The Top Ten Astronomical “Breakthroughs” of the 20Th Century CAPCAP Vol

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h 15 the In example. good a is position cosmic Earth’s The time. of period short relatively a over cally of the astronomical Universe changes dramati - concept our Here, tree.’ wrong the up ‘barking shifts, the realization paradigm that we have actually been major are These “breakthroughs”. have we hand, other the On spectra. and tures tempera- luminosities, masses, distances, lar and painstaking accumulation of accurate stel- astronomy.in slow this the of of ples think Just exam- many are of There data. accumulation and knowledge gradual the have one we the hand On ways. two in advances Science continue. will progress of rate this whether der won- might we sciences, today’s of many with standing of the Universe was revolutionized. As an epoch of enlightenment, in which our under in theearly19 started essentially astrophysics of science The sub-disciplines. its to added was astrophysics The progress of astronomy leapt forward when Introduction E-mail: [email protected] University ofSheffield Department ofPhysicsandAstronomy, David W. Hughes of the20 The Top Ten Astronomical“Breakthroughs” y h 17 the By Universe. the of centre the at Earth the placed fact we might suggest that the 20 century.last In the in so especially pace, great oms a cagd rmtcly n Earth and dramatically changed had cosmos 20 Research Breakthroughs Advancement Key Words th th Centuryastronomy etr te at aoiy f thinkers of majority vast the century th etr or nesadn o the of understanding our century th centuryandhasadvancedata th Century th century was -

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2 Applications , - proved sufficiently for the first stellar distance energy generation was also transformed from with the exception of the “aether”, astronomers to be measured. The star was 61 Cygni, and being a mere fuelling process. Not only were were convinced that space was empty. The 20th the measurement was made in 1838 by Frie- we producing energy, we were also manufac- century discovery that space contained consid- drich Wilhelm Bessel. (The distance of this star turing new, and heavier, elements. The over- erable amounts of dust and gas, and the dis- is now given as 3.496±0.007 pc.) This was the abundance of stellar helium was explained by covery of the influence of missing mass (“dark astronomical breakthrough, as it confirmed processes that occurred in the Big Bang. The matter”) was a considerable breakthrough. astronomers’ suspicions as to the enormity of metallicity of the Universe was explained by the Milky Way. As the 19th century progressed, Burbage, Burbage, Fowler and Hoyle (1957), Notice that we do not count techniques and more and more stellar distances were meas- evoking nuclear synthesis in stellar interiors instruments as breakthroughs, even though ured, this leading to the assessment of stellar and during supernova explosions. Here we new types of instruments and bigger and more luminosities and stellar masses, and eventually have another breakthrough; the chemistry of sensitive examples of old ones might lead to the foundation of astrophysics. the Universe was no longer a complete mys- breakthroughs. The invention of the telescope, tery. the spectroscope, the photographic process Distance is only one of a host of physical and and the silvering of glass mirrors are not break- chemical astronomical characteristics. Think (iii) Object breakthroughs can be divided into throughs, and neither is the construction of, briefly of the parameter “velocity”. Albert Ein- “new” and “similar” objects. So you might say, the 100 inch (2.54 m) Hooker Telescope, or stein regarded the Universe as static. Then flag a breakthrough if you discover something the Lovell radio dish at Jodrell Bank, or the mi- along came Edwin Hubble and his discovery completely new, something that you had no crowave horn antenna at Bell Telephone Com- that clusters of galaxies have non-random ve- idea existed. Examples might be Uranus, white pany, Holmdel, New Jersey, USA, or the Hubble locities, and that the Universe is expanding. dwarf stars, Cepheid variables, quasars and Space Telescope, or the Saturn rocket that took This was a breakthrough; the concept of the gamma-ray bursters. Then you have the ob- men to the Moon. The use of these certainly Universe was revolutionized. jects that are predicted theoretically but take resulted in a number of breakthroughs, such a considerable effort to find. Neptune, Pluto, as the discovery of planetary rings, asteroids, Consider the age of astronomical objects. asteroids, pulsars, black holes, the cosmic mi- external galaxies, stellar composition, interstel- Many thought of the Earth as being created in crowave background and the 21 cm radiation, lar hydrogen and dust, the exansion of the Uni- a Biblical fashion some 6000 years ago. Then, spring to mind. verse and the cosmic microwave background, we subsequently discover that the Earth is but they are not breakthroughs in themselves. actually