Themes and Concepts:

AnthropomorphismAnthropomorphism Action of a supreme craftsmanAction of a supreme craftsman Generation from a seedling/eggGeneration from a seedling/egg Imposition of order over “chaos”Imposition of order over “chaos” Life cycle dominates over eternal/unchanging: Life cycle dominates over eternal/unchanging:

there is a beginningthere is a beginning Hybrid schemes: act of creation, but supreme Hybrid schemes: act of creation, but supreme

being/chaos existed foreverbeing/chaos existed forever AnthropocentrismAnthropocentrism

Scientific “Creation” Story 2005:

In the beginning there was neither space nor time In the beginning there was neither space nor time aswe know them, but a shifting foam of strings aswe know them, but a shifting foam of strings and loops, as small as anything can be. Within the and loops, as small as anything can be. Within the foam, all of space, time and energy mingled in a foam, all of space, time and energy mingled in a grand unification. But the foam expanded and grand unification. But the foam expanded and cooled. And then there was gravity, and space and cooled. And then there was gravity, and space and time, and a universe formed. …time, and a universe formed. …

Is there a difference ?Is there a difference ?

The Scientific Method

specific instancesspecific instances

observationsobservations

inductioninduction

general principlegeneral principle

deductiondeduction

predictionprediction

individual eventsindividual events

revisionrevision

History:

Mythology vs the scientific methodMythology vs the scientific method Cosmos = Earth Cosmos = Earth solar system solar system Milky Way Milky Way

Hubble sphereHubble sphere

Edwin Hubble (1889-1953)Four major accomplishments Four major accomplishments in extragalactic astronomyin extragalactic astronomy The establishment of the The establishment of the

Hubble classification Hubble classification scheme of galaxiesscheme of galaxies

The convincing proof that galaxies are island The convincing proof that galaxies are island “universes”“universes”

The distribution of galaxies in spaceThe distribution of galaxies in space The discovery thatThe discovery that the universe is expanding the universe is expanding

Doppler effect (for light)

The light of an approaching source is shifted to the The light of an approaching source is shifted to the blue, the light of a receding source is shifted to the blue, the light of a receding source is shifted to the redred

Doppler effect

red shiftred shiftblue shiftblue shift

The light of an approaching source is shifted to the blue, The light of an approaching source is shifted to the blue,

the light of a receding source is shifted to the red.the light of a receding source is shifted to the red.

Doppler effect

redshift:redshift:

zz=0: not moving=0: not moving

zz=2: =2: vv=0.8=0.8cc

zz==: : vv==cc

cv

cvz

/1

/11

cv

cvz

/1

/11

The redshift-distance relation

Key results Most Most galaxiesgalaxies are moving are moving away from usaway from us The recession The recession speed vspeed v is is larger for more distant larger for more distant

galaxiesgalaxies. The relation between recess velocity. The relation between recess velocity vv and distanceand distance dd fulfills a linear relation: fulfills a linear relation: v = Hv = H0 0 d d

Hubble’s measurement of the constantHubble’s measurement of the constant HH00::

HH00 = 500 km/s/Mpc = 500 km/s/Mpc

today’s best fit value of the constant:today’s best fit value of the constant: HH00 = 71 km/s/Mpc = 71 km/s/Mpc (WMAP)(WMAP)

Question:If all galaxies are moving away from us,If all galaxies are moving away from us,

does this imply that we are at the center?does this imply that we are at the center?

Answer:Not necessarily, it also can indicate that the Not necessarily, it also can indicate that the universe is expanding and that we are at no universe is expanding and that we are at no special place.special place.

Einstein’s General Relativity +Einstein’s General Relativity +observation of expanding Universe:observation of expanding Universe:

Universe started from a point:Universe started from a point:

““Big Bang Model”Big Bang Model”

Example: static universe

R(t)

t

Example: expanding at a constant rate

R(t)

t

: expansion

is

slowing down

R(t)

t

Example: expansion slowing downExample: expansion slowing down

Example: expansion is accelerating

R(t)

t

Example: expansion accelerating Example: expansion accelerating

R(t)

t

Example: Collapsing UniverseExample: Collapsing Universe

Cosmological redshift

While a photon travels from a distance While a photon travels from a distance source to an observer on Earth, the source to an observer on Earth, the Universe expands in size fromUniverse expands in size from RRthenthen toto RRnownow..

Not only the Universe itself expands, but Not only the Universe itself expands, but also the wavelength of the photonalso the wavelength of the photon ..

emittedthen

nowreceived R

R emittedthen

nowreceived R

R

Cosmological redshift

General definition of redshift:General definition of redshift:

for cosmological redshift: for cosmological redshift:

emitted

emittedreceivedz

emitted

emittedreceivedz

then

now

emitted

received

R

Rz

1then

now

emitted

received

R

Rz

1

A large redshift z implies ...

The spectrum is strongly shifted toward red The spectrum is strongly shifted toward red or even infrared colorsor even infrared colors

The object is very far awayThe object is very far away We see the object at an epoch when the We see the object at an epoch when the

universe was much younger than the universe was much younger than the present day universepresent day universe

most distant astrophysical object discovered most distant astrophysical object discovered so far: z=5.8so far: z=5.8

z>5.8: “dark ages”z>5.8: “dark ages”

k>0k>0 k<0k<0k=0k=0

Are there any indications that this picture is correct?

Yes !Yes !

Primordial NucleosynthesisPrimordial Nucleosynthesis

Cosmic Microwave backgroundCosmic Microwave background

Primordial NucleosynthesisGeorgy Gamov (1904-1968)

If the universe is expanding, then If the universe is expanding, then there has been a big bangthere has been a big bang

Therefore, the early universe must Therefore, the early universe must have been very dense and hothave been very dense and hot

Optimum environment to breed the elements by Optimum environment to breed the elements by nuclear fusion (Alpher, Bethe & Gamow, 1948)nuclear fusion (Alpher, Bethe & Gamow, 1948) success: predicted that helium abundance is 25%success: predicted that helium abundance is 25% failure: could not reproduce elements more massive failure: could not reproduce elements more massive

than lithium and beryllium (than lithium and beryllium ( formed in stars) formed in stars)

The Cosmic Microwave Background (CMB)

Last scattering surface transparenttransparent

opaqueopaque

Penzias and Wilson 1965

Working at Bell labsWorking at Bell labs Used a satellite dish to measure radio Used a satellite dish to measure radio

emission of the Milky Wayemission of the Milky Way They found some extra noise in the They found some extra noise in the

receiver, but couldn’t explain itreceiver, but couldn’t explain it discovery of the background radiation discovery of the background radiation

Most significant cosmological observation Most significant cosmological observation since Hubblesince Hubble

Nobel prize for physics 1978Nobel prize for physics 1978

More results from the CMB

The Earth is moving The Earth is moving with respect to the with respect to the CMB CMB Doppler shift Doppler shift

The emission of the The emission of the GalaxyGalaxy

Fluctuations in the Fluctuations in the CMBCMB

•Fluctuations in CMB responsible for structure formation in the universe

Documents

General GeoAstro II: Astronomy