IB Astrophysics option part 2. See also http://nothingnerdy.wikispaces.com

1. presents
a production
STELLAR RADIATION
STELLAR TYPES
based on the IB Astrophysics option
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2. STELLAR RADIATION
STELLAR TYPES
Nuclear fusion
Luminosity
Apparent brightness
Black body radiation
Stefan-Boltzmann law
Wien’s law
Absorption spectra
Spectral classification
The HR diagram

3. Stars
Stars are formed by dust coming together over
a long time through mutual gravitational
attraction.
The loss of potential energy is responsible for
the initial high temperature necessary for
fusion.
The fusion process releases so much energy
that the pressure created prevents the star from
collapsing due to gravitational pressure.

4. Gravity
Radiation
Gravity pulls outer layers in, Pressure
Radiation Pressure
pushes them out. Gravitational
The more mass the pressure
star has, the greater
the gravitational
pressure and the
higher the central
pressure

5. Nuclear fusion
The energy
the Sun
emits is
generated
by the fusion
in its core.
In order to begin the fusion process
of hydrogen to form helium, a very
high temperature is needed: 107 K.

6. MASS
The most important variable
for a ‘hydrogen-burning’ star
Mass affects its
LUMINOSITY and TEMPERATURE

7. Luminosity
The LUMINOSITY of a star is how much
ENERGY it gives off per second
(aka Power) in watts
This light bulb has a
luminosity of 60 Watts
The Sun has a
luminosity of 3.90x1026 W
(often written as L0)

8. Apparent brightness
The energy that arrives at the Earth is
only a very small amount when compared
with the total energy released by the Sun.
d
b is called the apparent
brightness of the star
Unit is W/m2

9. Solar Constant
Solar luminosity, L0 = 3.90x1026 W
Sun-Earth distance, d=1.5 x 1011 m
4πd2 = 2.83 x 1023
b = 1378.1 Wm-2 or Js-1 m-2
The amount of energy arriving at the Earth
every second per square metre, although it
is not evenly spread over the sphere.

10. Black body
A black body is a perfect
thermal emitter and
absorber. Its spectrum
depends only on its
temperature.

11. Black body Spectrum
A black body with
a higher
temperature has
greater intensities
of all wavelengths
and its wavelength
of maximum
intensity is shorter.

12. Stefan-Boltzmann
The area under a black body radiation
curve is equal to the total energy emitted
per second (L) per unit of area (A) of the
black body. Stefan showed that this area
was proportional to the fourth power of
the absolute temperature (T)of the body.
where, σ = 5.67x10-8 W m-2 K-4

13. Wien displacement law
The wavelength of maximum
intensity is inversely
proportional to the absolute
temperature of the body

14. A Star’s Temperature
This can be calculated from its black body spectrum
For the Sun, max. intensity wavelength = 500 nm

15. Size of a star
We can use Wien’s Law to find the
temperature of a star from its spectrum
Then find the luminosity/ area
from the Stefan-Boltzmann relation
Then find the luminosity from the
apparent brightness and distance
Find the radius from the area of the star

16. Real stellar spectra
In the spectrum of a star is evidence for
the elements in its outer layers
Theoretical Emission
black body and
Spectrum Absorption
Lines

17. Absorption line strength
The relative strength of
hydrogen absorption lines
in stellar spectra depends
on the temperature of the
star. The first stellar
classification used this
method.
The Harvard team of ‘computers’
Williamina Fleming

18. Spectral classification
Nowadays, the classification is according to temperature
Hotter
Cooler

19. OBAFGKM
O Be A Fine Girl/Guy Kiss
Class Spectrum Me Color Temperature
O
ionized and neutral helium, bluish 31,000-49,000 K
weakened hydrogen
B
neutral helium, stronger blue-white 10,000-31,000 K
hydrogen
A
strong hydrogen, ionized white 7400-10,000 K
metals
F
weaker hydrogen, ionized yellowish white 6000-7400 K
metals
G
still weaker hydrogen, ionized yellowish 5300-6000 K
and neutral metals
K
weak hydrogen, neutral orange 3900-5300 K
metals
M
little or no hydrogen, neutral reddish 2200-3900 K
metals, molecules
L
no hydrogen, metallic red-infrared 1200-2200 K
hydrides, alkalai metals
T
methane bands infrared under 1200 K

20. The HR Diagram
This diagram shows a
correlation between the
luminosity of a star and
its spectral type.
The scale on the axes is
not linear.
Luminosity depends on
mass and size.
Colour depends on
temperature.

21. The HR Diagram
This diagram shows a
correlation between the
luminosity of a star and
its spectral type.
h ere
are The scale on the axes is
u
Yo not linear.
Luminosity depends on
mass and size.
Colour depends on
temperature.

22. Stars are not
randomly
distributed; they
form groups.
MAIN SEQUENCE
90% of all stars
GIANTS AND
SUPERGIANTS
Very large and
very cool for a star
WHITE DWARFS
Small and hot stars

23. Binary stars
Sirius A and B
Many star systems are
binary (double stars)
Sirius is a visual binary,
since we can see both
stars (Sirius B is at the
ESA/Hubble
bottom left).

24. Eclipsing binary
When the plane of orbit is end-on, each
star will pass in front of the other and dim it
Their orbital
period and
relative sizes can
be estimated
from the variation
in brightness

25. Spectroscopic binary
Due to the Doppler
effect, the spectral lines
of an approaching star
will be blue-shifted, while
the spectral lines of a
receding star will be red-
shifted leading to double
lines from which the
orbital speeds can be
calculated.

26. a production
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