Download Powerpoint of lecture 1

yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Document related concepts

Cosmic distance ladder wikipedia, lookup

Aquarius (constellation) wikipedia, lookup

Ursa Minor wikipedia, lookup

Corvus (constellation) wikipedia, lookup

Astronomical unit wikipedia, lookup

Boötes wikipedia, lookup

Perseus (constellation) wikipedia, lookup

Corona Australis wikipedia, lookup

Aries (constellation) wikipedia, lookup

Canis Major wikipedia, lookup

Auriga (constellation) wikipedia, lookup

Cygnus (constellation) wikipedia, lookup

Canis Minor wikipedia, lookup

Cassiopeia (constellation) wikipedia, lookup

Corona Borealis wikipedia, lookup

Timeline of astronomy wikipedia, lookup

Hipparcos wikipedia, lookup

Observational astronomy wikipedia, lookup

Lyra wikipedia, lookup

Serpens wikipedia, lookup

International Ultraviolet Explorer wikipedia, lookup

Dialogue Concerning the Two Chief World Systems wikipedia, lookup

Formation and evolution of the Solar System wikipedia, lookup

Rare Earth hypothesis wikipedia, lookup

CoRoT wikipedia, lookup

Space Interferometry Mission wikipedia, lookup

Ursa Major wikipedia, lookup

Stellar evolution wikipedia, lookup

Star formation wikipedia, lookup

Stellar kinematics wikipedia, lookup

Future of an expanding universe wikipedia, lookup

Dyson sphere wikipedia, lookup

Theoretical astronomy wikipedia, lookup

Constellation wikipedia, lookup

Stellar classification wikipedia, lookup

Star catalogue wikipedia, lookup

Star wikipedia, lookup

Malmquist bias wikipedia, lookup

H II region wikipedia, lookup

Open cluster wikipedia, lookup

Crux wikipedia, lookup

Stellar Structure
Section 1: Basic Ideas about Stars
Lecture 1 – Observed properties of stars
Relationships between observed properties
Outline of the life history of a star
A star is:
a “vast mass of gas”
supported by internal pressure
Some questions:
source of pressure?
energy source?
do they stay hotter than surroundings?
how long do they live?
Real and ideal stars
Ideal stars are:
• isolated
• Spherical
Real stars may be:
• Embedded in gas and/or dust
• In a double or multiple star system
• Connected to surrounding gas by
magnetic field lines
• Rotating rapidly
Factors affecting observational
properties of stars
Observed appearance depends on:
• Distance (and any gas/dust in the way)
• Initial mass
• Initial chemical composition
• Current age
How do we measure observed properties?
Distance measurement
Direct: trigonometric parallax
Earth (January)
1 AU
Earth (July)
p = ‘parallax’  0.76 arcsec
d = 1 parsec (pc) when p = 1 arcsec
d(in pc) = 1/p(in arcsec)
Light output
Deduce luminosity L (total power output) from
flux density F (Wm-2) measured on Earth and
distance d (when known): L = 4πd2F.
Spectrum gives surface temperature (from
overall shape of continuum – best fit to a black
body) and chemical composition (from relative
strengths of absorption lines).
Mass and Radius
• Directly, only from double star systems
• Indirectly, from surface gravity (from spectrum) and radius
• Interferometry
• Eclipse timings
• Black body approximation: L = 4π Rs2 Ts4, if L, Ts known.
Can also define the effective temperature Teff of a star by:
L  4π Rs2 Teff4
Typical observed values
0.1 M < M < 50 M
10-4 L < L < 106 L
10-2 R < R < 103 R
2000 K < T < 105 K
Stellar magnitudes
Hipparchus (~150 BC):
6 magnitude classes (1 brightest, 6 just visible)
Norman Pogson (~1850): defined apparent magnitude m by
m = constant – 2.5 log10F ,
choosing constant to make scale consistent with Hipparchus.
Absolute magnitude M is defined as the apparent magnitude a star
would have at 10 pc. If D = distance of star:
M = m – 5 log10(D/10pc).
[We can hence also define the distance modulus m-M by:
m - M = 5 log10(D/10pc).]
Hertzsprung-Russell diagram (HRD)
Relation between absolute magnitude and surface temperature
(Handout 1):
• Dominated by main sequence (MS) band (90% of all stars)
