Unit 1.6: Using radiation to investigate the stars

Question 1

Do stars have a definite surface?

Answer 1

No. The light we see comes from a layer of gas several hundreds of km thick

Question 2

What is the ‘surface’ of a star called?

Answer 2

Photo-sphere

Question 3

Define ‘a black body’

Answer 3

A black body is a body which absorbs all the electromagnetic radiation that falls upon it.

Question 4

Is there a better emitter of radiation than a black body?

Answer 4

Nothing is a better emitter of radiation at any wavelength than a black body at the same temperature.

Question 5

How do stars emit light?

Answer 5

Stars emit light as a continuous spectrum of radiation from the gases at the stars surface.

Question 6

How is an absorption spectrum formed?

Answer 6

The continuous spectrum of light that’s emitted by a star must pass through gases in the starts atmosphere. The atoms in the gas absorb some wavelengths of light leaving absorption lines to be superimposed on the spectrum.

Question 7

When the intensity of each small section of the absorption spectrum is studied, it is found to follow what law?

Answer 7

Wien’s law λ=W/T
λ=peak wavelength (m)
W=Wien’s constant (mK)
T=Surface temperature (K)

Question 8

On a black body spectrum graph, what is on the x axis?

Answer 8

Spectral intensity

Question 9

On a black body spectrum graph, what is on the y axis?

Answer 9

Wavelength

Question 10

What are the key aspects of a black body spectrum?

Answer 10

1. The shape of the curve is always
   similar but the peak can be shifted and higher due to different temperatures.
2. The peak intensity wavelength
   is inversely proportional to the temperature of the star.
3. The higher the temperature, the
   higher the peak intensity.
4. The gradient of the curve is 0 at a
   wavelength of 0.

Question 11

What is spectral intensity?

Answer 11

The amount of light energy striking each meter square per second.

Question 12

Why does the spectral intensity decrease the further you go from a source of light?

Answer 12

The light initially emitted is spread out over a larger and larger surface area as it travels away from the source

Question 13

State the equation for spectral intensity

Answer 13

I=P/4πx²
I=Spectral intensity
P=Power or luminosity
4πx²=surface area of sphere
x²=distance from source

Question 14

State the relationship between the intensity of light at the surface of a star and the fourth power of the surface temperature

Answer 14

Intensity∝T^4

Question 15

What is the relationship: Intensity∝T^4 known as?

Answer 15

Stefan’s law

Question 16

State the equation associated with Stefan’s law

Answer 16

P=AσT^4

Question 17

What can making observations of space outside of the visible light spectrum give us?

Answer 17

It can give more information about the processes which occur e.g observations using the microwave region of the spectrum revealed CMBR.

Question 18

What does a visible light image of a galaxy show?

Answer 18

The most detailed image of a galaxy. Spirals etc.

Question 19

What does a UV image of a galaxy show?

Answer 19

A UV image picks out the hotter regions and it shows knots of young giant stars and supernovas

Question 20

What does an X-ray image of a galaxy show?

Answer 20

An X-ray image shows only very high temperature regions

Question 21

What does an infrared image of a galaxy show?

Answer 21

clouds of dust and gas