Stellar Physics Flashcards
How can we deduce the width of the sun if we know its angular diameter α and the distance D from the observer?
For small angles, α≈2R⊙/D
α is in radians
What is one Astronomical Unit (a.u.) equal to?
The distance between the Earth and Sun
Who devised a method to measure the size of the Sun using transits?
Halley
Describe the transit method using the sun as an example
Venus forms two similar triangles with the Earth and the Sun at their opposite edges. Since the triangles are similar, the projected distance on the Sun can be found by comparing to a known baseline on Earth
How can the distance to the nearest stars can be found?
Using the parallax method
How far away (roughly) is the nearest star?
10^18m away
What is the diameter (roughly) of the sun?
10^9m
What is the Luminosity of a star?
It’s energy output per unit time
What units is luminosity is measured in?
Watts (W) Joules per second (J/s)
What does it mean if a star radiates isotropically?
The star radiates equally in all directions
What happens to the power flux if a star radiates isotropically?
The power flux, F, is equal across all parts of the sphere.
What are the units of flux?
Watts per meter squared (W/m^2)
What is the equation used to calculate flux?
[see equation sheet]
What is Photometry?
The measurement of the quantity of light from a star.
i.e. the flux of energy per unit time, per unit area, that we receive at Earth
What is absolute magnitude (M)?
A measure solely of the star’s intrinsic luminosity
What is apparent magnitude (m)?
A measure of the star’s apparent brightness
What is apparent brightness?
The flux received from the star by us on Earth1
What does apparent magnitude depend on?
The intrinsic luminosity and its distance ( r ) from Earth
If a star is at r=10 parsecs from us the apparent (m) and absolute (M) magnitudes are equal, what is the relationship between them?
M = m+5−5lnr
[see equation sheet]
If we have two stars with measured fluxes, F1 and F2 , what relationship will their magnitudes will be related by?
[see equation sheet]
If the two stars are at the same distance r (e.g. two stars in a binary), what relationship will their magnitudes will be related by?
[see equation sheet]
What is the absolute magnitude, M, equal to?
The apparent magnitude, m, at 10 pc distance
What is the most commonly quoted magnitude for stars?
The visual magnitude
What is the visual magnitude?
The magnitude in the visual part of the spectrum
What is bolometric magnitude?
The magnitude including the luminosity across the whole electromagnetic spectrum of wavelengths, whether observed or not
What kind of scale is the magnitude scale?
Logarithmic
What do we need to know to convert apparent to absolute magnitude?
The distance to the star in parsecs
What is a blackbody?
A body that totally absorbs all radiation that falls upon it
What happens if a blackbody absorbs energy?
It heats up giving it a temperature, T
What will happen if a blackbody is hotter than its environment?
It will cool down by re-radiating its energy
How is radiation is emitted from a perfect emitter?
Equally from the surface of an object, and isotropically over solid angle
What is spectral radiance?
The power emitted per unit wavelength λ , per unit area, per steradian
What does the black body spectrum across frequencies depend on?
Only on the temperature of the object
State the Rayleigh-Jeans law (equation)
[see equation sheet]
What is the problem with the Rayleigh-Jeans law?
It is only a good approximation to the observed spectrum at long wavelength (low frequencies)
In the Rayleigh-Jeans law, what happens at short wavelengths (higher frequencies)?
The radiance keeps increasing indefinitely
What is the ultraviolet catastrophe?
The failure of classical physics to explain the behaviour of thermal radiation
What is Wein’s law?
A good approximation to the observed spectrum at short wavelength (high frequencies)
State Wein’s law (equation)
[see equation sheet]
How was Planck able to to derive a spectrum that fit the data?
By assuming that energy comes in discrete quanta
State the Planck radiation spectrum for a black body’s spectral radiance (equation)
[see equation sheet]
Describe the Planck spectrum (3)
- has a clear peak at some wavelength, which decreases as temperature increases
- Bλ(λ;T)>0 for all wavelengths, but tends toward 0 at the extreme ends of the spectrum
- overall increases with temperature
What determines the apparent colour of a blackbody object?
The shape of the spectrum and the position of the peak
What is Wien’s displacement law?
The inverse relationship between the temperature T in kelvin and the peak emission wavelength λpeak in metres
State Wien’s displacement law (equation)
[see equation sheet]
What can we use Wien’s displacement law to determine?
The effective temperature of a star from the spectrum of its light (assuming it is a black body)
Converting between wavelength and frequency
[see sheet] (SHEET STILL TO BE MADE)
What is atmospheric extinction?
When light from the Sun is obscured by the atmosphere of the Earth, which absorbs some wavelengths more than others
What is the Stefan-Boltzmann Law used to find?
The total power radiated by a black body
What are the units of radiant emittance?
Watts per meter squared
State the Stefan-Boltzmann Law (equation)
[see equation sheet]
What equation is used to find the total luminosity of a spherical star of radius R?
[see equation sheet]
What do we need to know to find the absolute magnitude of a star?
The apparent magnitude and the distance d
What do we need to know to find the luminosity of a star?
The absolute magnitude
What do we need to known to find a star’s radius?
The luminosity and the temperature