around 4,570,000,000 yr old (see for In the context of “similar” objects one can think example Faul, 1966; Brush, 1996.) This was of galaxies. Astronomers spent the first few clearly a major paradigm shift and thus another thousand years of their scientific endeavour The Time Period breakthrough. being convinced that there was but one galaxy, In this paper we restrict ourselves to the 20th the one that contained our Sun and Solar Sys- century. Let us review a few of the changes that In the early 19th century we had no idea as to the tem. Then, in 1928, there was a breakthrough. occurred in this 100 year time interval. composition of the Sun. Even in the 1920s Sir The Universe did not just contain a single gal- Arthur Eddington thought that the solar com- axy; there were actually huge numbers of them. In 1900, astronomical calculations were carried position was similar to that of the Earth. Along (1999 Hubble Space Telescope observations out using logarithm tables and slide rules, but came Cecelia Payne (later Payne-Gaposchkin), led to an estimate of about 125 billion, and by 2000 we had the laptops and supercomput- who discovered that the solar mass is about more recent modelling programs indicate that ers. In 1900, it took three weeks to calculate a 74% hydrogen, 24% helium and 2% metals; an- the number might be as high as 500 billion.) cometary orbit from a limited data set. By the other breakthrough. Here, we are reminded of A second surprise was the realization that our year 2000 the job could be done in less than a further episode in the history of our subject, Galaxy was not very special but was rather sim- three minutes. In 1900, we had no idea what when, like Aristotle, we regarded the heavens ilar to many other large (non-dwarf) galaxies. was inside the atom. The electron and neutron as “perfect” and made of some “quintessence” had not been discovered, quantum mechan- completely unlike the mundane terrestrial earth, Turn to the Solar System. As soon as the Earth ics had not laid the foundation for the study of fire, air and water. The breakthrough was due had been demoted from its geocentric cos- spectroscopy and electromagnetic radiation, to the development of spectroscopy and the mic elevation, the normality of the Sun and there was no special or general relativity, no E discovery that the heavenly bodies consisted the profusion of planets led astronomers to = mc2, and no understanding of nuclear fusion of exactly the same elements as the Earth be- suggest that planetary systems were com- or fission. neath our feet. monplace. The breakthrough came when, in the mid-1990s, other planetary systems were In 1900, we were still wedded to the refracting (ii) Processes. An example of the “process” detected, by radial velocity measurements and telescope, and Lord Rosse’s reflecting Levia- breakthrough would be the mechanism of transit observations. A subsequent surprise than, in the middle of Ireland was regarded as stellar energy generation. As soon as astrono- was the realization that our Solar System was somewhat of an oddity. The great Yerkes re- mers had been convinced that the constant- rather unusual and might be way off the Gaus- fractor, near Chicago, with its 40 inch (1.01 m) luminosity Sun was more than 6000 years old, sian mean when it came to the distribution of lens, was a ‘thrusting research tool’ when it was they started worrying about its energy source. planetary system characteristics. Many of the commissioned in 1897. The largest reflecting Was it burning? Was it shrinking? Was it gain- newly discovered planetary systems had large telescope effectively working on astronomi- ing mass (and kinetic energy) by cometary and Jupiter-like planets very close to the central star cal research in 1900 was Ainsley Common’s meteoritic accretion? Was it radioactive and (see, for example, Croswell, 1997; Goldsmith, 36 inch (0.91 m) Crossley reflector, this being thus decaying? All these mechanisms proved 1997). the telescope that the Lick Observatory had to be inadequate. Then in 1905 Einstein intro- bought in 1885. In the first decade of the 20th duced E = mc2. Mass, m, could be converted Perhaps the term “object” can be stretched century the Americans were hard at work trying into energy, E, the discovery of this process be- slightly. In Newtonian times astronomers were to fund and built the 60 inch (1.5 m), Ritchey ing a breakthrough. All that then remained was convinced that space was Euclidean, and that and the 100 inch (2.5 m) Hooker telescopes at to decide what specific mass was being used. light always travelled in straight lines from emit- Mount Wilson, California. The former started to It was soon realized that atoms and electrons ter to observer. We now realize that this is far be used in 1908 and first light hit the Hooker were not being annihilated but merely convert- from the case, and the discovery of gravita- in November 1917 (Edwin Hubble joined the ed from one form into another. Hydrogen was tional lenses has led to an interesting break- Mount Wilson staff in 1919). By the year 2000 transformed into helium, helium into carbon, through, in essence showing that massive we had a 2.5 m telescope orbiting our planet, carbon into oxygen, and so on. These ideas bodies affect the geometry of the surrounding 600 km up, and giant 8 and 10 m telescopes in eventual led to our detailed understanding space, this leading to the bending of the rays of both hemispheres. Instrumentation had been of the proton-proton and CNO cycles. Stellar light that pass close by. Also in the 19th century, further augmented by the replacement of the

