Stars &EA 5/6 Flashcards
What is Planck’s function?
Planck’s function is the relationship between intensity and wavelength at different temperatures.
What is Wien’s law?
Wien’s law is the relationship between the wavelength that radiates the maximum energy and the temperature.
What is the equation that connects luminosity, temperature and size of stars?
L= 4(pi)R^2(sigma)Teff^4 (W)
How is magnitude and colour connected?
Use B-V as an example
Yo can measure the magnitude of different regions of th spectra using filters e.g. U,B,V,R,I.
These different regions are called bands and colour is derived by looking at the difference between the magnitudes in two bands.
Example
Hot stars (B-V) < 0 - It is bright in the blue band so Mb is small, therefore (B-V) is low.
Coo stars (B-V) > 0 - It is faint in the blue band so Mb is large, therefore (B-V) is high.
Which graph is used to determine colour and temperatures?
Planck’s function graph
What is a bolometric magnitude? How do we use it?
A bolometric magnitude is the measure of the total radiation of a star emitted across all wavelengths of the electromagnetic spectrum.
We can’t measure this as we can only see the visible region, but we ca use a bolometric correction for a particular T, to get from Mv to Mbol.
Describe the radiation that stars emit?
Stars emit continuous radiation, which is described as blackbody radiation, however stars are not entirely blackbodies due to surrounding gases.
Describe what spectra of stars look like and why?
A continuous spectrum with absorption lines in specific regions where less light is emitted. It is produced by a hot opaque body seen through a transparent layer of cool gas which absorbs light at specific wavelengths.
What is an emission line and how is it produced?
An emission line spectrum is produced by hot transparent gas. The light is only emitted at specific wavelengths.
e.g When clouds of gas are heated by nearby hot stars.
What is spectral type and what system is used now compared to previously?
Spectral type is an observable property related to colour and temperature.
Originally there were 4 types but we now use the MK system with types: O,B,A,F,G,K,M. Which each have 10 sub-types.
What causes spectral lines?
Spectral lines are caused by absorption or emission of electromagnetic radiation. It does this when atoms absorb or emit photons of a certain wavelength when electrons jump from one energy level to another. The change is energy is proportional to the frequency of the photon. E = hv= hc/(lambda)
Use hydrogen as an example to explain spectral lines. Including the Rydberg equation.
The energy difference between the ground state and the first excited state is E12 =10.2 eV so the wavelength is 121.6nm.
The Ryberg equation is used to describe transitions of the hydrogen atom.
1/(lambda) = R ( 1/n^2.L - 1/n^2.U)
R = 1.097x10^7 m^-1
n^2.L &n^2.U are quantum numbers of lower and upper level in a specific transition.
Lyman - UV; n^2.L = 1; n^2.U >1
Balmer - Optical/Visible; n^2.L = 2; n^2.U >2
Paschen - Infrared; n^2.L = 3 ; n^2.U > 3
What is equivalent width?
This is used to describe how strong absorption lines are.
It is calculated using the width of a rectangle from 0 to the continuum line with the same area as the absorption line. The units are nm, use a graph of wavelength vs flux.
Describe the types of chemicals found in hot stars, sun-like stars and cold stars, how is this relevant to spectra?
Spectra is determined by electron configurations which is dependent on the surface temperature. Electron configurations is another way of saying elements/chemicals/etc.
Hot stars - Mostly have ionised gas, this means you see helium and hydrogen lines in the spectra.
Sun-like stars - These have neutral gas, meaning the spectra show metal lines.
Cool stars - These have molecules. This means there are broad absorption band due to metal oxides
Name some other factors that can affect absorption lines
Size of stars
Surface gravities
Luminosity
Atmospheric pressure
