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DEPARTMENT OF PHYSICS AND ASTRONOMY
LIFECYCLES OF STARS
Option 2601
M.R.Burleigh 2601/Unit 1
Books
 Introductory Astronomy and
Astrophysics
– Zeilik and Gregory
 Astrophysics I: stars
– Bowers and Deeming
 The Physics of Stars
– A.C. Phillips
M.R. Burleigh 2601/Unit 1
Stellar Lifecycle
M.R. Burleigh 2601/Unit 1
Starbirth
M.R. Burleigh 2601/Unit 1
Young Stars
M.R. Burleigh 2601/Unit 1
Globular Clusters
M.R. Burleigh 2601/Unit 1
Star Death
M.R. Burleigh 2601/Unit 1
Star Death
M.R. Burleigh 2601/Unit 1
Star Death
M.R. Burleigh 2601/Unit 1
Star Death
M.R. Burleigh 2601/Unit 1
Aims and Objectives
To introduce you to the underlying
physics governing the properties of
stars and their evolution
M.R. Burleigh 2601/Unit 1
Lifecycles of Stars
 Unit 1 - Observational properties of
stars
 Unit 2 - Stellar Spectra
 Unit 3 - The Sun
 Unit 4 - Stellar Structure
 Unit 5 - Stellar Evolution
 Unit 6 - Stars of particular interest
M.R. Burleigh 2601/Unit 1
DEPARTMENT OF PHYSICS AND ASTRONOMY
Unit 1
Observational Properties of Stars
M.R.Burleigh 2601/Unit 1
Observational Properties of Stars





The electromagnetic spectrum
Radiation
Flux, intensity and luminosity
Stellar magnitudes and photometry
Temperatures masses and radii
M.R. Burleigh 2601/Unit 1
The Electromagnetic
Spectrum
M.R. Burleigh 2601/Unit 1
Atmospheric absorption
M.R. Burleigh 2601/Unit 1
The Nature of EM radiation
Wave nature:
c = ln
Wavelength
Energy
M.R. Burleigh 2601/Unit 1
Frequency
Planck’s constant
Energy Conversions
1keV = 2.418  1018Hz
1keV = 11.60  106K
1keV = 1.24Å
1keV = 1.6  10-9erg
1J = 107erg
M.R. Burleigh 2601/Unit 1
I
Normal
Solid angle
Da
DW= 2
r


