Post on 26-Feb-2019
Straling
Reynier PeletierKapteyn Astronomical Institute
Groningen
Blackbody Radiation(thermal radiation):
Converting this to the intensity as a function of frequency:
For low frequencies ( ), we have:
Here h is the constant of Planckand k the constant of Boltzmann
h ν ≪ kT
(Rayleigh-Jeans law = the classical approximation)
Blackbody Radiation:
Hotter B.B. emitters “emit” more total radiation per unit area.However, a big cold object can emit the same or more energy (depending
on how big it is) than a small, hotter one
Cold Hot
Integrating the radiation of a blackbody gives: Stefan-Boltzmann Law: Emitted power per square meter = σ T4
σ = 5.7 x 10-8 W/(m2K4)
Total emitted power: E = 4 R2 σ T4
Two Laws of Black Body Radiation
2. The peak of the black body spectrum shifts towards shorter wavelengths when the temperature increases.
Wien’s displacement law:
max ≈ 3,000,000 nm / TK
(where TK is the temperature in Kelvin).
1. The hotter an object is, the more luminous it is.
The Color Index (I)B band
V bandThe color of a star is measured by
comparing its brightness in two different wavelength bands:
The blue (B) band and the visual (V) band.
We define B-band and V-band magnitudes just as we did before for total magnitudes (remember: a larger number indicates a fainter
star).
The Color Index (II)
We define the Color Index
B – V(i.e., B magnitude – V magnitude)
The bluer a star appears, the smaller the color index B – V.
The hotter a star is, the smaller its color index B – V.
Stars come in different colors
Color and Temperature
Orion
Betelgeuse
Rigel
Stars appear in different colors,
from blue (like Rigel)
via green / yellow (like our sun)
to red (like Betelgeuse).
If the spectra of stars are black bodies, then these colors tell us about the star’s temperature.
M 51 – HST ACSDistance: 8 Mpc
A Galaxy: Messier 51
In spiral arm
Between spiral arms
M 330.9 Mpc
M33 -Distance 700 kpc
Near a spiral arm
NGC 2050.8 Mpc
NGC 205
The Andromeda Galaxy (M31) with its 2 satellites
M 32
NGC 205
M 32
NGC 89110 Mpc
NGC 891
NGC 891 – thick disk
M51 - In spiral arm
Baade (1944)'s stellar population paradigm:
- Population I is a stellar population similar to the population of stars in the solar neighbourhood, with bright red and blue stars
- Population II is a stellar population similar to stars in globular clusters, where the brightest stars are red.
POP II
POP I
What do we know about colours of stars?
The HR (Hertzsprung-Russell) diagram
B - V
Lum.
Stars populate distinct regions of this plane, corresponding to a particular evolutionary phase.
Terminology:
HRD is “theory” plane: temp. vs. bolometricluminosity.CMD (colour-magnitudediagram) is its emipiricalanalogue.
Stellar Evolutionary Phases
Main Sequence (MS)
Core Hydrogen burning. Longest phase.
Red Giant Branch (RGB)
Hydrogen burning in shell around He core. He core is growing.
When He core is massive and hot enough, it will ignite, starting He-burning (He-flash).
Stellar Evolutionary Phases
Horizontal Branch (HB)
Core He burning. Position stronglymetallicity-dependent.
Asymptotic Giant Branch (AGB)
He-burning in shell around inert CNO core (+ H-burning). Evolution dependenton mass-loss. Pulsating phase.
White Dwarf Sequence (WD)
Remains of the star after envelopehas been ejected (low-mass stars)
Tracks & Isochrones
Tracks:
trajectories of individual stars in the HRD.
Depend on initial mass.
Examples:
BaSTIPadovaGenevaYonsei-Yale
Tracks & Isochrones
Isochrones:
Loci on the HRD populated by stars of the same age, butwith different masses.
(Padova example, Marigo)
Tracks & Isochrones
Isochrone comparison – Bertelli (Padova) vs. BaSTI
Globular Clusters
De-populated upper MS. Strong RGB and HB.
1,000s to 100,000s of stars.
Open Clusters
Open cluster: MS dominant, few RGB or other evolved stars. Only few 100s or 1000s of stars.
Globular cluster: de-populatedMain Sequence, Strong HB and RGB.1000s to 1.000.000s of stars.
Spectra
Encoded in an object’s spectrumis information about the emitter/absorber. This is how we learn what the Universeis made of!
Kirchhoff’s Laws of Radiation (I)
1. A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum.
Kirchhoff’s Laws of Radiation (II)2. If light comprising a continuous spectrum passes
through a cool, low-density gas, the result will be an absorption spectrum.
