Stars & EA 2

Question 1

If a star has a parallax of 0.1’’, what is it’s distance?

Answer 1

10 parsecs

Question 2

If a star has a distance of 1000pc, how small is the paralllax?

Answer 2

0.001’’

Question 3

Describe 2 features of a property of standard candles.

Answer 3

• Should not very with distance

- Is related to its brightness

Question 4

Define luminosity and its units.

Answer 4

How much energy is emitted from something per second.

Joules/sec or Watts

Question 5

How is light emitted?

Answer 5

Isotropically - the same amounts in all directions

Question 6

Define flux and its units.

Answer 6

The energy per second that an observer on earth measures.
W m^-2
Or for small fluxes a Jansky: 1 Jy = 10 ^-26 W m-2 Hz -1

Question 7

What use are cepheids?

Answer 7

They are standard candles. The brightness varies so you can measure the period to find the luminosity which can be used to find the distance.

Question 8

What is particularly useful about the flux?

Answer 8

You always know it!!!!! Because you’re on earth, you muppet!

Question 9

How are flux, luminosity and distance related?

Answer 9

f = L / 4 (pi) d^2

The flux drops with inverse square of the distance.

Question 10

Numerically describe what happens as distance increases e.g. twice the distance

Answer 10

Emitted light spreads out over the surface of a sphere.
At twice the distance the light has spread out to cover 4 times the area. This means a collecting space receives 1/4 as much light.

Question 11

Why do we use magnitude instead of flux?

Describe the 2 mathematical equations showing the conversion.

Answer 11

Flux e.g. Jansky is a linear scale whereas magnitude is a logarithmic based scale which is more useful.

m1 -m2 = -2.5 log (f1/f2)
(f1/f2) = 10 ((m1-m2)/-2.5)

Question 12

Describe the history involved in getting to magnitudes.

Answer 12

1) Stars were originally assigned 1 of 6 classes; 1 is brightest, 6 is just visible with the naked eye.
2) The system was extended to include what could be seen with telescopes
3) It was decided that 5 magnitudes = 100 flux or 1 magnitude - 2.512 in flux.
4) Fechner’s law states eyes see logarithmically, providing evidence magnitudes are a good idea
5) The star Vega was chosen as a zeropoint m=0.0

Question 13

Define apparent magnitude and absolute magnitude, and their uses.

Answer 13

Apparent magnitude (m) - related to the flux and distance
Absolute magnitude (M) - relates to luminosity but not distance.
Absolute magnitude can be used to compare stars as distance is not considered.

Question 14

Define the distance modulus

Answer 14

m - M = 5log (d/10pc)

Question 15

Derive the distance modulus.

I’d get some paper out my dear.

Answer 15

Use inverse square law and the definition of magnitudes.
fd / f10pc = (10 / d)^2
m1 - m2 = m - M = -2.5 log10 (fd / f10pc)
m - M = -2.5 log10 ( (10 / d)2) = -2.5 x -2 log10 (d / 10)
m - M = 5log (d/10pc)