Physics › Astrophysics › Quasars
Quasars
A quasar is an active galactic nucleus powered by matter falling onto a supermassive black hole, radiating more than a galaxy of stars. Its large red shift puts it at a cosmological distance, and its brightness varies over days, so the emitting region is no larger than a solar system.
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Quasars and exoplanets, part 1 of 2. Part 2 is Detecting exoplanets.
Builds on The Doppler effect and red shift and Supernovae, neutron stars and black holes.
IN THIS TOPIC
- Describe the discovery and nature of quasars, and estimate their distances and power outputs from red shift.
COMMON MISCONCEPTION
Quasars outshine whole galaxies because they are far bigger than galaxies.
Quasars are tiny: brightness that flickers in hours limits the engine to light-hours across. The power is accretion onto a supermassive black hole, a compact engine outshining its whole galaxy from a region smaller than the solar system.
Quasars: luminosity, size and distance
In the 1950s radio surveys turned up strong sources that matched nothing obvious in the visible sky. When optical counterparts were finally pinned down they looked like faint blue stars, hence the name quasar, from quasi-stellar radio source. Then their spectra came in and the puzzle deepened, because the lines showed enormous red shifts, far beyond any star's.
Describe the drawing
Two spectrum strips share a wavelength axis that runs on into the infrared. The upper strip carries the four visible hydrogen Balmer lines at their laboratory wavelengths; the lower carries the same pattern from a quasar at red shift 0.5, every line stretched by the same factor 1.5, so the red end of the pattern has slid past the edge of the visible. Dashed leaders join each line to its shifted partner.
Take the red shift at face value and Hubble's law puts quasars billions of parsecs away. That has a striking consequence. To appear even faintly visible from there, a quasar must outshine an entire galaxy of stars. Yet their brightness flickers over days, and nothing can coordinate a flicker faster than light can cross the source, so quasars are roughly solar-system sized. That is a galaxy's power output from a region that small.
Describe the drawing
Above, a brightness trace dips and recovers, with the eight hours from trough to crest marked between dashed lines. Below, two bars on one scale: Neptune's orbit, 9.0 billion km across with the Sun at its centre, and just inside it the 8.6 billion km that light crosses in eight hours, the most the flickering source can span. A galaxy's power comes from inside a solar system.
The specification calls quasars the most distant measurable objects, and for the 1960s that was fair. It is no longer true. Ordinary galaxies have since been found at red shifts far beyond any quasar's, and gamma-ray bursts have been measured further still. Quasars remain among the most distant objects bright enough to study in detail, and that is the defensible version. Write the spec's phrase if a question wants it, but know what it now means.
Only one mechanism fits. An active supermassive black hole of millions to billions of solar masses sits at the centre of a young galaxy, and matter spiralling in heats by friction to millions of kelvin, blazing across the spectrum before it crosses the horizon. The supermassive black holes met with the HR diagram are the quiet survivors of that era. A quasar is one still feeding.
WORKED EXAMPLE
Distance and power from a spectrum
A quasar's 656.3 nm hydrogen line arrives at 984.5 nm. Use the low red shift approximations to estimate its distance (H = 65 km s−1 Mpc−1), and its absolute magnitude given an apparent magnitude of +17.0.
z = (984.5 − 656.3) / 656.3 = 0.50. The specification's route is v = zc = 1.5 × 108 m s−1, then d = v/H = 150 000 / 65 ≈ 2300 Mpc.
Now say plainly what that is. Both v = zc and d = v/H are low red shift approximations, comfortable below about z = 0.1 and badly stretched at z = 0.50. At red shifts this large the shift is cosmological, space stretching while the light travels, rather than a source rushing through space, so no single corrected speed replaces zc; a modern distance needs a specified cosmological model and a named distance measure, and different measures legitimately give different figures at the same z. The exam's route stays v = zc then d = v/H, so give its answer and call it the estimate it is. Two significant figures is generous, and the word estimate belongs in the answer.
M = m − 5 log(d/10 pc) = 17.0 − 5 log(2.3 × 108) ≈ −25. The distance fed in came from the low-red-shift approximations, and reading d as an ordinary Euclidean distance is itself approximate out here, so the figure carries an error of its own at this red shift; the modulus relation stays exact only when d is the luminosity distance cosmologists use. Feed in 3000 Mpc instead and it still rounds to −25, so the conclusion below survives the wobble.
The conclusion holds despite the approximations, which is the point of doing it at all. Against a bright galaxy at M ≈ −21 the quasar is about four magnitudes brighter, a factor of a few tens, and it manages that from a source the size of the solar system.
Exoplanets: why looking fails has moved to the next part of this lesson.
ASSESSMENT FOCUS
- Give the quasar argument in order. Strong radio source, star-like optical appearance, very large red shift, therefore very distant by Hubble's law, therefore galaxy-scale power from a solar-system-sized region, powered by an active supermassive black hole.
- Quasar distance estimates run z, then v = zc, then d = v/H. Flag that a large z strains the approximation. Both formulas hold for v much less than c, so past z of about 0.1 every figure that follows is an estimate, and at z = 0.5 the speed is out by around thirty per cent.
- The specification's phrase is that quasars are the most distant measurable objects. It is a historical statement. Galaxies and gamma-ray bursts are now measured at greater red shifts, so if a question invites comment, say that quasars are among the most distant objects bright enough to study.
CHECK YOURSELF
A quasar's brightness varies noticeably over about two days. Explain what this says about its size, and why that observation, combined with its red shift, forces an extraordinary power source.
Show a hint
Nothing can synchronise a change faster than light can cross the object.
Show the answer
A source cannot brighten as a whole faster than light can cross it, so its diameter is at most about two light-days, roughly 5 × 1013 m. Solar-system sized, not galaxy sized.
The large red shift places the quasar billions of parsecs away by Hubble's law, and to be visible at all from there it must radiate more power than a whole galaxy.
Ordinary starlight cannot deliver galaxy-scale power from a solar-system-scale volume. Only matter heating violently as it falls towards a supermassive black hole fits both facts at once.
A quasar is an active supermassive black hole, giving galaxy power from a solar-system volume.
Its flicker time caps its size, and its red shift estimates its distance.
WORKBOOK
Printable practice for this topic: original exam-style questions with room to work, and a fully worked answer book. Free to use; please do not redistribute or sell.
Or read them with their mark schemes on the quasars questions page.
WHERE TO GO NEXT
- Standard form and prefixes is the maths this lesson leans on, worked through step by step.
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- Describe the discovery and nature of quasars, and estimate their distances and power outputs from red shift.
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