General GeoAstro II: Astronomy

1
General GeoAstro II Astronomy

• The name of the game
• Not all info on slides attend the lectures, take
  notes !
• sugg. reading - Universe (Kaufmann
  Freedman) basic reference
• - Astronomy-fun,
  animations etc
• 
  http//www.opencourse.info/astronomy/introduction/
  
• 
  astronomy_links.html
• - more advanced
• 
  http//ocw.mit.edu/OcwWeb/Physics/8282JSpring2003/
  
• 
  StudyMaterials/index.htm
• no laptops, no mobiles during class
• classes are not complicated, but please repeat
  them regularly
• only few formulae, but you have to
  know/understand them
• Statistics from previous years attendance good
  grade
• Preparing the night before exam will
  not work !
• http//www.faculty.iu-bremen/course/spring06/Gener
  alGeoAstro2/astro

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General GeoAstro II Astronomy

• Stars
• Nature of stars
• Birth of stars
• Stellar evolution
• Endpoints
• White Dwarfs
• Neutron Stars
• Black Holes

• Galaxies
• - Milky Way
• - Other galaxies
• - Supermassive
• black holes

3
Distance to the stars

• From brightness? No!
• Parallax-experiment
• full circle 360 deg
• 1 deg 60 arcmin
• 60 '
• 6060 arcsec
• 3600 ''
• Stellar parallax

d 1/p
4

Distance to the stars

• Definition star has a distance of 1 parsec
  (pc) if
• its parallax is
  one arcsecond
• 1 pc 3.26 light years
• Brightest stars on the night sky too far to
  measure parallax
• Blurring of atmosphere parallaxes lt 0.01 arcsec
  extremely hard to measure, reliable out to
• d 1/p 1/0.01 100 pc

5

Distance to the stars

• Hipparcos High Precision Parallax Collecting
  Satellite
• (Hipparchus greek astronomer)
• Parallaxes still important to gauge other
  distance indicators

6
Stellar velocities

• Important tool Doppler shift
• in words
• formula (l-lo) / lo vr/c
• vr radial motion

7
Stellar velocities

• Proper motion m Which angle is travelled per
  time?
• Radial motion vr measured via Doppler-shift
• True velocity

8
Brightness and Distance (Inverse
square law)

• Distance and brightness
  luminosity
• Stars have different masses
• different luminosities
• luminosity energy/time J/s
• brightness energy/(time surface area) J/s
  m2

9
Brightness and distance
10


luminosities

• huge variety of stellar luminosities
• Lmax 1010 Lmin
• (1010 number of all people that ever lived
  on earth)

11
The Magnitude system

• System to classify stellar brightness
• Very old Hipparchus (200 B.C.)
• brightest stars first magnitude
• half as bright second magnitude
• half as bright third magnitude
• apparent magnitudes
• Attention scale backwards

12

Magnitude system

• 19th century astronomers first magnitude stars
  shall be 100 times brighter than sixth magnitude
  stars
• difference of 5 mag corresponds to a
• factor of 100 in brightness,
• i.e. x5 100 x
  2.512
• half as bright
  1/2.512 as bright

13

Magnitude
system

• Scales backwards the brighter the more
• negative
• Examples
• Venus m - 4
• Full moon m - 13
• Our sun m - 26.8
• Relation brightness magnitudes...
• m2-m1 2.5 log(b1/b2)

14
Absolute magnitudes

• Definition absolute mag. relative mag. as seen
  from a distance of 10 pc
• Distance modulus (m-M)
• m - M 5 log(dpc) 5
• dpc distance in pc
• 
  m apparent magnitude
• 
  M absolute magnitude

15
Stellar colours

• Stellar colours depend
• on the surface temperature !
• Wiens law ?max T const

16
For your information

• Geo-Astro helpdesk
• Tuesday 1900 2100, East Hall 5
• 1st session February 14th

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Spectra of stars

• How do we know the same laws of physics hold in
  the observable universe?
• Sun absorption line spectrum (continuum dark
  lines)
• Spectral classification O B A F G K M
• Oh be a fine girl/guy kiss me
• hot Tsurf 25 000 K
• 
  
  Sun
• cool Tsurf 3000 K

18

Spectra of
Stars

• Quantum mechanics
• Interpretation of absorption lines in terms
  of atomic energy levels

19
Stefan-Boltzmann law for black body radiation

• F s T4
• - F energy flux from star,
• Joules per
  square meter per second
• - s a constant
  (Stefan-Boltzmann constant)
• - T temperature
• Luminosity of a star L 4pR2 s T4

20
Stellar sizes

• impossible to measure with telescopes
• measure i) brightness
• ii) distance (parallax)
• iii) surface temperature
  (spectral type)
• .
• .

