Week 3 Luminosity distances, telescopes & optics Flashcards
define apparent magnitude and what is it dependent upon
magnitude of a star/object as seen from Earth
it is dependent upon distance
define absolute magnitude
magnitude which would be seen if the star were at a distance of 10pc
what is the absolute magnitude equation
m - M = -5 + 5log[d]
what does the distance modulus assume and why is it a problem
it assumes that we know the absolute magnitude of a star
the problem is that this is rarely the case
what is a Cepheid variable
commonly used stars that are easily recognisable and have known absolute magnitudes where the brightness changes periodically (they pulsate)
what is true of Cepheid variables
the longer the pulsating period the more luminous the star
what do we use once Cepheid variables are no longer visible at longer distances and why
supernovae because they are extremely bright and visible
what are the wavelength and frequency ranges for Gamma rays
λ<10^-10
f> 3 x 10^18
what are the wavelength and frequency ranges for Xrays
10^-8 > λ > 10^-10
3 x 10^16 < f < 3 x 10^18
what are the wavelength and frequency ranges for UV
3 x 10^-7 > λ > 10^-8
10^15 < f < 3 x 10^16
what are the wavelength and frequency ranges for Optical waves
3 x 10^-7 < λ < 7 x 10^-7
4 x 10^14 < f < 10^15
what are the wavelength and frequency ranges for Infrared
7 x 10^-7 < λ < 10^-4
3 x 10^12 < f < 3 x10^14
what are the wavelength and frequency ranges for Microwave
10^-4 < λ < 10^-2
3 x 10^10 < f < 3 x 10^12
what are the wavelength and frequency ranges for Radiowaves
λ > 10^-2
f < 3 x 10^10
define a blackbody
object that emits and absorbs radiation with perfect efficiency
what is Planck’s function/law equation
Bλ(T) = 2hc^2 / λ^5 x 1 / e^(hc/kTλ) -1
B is the energy emitted per second
k is the boltzmann constant
what does the Planck function tell us about a blackbody graph
there is a sharp peak then a gradual decline
the gradient either side of the peak and the position of the peak changes as temperature changes
what does Wien’s displacement law tell us and what is the equation
it tells us the strict relation between the peak of the blackbody curve and the temperature
λpeak = b/T
b is Wien’s constant
how is the flux obtained from the Planck function curve
integrating under the curve
what is the Stefan-Boltzmann law equation and what is it the result of
F = σT^4
it is the result of the integration of the Blackbody curve
what are the two ways to define the temperature of a star and describe each
colour temperature - the blackbody curve that most closely fits the curve at a selected wavelength range
effective temperature - use a combination of stefan law, known luminosity and known surface area
what is the effective temperature equation
Teff = [ L / σ4πR^2 ]^1/4
what are the two types of telescope and define each
refractors - use lenses to bring the incoming light into focus
reflectors - use a curved mirror to focus the light
what are the two types of lens and define each
convex/converging - rays are focused to a point behind the lens
concave/diverging - rays are focused to a point in front of the lens
what is the lens power equation
p = 1 / f
f is the focal length
what is the lens equation
1 / f = 1 / u + 1 / v
u is the perpendicular distance from the lens to the object
v is the perpendicular distance from the lens to the image
what is the magnification equation
M = -v / u
what can we say about light from distant objects
they enter the telescope as a parallel beam
why are most modern telescopes reflectors
they are safer and easier to use
