The following notes were taken primarily from Physics for IB by Chris Hamper and Physics Course Companion by Tim Kirk

E.2: Stellar Radiation and Stellar Types

E.2.1State that fusion is the main energy source of starsStudents 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.2.2Explain 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 sunHappens on the inside of the sun (Yes, there are different

layers)Produces neutrinos that leave the sun and travel to Earth

“H” is Fuel

This is the same reaction discussed in Topic 7. Each complete chain reaction produces 26.7MeV.

The proton-proton chain

Remember you need 4 H to end up with one He

See simplified equation:

Gravity pulls inwardSo much the sun should collapse.

Nuclear explosions push outwardThese two have to balance out to be at pressure

equilibrium Ex. Balloon.

Rubber is like gravityAir is like the explosionsIf the temp changes the inside pressure will change and

won’t be stable

Star Stability

E.2.3Define the luminosity of a star.

E.2.4Define 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 energySun’s luminosity of 3.839 x 1026W

Luminosity

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

Apparent brightness(b)

E.2.5Apply the Stefan–Boltzmann law to compare the

luminosities of different stars.

E.2.6State 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 levelsMeans more photons released, and their average energy is

greater.E = hf, Higher energy means higher frequency/shorter

wavelength

Black Body Radiation

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)

Stefan-Boltzmann

If a star has a surface area A and temperature T then the total power emitted (luminosity), L is given by:

L = σAT4

Stefan-Boltzmann

At the temperature increases, the peak wavelength is shorterRelationship between peak wavelength and temp is Wien

displacement law:

λmax = (2.90 x 10-3km) / T

Stefan-Boltzmann

ExampleThe 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.2.7Explain 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.2.8Describe 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 levelsWhen excited only produce specific wavelengths.

(Emission Spectrum)When white light passes through same gas these

wavelengths are absorbed. (Absorption spectrum)

Stellar Spectra

Stars emit a continuous spectrum of EMPeak 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.

Stellar Spectra

The absorption spectra also helps us to calculate the temperature of the gas.

Hot gasMost electrons are already in higher energy levelsMeaning they can’t make the biggest jumpSee “Energy Levels” diagramMeans the higher energy photons will not be absorbed.Means a weak absorption lineWhich can let us find the temp

Stellar Spectra

E.2.8Describe 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 colorOh Be A Fine Girl Kiss Me

Spectral Classification of Stars

Class Temperatrue ColorO 30k - 60k BlueB 10k - 30k Blue-WhiteA 7.5k - 10k WhiteF 6k - 7.5k Yellow-WhiteG 5k - 6k YellowK 3.5k - 5k OrangeM 2k - 3k 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 awayBlue shift – shorter λ – star moving closer

Doppler Shift