21
Hertzsprung-Russel diagram

• Idea plot luminosity vs. temperature
• 
  (spectral type)

22

Hertzpsrung-Russel diagram

• not random, just a few classes
• most stars on Main Sequence (hydrogen burning)
• White dwarfs same temperature, but lower
• luminosity
  small radius
• RWD 10 000 km
  Rearth
• Giants same temperature, but higher
• luminosity
  large radius
• Rgiant 10 - 100
  Rsun
• 
  Tsurf 3000 6000 K
• Supergiants up to 1000 Rsun

23
Stellar Masses

• need binary stars ! (50 of all stars in
  binaries)
• double stars either i) optical double
  stars
• 
  ii) true binary star
• How to get masses???
• Kepler III ????G??M1M2)/a3
• 
  M1 mass star 1
• M2 mass star
  2
• 
  a separation between stars
• 
  G gravitational constant
• 
  ? 2 ?/T, T orbital period
• measure a and T total system
  mass

24

Stellar
masses

• individual masses?
• i) find center of mass (CM)
• ii) distances from CM to stars, a1
  a2
• a1 (M2/Mtot) a
• a2 (M1/Mtot) a

25
Mass-luminosity relation

• Observation
• L
  M3.5 ..
• 
  proportional to
• 
  
• Stellar lifetime??????????????????????????????
• fat blokes die young
• 
  

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The Birth of Stars

• We see a region of space extending from the
  centre of the sun to unknown distances contained
  between two planes not far from each other
• (Immanuel Kant Allgemeine Naturgeschichte
  und Theorie des Himmels)
• Nuclear burning in the sun (hydrogen to
  helium)
• consumes 6 1011 kg/s of hydrogen
• no infinite fuel resources finite life time
• stellar evolution (birth,
  evolution, death)

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Birth of Stars

• snapshot problematic
• stellar gtgt human lifetime
• Derive evolutionary sequence from a set of
  snapshots

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Stellar Birth

• Stars are born in the
• gravitational collapse of
• giant molecular clouds

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Stellar Birth

• computer-simulation of
• the collapse of a giant
• molecular cloud by
• Mathew Bate
• very dynamic process
• stars form in groups
• many binary/multiple
• star systems form
• observation
• 50 of stars are in
• binary systems

30
Stellar birth

• Where does star formation take place?
• in the spiral arms of galaxies

31
Interstellar Medium (ISM)

• ISM provides matter of which stars are made
• ISM consists of a combination of gas and dust
• Interstellar
  gas
• Very low density 1 H atom/ccm (air
  1019 atoms/cm3 )
• but still 20-30 of mass of galaxy
• mainly Hydrogen Helium, mixed with cosmic rays,
  magnetic fields and radiation

32
Interstellar medium

• Interstellar dust
• - mainly H,C,O,Mg and Fe
• - size lt 1/1000 mm
• wavelength blue light
• blue scattered in all
  directions,
• intensity reduced
• interstellar reddening
• - also absorbs light, heats up, emits
  infrared radiation
• insterstellar extinction

33
Interstellar medium

• For historical reasons interstellar clouds are
  called
• Nebulae
• Three kinds of nebulae
• Emission N. Reflection N.
  Dark N.

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Interstellar medium

• Emission nebulae
• - contain hot, young stars
• (O and B stars with Tsurf gt 10
  000K)
• - temperatures 10 000 K
• - masses 10 10 000 Msolar
• - density n few 1000 atoms/cm3
• (compare with air 1019 atoms/cm3
• 
  ISM 1 atom/cm3)

35
Interstellar medium emission nebulae

• Interstellar hydrogen found in two forms
• HI-region neutral hydrogen
• HII-region ionized hydrogen (i.e. protons
• 
  and
  electrons)

36
Interstellar medium emission nebulae

• Emission mechanism HII-region
• - Hydrogen ionized by
• UV-radiation from hot stars
• - recombination
• (proton captures electron, emits light
• as it cascades down)
• - most important transition
• from n3 to n2 (Ha-photons)
• reddish colour

37
Reflection nebulae

• Lots of fine-grained dust, low density
• reflects short-wavelengths more
• efficiently than long ones
• blue colour

38
Dark Nebulae

• High density of dust grains
• block view to the stars
• Temperature 10 100 K
• hydrogen molecules
• Density n 104 109 atoms/cm3

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Stellar Evolution

• Protostars
• Gravity has to overcome gas pressure
• dense cold regions preferred
• dark nebulae (stellar nurseries)
• standard cosmic abundances
• 75 Hydrogen
• 24 Helium
• 1 heavier elements

40
Protostars

• young protostars more luminous than later on the
  main sequence (gravitational energy)
• Decrease of luminosity at almost constant surface
  temperature,
• but central temperature
• rises
• Evolutionary path in
• HR-diagram

41
Protostars

• At Tcentral 106 K thermonuclear reactions
• (H He)
  set in
• produce
  energy/pressure
• stop
  contraction
• hydrostatic equilibriumnuclear burning
• Main sequence (MS) reached
• Exact position on MS determined by stellar mass

