Flashcards in Stars & EA 4 Deck (26):

1

## What 2 types of properties do stars have and how are they connected?

### Stars have physical and observable properties. We must use the observable properties to find the physical properties.

2

## List some physical properties.

###
Luminosity

Absolute magnitude

Size

Mass

Temperature

Velocity

3

## List some observable properties.

###
Position

Flux

Magnitude

Colour

Spectral type

Lightcurve

4

## Describe 3 characteristics of electromagnetic radiation.

###
All EMR travels at the speed of light (c = 3x10^8 m/s)

It has wave-like properties; described by wavelength (lambda) or frequency (f), e.g interference.

It has particle-like properties; described in photons as 'packets' of energy, e.e photoelectric effect

5

## Write an equation to describe wave-like properties of EMR.

### c = f * (lambda)

6

## Write an equation to describe particle-like properties of EMR.

###
E = hf or E =hc/lambda

h -> Planck's constant

7

## If a body has a temperature higher than 0K what does this mean?

### It is higher than -273C and it emits radiation.

8

## What depends on the temperature of the emitting body?

### The wavelength, colour and intensity.

9

## Define blackbodies

### An idealised body in thermodynamic equilibrium with surroundings. It absorbs all radiation incident and re-radiates it.

10

## Give 3 examples of blackbodies

###
-Toaster

-Black tarmac on a hot day

-An oven with a hole

11

## How is Planck's function and blackbodies related?

###
A blackbody emits some energy at all wavelengths. This spectrum is described by Planck's function which is the emitted flux as a function of frequency or wavelength.

At higher temperatures the peak of Planck's function shifts towards shorter wavelengths.

12

## Describe the difference between hot and cold blackbodies.

### At every wavelength a hotter blackbody emits more energy than a cooler one.

13

## Draw a graph showing Planck's function

###
X-axis - wavelength

Y-axis - Flux or intensity

higher temperature line, peaks at a shorter wavelength and has a higher peak that lower temperatures.

14

## What is Wien's law equation?

### (lambda)Max . T = 0.0029mK

15

## Describe Wien's law and what this means?

### For a blackbody at temperature T, there is a wavelength (lambda)Max at which it radiates its maximum amount of energy. This relationship is WIen's law.

16

## Are stars described as blackbodies?

###
Yes, but

-Energy can only escape from the very outer layer

-Stellar interior is very opaque to virtually all EMR.

17

## What is an effective temperature?

### An effective temperature (Teff) is the temperature of a blackbody that wold emit the same amount of radiation as the star, allowing star temperatures to be calculated.

18

##
Calculate example (lambda)Max values:

Sun = Teff 5800K

Hot star = Teff 12000K

Cold star = Teff 3000K

###
Sun 500nm

Hot star 250nm

Cold star 1000nm

19

## Why do stars have different colours?

### Stars show different colours depending on their temperatures which affect the appearance of their Planck's function graph which is the easiest way to visualise this.

20

##
What colour is:

Sun

Hot star

Cold star

###
Sun - yellow

Hot star - blue

Cold star - red

21

## What are the units for Planck's function?

### W m-2 Hz-1 Ster-1

22

## What is the equation for Planck's function?

### B (V, T) = (2hv^3/c^2)/(e^hv/kT -1)

23

## Why do we use the Rayleigh-Jeans approximation and what is the equation?

### For long wavelengths and small frequencies this approximation is used e.g it could be used to predict infinite energy towards large frequencies (UV catastrophe)

24

## Describe the Stefan-Boltzmann Law

###
This law helps to calculate the total energy emitted by a blackbody over all wavelengths, per second, per square metre of the surface of the blackbody.

Basically qualitatively this equates to energy emitted at every wavelength depends strongly on temperature.

25

## Explain the maths that occurs to allow the Stefan-Boltzmann law to occur

###
-Use Planck's function

-Intergrate it and substitute hV/kT to X

-This eventually gives

E (T) = (sigma) T^4

(sigma) = 5.67 x10^-8 W m-2 K-4

26