Discovering Quasars: Bright Giants of the Universe Revision Notes | A Level AQA Physics

1. The Mystery That Baffled Astronomers

In the late 1950s, radio telescopes were picking up powerful signals from what appeared to be faint stars. Nothing unusual there — except that when astronomers looked at the optical spectra of these objects, the spectral lines were in completely the wrong positions. These were not stars. They were something nobody had ever seen before.

The objects were named quasi-stellar radio sources — shortened to quasars. The name captures the original confusion: they looked like stars in optical images, yet they were broadcasting enormous amounts of radio energy. Resolving that contradiction took years of careful work and led to one of the most significant discoveries in modern astrophysics.

2. Discovery: Bright Radio Sources That Looked Like Stars

Radio astronomy expanded rapidly after World War II. By the early 1960s, catalogues of radio sources — most notably the Third Cambridge Catalogue (3C) — listed hundreds of objects emitting strongly at radio wavelengths. When optical telescopes were pointed at the positions of some of these sources, they found what appeared to be ordinary blue-white stars.

The breakthrough came in 1963 when astronomer Maarten Schmidt examined the spectrum of the radio source 3C 273. He noticed that the hydrogen emission lines were present, but shifted dramatically towards longer (redder) wavelengths. Once he recognised the pattern, the implication was staggering: 3C 273 was receding from us at roughly 16% of the speed of light, placing it at an enormous distance — far beyond any ordinary star.

Not all quasars are strong radio emitters — later surveys found that radio-quiet quasars are actually more common. However, the name stuck. What unites all quasars is their extraordinary optical luminosity and their large redshifts, both of which point to objects at cosmological distances powered by processes far more energetic than ordinary stellar fusion.

3. Quasars as the Most Distant Measurable Objects

Because light travels at a finite speed ($c = 3.0 \times 10^8$ m/s), looking at a distant object means looking back in time. A quasar with a redshift of $z = 6$ is being observed as it was when the universe was less than a billion years old. This makes quasars uniquely valuable: they are the most distant objects whose distances can be measured reliably, using the relationship between redshift and recession velocity.

The redshift parameter $z$ is defined as the fractional change in wavelength: $z = \frac{\Delta \lambda}{\lambda_0}$, where $\Delta \lambda$ is the shift in wavelength and $\lambda_0$ is the rest wavelength. For small redshifts, recession velocity $v \approx zc$. Combined with Hubble's law ($v = H_0 d$), this gives a distance estimate: $d \approx \frac{zc}{H_0}$.

Quasars with redshifts exceeding $z = 7$ have been detected, corresponding to distances of over 13 billion light-years. No other class of object has been observed at such extreme distances with measurable spectra, which is why quasars hold the record as the most distant measurable objects in the observable universe.

4. Large Optical Redshifts: Reading the Spectrum

When you look at a quasar spectrum, you see familiar emission lines — hydrogen Lyman-alpha at a rest wavelength of 121.6 nm, for example — but shifted to much longer wavelengths. A quasar at $z = 3$ would show this line at $121.6 \times (1 + 3) = 486.4$ nm, placing it in the visible blue-green range rather than the ultraviolet. This is why high-redshift quasars can actually be studied with optical telescopes even though their emission originates in the ultraviolet.

The size of the redshift is what makes quasars stand out. Typical galaxies in the local universe have redshifts of $z < 0.1$. Quasars routinely show $z > 1$, and the highest confirmed quasar redshifts exceed $z = 7$. These are not small Doppler shifts from orbital motion — they are cosmological redshifts caused by the expansion of space itself stretching the wavelength of light during its journey to us.

Worked example: Estimating Quasar Distance from Redshift

A quasar shows a hydrogen emission line at an observed wavelength of 486 nm. The rest wavelength of this line is 121.6 nm. Estimate the distance to this quasar. Use $H_0 = 70$ km/s/Mpc and $c = 3.0 \times 10^5$ km/s.

1. $z = \frac{\lambda_{obs} - \lambda_0}{\lambda_0} = \frac{486 - 121.6}{121.6} = \frac{364.4}{121.6} \approx 2.997 \approx 3.00$ — The redshift z is the fractional increase in wavelength. Here $\Delta\lambda = 486 - 121.6 = 364.4$ nm and $\lambda_0 = 121.6$ nm (the accepted rest wavelength of hydrogen Lyman-alpha). [M1]
2. $v \approx zc = 3.00 \times 3.0 \times 10^5 \text{ km/s} = 9.00 \times 10^5 \text{ km/s}$ — For large redshifts this is an approximation; the full relativistic formula would be needed for precision, but this approach is acceptable at A-level. [M1]
3. $d = \frac{v}{H_0} = \frac{9.00 \times 10^5 \text{ km/s}}{70 \text{ km/s/Mpc}} \approx 1.29 \times 10^4 \text{ Mpc}$ — Dividing recession velocity by the Hubble constant gives distance in megaparsecs (Mpc). [M1] [A1]
4. $1 \text{ Mpc} \approx 3.26 \times 10^6 \text{ light-years}$, so $1.29 \times 10^4 \text{ Mpc} \approx 4.2 \times 10^{10}$ light-years — This is tens of billions of light-years — consistent with a high-redshift quasar near the edge of the observable universe. A result in this range confirms the working is reasonable.

