5.5 Astrophysics (EM Radiation and Distances) Flashcards
Can electrons bound to an atom exist in any energy level?
No. Only certain discrete energy levels.
Can electrons have an energy value between two energy levels?
No.
Are all energy levels the same for each element?
No. Each element has its own set of energy levels.
What does it mean when an electron becomes ‘excited’?
It has moved from a lower energy state to a higher energy state.
What is required for an electron to become ‘excited’?
The input of external energy (eg. heat, absorption of photon)
What occurs when an electron is de-excited?
It moves towards the ground state. It releases energy in the form of a photon with a specific wavelength.
All energy level values are negative, with the ground state being the most negative. Why use the negative sign?
To represent the energy required to be inputted to remove the electron from the atom.
(Energy levels) An electron which is completely freed from an atom has an energy equal to what?
0
Emission Line Spectra
A series of coloured lines on a black background.
Continuous Line Spectra
All visible wavelengths of light are present.
Absorption Line Spectra
A series of dark spectral lines against the background of the continuous spectrum, with each line corresponding to a wavelength of light used to excite atoms of that element.
Why are the wavelengths of light produced by de-excited electrons different for each element?
Each element has a unique set of discrete energy levels.
What is spectroscopy?
The technique used to identify elements based on the wavelengths of light emitted when atoms in a gas are excited.
Diffraction grating formula
dsinΘ = nλ
d: diffraction slit distance
λ: wavelength
n: order of maximima
Θ: angle of diffraction
What is the colour of a star affected by?
Its surface temperature
For any object above 0K, objects emit what? [Weins law]
Electromagnetic radiation of varying wavelength and intensity.
What can stars be modelled as?
Idealised black bodies that emit radiation across a range of wavelengths, with a peak in intensity at a specific wavelength corresponding to the colour of the star.
State Wein’s Law
The black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature of the object.
Wein’s Law Equation
λmax T = Wein’s Constant
λmax ∝ 1 / T
λmax: Wavelength of light produced with max intensity (peak wavelength)
T: Absolute surface temperature of the object.
What is the luminosity of a star?
The radiant power output of the star.
What is the luminosity of a star proportional to?
The surface area of the star, or the surface temperature.
State Stefan’s Law
For a black body, the luminosity is proportional to the fourth power of its absolute surface temperature.
Give the equation for Stefan’s law.
L ∝ 4πr²T⁴
L = 4πr²T⁴σ
(σ = stefan’s constant)
If we know the temperature and luminosity of a star, what can we determine, using what law?
The radius, using Stefan’s law.
Stefan’s law relates the luminosity to what?
The temperature and radius.
When dealing with Stefan and Weins laws, do we use C or K for temperature?
K.
(Remember to convert in the exam)
What is 1AU?
One astronomical unit is the average distance from the earth to the sun (1.5x10¹¹m).
What are astronomical units mostly used for?
Expressing the distance of planets from the sun.
What is 1ly?
1 light year is the distance light travels in one year. It is given using the speed of light x time of 1 year (in seconds) = 9.46x10¹⁵m.
What are light years mostly used for?
Expressing distance to stars and other galaxies.
For angles which are only a small fraction of a degree, what units can be used?
Arcminutes and arcseconds.
How many arcminutes and arcseconds are there in one degree?
60 arcminutes
3600 arcseconds.
Define parsec
The distance at which a radius of 1AU subtends an angle of 1 arcsecond. 1pc = 3.1x10¹⁶.
Stellar Parallax
The change in position of an object depending on the viewing angle.
Up to what distance is stellar parallax accurate for? Why?
Up to 100pc. Beyond this point, the angles involved are so small they are hard to accurately measure.
To use parallax to calculate distance, what formula can be used?
Where is this relationship only true?
d = 1/p
d: distance between observer and object
p: parallax angle
This relationship is only true where d is measured in parsecs, and p in arcseconds.
Black body radiation
EM radiation with a spectrum that peaks depending on the temperature of the emitter.
What can stellar parallax be used for?
Estimating the distance of a star, based on how much it moves relative to the background of stars in the time it takes for the earth to move half an orbit.
Why does each element produce a unique emission line spectrum?
Because of the unique set of energy levels associated with its electrons.
What are continuous line spectrums produced by?
They are produced by atoms of solid heated metals.
Comparing the absorption and emission line spectrums of the same element, where are the dark lines compared to the coloured lines?
The dark lines are at the same wavelengths as the coloured lines produced when the atoms are de-excited.
