Transcript Topic E: Astrophysics

The following notes were taken primarily from Physics for IB by
Chris Hamper and Physics Course Companion by Tim Kirk
E.2.1
 State that fusion is the main energy source of stars
 Students should know that the basic process is one in
which hydrogen is converted into helium. They do not
need to know about the fusion of elements with higher
proton numbers.
E.2.2
 Explain that, in a stable star (for example, our Sun), there
is an equilibrium between radiation pressure and
gravitational pressure.
 How does our sun work?
 Fusion of hydrogen into helium that provides the energy, for
our sun
 Happens on the inside of the sun (Yes, there are different
layers)
 Produces neutrinos that leave the sun and travel to Earth
 This is the same reaction discussed in Topic 7. Each
complete chain reaction produces 26.7MeV.
 Remember you need 4 H to end up with one He
 See simplified equation:
 Gravity pulls inward
 So much the sun should collapse.
 Nuclear explosions push outward
 These two have to balance out to be at pressure
equilibrium
 Ex. Balloon.
 Rubber is like gravity
 Air is like the explosions
 If the temp changes the inside pressure will change and
won’t be stable
E.2.3
 Define the luminosity of a star.
E.2.4
 Define apparent brightness and state how it is measured.
 Light measurements give us information about the
temperature, size and chemical composition of a star.
 Luminosity(L) is the total amount of energy emitted by the
star per second.
 Unit is watt (same as power)
 Depends on the temp.
 Ex. Two stars have same temp, the bigger one will give out
more energy
 Sun’s luminosity of 3.839 x 1026W
 Some stars appear brighter than others.
 Brightness depends on:
 How much energy is radiated (luminosity)
 How far away it is located
 Apparent brightness is the amount of energy per second
received per unit area.
 Unit is W/m2
 b = (L) / 4πd2
 d is distant to the star
E.2.5
 Apply the Stefan–Boltzmann law to compare the
luminosities of different stars.
E.2.6
 State Wien’s (displacement) law and apply it to explain
the connection between the color and temperature of stars.
 Black bodies absorbs all wavelengths of light and reflects
none. It also is a perfect emitter of radiation.
 If temp is increased the energy available is increased.
 Means the electrons can gain more energy and move into
higher energy levels
 Means more photons released, and their average energy is
greater.
 E = hf, Higher energy means higher frequency/shorter
wavelength
 Each peek represents the intensity(apparent brightness) of
radiation at different wavelengths.
 Total intensity is the area under the curve.
 Power per unit area = σ T4
 σ = 5.6 x 10-8 W/m2K4 (Stefan-Boltzmann constant)
 If a star has a surface area A and temperature T then the total
power emitted (luminosity), L is given by:
 L = σAT4
 At the temperature increases, the peak wavelength is shorter
 Relationship between peak wavelength and temp is Wien
displacement law:
 λmax = (2.90 x 10-3km) / T
Example
 The maximum in the black body spectrum of the light
emitted from the sun is at 480 nm. Given that the Sun’s
radius is 7.0 x 108m, calculate the temperature of the sun,
the power emitted per square meter, and the luminosity.
 Answers: 6000K, 7.3 x 107 W/m2, 4.5 x 1026W
E.2.7
 Explain how atomic spectra may be used to deduce chemical
and physical data for stars.
 Students must have a qualitative appreciation of the Doppler
effect as applied to light, including the terms red-shift and
blue-shift.
E.2.8
 Describe the overall classification system of spectral classes.
 Students need to refer only to the principal spectra classes
(OBAFGKM).
Remember:
 Electrons only exist in certain energy levels
 When excited only produce specific wavelengths.
(Emission Spectrum)
 When white light passes through same gas these
wavelengths are absorbed. (Absorption spectrum)
 Stars emit a continuous spectrum of EM
 Peak intensity depends on the temp.
 As this EM pass through the outer layer of the star, some
is absorbed.
 The absorption spectrum of a star tells us what elements
are present because of the missing lines.
 The absorption spectra also helps us to calculate the
temperature of the gas.
 Hot gas
 Most electrons are already in higher energy levels
 Meaning they can’t make the biggest jump
 See “Energy Levels” diagram
 Means the higher energy photons will not be absorbed.
 Means a weak absorption line
 Which can let us find the temp
E.2.8
 Describe the overall classification system of spectral
classes.
 Students need to refer only to the principal spectra classes
(OBAFGKM).
 The spectrum of a star is related to it’s temp and chemical
composition.
 Also the color. The peak points to it’s color
 Oh Be A Fine Girl Kiss Me
Class
O
B
A
F
G
K
M
Temperatrue
30k - 60k
10k - 30k
7.5k - 10k
6k - 7.5k
5k - 6k
3.5k - 5k
2k - 3k
Color
Blue
Blue-White
White
Yellow-White
Yellow
Orange
Red
 As objects move, the wave lengths they produce is either




pushed together or spread apart.
Called doppler effect.
Applies to all waves including light from stars.
Red shift – longer λ – star moving away
Blue shift – shorter λ – star moving closer