Astrophysics Flashcards
Describe the links between galaxies, black holes and quasars.
Quasars are produced by (supermassive) black holes. ✔
These black holes are at the centre of (active) galaxies (active galactic nuclei.) ✔
A typical quasar is believed to be approximately the size of the solar system, with a power output similar to that of a thousand galaxies.
Estimate, with reference to the inverse-square law, how much further the most distant visible quasar is likely to be compared to the most distant visible galaxy.
Both quasar and galaxy should have same brightness (and therefore similar received power) ✔
Use of Inverse square law eg
Power of quasar/(distance to quasar)2 = power of galaxy / (distance to galaxy)2 ✔
Or 1000/d2 = 1/1
So distance to quasar = (1000)½ = about 30 times greater than distance to galaxy ✔
Evidence to support the Big Bang theory comes from cosmological microwave background radiation and the relative abundance of hydrogen and helium in the Universe.
(a) Explain what is meant by cosmological microwave background radiation and how its existence supports the Big Bang theory.
It is the radiation coming from all parts of the Universe ✔
When the Universe cooled sufficiently for matter and radiation to ‘decouple’, with the combination of protons and electrons to form neutral atoms ✔
This radiation has been red-shifted into the microwave region as the Universe has expanded ✔
OR
This is (em) radiation from all parts of the Universe, ✔
the spectrum has a peak in the microwave region / corresponds to a temperature of 2.7 K ✔
It can be interpreted as the radiation left over from the Big Bang / the photons having been stretched to longer wavelengths and lower energies ✔
Explain how the relative abundance of hydrogen and helium supports the Big Bang theory.
(The Big Bang theory suggests that a very brief period of) fusion occurred (when the Universe was very young), resulting in the production of helium from fusing hydrogen. ✔
Fusion stopped as the Universe then expanded and cooled ✔
Resulting in a relative abundance of hydrogen and helium in the ratio of 3:1/ cooled too rapidly for the creation of larger nuclei,
Or suitable relevant observation ✔
The telescopes are used to view the same object.
Suggest which telescope in the table produces the brighter image.
Support your answer with a suitable calculation.
both refracting but B has larger Diameter
B brighter with support (eg diameter of B bigger) ✔
(The brightness of the image is determined by the collecting power and) collecting power related to D2 or area ✔
Calculation of areas or d2 ✔
Explain what is meant by the Rayleigh criterion.
Two objects will just be resolved when the first minimum/edge of the airy disc in the diffraction pattern of one image ✔
Correct diagrams can gain both marks
1
Coincides with central maximum/centre of the airy disc of the other. ✔
Discuss which of the two telescopes in the table would be better at resolving the images of two objects that are close together. b has larger D
B is better because it has a larger diameter ✔
Minimum angular separation/angular resolution depends on 1/D
Define the parsec.
Distance at which 1AU ✔ subtends an angle of 1/3600th degree ✔
Type 1a supernovae can be used as standard candles.
Explain what is meant by a standard candle.
Object whose absolute magnitude is known (and whose apparent magnitude can be measured.) ✔
Measurements of type 1a supernovae in 1999 led to a controversy concerning the behaviour of the Universe.
Describe this controversy and how the measurements led to it.
Measurements of supernovae do not agree with predictions (from Hubble’s Law) ✔
1
So Universe must be expanding at increasing rate/accelerating ✔
1
(Controversial as) no known energy source for expansion or reference to dark energy ✔
Most modern optical observatories make use of reflecting telescopes rather than refracting telescopes.
Discuss the principal optical advantages of reflecting telescopes.
Mirrors do not suffer from chromatic aberration, ✔
Mirrors can be larger and therefore have greater collecting powers ✔
Mirrors can be larger and therefore have smaller minimum angular resolution ✔
Parabolic mirrors with axial rays do not suffer from spherical aberration ✔
4
Explain what is meant by apparent magnitude and describe the main features of the Hipparcos scale.
The following statements could be present
• apparent magnitude is based on how bright a star appears on the Earth
• this depends on how bright a star actually is and how far away it is
• larger values of apparent magnitude are given to dimmer stars
• the difference of 1 on the apparent magnitude scale is associated with a difference in brightness of a factor of 2.51.
The three stars belong to different parts of the Hertzsprung−Russell diagram.
Deduce which star is a white dwarf.
The white dwarf must have the lowest (dimmest) absolute magnitude due to position on HR diagram (or ref to size) ✔
Omicron 2 has the dimmest apparent magnitude, and is the closest so must have dimmest absolute magnitude (or ref to m-M) ✔
Hence Omicron 2 is the white dwarf ✔
In 2013 a gamma-ray burst was detected from a region of space between the constellations of Leo and Ursa Major.
(a) State the event that was the likely cause of this gamma−ray burst.
