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The Origin of the Elements – part 1

One of the outstanding problems in astrophysics in the 1940s was how were the elements created. In the 1920s it was realised by Cecilia Payne-Gaposchkin in her PhD work that the Sun was mainly composed of hydrogen. Then, spectral analysis of others stars and the gases of the interstellar medium led astrophycisits to realise that the Universe was composed mainly of hydrogen (about $75\%$), with the remainder being helium (about $24\%$), and only $1\%$ or so being all the other elements (oxygen, nitrogen, carbon etc.)

In the mid 1940s Russian-American physicist George Gamow started thinking about how the elements originated, and he developed a theory with his student Ralph Alpher that they were all created in the early Universe when, he argued, it would have been hotter and denser than it currently is. He published his famous paper “The Origin of Chemical Elements” in Physical Review in 1948, adding renowned physicist Hans Bethe’s name to the author list as a joke so that the paper would have the author list Alpher, Bethe, Gamow (alpha,beta, gamma, geddit? There is also a story that he tried to get his post-doctoral researcher Bob Herman to change his last name to “Delta” 😛 )

The famous Alpher, Bethe, Gamow paper, “The Origin of Chemical Elements”, which appeared in Physical Review in 1948.

In fact, Bethe played no part in writing the paper, but he was happy for Gamow to include his name for Gamow’s little joke. In this paper, Gamow and Alpher argued that all the elements were created in the early Universe. However, when others went through the details it was realised that the numbers did not add up, the Universe expanded and cooled too quickly for all the elements to be created in this way. Although it was possible for hydrogen and helium to be created in the first few minutes of the Universe, by the time the Universe was a few minutes old it had become too cool and the density too low to form the heavier elements beyond helium. Part of the reason for heavier elements not being built up in these first few minutes was due to something called the deuterium bottleneck, which I will explain in a future blog.

In the 1950s an alternative theory for the origin of the elements was put forward by Fred Hoyle and his collaborators Willy Fowler and Geoffrey and Margaret Burbidge. In a series of papers they argued that the elements had been built up in the interior of stars, the most famous of this series of papers was a 1957 paper entitled “Synthesis of the Elements in Stars” which appeared in Reviews of Modern Physics. Hoyle was the main advocate of a theory called the Steady State Theory which he had first proposed in 1948. This was a competing theory to the hot big bang theory, and so of course Hoyle did not believe any elements had been formed in a hot, dense early Universe as he did not believe such a Universe ever existed.

The first page of Burbidge teal.’s famous paper “Synthesis of the Elements in Stars” published in Reviews of Modern Physics in 1957

Again, as with Alpher and Gamow’s theory, detailed calculations found flaws in the Burbidge etal. theory. Although it could explain the creation of elements beyond helium, it was not possible to create enough helium in stars to account for the approximately $25\%$ found to be present in the Universe today. In part 2 of this blog, I will explain what our current understanding is of the origin of the elements in the proportions we observe in the Universe, and what the deuterium bottleneck is and why it is important.

The discovery of quasars

The discovery of quasars in the 1960s played a crucial role in helping show that the Universe was different in the past. This had important implications for the testing of the two competing cosmological theories of the time – the “big bang theory” and the “steady state theory”. The steady state theory, whose main proponent was Fred Hoyle, argued that the Universe had always existed and did not change in time. The big bang theory, on the other hand, argued that the Universe had a finite beginning in time, and that since this beginning had expanded, cooled and evolved.

The discovery of quasars was made using radio astronomy, and in fact the word “quasar” stands for “quasi-stellar radio source”. Later similar objects would be found that did not emit strongly in the radio, and so the term “QSO” (quasi-stellar object) was suggested. Both acronyms are used today, pretty much interchangeably. The term “quasar” is often used incorrectly for QSOs which do not have strong radio emission.

There was a little radio astronomy done before the 2nd World war. Karl Jansky accidentally discovered radio waves coming from space in 1931. After a little research he realised that the signal he was detecting with his radio receiver was coming from the centre of our Milky Way galaxy. However, Jansky was an engineer working for Bell Labs, and his request for funding to follow up this discovery and do a more complete survey of radio emissions from space was rejected, and Jansky re-assigned to another project and not given the freedom of following up on this discovery. The unit of flux in radio astronomy, the Jansky, is named after him.

In 1944 Dutch PhD student Hendrik van de Hulst predicted the existence of an emission line from neutral hydrogen, due to a hyperfine splitting in the ground state. I will explain in a separate blog the theory behind this, but basically the emission line comes about when an electron jumps between two very closely spaced energy levels in the ground state of neutral hydrogen. Because the energy difference is so small, the wavelength of the ensuing photon is extremely long – 21cm to be precise. This is in the radio part of the electromagnetic spectrum.

With a combination of this prediction, and the developments made in radar during the 2nd World war, the post-war years saw a boom in radio astronomy. One of the first groups to be established was at Cambridge University. The group was initially led by Martin Ryle, who had worked during the war with the Telecommunications Research Establishment on the design of antennae for airborne radar equipment. After the war, Ryle got a fellowship at the Cavendish Laboratories, and it was there that he established what became known as the Cambridge Radio Astronomy group.

The first Cambridge Radio catalogue (1C)

In 1950 he and his group published the first ever catalogue of radio sources. The paper, published in Monthly Notices of the Royal Astronomical Society (Ryle etal. 1950, MNRAS, 110, 508-523) was entitled “A preliminary survey of radio stars in the Northern Hemisphere”. In this paper they listed the positions of 50 discrete “radio stars”, along with the intensity of each source. The positions were not known very accurately, because the long wavelength used (3.7 metres) meant the resolution of their antenna array could not locate the sources’ exact positions to better than about 1 degree of arc. This was a major problem in identifying which astronomical objects the radio sources were.

