History of Astronomy Example Exam Question Hints and Suggestions

The main points to make are the following:

Before WWII:

Jansky (1932) discovers emission from Milky Way while studying radio interference. Reber builds a parabolic antenna and maps the radio sky (admittedly with very poor resolution and sensitivity). Both of these are radio engineers: the astronomy is a by-product of other work (Jansky) or a hobby (Reber). Of the professionals, only Jan Oort shows any interest (sets a student the task of predicting any radio spectral lines).

During WWII:

Many academic physicists employed in developing radar, which coincidentally uses exactly the right technology for radio astronomy. Hey (1942) identifies the Sun as a radio source. "Sporadic echoes" are noted as natural rather than artificial signals and prompt Bernard Lovell to become interested in prospects for radio astronomy (the source is subsequently identified as meteor trails).

Post-WWII (1940s):

Return of radar physicists to academic life and availability of surplus radar equipment prompts birth of professional radio astronomy. At this point radio astronomers are generally physicists and radio engineers, not professionally trained research astronomers - expertise with the new technology is more important than pre-existing astronomical knowledge - although Oort's student van de Hulst has contributed by predicting the 21 cm line of neutral hydrogen.

Various radio astronomy groups set up: Sydney group under John Bolton (sea interferometer), Cambridge group under Ryle (conventional interferometer), Jodrell bank under Lovell (big parabolic antenna). First discrete sources discovered, but only very few identified with optical counterparts (Taurus A = Crab, Sydney; Cygnus A = peculiar galaxy, Cambridge). Most believed to be unidentified class of Galactic sources ("radio stars", Ryle, 1950).

Early modern era (1950s):

More identifications produce more interest by professional astronomers (Baade, Minkowski) and recognition that most discrete sources are luminous and distant, not faint and local. Radio astronomy now recognised as valuable branch of observational astronomy.

The contrast between pre- and post-war radio astronomy is clear from the above history. Pre-war, radio astronomy had essentially no professional status - it was a hobby for radio engineers, and attracted little interest from professional astronomers. After the war, the availability of equipment and expertise derived from radar, together with the essentially accidental discoveries made during the war, drove the development of a professional/academic radio astronomy community (albeit initially rather detached from the optical astronomy community). It is not clear that this would have happened without the war effort as a driver for technological advance.

1. The identification of a discrete radio source with an anomalously blue star-like object.
2. The measurement of the redshift of the source, and hence the introduction of the idea that these are highly luminous, extragalactic objects, not in fact stars.
3. The discovery of multiple such objects, which therefore establishes this as a class of radio sources, not a single anomalous object.

Having identified these key steps, we can now flesh them out.

Identification of the optical counterpart

Hazard, 1962, observes lunar occultation of radio source 3C273. Lunar occultation is key because lunar limb provides precise position (remember, radio astronomy angular resolution at this time is not very good). Source is found to be double, and coincides with blue star-like object which is also double, having a faint but visible nebulous extension (now recognised as a typical AGN jet). The coincidence of the two components makes the identification absolutely solid (it can't be an accidental coincidence).

Redshift measurement

Spectrum of 3C273 initially uninterpretable, but in 1963 Maarten Schmidt realises that it can be interpreted as hydrogen lines with an extremely high (for the time) redshift of 0.14.

Multiple objects

Source 3C48 immediately recognised as a second member of the class. In fact it had been identified with a blue star before 3C273, in 1960 (Sandage), but the association was less solid because there was no occultation (hence, poorer position) and the source is not double (so coincidence is less unlikely). Its redshift was measured as 0.37 immediately after Schmidt's interpretation of 3C273 made it "respectable" to look for high redshifts.

Additional identifications followed quickly: by April 1964 the number was into double figures, and by 1965 quasars (and their radio-quiet analogues) were already established as "a major new constituent of the universe" (title of a paper by Sandage).

• Theory: Alpher and Herman predict blackbody background at ~5K (1950), based on the "αβγ" model of a universe expanding from a hot dense initial state. This prediction receives little attention, probably because the technology to test it is not available.
• Theory: Dicke et al. independently predict a blackbody background of <50K (1965), working from the assumption of an oscillating universe (but acknowledging that a "big bang" model will give a similar result). They set out to build a microwave horn antenna to look for this.
• Observation: Penzias and Wilson detect excess noise corresponding to an "antenna temperature" of 3K in their microwave horn antenna (1965).
• Observation: Wilkinson et al. (of Dicke's Princeton group) make observations at several additional wavelengths, confirming that the spectrum of the radiation follows the Rayleigh-Jeans law for the long-wavelength tail of a blackbody.

These are the key points in the discovery itself. The wording of the question is a little ambiguous: should we consider the theoretical consequences of the discovery (i.e. the disproving of the Steady State model) as a "theoretical aspect"? Also, is the 1941 "pre-discovery" by Adams and McKellar worth mentioning? (It is a valid observation - the technique was later used, e.g. by Meyer and Jura, to probe the high-frequency part of the blackbody spectrum, which is not directly observable from the ground - but in fact the observation was essentially forgotten and played no role in the establishment of the reality of the CMB.) Generally, provided you have the time, adding "possibly relevant" items such as these can't hurt, and in fact both would have got credit - but you would not get full marks unless you had discussed all four of the key items listed above.

For a 3 mark question, a simple list of the four key events would not provide enough detail: you would need to provide some level of detail (names, dates, discussion), or some additional information (list of key events plus Adams and McKellar plus effect on Steady State probably would be enough for 3 marks, unless the list was particularly terse and uninformative).