Answer: $z \approx 3.00$; distance $\approx 1.29 \times 10^4$ Mpc (approximately $4.2 \times 10^{10}$ light-years)

Notice that the step above uses $v \approx zc$, which is a non-relativistic approximation. At $z \approx 3$, this gives a recession speed nearly three times the speed of light — which sounds impossible, but is permitted in an expanding universe where it is space itself that is stretching, not objects moving through space. AQA does not require the relativistic correction, but you should be aware the approximation breaks down at large $z$.

5. Power Output: Why Quasars Cannot Be Ordinary Stars

Once the distances to quasars were established, astronomers could calculate their luminosities. The result was extraordinary. Using the inverse-square law for flux: $F = \frac{L}{4\pi d^2}$, where $F$ is the observed flux (power per unit area received at Earth) and $d$ is the distance, rearranging gives $L = 4\pi d^2 F$.

Quasar 3C 273, the first to be identified, has a luminosity of roughly $4 \times 10^{39}$ W. For comparison, the entire Milky Way galaxy — containing hundreds of billions of stars — has a luminosity of approximately $4 \times 10^{36}$ W. That means 3C 273 alone outshines our entire galaxy by a factor of about 1000. No collection of stars could produce this from a region as compact as a quasar's core.

Worked example: Estimating Quasar Luminosity from Flux and Distance

A quasar is observed to have a flux of $F = 1.0 \times 10^{-16}$ W/m² at Earth. Its distance is estimated as $d = 2.0 \times 10^{25}$ m. Estimate the luminosity of the quasar.

1. $L = 4\pi d^2 F$ — This is the rearrangement of the inverse-square law. [M1]
2. $d^2 = (2.0 \times 10^{25})^2 = 2.0 \times 2.0 \times 10^{25} \times 10^{25} = 4.0 \times 10^{50} \text{ m}^2$ — Squaring the distance: $2.0 \times 2.0 = 4.0$ and $10^{25} \times 10^{25} = 10^{50}$. Always expand the square explicitly to avoid errors. [M1]
3. $L = 4\pi \times 4.0 \times 10^{50} \times 1.0 \times 10^{-16}$ — Bringing together all known values with their units.
4. $4\pi \approx 12.57$ $L \approx 12.57 \times 4.0 \times 10^{50} \times 1.0 \times 10^{-16} = 12.57 \times 4.0 \times 10^{34}$ $= 50.3 \times 10^{34} \approx 5.0 \times 10^{35} \text{ W}$ — $12.57 \times 4.0 = 50.28$, and $10^{50} \times 10^{-16} = 10^{34}$. So $L \approx 5.0 \times 10^{35}$ W. [A1]
5. Milky Way luminosity $\approx 4 \times 10^{36}$ W; this quasar $\approx 5 \times 10^{35}$ W — This quasar is roughly one-eighth the luminosity of the whole Milky Way — still an enormous output for what appears to be a point source. More luminous quasars exceed the Milky Way by factors of hundreds to thousands.

Answer: $L \approx 5.0 \times 10^{35}$ W

The compactness of quasars is just as striking as their luminosity. Brightness variations on timescales of days to weeks indicate that the emitting region is no larger than a few light-days across — comparable to the size of our solar system. Packing the power output of thousands of galaxies into a region that small rules out any stellar explanation and points firmly towards accretion onto a supermassive black hole as the energy source.

6. Comparing Quasars with Ordinary Galaxies

Earlier in this lesson, we established that quasars have enormous redshifts placing them at cosmological distances. Now that we also know their power outputs, we can make a direct comparison with ordinary galaxies — and the contrast is stark.

The table makes clear why quasars were so puzzling at first. Their point-like appearance suggested stars, but their luminosities and distances are galactic — or rather, super-galactic. The current consensus is that quasars are the active nuclei of young, distant galaxies, seen at a time when the universe was much younger and denser, and when supermassive black holes were actively consuming surrounding matter at prodigious rates.

7. Exam Technique and Common Pitfalls

Quasar questions on AQA papers tend to cluster around three skills: describing the discovery, interpreting redshift data, and performing or evaluating luminosity calculations. Each has its own set of traps that cost marks unnecessarily.

For extended-response questions asking you to evaluate the evidence that quasars are the most distant objects, structure your answer around three pillars: the large redshifts observed in their optical spectra, the application of Hubble's law to convert redshift to distance, and the extreme luminosities that confirm these are not local objects. A conclusion that explicitly states quasars are found at the edge of the observable universe, corresponding to the earliest epochs of cosmic time, will secure the top mark band.

Quasars tie together several strands of the AQA astrophysics specification: redshift and Hubble's law from cosmology, the inverse-square law from stellar physics, and the concept of lookback time from the expanding universe. Recognising those connections in an exam question — and deploying the right formula for the right part of the problem — is what separates a good answer from a full-marks answer.