The collapse of a (super) massive star into a neutron star or black hole ✔
Explain what is meant by the transit method of detection.
Apparent magnitude of star is measured over a long period of time ✔
When planet passes in front of star (as seen from Earth), some of the light from star is absorbed and therefore the amount of light reaching Earth reduced ✔
This produces a light curve showing constant value with a dip periodically as the planet passes in front of the star ✔
Two methods involved in the detection of exoplanets are the radial velocity method and the transit method.
(a) Explain what is meant by the transit method of detection.
(b) Explain why it is important that there is more than one method of detection.
Dip in light curve can be caused by other effects ✔
Except for planets very close to star, periods likely to be very long and may take many years of observation using transit method alone ✔
Explain what is meant by quantum efficiency and compare the quantum efficiency of a CCD with that of the eye.
QE. = number of photons arriving at the detector and be detected / number of phons arriving at the detector
1
For CCD QE> 80%✓
For eye QE = 1%✓
Deduce which of the three stars appears brightest.
Lowest value of apparent magnitude indicates the brightest star. ✓
1
Vega has the lowest apparent magnitude (so is brightest) ✓
State what is meant by a supernova.
An object that produces a rapid increase in brightness ✓
State the defining properties of a neutron star.
Extremely dense ✓
Made up of neutrons ✓
Some scientists are concerned about the consequences for the Earth of a supernova occurring in a nearby part of the galaxy.
Explain the cause of this concern.
Collapsing star can produce gamma ray bursts with energy similar to total output of Sun ✓
Highly collimated – if in direction of Earth, could cause mass extinction event ✓
In 1999 a planet was discovered orbiting a star in the constellation of Pegasus.
(a) State one reason why it is difficult to make a direct observation of this planet.
Star much brighter than reflected light from planet ✓
Or
Planet very small and distant – subtends very small angle compared to resolution of telescopes
Explain how an orbiting planet causes a Doppler shift in the spectrum of a star.
Planet and star orbit around common centre of mass that means the star moves towards/away from Earth as planet orbits ✓
Causes shift in wavelength of light received from star ✓
Explain how an orbiting planet causes a change in the apparent magnitude of a star.
When planet passes in front of star (as seen from Earth), some of the light from star is absorbed and therefore the amount of light reaching Earth reduced ✓
Apparent magnitude is a measure of the amount of light reaching Earth from the star ✓
State which property of the first identified quasar led to its discovery.
The quasar is a bright radio source. ✔
Measurements of the red shift of the quasar suggest the expansion of the Universe has accelerated since the detected light left the quasar.
State the cause of this acceleration.
Dark energy
State the difference between apparent magnitude and absolute magnitude.
App magnitude is how bright object appears in sky
Abs magnitude is how bright object would appear if 10 parsecs away
State what is meant by a black hole.
an object with an escape velocity greater than the speed of light ✓
State what is meant by the Hubble constant.
Gives the ratio of the (recessional) velocity (of galaxies) to distance from Earth
Early telescopes had very small diameter objective lenses. State two advantages of using an astronomical telescope that has a large diameter objective lens when making observations.
Large diameter allows fainter objects to be viewed,
(as the collecting power is proportional to d2) ✓
Larger diameter allows better resolution (as smallest resolvable angle is proportional to 1 / d)✓
Define the term absolute magnitude.
Apparent magnitude at a distance of 10pc
Explain what is meant by a parsec.
1 pc is the distance at which 1AU subtends an angle of 1 arc second
The lens was used as one of the components of a simple refracting astronomical telescope. State whether the lens formed the eyepiece or objective, giving reasons for your answer.
Objective.
No credit for unsupported answer.
As M = fo / fe, for magnification fo > fe
As telescope length = fo + fe, lens must be objective (so that telescope not too long.)
With reference to the diffraction patterns formed, explain what is meant by the Rayleigh criterion.
Two sources will be ( just) resolved if the central maximum of the diffraction pattern of one coincides
Central max and first min may be labelled on diagram in ai
with the first minimum of the other.
Many astronomical observations rely on a Charge Coupled Device (CCD) to obtain an image. Describe the structure and operation of the CCD and discuss the advantages of using a CCD for astronomical observations.
The quality of your written communication will be assessed in this question.
Points that can be used to support the explanation:
• The CCD is a silicon chip
• The chip is divided into picture elements
• Each picture element is associated with a potential well in the silicon
• Incident photons are focused on the CCD
• The photons cause the release of electrons within the semiconductor
• The number of electrons liberated is proportional to the intensity of the light.
• Electrons are trapped in the potential wells
• An electron pattern is built up which is identical to the image formed on the CCD
• When exposure is complete the charge is processed to form an image.
Advantages:
High quantum efficiency > 70%
Light integration – using long exposure times to capture faint images.
Device can be directly linked to computer for capture and analysis.