Light excites electrons in atoms to higher energy states
Frequencies corresponding to the transition energies are absorbed from
the continuous spectrum.
Kirchhoff’s Laws of Radiation (III)3. A low-density gas excited to emit light will do so at
specific wavelengths and thus produce an emission spectrum.
Light excites electrons in atoms to higher energy states
Transition back to lower states emits light at specific frequencies
The Spectra of StarsInner, dense layers of a star produce a continuous (black
body) spectrum.
Cooler surface layers absorb light at specific frequencies.
=> Spectra of stars are absorption spectra.
Most prominent lines in many astronomical
objects: Balmer lines of hydrogen
Lines of Hydrogen
The Balmer Lines
n = 1
n = 2
n = 4
n = 5n = 3
H H H
The only hydrogen lines in the visible wavelength range.
Transitions from 2nd to higher levels of hydrogen
2nd to 3rd level = H (Balmer alpha line)2nd to 4th level = H (Balmer beta line)
…
Absorption spectrum dominated by Balmer lines
Modern spectra are usually recorded digitally and represented as plots of
intensity vs. wavelength
Emission nebula, dominated by the red H line.
The Balmer Thermometer
Balmer line strength is sensitive to temperature:
Measuring the Temperatures of Stars
Comparing line strengths, we can measure a star’s surface temperature!
Spectral Classification of Stars (I)
Tem
pera
ture
Different types of stars show different characteristic sets of absorption lines.
The Spectrum of a star (the Sun)
There are similar absorption lines in the other regions of the electromagnetic spectrum. Each line exactly corresponds to chemical elements in the stars.
The spectrum of a star is most determined by
1. The temperature of the star’s surface
2. The star’s distance from Earth
3. The density of the star’s core
4. The luminosity of the star
Spectral type• Sequence is: O B A F G K M• O type is hottest (~25,000K), M type is coolest (~2500K)• Star Colors: O blue to M red• Sequence subdivided by attaching one numerical digit,
for example: F0, F1, F2, F3 … F9 where F1 is hotter than F3 . Sequence is O … O9, B0, B1, …, B9, A0, A1, … A9, F0, …
• Useful mnemonics to remember OBAFGKM:– Our Best Astronomers Feel Good Knowing More– Oh Boy, An F Grade Kills Me– (Traditional) Oh, Be a Fine Girl (or Guy), Kiss Me
Spectral Classification of Stars (II)
Oh Oh Only
Be Boy, Bad
A An Astronomers
Fine F Forget
Girl/Guy Grade Generally
Kiss Kills Known
Me Me Mnemonics
Mnemonics to remember the spectral sequence:
Stellar spectra
OB
A
F
G
KM
Surface tem
pera ture
The Composition of StarsFrom the relative strength of absorption lines (carefully
accounting for their temperature dependence), one can infer the composition of stars.
Examples of the use of colours in stars and galaxies
1. Multiple main sequences
2. Star formation histories of Local Group dwarf galaxies
3. Galaxies at various wavelengths
4. Redshifts of galaxies
1. Multiple main sequences
Multiple main sequencesMilone et al. (2007)
Multiple main sequence in NGC 2808, Omega Cen, and a double SGB in NGC 1851 (Mackey & Broby Nielsen) impliesthat globular clusters don't have single age/metallicities!
NGC2808 Piotto et al. (2007)
2. Star formation histories of Local Group dwarf galaxies
Determining Star Formation Histories of
Local Group dwarfs
Mateo et al. 1998
Determining Star Formation Histories of Local Group dwarfs
Tucana – Gallart (part of LCID project)
All work based on Hubble Space Telescope data
Draco (from Dolphin 2005)
Some examples:
M32
NGC 147
Charts from Dolphin 2005
The Magellanic Clouds
LMC SMC
SMC
dIrr SFHs – Summary
All dIrrs have an old population. Continuous star formation over
the entire lifetime of the galaxy. Recent star formation often
dominates. No two histories look the same. Different regions can have
different star formation histories (Hodge et al 1991).
3. Galaxies at various wavelengths
4. Redshifts of galaxies
The Doppler Effect: stars/galaxies that move away from us become redder.
Galaxies
What happens to colors of redshifted galaxies?
HUDF–JD2 (z=6.5) (Mobasher et al. 2005)
i > 30.88 z > 30.26
B > 30.61V > 31.02
HST/ACS
VLT/ISAAC
SST/IRAC
Example of a very redshifted object
0.45 0.55 0.70 0.90
2.101.601.20
JD2 (J-dropout) in HUDF (z=6.5)