Unit 1.6)Using radiation to investigate stars Flashcards
What are emission line spectra and how are they formed?
A series of coloured lines on a black background.
When light passes through the outer layers of a star, the electrons in the atom absorb photons and become excited. They then de-excite, releasing photons of specific wavelengths. These photons are detected on Earth and wavelengths characteristic of the elements in the outer layers, shown as emission line spectra.
What are continuous line spectra
All visible wavelengths of light are present. They are produced by the atoms of heated metals.
(All colours, all wavelengths, its the colourful spectrum no lines on it unless its absorption line spectra)
All stars emit light as a continuous spectrum of radiation.
What are absorption line spectra
A series of dark lines against the background of the continuous spectrum, with each line corresponding to a wavelength of light absorbed by atoms in the outer layers of a star. The dark lines are at wavelengths that are characteristic of the elements in the outer layers (as with emission spectra).
What is meant by black body?
A perfect absorber and emitter of electromagnetic radiation-it absorbs all EM radiation incident upon it.
Stars can be approximated as black body’s.
State Wiens’s displacement law
Wien’s law states that the wavelength of peak emission from a black body is inversely proportional to the absolute (kelvin) temperature of the body:
λ max= T/W
where:
λ
max
= wavelength of peak emission (m)
T = absolute temperature (K)
W = Wien constant = 2.90×10
−3
mK.
wavelength max T = 2.9x10^-3 metre-kelvins.
State Stefan’s law
The power output of a star is directly proportional to its surface area and to its (absolute temperature)^4
P= AσT^4 (where σ(the constant of proportionality) is the Stefan-Boltzmann constant.
σ = Stefan constant = 5.67×10^-8Wm^-2k^-4
The law can also mean-The total electromagnetic radiation energy emitted per unit time by a black body is given by
P= AσT^4
The luminosity of a star is the total energy it emits per unit time in the form of electromagnetic radiation.
Therefore, luminosity is equivalent to the power of the star.
This means we could have written luminosity instead of power in Stefan’s law (above).
What is the inverse square law?
the intensity of radiation from a star is inversely proportional to the distance from the star squared given by:
I = P / 4πr^2
I=intensity(W m^-2), P=source power output(W), r = distance from the source (m)
or p = I x 4πr^2.
How can the distance to nearby stars be measured.
If the radius of the star and its temperature are known, you can calculate the luminosity/power of the star.
Then by measuring the intensity of the star from the Earth, you can use the inverse square law to find the distance.
How can we work out the surface temperature of a star?
By using the equation for luminosity/power:
p = 4σπr^2T^4
(from Stefan’s law , converting area to 4πr^2
r= radius of the star, σ= Stefan Boltzmann’s constant, T = surface temp.
What is multiwavelength astronomy
A galaxy can be studied using different telescopes which are sensitive to different wavelengths of the electromagnetic spectrum. Many astronomical objects emit radiation outside of the visible spectrum.
With the exception of visible light astronomy, the colours are ‘false colours’, i.e. the colour is an intensity code rather than an actual colour.
Black body spectrum what is the shape of the curve and the link between peak wavelength and absolute temperature.
Look at blended learning unit 1.6 for shape.
The peak wavelength is inversely proportional to the absolute temperature(i.e the higher the temperature the smaller the wavelength is before it reaches the peak intensity)
Intensity equation?
I=p/4pier^2
Luminosity equation(with just radiation intensity and distance)
F = L/4pie d^2
L = F x 4pie d^2
F = radiant flux intensity, or observed intensity on Earth (W m-2)
L = luminosity of the source (W)
d = distance between the star and the Earth (m)
How do you find the radius of a star?
R = L/4pie stefans constant T^4 all sqrt
