Practise › Questions › Quasars and exoplanets
Quasars and exoplanets questions
Quasars are the most luminous objects known, outshining whole galaxies from billions of parsecs away, and exoplanets are among the faintest things detected at all. Both are found indirectly, from changes in the light received: a large red shift and a strong radio output for one, a periodic dip in brightness or a wobble in a spectrum for the other.
18 original questions · 54 marks · the quasars and exoplanets notes · Astrophysics
On this topic the library is tagged for AQA.
Every question here is written for this library rather than taken from a past paper. Write your answer out before opening a mark scheme: the schemes award marks point by point, and the marks are easier to see when you have something of your own to compare against.
Explain the origin of the name “quasar”, and how the first quasars were discovered.
Mark scheme
Quasi-stellar radio source (1): they were identified in the early 1960s as strong radio sources whose optical counterparts looked like faint star-like points, not galaxies (1).Give two reasons why an exoplanet is extremely difficult to photograph directly.
Mark scheme
Its star outshines it by a factor of order a billion (1), and at interstellar distances the angular separation between planet and star is below the diffraction-limited resolving power of telescopes (1).In the transit method, state what the depth of the dip in the light curve measures, and what its repeat interval gives.
Mark scheme
The fractional dip equals (rp/rs)2, the squared ratio of planet radius to star radius, so it measures the planet's relative size (1). The interval between dips is the planet's orbital period (1).State the type of object now believed to power a quasar, and where quasars are found.
Mark scheme
An active supermassive black hole, of millions to billions of solar masses, fed by infalling matter (1). At the centres of galaxies in the early universe, at very large distances (1).Explain why the first exoplanets discovered by the radial velocity method were massive planets orbiting close to their stars.
Mark scheme
A more massive planet drags its star through a faster counter-orbit about their common centre of mass, so the Doppler shift is larger (1); a close orbit makes the star's speed higher still and the period short, so the wobble is easiest to detect (1).State why only a small fraction of planetary systems can be detected by the transit method.
Mark scheme
A transit is seen only when the orbit lies edge-on to us, so that the planet crosses the star's disc along our line of sight (1).A quasar shows a red shift of z = 0.40. Estimate its recession speed and its distance, taking H = 65 km s−1 Mpc−1, and state why both figures are only estimates.
Mark scheme
v = zc = 0.40 × 3.0 × 108 = 1.2 × 108 m s−1 (1). d = v/H = 120 000/65 ≈ 1800 Mpc (1). Both v = zc and d = v/H are low red shift approximations, valid for v much less than c (1); at z = 0.40 the relativistic Doppler speed is 0.32c, so v = zc runs about 23 per cent high, and the distance should not be quoted to more than two figures (1).A quasar's brightness varies noticeably over about three days. Estimate the maximum size of the emitting region, and comment on the scale.
Mark scheme
An object cannot brighten as a whole faster than light crosses it (1)
size ≤ ct = 3.0 × 108 × 3 × 86 400 ≈ 7.8 × 1013 m (1)
Comparable with the size of the solar system despite a galaxy-scale power output (1)During each transit of an exoplanet, its star (radius 6.96 × 108 m) dims by a fraction 2.5 × 10−3. Calculate the planet's radius.
An orbiting planet makes its star move along our line of sight at up to 55 m s−1. Calculate the maximum shift this produces in the star's 656.3 nm line.
A space survey can detect any transit that dims a star by more than 1.0 × 10−4 of its brightness. A star of radius 6.96 × 108 m is orbited by a gas giant of radius 7.0 × 107 m and by a rocky planet of radius 6.37 × 106 m, and both planets transit. Deduce which of the two planets the survey will detect.
A quasar radiates a total power of 4.0 × 1039 W. The Sun radiates 3.8 × 1026 W. Calculate the number of Sun-like stars that would be needed to match the quasar, and comment on the result, given that a large galaxy contains about 1011 stars.
The 656.3 nm line of a star is observed to swing by up to 2.6 × 10−4 nm either side of its laboratory wavelength. Calculate the maximum line-of-sight speed of the star, and state what the periodic swing reveals.
A quasar of apparent magnitude 16.1 has an estimated distance of 1800 Mpc, obtained from its red shift through v = zc and d = v/H. Calculate its absolute magnitude, compare its output with a bright galaxy of absolute magnitude −21, and state one reason the absolute magnitude is approximate.
Explain how an active supermassive black hole accounts for both defining properties of a quasar: enormous power output and small size.
A star's light curve shows periodic dips, and its spectrum shows a periodic Doppler wobble with the same period. Explain what each measurement contributes to the picture of the orbiting planet.
A transit survey finds that a star of radius 6.5 × 108 m dims by a fraction 1.1 × 10−4 during each transit. Radial velocity measurements give the orbiting planet's mass as 7.3 × 1024 kg. The Earth's mean density is 5.5 × 103 kg m−3. Determine the planet's density, and suggest whether the planet is rocky or gaseous.
Stars V and W are similar stars, each orbited by a single planet in a similar orbit, and both systems are observed in the plane of the orbit. Star V's 550 nm line swings by up to 3.2 × 10−4 nm; star W's swings by up to 8.0 × 10−5 nm. Deduce which planet is the more massive.
The same practice on paper: the printable workbook for this topic, questions and a worked answer book.
Practise quasars and exoplanets one question at a time
The player marks nothing for you. It shows one question, waits, then shows the scheme so you can mark yourself, and brings a question back sooner when it went badly.