5.5 Astrophysics (EM Radiation and Distances)

Question 1

Can electrons bound to an atom exist in any energy level?

Answer 1

No. Only certain discrete energy levels.

Question 2

Can electrons have an energy value between two energy levels?

Answer 2

No.

Question 3

Are all energy levels the same for each element?

Answer 3

No. Each element has its own set of energy levels.

Question 4

What does it mean when an electron becomes ‘excited’?

Answer 4

It has moved from a lower energy state to a higher energy state.

Question 5

What is required for an electron to become ‘excited’?

Answer 5

The input of external energy (eg. heat, absorption of photon)

Question 6

What occurs when an electron is de-excited?

Answer 6

It moves towards the ground state. It releases energy in the form of a photon with a specific wavelength.

Question 7

All energy level values are negative, with the ground state being the most negative. Why use the negative sign?

Answer 7

To represent the energy required to be inputted to remove the electron from the atom.

Question 8

(Energy levels) An electron which is completely freed from an atom has an energy equal to what?

Answer 8

0

Question 9

Emission Line Spectra

Answer 9

A series of coloured lines on a black background.

Question 10

Continuous Line Spectra

Answer 10

All visible wavelengths of light are present.

Question 11

Absorption Line Spectra

Answer 11

A series of dark spectral lines against the background of the continuous spectrum, with each line corresponding to a wavelength of light used to excite atoms of that element.

Question 12

Why are the wavelengths of light produced by de-excited electrons different for each element?

Answer 12

Each element has a unique set of discrete energy levels.

Question 13

What is spectroscopy?

Answer 13

The technique used to identify elements based on the wavelengths of light emitted when atoms in a gas are excited.

Question 14

Diffraction grating formula

Answer 14

dsinΘ = nλ
d: diffraction slit distance
λ: wavelength
n: order of maximima
Θ: angle of diffraction

Question 15

What is the colour of a star affected by?

Answer 15

Its surface temperature

Question 16

For any object above 0K, objects emit what? [Weins law]

Answer 16

Electromagnetic radiation of varying wavelength and intensity.

Question 17

What can stars be modelled as?

Answer 17

Idealised black bodies that emit radiation across a range of wavelengths, with a peak in intensity at a specific wavelength corresponding to the colour of the star.

Question 18

State Wein’s Law

Answer 18

The black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature of the object.

Question 19

Wein’s Law Equation

Answer 19

λmax T = Wein’s Constant
λmax ∝ 1 / T

λmax: Wavelength of light produced with max intensity (peak wavelength)
T: Absolute surface temperature of the object.

Question 20

What is the luminosity of a star?

Answer 20

The radiant power output of the star.

Question 21

What is the luminosity of a star proportional to?

Answer 21

The surface area of the star, or the surface temperature.

Question 22

State Stefan’s Law

Answer 22

For a black body, the luminosity is proportional to the fourth power of its absolute surface temperature.

Question 23

Give the equation for Stefan’s law.

Answer 23

L ∝ 4πr²T⁴
L = 4πr²T⁴σ
(σ = stefan’s constant)

Question 24

If we know the temperature and luminosity of a star, what can we determine, using what law?

Answer 24

The radius, using Stefan’s law.

Question 25

Stefan’s law relates the luminosity to what?

Answer 25

The temperature and radius.

Question 26

When dealing with Stefan and Weins laws, do we use C or K for temperature?

Answer 26

K.
(Remember to convert in the exam)

Question 27

What is 1AU?

Answer 27

One astronomical unit is the average distance from the earth to the sun (1.5x10¹¹m).

Question 28

What are astronomical units mostly used for?

Answer 28

Expressing the distance of planets from the sun.

Question 29

What is 1ly?

Answer 29

1 light year is the distance light travels in one year. It is given using the speed of light x time of 1 year (in seconds) = 9.46x10¹⁵m.

Question 30

What are light years mostly used for?

Answer 30

Expressing distance to stars and other galaxies.

Question 31

For angles which are only a small fraction of a degree, what units can be used?

Answer 31

Arcminutes and arcseconds.

Question 32

How many arcminutes and arcseconds are there in one degree?

Answer 32

60 arcminutes
3600 arcseconds.

Question 33

Define parsec

Answer 33

The distance at which a radius of 1AU subtends an angle of 1 arcsecond. 1pc = 3.1x10¹⁶.

Question 34

Stellar Parallax

Answer 34

The change in position of an object depending on the viewing angle.

Question 35

Up to what distance is stellar parallax accurate for? Why?

Answer 35

Up to 100pc. Beyond this point, the angles involved are so small they are hard to accurately measure.

Question 36

To use parallax to calculate distance, what formula can be used?
Where is this relationship only true?

Answer 36

d = 1/p
d: distance between observer and object
p: parallax angle

This relationship is only true where d is measured in parsecs, and p in arcseconds.

Question 37

Black body radiation

Answer 37

EM radiation with a spectrum that peaks depending on the temperature of the emitter.

Question 38

What can stellar parallax be used for?

Answer 38

Estimating the distance of a star, based on how much it moves relative to the background of stars in the time it takes for the earth to move half an orbit.

Question 39

Why does each element produce a unique emission line spectrum?

Answer 39

Because of the unique set of energy levels associated with its electrons.

Question 40

What are continuous line spectrums produced by?

Answer 40

They are produced by atoms of solid heated metals.

Question 41

Comparing the absorption and emission line spectrums of the same element, where are the dark lines compared to the coloured lines?

Answer 41

The dark lines are at the same wavelengths as the coloured lines produced when the atoms are de-excited.