Week 3 Luminosity distances, telescopes & optics

Question 1

define apparent magnitude and what is it dependent upon

Answer 1

magnitude of a star/object as seen from Earth

it is dependent upon distance

Question 2

define absolute magnitude

Answer 2

magnitude which would be seen if the star were at a distance of 10pc

Question 3

what is the absolute magnitude equation

Answer 3

m - M = -5 + 5log[d]

Question 4

what does the distance modulus assume and why is it a problem

Answer 4

it assumes that we know the absolute magnitude of a star

the problem is that this is rarely the case

Question 5

what is a Cepheid variable

Answer 5

commonly used stars that are easily recognisable and have known absolute magnitudes where the brightness changes periodically (they pulsate)

Question 6

what is true of Cepheid variables

Answer 6

the longer the pulsating period the more luminous the star

Question 7

what do we use once Cepheid variables are no longer visible at longer distances and why

Answer 7

supernovae because they are extremely bright and visible

Question 8

what are the wavelength and frequency ranges for Gamma rays

Answer 8

λ<10^-10

f> 3 x 10^18

Question 9

what are the wavelength and frequency ranges for Xrays

Answer 9

10^-8 > λ > 10^-10

3 x 10^16 < f < 3 x 10^18

Question 10

what are the wavelength and frequency ranges for UV

Answer 10

3 x 10^-7 > λ > 10^-8

10^15 < f < 3 x 10^16

Question 11

what are the wavelength and frequency ranges for Optical waves

Answer 11

3 x 10^-7 < λ < 7 x 10^-7

4 x 10^14 < f < 10^15

Question 12

what are the wavelength and frequency ranges for Infrared

Answer 12

7 x 10^-7 < λ < 10^-4

3 x 10^12 < f < 3 x10^14

Question 13

what are the wavelength and frequency ranges for Microwave

Answer 13

10^-4 < λ < 10^-2

3 x 10^10 < f < 3 x 10^12

Question 14

what are the wavelength and frequency ranges for Radiowaves

Answer 14

λ > 10^-2

f < 3 x 10^10

Question 15

define a blackbody

Answer 15

object that emits and absorbs radiation with perfect efficiency

Question 16

what is Planck’s function/law equation

Answer 16

Bλ(T) = 2hc^2 / λ^5 x 1 / e^(hc/kTλ) -1
B is the energy emitted per second
k is the boltzmann constant

Question 17

what does the Planck function tell us about a blackbody graph

Answer 17

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

Question 18

what does Wien’s displacement law tell us and what is the equation

Answer 18

it tells us the strict relation between the peak of the blackbody curve and the temperature
λpeak = b/T
b is Wien’s constant

Question 19

how is the flux obtained from the Planck function curve

Answer 19

integrating under the curve

Question 20

what is the Stefan-Boltzmann law equation and what is it the result of

Answer 20

F = σT^4

it is the result of the integration of the Blackbody curve

Question 21

what are the two ways to define the temperature of a star and describe each

Answer 21

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

Question 22

what is the effective temperature equation

Answer 22

Teff = [ L / σ4πR^2 ]^1/4

Question 23

what are the two types of telescope and define each

Answer 23

refractors - use lenses to bring the incoming light into focus
reflectors - use a curved mirror to focus the light

Question 24

what are the two types of lens and define each

Answer 24

convex/converging - rays are focused to a point behind the lens
concave/diverging - rays are focused to a point in front of the lens

Question 25

what is the lens power equation

Answer 25

p = 1 / f

f is the focal length

Question 26

what is the lens equation

Answer 26

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

Question 27

what is the magnification equation

Answer 27

M = -v / u

Question 28

what can we say about light from distant objects

Answer 28

they enter the telescope as a parallel beam

Question 29

why are most modern telescopes reflectors

Answer 29

they are safer and easier to use