AQA A Level Physics Astrophysics
question with answers
Definition parsec - - the distance to an object subtending 1 second of arc to the
radius of the Earth's orbit.
(1 second of arc (arcsecond) = 1/3600 of a degree).
-Units parsec - - pc
-Definition light year - - Distance light travels in a vacuum in 1 year.
(number of seconds in 1 year = 365 x 24 x 60 x 60 = 3.15x107s
-Units light year - - ly
-Definition astronomical unit - - The mean distance between the Earth and the Sun
-Units astronomical unit - - AU
-Definition of absolute magnitude - - The brightness an object would appear if it
was 10 parsecs from the Earth.
-Definition of apparent magnitude - - The brightness of an object as seen from
Earth
-What is black body radiation? - - The electromagnetic radiation emitted by an
object because of its temperature
-General shape of black body curves - - Temperature is DECREASING going from P
to R.
Area under curve is total power output per m2 (as given by Stefan's Law)
You need to able to sketch these curves
-Definition λmax - - The wavelength at which maximum emission occurs.
-Assumption about a star to use Wein's displacement law (λmax) - - Star is acting
as a black body.
-Effect of atmosphere on estimate of a star's temperature - - • Ozone in
atmosphere preferentially absorbs UV part of spectrum
• This increases observed λmax (longer wavelength)
• Resulting in an underestimate of the star's temperature
-Definition of intensity - - energy arriving every second on a 1 m² surface
orientated perpendicular to the direction of radiation
-Units of intensity - - Wm-2
, -Relation between intensity and apparent magnitude - - Each change in
magnitude of 1 (on the magnitude scale) corresponds to a change in intensity of
2.51 .
So a star which has a magnitude that is 5 times brighter than another star, has an
intensity which is 100 times greater (2.515 = 100).
-Inverse square law and assumptions in its application - - the intensity I at a
distance r from a star with a total power output P, is given by
Intensity I = P/4πr²
Inverse square law assumes that:
• no light is scattered or absorbed between source and observer
• the source can be treated as a point
-spectral class information - - O Blue 50,000-25,000K He+,He,H
B Blue 25,000-11,000K He,H
A Blue-white 11,000-7,500K H(strongest) ionised metals
F White 7,500-6,000K Ionised metals
G Yellow-white 6,000-5,000K Ionised & neutral metals
K Orange 5,000-3,500K Neutral metals
M Red 3,500-2,500K Neutral atoms, TiO
-How are Hydrogen Balmer lines produced? - - • The atmosphere of the star has
hydrogen atoms with electrons in the n=2 state
• Light from the star passes through the atmosphere of the star
• Electrons (at the n=2 level) are excited into higher energy states
• They can only absorb certain amounts of energy
• These certain energies are related to specific frequencies (E=hf)
• The electrons then de-excite
• The electrons may de-excite through different energy level changes
• When the electrons de-excite the light is radiated in all directions
• This means that the intensity of the light at particular frequencies is reduced,
resulting in absorption lines
-Hertzsprung Russell (HR) diagram - - Y axis - absolute Magnitude -10 at top to
+15 at bottom
X axis - Temperature K or spectral class
You need to:
• Know names of different star categories
• Sketch the position of different star categories
• Add appropriate scales to axes (absolute magnitude, temperature and/or spectral
class).
question with answers
Definition parsec - - the distance to an object subtending 1 second of arc to the
radius of the Earth's orbit.
(1 second of arc (arcsecond) = 1/3600 of a degree).
-Units parsec - - pc
-Definition light year - - Distance light travels in a vacuum in 1 year.
(number of seconds in 1 year = 365 x 24 x 60 x 60 = 3.15x107s
-Units light year - - ly
-Definition astronomical unit - - The mean distance between the Earth and the Sun
-Units astronomical unit - - AU
-Definition of absolute magnitude - - The brightness an object would appear if it
was 10 parsecs from the Earth.
-Definition of apparent magnitude - - The brightness of an object as seen from
Earth
-What is black body radiation? - - The electromagnetic radiation emitted by an
object because of its temperature
-General shape of black body curves - - Temperature is DECREASING going from P
to R.
Area under curve is total power output per m2 (as given by Stefan's Law)
You need to able to sketch these curves
-Definition λmax - - The wavelength at which maximum emission occurs.
-Assumption about a star to use Wein's displacement law (λmax) - - Star is acting
as a black body.
-Effect of atmosphere on estimate of a star's temperature - - • Ozone in
atmosphere preferentially absorbs UV part of spectrum
• This increases observed λmax (longer wavelength)
• Resulting in an underestimate of the star's temperature
-Definition of intensity - - energy arriving every second on a 1 m² surface
orientated perpendicular to the direction of radiation
-Units of intensity - - Wm-2
, -Relation between intensity and apparent magnitude - - Each change in
magnitude of 1 (on the magnitude scale) corresponds to a change in intensity of
2.51 .
So a star which has a magnitude that is 5 times brighter than another star, has an
intensity which is 100 times greater (2.515 = 100).
-Inverse square law and assumptions in its application - - the intensity I at a
distance r from a star with a total power output P, is given by
Intensity I = P/4πr²
Inverse square law assumes that:
• no light is scattered or absorbed between source and observer
• the source can be treated as a point
-spectral class information - - O Blue 50,000-25,000K He+,He,H
B Blue 25,000-11,000K He,H
A Blue-white 11,000-7,500K H(strongest) ionised metals
F White 7,500-6,000K Ionised metals
G Yellow-white 6,000-5,000K Ionised & neutral metals
K Orange 5,000-3,500K Neutral metals
M Red 3,500-2,500K Neutral atoms, TiO
-How are Hydrogen Balmer lines produced? - - • The atmosphere of the star has
hydrogen atoms with electrons in the n=2 state
• Light from the star passes through the atmosphere of the star
• Electrons (at the n=2 level) are excited into higher energy states
• They can only absorb certain amounts of energy
• These certain energies are related to specific frequencies (E=hf)
• The electrons then de-excite
• The electrons may de-excite through different energy level changes
• When the electrons de-excite the light is radiated in all directions
• This means that the intensity of the light at particular frequencies is reduced,
resulting in absorption lines
-Hertzsprung Russell (HR) diagram - - Y axis - absolute Magnitude -10 at top to
+15 at bottom
X axis - Temperature K or spectral class
You need to:
• Know names of different star categories
• Sketch the position of different star categories
• Add appropriate scales to axes (absolute magnitude, temperature and/or spectral
class).
