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SIR FRED HOYLE: HIS WORKS

- Thamayanthi Giritharan -

The stars in the sky have consistently proved the enigma of the universe. Humans have always searched the skies above them out of both curiosity and wonder. Throughout the centuries, several people have studied celestial objects in hopes of discovering a bit more of not only the world but also the universe they live in. The fascination for such a topic has undeniably continued well into the 1900s. One of the most innovative astronomers of the twentieth century was an English astronomer, Sir Fred Hoyle. With a career spanning over six decades, Hoyle has become an eminent astronomer and recognized for his contributions to the field. He is principally remembered today for his involvement in the creation of the theory of stellar nucleosynthesis and his views opposing the theory of the “Big Bang” – an expression he himself had created. The purpose of this paper is to inform the reader of some of the main astronomical theories and works of Sir Fred Hoyle, > such as his work on star formation and interstellar structure, stellar nucleosynthesis, and his works on stellar structure and evolution, particularly those of red giants. It is also imperative to consider his cosmological work with the controversial Steady State Theory, which shall be done at the end of the paper. In addition to this, a brief overview of his life will be shortly discussed with other inputs not directly related to the astronomical field.

Fred Hoyle was born and raised in Gilstead, a village near the town of Bingley in West Yorkshire, England. Hoyle was born as the first son on June 24th, 1915, just before his father was conscripted into the British Army for World War One. His mother had a great passion for music, which allowed her to earn a living by working as the piano player for silent films at the local theatre. Although Hoyle was born into a modest family, his parents worked hard to earn money for the family.

During his earlier childhood years, Hoyle had some difficulties in school. After concluding his teacher was unable to count past five (for he brought in a flower with six petals and his teacher insisted there were only five), Hoyle shirked his responsibility of going to school. However, he was still keen on learning and taught himself concepts from a Chemistry book that the family had owned. His mother was adamant in giving a proper education for Hoyle and soon decided to enrol him at a primary school in a nearby village, Eldwick. He had flourished there and was granted acceptance into a well-known Bingley grammar school in 1926. During his years at the school, he prepared for the Cambridge scholarship exam, with the aid of a professor, Alan Smailes. Although he received no financial support, he did gain acceptance into Cambridge at the Emmanuel College in 1933, where he went on to complete his Bachelor of Arts in Mathematics three years later. Within those three years, Hoyle was awarded the Mayhew Prize for his top performance in applied mathematics. In July 1936, Hoyle was now a research student. He worked on an essay on Beta (β) decay, for which he was honoured with the Smith’s Prize in 1938. Seeing that this was a monetary award as well, it was sure to eradicate a good portion of his financial burden. It was also during this period when Hoyle met Raymond Lyttleton, a student at the St. John’s College working in astrophysics. Lyttleton at the time was researching the accretion of stellar matter. During one of Lyttleton’s seminars, Hoyle had met the senior student and through this episode, he had become interested in astrophysics. The concept of accretion was the first topic in the field that Hoyle had delved into for further research. In 1939, both the events of his marriage and World War Two took place. Hoyle and Barbara Clark married in December 1939, and shortly after went to work for a naval radar research establishment. Here he met another two of his future colleagues, Thomas Gold and Hermann Bondi. Bondi, Gold and Hoyle met while working at a naval radar research establishment in Chichester, England. It appeared that they shared similar principles that funded their involvement in the creation of the Steady-State Theory. Subsequent to the war, Hoyle returned to Cambridge and continued his research. The period following the war was one where Hoyle would have propounded some of his most important theories and works.

One of Hoyle’s first ventures in astronomy was through the study of accretion in the 1940s. Accretion is the term given to describe the creation of a celestial object by pulling together the encompassing objects and gases through gravity. Both Hoyle and Lyttleton had developed the concept that the accretion of stellar matter was vital to stellar evolution in two papers in 1939 and 1940. They had determined that if a star were to move in rectilinear motion through a cloud of gas initially at rest, the orbits of the gas particles would meet with the “accretion axis,” a line behind the star. Collisions on this line would reduce the momentum of the particles, which would leave them with negligible kinetic energy. With little energy, the particles are now unable to avoid being captured by the star and thus they become a part of that star. Bondi had also later worked on the concept of accretion with Hoyle, and because of this the Bondi-Hoyle-Lyttleton accretion problem was named after the three of them. The Bondi-Hoyle-Lyttleton accretion problem deals with the accretion rate of a body of particular mass that is moving at a constant velocity through a regular cloud of gas. Accretion is useful in possibly comprehending the creation of the first massive stars of the Universe, which were formed at redshifts of around z = 6-15 (z being the ratio and the 6-15 showing that they are of a great distance from us as a redshift indicates the object is moving away from us since a longer, red wavelength is detected). These stars are of interest as they probably produced the first heavy elements. Through computer simulation analysis, it was found to be probable that these stars first formed as a nucleus and then grew from the accumulation of intergalactic gas through accretion.