• Giants & supergiants (plus a few white dwarfs): ~10%
• L  R2 – so most luminous stars are also the largest
• 90% of all stars are MS stars for all their lives
• All stars spend 90% of their lives on the MS 
Mass-luminosity relation (MS stars)
• Strong correlation between mass and luminosity (Handout 2)
• Main-sequence stars only
• Calibrated from binary systems
• Slope steepest near Sun (L  M4)
• Less well-determined for low-mass stars (hard to observe) …
• … and high-mass stars (rare)
Indirect ways of finding stellar
• Spectrum: absorption line strengths depend on
Chemical composition
• Chemical composition similar for many stars …
• … so Teff, L can be deduced
• Variability:
• some pulsating variables show period-luminosity relation
• Measure P  L  M; plus measure m  distance
Star clusters
Gravitationally bound groups of stars, moving together
Globular clusters:
• compact, roughly spherical, 105-106 stars;
• in spherical halo around centre of Galaxy
Galactic (or open) clusters:
• open, irregular, 102-103 stars;
• concentrated in plane of Galaxy
Small compared to distance  all stars at ~same distance
 Apparent magnitude/temperature plot gives the shape of the
HR diagram
Globular cluster HR diagrams
(Handout 3)
All globular cluster HR diagrams are similar:
• short main sequence
• prominent giant branch
• significant horizontal branch (containing RR Lyrae variables)
Find distances by comparing apparent magnitudes of
• main sequence stars
• red supergiant stars
• RR Lyrae variable stars
with those of similar nearby stars of known absolute magnitudes
Galactic cluster HR diagrams
(Handout 3)
Much more variety, but all diagrams show
• Dominant main sequence, of varying length
• Some giant stars, in variable numbers
If all main sequences are the same (i.e. have the same absolute
magnitude at a given temperature), then can create a composite
HR diagram (Handout 3) – plausible if all stars formed at same
time out of same gas cloud  same age and composition
Then find distances to all, if know distance of one, by this “mainsequence fitting” procedure
Mean MS is narrow – suggests it is defined by a single parameter
– the mass increases from faint cool stars to hot bright ones
Life history of stars:
Interstellar cloud of dust and cool gas:
• Perturbed by external event: self-gravity starts contraction
• If spinning, contraction leads to faster spin
• High angular momentum material left behind in disc
• Disc may form planets, and may also eject jets
• Central blob radiates  initial isothermal collapse
• When blob opaque, radiation trapped and temperature rises
• Thermal pressure slows collapse
• “Proto-star” – hot interior, cool exterior
• Contraction releases just enough energy to balance radiation
Life history of stars:
Energy sources
Gravitational energy, from contraction – if sole energy source for
Sun (Kelvin, Helmholtz, 19th century), then timescale ~ E/L where
E = gravitational energy of star, L = luminosity:
tKH = GM2/LR ~ 3107 yr for Sun.
But geology requires much longer timescale – only nuclear fuel
provides this; nuclear binding energy releases up to ~1% of rest
mass energy: EN ~ 0.01Mc2, so
tN ~ 0.01Mc2/L ~ 1.5 1011 yr for Sun.
Over-estimate, because not all mass of Sun is hot enough to be
transformed. Strong mass dependence, because L  M4 – so, for
50 M, tN ~ 108 yr – massive stars were born recently.
Life history of stars:
Life and death
• Proto-star contracts until centre hot enough for hydrogen to
fuse to helium
• Nuclear energy source enough to balance radiation, and
contraction ceases (no more need for gravitational energy)
• Very little change for a nuclear timescale – i.e. until nuclear fuel
• Series of phases of alternating contraction (releasing
gravitational energy until centre hot enough) and further nuclear
reactions (helium to carbon, etc, possibly up to iron)
• After all possible nuclear fuels exhausted, star contracts to a
dead compact object: white dwarf, neutron star or black hole.