r
A
a
Spherical
surface
Total energy flow from the surface of a star
¥
L = 4pAò I (n )dn
(
)
I
n
d
n
dAd
W
ò
0
Stellar Absolute Luminosity
Monochromatic intensity
L
l=
2
4pr
M.R. Burleigh 2601/Unit 1
Stellar distance
Surface flux of star (F): radiant
energy
Output per unit area of source per second over the whole spectrum:
L = 4pR F
2
Flux
Stellar radius
Can also be considered as monochromatic
luminosity or flux i.e. L(), F()
Brightness (apparent luminosity) is sometimes termed
as ‘flux at the Earth’:
Inverse square law
M.R. Burleigh 2601/Unit 1
F=
L
4pR 2
Inverse Square Law
M.R. Burleigh 2601/Unit 1
Magnitude System
 Comparison of stars wrt one another
 Introduced by Hipparchus ~120BC
 Catalogued >1000 naked eye stars in
order of importance (brightness)
 1st magnitude = 1st importance
 Extended by Ptolemy 180AD
M.R. Burleigh 2601/Unit 1
Magnitude System
 Modern scale dates from 1854, by
Pogson
 Showed that brightness scale is
logarithmic, 1st mag ~100 x 6th mag
 A step of 1 mag = 10 2/5 = 2.512
 Can easily calculate differences
M.R. Burleigh 2601/Unit 1
Magnitude system
m µ log10 L = -2.5 log10 l + K
Constant
Apparent magnitude (m):
Absolute magnitude (M):
æ la ö
ma - mb = -2.5 log10 çç ÷÷
è lb ø
æ L ö
2
ç
2 ÷
é
ù
æ ld ö
10
æ
ö
4
p
d
÷ = -2.5 log10 êç ÷ ú
m - M = -2.5 log10 çç ÷÷ = -2.5 log10 ç
èdø û
çç L ÷÷
è l10 ø
ë
è 4p 10 2 ø
Þ m - M = -5 log10 10 + 5 log10 d = 5 log10 d - 5
Distance modulus
M.R. Burleigh 2601/Unit 1
d=
1
p
Parallax
Stellar Magnitudes
 Any detector (i.e the eye) is only
sensitive to a limited wavelength range
 Only sampling part of radiation from a
star
 Photographic film centred on ~420nm
(mpg)
 Visual (eye) most sensitive ~550nm
(mv)
M.R. Burleigh 2601/Unit 1
Magnitude Definitions
Johnson system:
U
365
B
V
R
I
J
H
K
440 550 700 900 1250 1.65 2.2
nm
M.R. Burleigh 2601/Unit 1
L
3.6
m
M
N
4.8 10.2
Colour Index
CI = M B - M V
(B – V)
(also U – B etc.)
B-V
-ve for
20,000K
0
10,000K (A0) e.g. Vega
For
+ve for
M.R. Burleigh 2601/Unit 1
3,000K
Bolometric Magnitude
¥
ò0 I l dl
I bol =
mbol = -2.5 log lbol + const
Cannot observe mbol directly so use bolometric
correction…
BC = M bol - M V
BC = -0.07 for the Sun (Teff = 6,500K)
M.R. Burleigh 2601/Unit 1
Temperature Definitions
Wien displacement law:
Stephan-Boltzmann law:
c
lm =
T
c = 0.2898 cm deg
Lbol = 4pR*2sT 4
Stefan’s constant
Planck law:
E = hn =
Effective
temperature Te
hc
l
Blackbody:
I lT =
2hc 2
l5
1
e
hc
l kT
-1
h is the Planck constant, k is Boltzmann’s constant
M.R. Burleigh 2601/Unit 1
M.R. Burleigh 2601/Unit 1
M.R. Burleigh 2601/Unit 1
Magnitude system
m µ log10 L = -2.5 log10 l + K
Constant
Apparent magnitude (m):
Absolute magnitude (M):
æ la ö
ma - mb = -2.5 log10 çç ÷÷
è lb ø
æ L ö
2
ç
2 ÷
é
ù
æ ld ö
10
æ
ö
4
p
d
÷ = -2.5 log10 êç ÷ ú
m - M = -2.5 log10 çç ÷÷ = -2.5 log10 ç
èdø û
çç L ÷÷
è l10 ø
ë
è 4p 10 2 ø
Þ m - M = -5 log10 10 + 5 log10 d = 5 log10 d - 5
Distance modulus
M.R. Burleigh 2601/Unit 1
d=
1
p
Parallax
Stellar Distances
The “Parallax” method of measuring
distance…
For nearest
stars only
(<100pc)
M.R. Burleigh 2601/Unit 1
Stellar Distances
d

a = 1AU

(radians)
= a/d

= 1 / d (pc) 1rad = 206,265
M.R. Burleigh 2601/Unit 1
1pc = 206,265AU
M.R. Burleigh 2601/Unit 1
Distances > 100pc
 Use Sun’s motion through the nearby
stars
 Motions of relatively nearby star clusters
M.R. Burleigh 2601/Unit 1
Measurement of Radii
Occultations/eclipses in binary stars
Occultations of stars by the Moon
Michelson stellar interferometer
Interference pattern depends upon angle between
wavefronts from opposite limbs of the star
D
a=
d
Angle in
radians
M.R. Burleigh 2601/Unit 1
Stellar
physical
diameter
Distance
So What is a Star?
 Self-gravitating ball of gas radiating
energy
 Energy produced by
– Thermonuclear reactions
– + gravitational/stellar collapse
 Star must produce enough energy to
maintain internal pressure to counter
gravitational field
M.R. Burleigh 2601/Unit 1
Physical Principles
 Atomic physics  radiation
processes/spectral lines
 Thermodynamics  behaviour of
gas/stellar structure
 Nuclear physics  energy
generation/creation of heavy elements
M.R. Burleigh 2601/Unit 1
Atmospheres
Interior
Unit 1 Slides and Notes
 Can be found at…
– www.star.le.ac.uk/~mbu/lectures.html
 In case of problems see me in lectures
or email me… [email protected]
M.R. Burleigh 2601/Unit 1