In the 1940 paper on accretion (Hoyle and Lyttleton), the problem that was dealt with was the cooling system of interstellar gas. Using a calculation by Arthur Eddington and the assumption that the interstellar matter had no chance for additional emission, the temperature of a pure hydrogen cloud was estimated to be 10000 K. This was too high for any significant accretion and through the paper, it was suggested that its cooling occurred through molecular hydrogen, H2. Two > options of cooling were considered: the free-free transitions (or Bremsstrahlung) in electron-to-proton collisions and the infra-red emission as a result of excited hydrogen molecules. The Bremsstrahlung (roughly translating to “braking rays” in German) is a term used to represent the process when a charged particle
decelerates when deflected by another charged particle, in which electromagnetic radiation is emitted. In the paper, it is explained that Bremsstrahlung does not reduce the temperature of the cloud to be of much consequence (after calculating the cross-section or the probability of the interaction to take place between those particles, as measured in barns). Through additional research, Hoyle and Lyttleton have concluded that between consecutive excitations of the hydrogen molecules, the molecules would have enough time to radiate away the energy that was obtained in the first excitation, thereby producing a cooling effect. Hoyle has been praised for his ability to have formulated the problem without laboratory assistance in determining cross sections. This paper was also the first to acknowledge cooling by molecular hydrogen.

During the 1960s and 1970s, Hoyle collaborated with Chandra Wickramasinghe in tackling the issue of the composition of interstellar grains. In 1962, their first paper on the subject proposed that the grains were composed of graphite formed in carbon stars and then, through pressure from radiation, emitted into the interstellar medium (ISM). This idea challenged the customary idea that the grains were mostly made of ice. A convincing argument to this theory is shown through the extinction curves for graphite and the normalized extinction curve (for interstellar grains). An extinction curve is also known as the interstellar reddening curve. Interstellar reddening is connected to the absorption (when the energy is consumed by matter) and the scattering (the deviation of the radiation when passing through certain objects) of electromagnetic radiation emitted by some object between the object emitting the radiation and the spectator. Interstellar reddening deals with the changes in the properties of the electromagnetic radiation spectrum (the range of all frequencies possible) and reddening will occur if light scatters off matter in the ISM. In the paper, an interstellar reddening curve for graphite flakes was calculated theoretically, which was shown to be quite similar to the observed interstellar reddening curve. This similarity suggests that the interstellar grains may indeed be composed of graphite, and not ice.

Both Hoyle and Wickramasinghe went on to further suggest that even more complex material composes interstellar grains and comets, including life in the form of bacteria. Hoyle was adamant in his belief that life did not originate on Earth as he viewed the probability of forming life from non-living materials on Earth is too small. The two also worked on the theory of panspermia, which states that the “seeds of life” already exist across the universe and life on Earth may have started through these seeds. Both Hoyle and Wickramasinghe have supported the theory and argued that the Earth is continued to be bombarded with various life forms, which may be responsible for epidemics and diseases.

Hoyle was the first to recognize that heavy elements can be produced in stars and then released into the interstellar medium by stellar winds or through various explosive methods from the stars. He also theorized that the massive stars, which have developed to have quite hot and dense centres, would generate the iron- peak elements (elements with 56 protons like Fe). This process was later named the “e-process,” with the “e” standing for equilibrium. It states that at temperatures greater than 5,000,000,000 K and with densities greater than 3,000,000 g cm-3, several collisions between the nuclei and the photons with great energy would occur. These collisions would disintegrate the nuclei and these pieces would then combine with other particles. In the long run, particles with iron-like properties would probably be trapped in the nuclei since the iron group of elements will have the largest binding energies. These elements would then be ejected into the interstellar medium by the means of an explosion (supernova explosion) as discussed in Hoyle’s 1946 paper.

