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Flashcards in Stars & EA 2 Deck (15):
1

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

10 parsecs

2

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

0.001''

3

Describe 2 features of a property of standard candles.

-Should not very with distance
-Is related to its brightness

4

Define luminosity and its units.

How much energy is emitted from something per second.
Joules/sec or Watts

5

How is light emitted?

Isotropically - the same amounts in all directions

6

Define flux and its units.

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

7

What use are cepheids?

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.

8

What is particularly useful about the flux?

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

9

How are flux, luminosity and distance related?

f = L / 4 (pi) d^2
The flux drops with inverse square of the distance.

10

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

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.

11

Why do we use magnitude instead of flux?
Describe the 2 mathematical equations showing the conversion.

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)

12

Describe the history involved in getting to magnitudes.

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

13

Define apparent magnitude and absolute magnitude, and their uses.

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.

14

Define the distance modulus

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

15

Derive the distance modulus.
I'd get some paper out my dear.

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)