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