Hoyle’s theory of nucleosynthesis with the triple-alpha process is perhaps one of his greatest triumphs. The triple-alpha process describes the process of converting three helium nuclei, or three alpha particles, into carbon. Hoyle was the first to predict the existence of the hydrogen-to-carbon reaction (the concept of producing carbon was earlier worked on by E.E. Salpeter). The reaction started off with two helium nuclei and then the addition of another helium nucleus after the first reaction: 4He + 4He ↔ 8Be (- 92 keV) and 8Be + 4He → 12C +γ (7.367 MeV). The net energy produced is 7.275 MeV. It is possible for the 8Be to revert back into the two alpha particles as it is quite unstable (if it does revert, it would do so in 0.0000026 seconds). However, it is said that, because of the conditions present during the fusion of helium, there will still be a significant amount of 8Be that will remain to proceed to the following reaction with another alpha particle and produce carbon. The process can continue to 12C + 4He → 16O + γ. Hoyle stated that the reaction rate to turn the helium into carbon was much higher than the carbon-to-oxygen reaction. This prevents the universe from becoming mostly composed of oxygen, yet leaves a considerable amount of carbon to produce life. The reaction can continue to fuse two carbon particles to produce neon and an alpha particle or sodium and a proton particle. The neon and the alpha particle can then produce magnesium. The whole process was collectively referred to as the “alpha process” by Hoyle. In the 1954 paper, Hoyle also extends his work to include the idea that, through continuous nuclear fusion of hydrogen in extremely hot stars, the synthesis of carbon to nickel is feasible. The alpha process can only occur in temperatures of 100,000,000 K and in stellar cores that have great helium abundance. This is the case for many older stars after helium has gathered through the proton-proton chains or the Carbon-Nitrogen-Oxygen cycle, which are the two methods by which stars convert hydrogen into helium. Therefore the present Sun is no likely candidate for such a reaction (but it is possible after it reaches its red giant phase).

In 1957, the reputed and eminent B2FH paper was published. It is originally titled the Synthesis of Elements in Stars, and was a collaborative venture between Geoffrey and Margaret Burbidge, William Fowler and Hoyle. The paper focussed and extended on many concepts Hoyle had put forth and is a paper of great impact to the subject, even to this day. In addition to describing the e-process, the r-process, s-process and the x-process were also discussed in this paper. The r-process describes the process of the addition of neutrons to iron-peak elements in rapid progression, which would only occur in supernovae. This theory was constructed to explain the existence of about half the elements after iron. The s-process is the same as the r-process but the neutrons are added in a much slower progression and occur in red giants; it was created to explain the existence of most of the other half of elements after iron. The x-process is an unknown process that the paper proposed to explain the creation of the light elements lithium, beryllium and boron.

Although the Nobel Prize for nucleosynthesis in stars was awarded to S. Chandrasekhar and Fowler in 1983, some believed that Hoyle should have been awarded it as well for his noteworthy contributions (including Fowler, an amiable colleague of Hoyle). Nonetheless, fourteen years later, the Royal Swedish Academy of Sciences awarded the 1997 Crafoord Prize in astronomy to both Hoyle and Salpeter for their contributions to stellar nucleosynthesis, a prize comparable to the Nobel Prize. Hoyle had initially done some work on red giants with Lyttleton throughout the 1940s. In their 1949 paper, the pair discussed the structure of inhomogeneous stars (stars without a consistent composition). However, Hoyle’s most celebrated collaboration on the subject was with Martin Schwarzschild in 1955. The pair calculated  the development of Population II stars from the stars in the main sequence (like the Sun) to red giants, as shown through Hertzsprung-Russell diagram. Population II stars are stars that are said to have comparatively little metal, or “metal-poor” and are some of the oldest stars in the Galaxy (the Sun is considered a Population I star because it is metal-rich, therefore depicting that it is a younger star). These stars are found in spherical clusters and are more common near the centre of the galaxy where the bulge is located. When the two met at Princeton University, they realized that the problem in the past works of stellar evolution was that convection was disregarded in the previous calculations of red giant models. Convection is the transport of heat by grand-scale shifts of gas. The currents flow upward and downward while carrying the hot gas outward and the cool gas inward. The cooler gas is reheated and then carried upward again. This process was already thought to occur in red dwarfs but Hoyle and Schwarzschild revealed that it was also effective in the structure of both red giants and supergiants. In fact, their first models that were calculated with the convection considered were shown to be more analogous to the observed spherical-cluster colour-magnitude diagrams than any of the previous models, which did not consider convection. Colour-magnitude diagrams are graphs that show the relation between the brightness (magnitudes) and colours of stars, which correlate to their temperatures and spectral variations. As a result of Hoyle’s and his collaborator’s work, red giants are often described as stars that have inert, constant-temperature cores, thin hydrogen-burning shells (where helium is formed), extended regions inside it where convection occurs, and are about 10 billion years old.

Later in the early 1960s, Hoyle and Fowler noted that heavy elements are emitted through Type I and Type II supernovae. They correctly deduced in their 1960 and 1964 papers that Type I supernovae are results of explosions of degenerate matter (highly compressed, dense matter where its normal atomic structure has broken down, such as the kind found in white dwarfs) and Type II supernovae occur due to the implosion and then explosion of non-degenerate stellar cores. For further classification, Type I supernovae have three subsets: Type Ia, which are explosions of white dwarfs, Type Ib and Type Ic are similar to Type II, in which they are explosions of massive stars but the Type Ib and Ic supernovae are caused by stars that are stripped of their outermost hydrogen layer (and most of the helium layer as well for the Type Ic).