The second Cambridge radio catalogue (2C)

The second catalogue by the Cambridge group was published in 1955. Entitled “A survey of radio sources between declinations -38 degrees and +83 degrees”, the lead author was John Shakeshaft of the group, with Ryle as second author. It was published in the Memoirs of the Royal Astronomical Society, (MmRAS 1955, 67, 106-154).

Just like the first catalogue, the second one was done at a wavelength of 3.7 metres. In this second catalogue, 1936 radio sources were found. Of these, 500 of the most intense could have their positions determined to an accuracy of about +/- 2 arc minutes in Right Ascension, and about +/- 12 arc minutes in declination. The team found most of the sources were of small angular diameter, and were distributed isotropically over the sky (that is to say not in any particular direction).

About 30 of the sources were of larger angular diameter, between 20 and 180 arc minutes, but the majority of these larger sources were close to the plane of the Mily Way galaxy and so the authors suggested that they represented a “rare class of galactic object”. They then went on to say that about 100 of the sources appeared to be related to objects which were in the New General Catalogue or the Index Catalogue; both optical catalogues of nebular objects which had been put together in the 1800s and the first decade of the 1900s.

The Third Cambridge radio survey (3C catalogue)

In 1959 the Radio Astronomy group at Cambridge produced their 3rd catalogue, using an upgraded antenna array. This time the observations were done at a frequency of 159 MHz (which corresponds to a wavelength of 1.9 metres). The paper, entitled “A survey of radio sources at a frequency of 159 Mc/s” was published in the Memoirs of the Royal Astronomical Society with D.O. Edge as lead author, and listed 471 sources (MmRAS 1959, 68, 37-60). It was revised in 1962 by Bennett, so the revised 3C catalogue had 470 sources (Bennet, 1962, MNRAS, 125, 75-86). Initially astronomers used the 3C catalogue to try to find optical counterparts to these radio sources. After 1962, with Bennet’s improved catalogue, the revised catalogue (3CR) was used.

The discovery of quasars

The first object in the 3C catalogue to which an optical counterpart was found was the object 3C 48, in 1960 by Thomas Matthews and Allan Sandage (both of Caltech). Using radio interferometry to narrow down its position, and then subsequent direct optical photographs, they found that 3C 48 corresponded to a faint blue star-like object. When its spectrum was taken, it looked unlike the spectrum of any star. First of all it contained emission lines (the spectra of stars usually show absorption lines), and these lines were broad not narrow as is usually the case with blue stars. But, most puzzlingly, the pattern of lines did not seem to fit any pattern that astronomers had seen before.

By 1963 Matthews and Sandage had found three starlike counterparts to three sources in the 3C catalogue, and published this in the Astrophysical Journal (“Optical Identification of 3c 48, 3c 196, and 3c 286 with Stellar Objects”, 1963, ApJ, 138, 30-56). The nature of the three sources was not known, but at least it seemed that they had been identified.

A breakthrough happened in 1962. One of the other 3C sources, 3C 273, was predicted to pass behind the Moon on several occasions. Using the Parkes Radio Telescope in Australia, Cyril Hazard and John Bolton were able to make measurements which allowed Caltech astronomer Maarten Schmidt to find its optical counterpart. Using the Mount Palomar 200-inch telescope, Schmidt obtained a spectrum of the star-like object. The spectrum was as confusing as that of 3C 48, he could see emission lines but was not able to identify them, the pattern just didn’t seem to make any sense.

After much head scratching, Schmidt realised that the lines corresponded to hydrogen emission lines, but they were redshifted to such an extent that he had failed to recognise them. The redshift he measured for 3C 273 was nearly 16% of the speed of light, an unheard of redshift at that time. Assuming Hubble’s law which relates redshift to distance, this put 3C 273 at a huge distance from Earth, much further away than any galaxy ever seen. This work was published in a one page letter in Nature in 1963 – “3C 273: a star-like object with large red-shift”. Nature 197, 1040-1040.

Caltech astronomer Maarten Schmidt, who in 1963 discovered quasars.

Very soon afterwards, Jesse Greenstein and Matthews at Caltech identified the redshift of 3C 48, and found it to be 37% of the speed of light, over twice as far away as 3C 273! (“Red-Shift of the Unusual Radio Source 3C48”, 1963, Nature 197, 1041–1042).

The acronym “quasar” was coined by astrophysicist Hong-Yee Chiu. In 1964, he said in an article in Physics Today magazine

So far, the clumsily long name ‘quasi-stellar radio sources’ is used to describe these objects. Because the nature of these objects is entirely unknown, it is hard to prepare a short, appropriate nomenclature for them so that their essential properties are obvious from their name. For convenience, the abbreviated form ‘quasar’ will be used throughout this paper.

As more research was conducted it was found that not all these quasi-stellar objects with extremely high redshifts had strong radio emission. some were “radio quiet”, so the name “QSO” was coined, but because the term “quasar” had been in use for quite a while by this time, many or most astronomers refer to these objects as quasars whether they are radio-loud or radio-quiet.

An evolving Universe

I will go into more detail in a future blog about what we think quasars are. It was in the early 1980s, using the Hubble Space Telescope, that their host galaxies were observed for the first time. They are the extremely active core of galaxies, but are only found in the distant universe. The lowest redshift quasar is at a redshift of 0.056. To put this into context, the redshift of the Norma Cluster is 0.01570, so the nearest quasar lies some 3 times further away. The further away one looks, the more common quasars are. As they are not found in the local Universe, and as looking far away means looking back in time, quasars are very clear evidence of the evolution of the Universe. They are one of the strongest pieces of observational evidence for an evolving Universe, and thus one of the pieces of evidence which helped show that the steady state theory of the Universe was wrong.