42
Main sequence masses

• Extreme cases
• Mass too small (lt0.08 Msol)
• no ignition of hydrogen, no main sequence
  stage
• Brown Dwarf
• Mass too big (gt100 Msol)
• violent winds
• disruption of the star
• Main sequence 0.08 lt MMS lt 100 Msol

43
Young stellar objects (YSOs)

• Accretion disks
• Jets

44
Young stellar objects

• examples of accretion disk jet connection
• interaction of these outflows with surrounding
  matter
• Herbig-Haro objects
• Jets are usually short-lived 104 years,but can
  eject large masses (1 Msol) during this
  time
• many young stars lose mass
• via strong winds mass loss 10-7 Msol/year
• (our sun 10-14 Msol/year)
• Example gas ejection from XZ-Tauri

45
Young stellar objects

• young stars like to hang around in groups
• (see previous movie)
• open clusters
• fastest stars may leave
• evaporation of open
  clusters

46
Stellar evolution overview

• once formed, evolution of stars depends on their
  masses
• M lt 0.08 Msol no nuclear fusion
• Brown
  dwarfs
• 0.08 lt M lt 8 Msol i) Main sequence
• ii) Giant
  phase
• iii) White
  Dwarf
• 
  planetary nebula

47
Stellar evolution overview

• 8 lt M lt 25 Msol i) Main Sequence
• ii) Giant
  phase
• iii)
  supernova
• 
  explosion
• neutron star
• M gt 25 Msol i) Main Sequence
• ii) Giant
  phase
• iii)
  supernova
• 
  explosion
• black hole

48
Evolution of a M lt 8 Msol star

• our sun - MS-star, H-burning in core
• - Red Giant H
  in core
• exhausted,
  H-burning in
• shell
• - Red GiantHe
  ignites in
• stellar
  core, radius 1 AU
• 
  earth swallowed
• (
  5 109 years from now)
• - final
  stages hot, cooling
• 
  Carbon-Oxygen core,
• 
  eject envelope
• 
  
• White
  dwarf planetary nebula

49
8 Msol -star

• Planetary nebulae

50
8 Msol -star

• Evolution in the HR-diagram

51
Testing stellar evolutionGlobular Clusters

• Globular Clusters
• 105 stars
• in halo of galaxy
• Old about same age as
• galaxy

52
Globular clusters and HR-diagrams

• Basic idea - formed at the same time
• - heaviest
  stars have already evolved away from
• main
  sequence
• - lightest stars
  still on main sequence
• 
  
  age of cluster

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Evolution for M gt 8 Msol

• Stages
• Main sequence
• Giant stage
• Final stage

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Evolution for M gt 8 Msol

• No more nuclear fuel (beyond iron)
• core-collapse
• supernova explosion (type II)

55
Evolution for M gt 8 Msol

• Supernova explosion results in
• either
• i) a neutron star (M lt 25 Msol)
• or
• ii) a black hole (M gt 25 Msol)

56
End stages of stellar evolution

• White dwarfs
• Left behind in center of planetary nebula
• No more nuclear burning just cools
  until it fades away
• Masses 0.2 1.4 Msol
• above 1.4 Msol
  collapse to neutron star
• Densities 106 108 g/cm3 (earth 5
  g/cm3)
• Equilibrium between gravity and degeneracy
  pressure

57
White dwarfs

• Degeneracy pressure
• purely quantum mechanical effect
• Electrons are Fermions (spin ½)
• dont want to be in the same state
• (Pauli-exclusion principle)
• resist compression even at zero temperature

• all mass from neutrons and protons
• all pressure from electrons

58
white dwarfs

• Mass-Radius relationship R M -1/3
• More massive WDs are smaller

59
End Stages of stellar evolution
Neutron Stars

• Masses 1.4 Msol
• Radius 10 - 15 km
• Density few 1014 g/cm3
• 
  
  observed neutron star mass
  distribution
• Magnetic field 1012 - 1015 G (earth 0.5 G)

60
Neutron stars

• hard to detect
• new-born neutron
• star in Supernova
• remnant

61
Neutron Stars

• Internal structure
• mostly neutrons
• (90 neutrons, 10 protons)
• crust iron-like
• nuclei
• center exotic
• particles?

62
End Stages of stellar evolution
Black holes

• neutron star has limiting mass, above that mass
  collapse to a black hole
• not even light can escape from a black hole
• How can a black hole be detected?

63
Black holes

• Black hole accretes mass from
• companion star
• X-ray binary

64
Further reading

• KaufmanFreedman Universe, part III
• http//observe.arc.nasa.gov/nasa/space/stellardea
  th/stellardeath_intro.html
• very simple, still interesting
• http//chandra.harvard.edu/edu/formal/stellar_ev/
• good overview
• http//www-ssg.sr.unh.edu/ism/links.html,
• good collection of links