It is wise to now note the “onion-skin” model, as explained by Hoyle in 1946. It is a model that is still used today and shows the composition of a star in its pre-supernova stage. There are seven layers to the star, with the outermost layer being hydrogen, the second layer being helium, and the innermost layer (or core) consisting of the iron group of elements. This shows that the star is inhomogeneous and, therefore, a very old star.

In 1948, Hoyle, Bondi and Gold developed the Steady-State theory, which opposes the Big Bang Theory. Although largely disregarded today (especially after the discovery of cosmic microwave background radiation in 1964), it was a strong idea in the mid-20th century. The theory relies on the perfect cosmological principal stating that the universe does not change with time or space. The theory allows the universe to exist for an infinite time as the same universe. Since the theory was created after E. Hubble discovered that the universe was expanding, it did take into account this expansion through what Hoyle called the “C-field” (“creation field”). This field was one that created matter from nothing at exactly the required rate to keep the density of the universe constant, even amidst expansion (but at only about 1 atom per m3, every 10 billion years). The theory had obvious flaws but was revised (as evidence supporting the Big Bang grew) during the early 1990s. The new Quasi-Steady-State theory was created by Hoyle, J. Narlikar and G. Burbidge, which suggested a continuous series of big bangs (known as “little bangs”) instead of one, initial Big Bang. Although this theory takes into account the ejection from active galaxies (as the Big Bang Theory does not), it still does not have much support due to lack of cogent evidence.

Sir Fred Hoyle died on August 20th, 2001 in Bournemouth, England at the age of 86. He started his career as a lecturer at his alma mater, Cambridge University, and then as the Plumian Professor of Astronomy. In 1967, he created the Institute of Theoretical Astronomy at the university. Hoyle was also an author of science fiction and has written many books alongside his son, Geoffrey. One of his most familiar novels is The Black Cloud (1957), a tale concerning the threat of a large cloud in the solar system blocking the sun’s radiation and foreshadowing the demise of the Earth’s creatures. He also branched out his creativity into television and wrote the popular British science-fiction drama, A for Andromeda (1961). Hoyle has had a profound effect on many aspects of astronomy and this paper only provides some of his more major works during his life. There are still many fascinating concepts he helped develop and after going through many of them, one truly  realizes the passion and dedication of the astrophysicists in the world. Hoyle has indeed left a legacy not to be forgotten.

COLLECTED INFORMATION – BOOKS
I. Gough, Douglas, ed. The Scientific Legacy of Fred Hoyle. New York: Cambridge UP, 2005.
II. Mitton, Simon. Conflict in the Cosmos: Fred Hoyle's Life in Science. New York: Joseph Henry P,
2005.
III. Osterbrock, Donald E. Walter Baade - A Life in Astrophysics. New York: Princeton UP, 2001.

COLLECTED INFORMATION – WEBSITES
I. http://astronomy.swin.edu.au/cosmos/T/Type+Ib+Supernova
II. http://astrophysics.suite101.com/article.cfm/steady_state_cosmology
III. http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Hoyle.html
IV. http://www.chemistrydaily.com/chemistry/Triple-alpha_process
V. http://csep10.phys.utk.edu/astr162/lect/energy/triplealph.html
VI. http://www.aps-pub.com/proceedings/1474/470411.pdf
VII. http://www.pas.rochester.edu/~rge21/research/bhl/
VIII. https://www.amazines.com/B%C2%B2FH_related.html
IX. http://www.csi.uottawa.ca:4321/astronomy/index.html#nucleosyntheticreaction
X. http://csep10.phys.utk.edu/astr162/lect/supernovae/type1.html
XI. http://www4.nau.edu/meteorite/Meteorite/Book-GlossaryD.html
XII. http://www.britannica.com/EBchecked/topic/126761/colour-magnitude-diagram
XIII. http://adsabs.harvard.edu/abs/1977AJ.....82..337S
XIV. http://astro.elte.hu/astro/en/library/padeu/padeu_vol_17/padeu_vol17_sipos_etal.pdf
XV. http://www.shodor.org/refdesk/Resources/Activities/InterstellarExtinction/
XVI. http://www.ndt-ed.org/EducationResources/HighSchool/Radiography/bremsstrahlung_popup.htm
XVII. http://www.lbl.gov/Science-Articles/Archive/cosmic-microwave-background-anisotropy.html

February 1, 2009