Quasars (A-level only) Flashcards – AQA A-level Physics (7408)

What observations led to the discovery of quasars?

They were found as bright radio sources whose optical counterparts showed large redshifts.

How is optical redshift calculated from observed and emitted spectral-line wavelengths?

$z=\frac{\lambda_{\mathrm{observed}}-\lambda_{\mathrm{emitted}}}{\lambda_{\mathrm{emitted}}}$

What do quasars' large cosmological redshifts indicate about their distances?

Quasars are extremely distant, among the most distant objects whose distances can be measured.

For isotropic emission, how is luminosity $L$ related to observed flux $F$ and luminosity distance $d_L$?

$L=4\pi d_L^2F$

What do quasars' great distances imply about their luminosities, given their observed brightness?

Their luminosities are exceptionally high.

What is needed to estimate distances to quasars at large redshifts?

A cosmological model.

At what redshifts is the simple Hubble-law approximation appropriate for estimating distance?

At sufficiently small redshifts.

What powers a quasar’s immense luminosity?

Matter accreting around a supermassive black hole at a galaxy’s centre, not light escaping the black hole itself.

What structure does gas falling towards a quasar’s black hole form?

An accretion disc.

What energy conversion occurs as material moves inward through a quasar’s accretion disc?

Gravitational energy is converted into thermal energy and radiation.

When does a quasar phase occur?

When a supermassive black hole receives enough infalling material to produce an exceptionally luminous active galactic nucleus.