The Top Ten Astronomical “Breakthroughs” of the 20th Century CAPCAP Vol. Vol. 1, 1, No. No. 1, 1, October October 2007 2007 Page 12 The Top Ten Astronomical “Breakthroughs” of the 20th Century — Continued

ries. This dropped to about 1000 after the rav- ing observed over a multitude of wavelengths 20th Century ages of the First World War. By the year 2000, from the gamma- and X-ray end of the spec- Top Ten Breakthroughs the world groaned under the efforts of around trum through to the long wavelength radio. 20,000 astronomers, each publishing, on aver- Listed using the Eurovision Song age, 2 research papers a year. Today, the world The middle orders of both our final lists are Contest approach to positioning has 32 telescopes with mirror diameters, D, in dominated by stellar astrophysics. There is the range 2.0 m < D < 3.0 m, fourteen in the considerable agreement in the ordering of 1. Expanding Universe range 3.0 m < D < 4.0 m, eight in the range 4.0 these breakthroughs. The most important was 2. The multitude of galaxies m < D < 8.0 m and eleven with D > 8.0 m. the discovery of the sources of stellar energy. The fact that there is a variety of nuclear “fu- 3. Cosmic microwave background els”, coupled with the possibility of simply uti- 4. Exotics (quasars/AGN) The Process lizing potential energy, means that there are a range of different star types. So the second 5. Stellar energy sources and evolution In our original letter to Astronomy & Geophys- ics (de Grijs & Hughes, 2006), we overviewed major “stellar” breakthrough concerns the divi- 6. Hertzprung-Russell diagram and stellar the huge advances in enlightenment and in- sion of the stellar population into dwarf stars, diversity strumentation that had occurred in the 20th cen- giant stars and white dwarf stars, exemplified 7. Exoplanets tury, and pointed to some of the ways in which by their positions on the Hertzsprung-Russell diagram. This advance was extremely fruitful, 8. Stellar chemical composition the understanding of our planetary, stellar and galactic neighbours had changed between AD leading as it did to the recognition of both a 9. Dark matter 1900 and 2000. We then decided to ask both mass-dependent stellar evolutionary sequence and a host of subspecies stellar types. The final 10. Galaxy mapping and structure the readers of Astronomy & Geophysics and our colleagues at the University of Sheffield to “stellar” breakthrough concerned composition. produce lists of what they considered to be Maybe we can couple this with physical state 20th Century the significant astronomical and astrophysical too. Clearly, we are dealing with a triumph for the spectroscopists and a transition from an Top Ten Breakthroughs breakthroughs that had occurred in this time interval, and to place these breakthroughs in era when we had no idea what a star was made order of significance. of or how stellar structure and composition Listed using the “horse racing form” varied from surface to centre to today’s deep approach to positioning Many contributions were received and the sug- understanding of elemental nuclear synthesis and stellar interiors. 1. The multitude of galaxies gested breakthroughs were then analysed and ordered in two ways. All the breakthroughs 2. Expanding Universe suggested by all the respondents were consid- It is interesting to note the lowly position of planetary astronomy in both Lists 1 and 2. 3. Stellar energy sources and evolution ered, even though some respondents put forward fewer than the ten requested. First choic- Despite the dawn of the space age, no char- 4. Hertzprung-Russell diagram and stellar acteristic of our Solar System makes the top diversity es were given ten points, second choices nine, third choices eight and so on. List 1 shows the ten. Exoplanets have a somewhat contentious 5. Stellar chemical composition results using the Eurovision Song Contest ap- breakthrough status considering that the discovery of well over a hundred planets orbiting 6. Exotics (quasars/AGN) proach. Here, the points given to each breakthrough are added up, and the breakthrough stars other than the Sun simply underlines that 7. Cosmic microwave background with the most points wins, the one with the next fact that we really have little idea where our So- 8. Dark matter highest tally coming in second, and so on. In lar System came from, or how cosmogonical this approach all the “judges” considered all processes fit in with general star birth. 9. Exoplanets the “entries”. 10. Solar probing using neutrinos/helioseis- A More Detailed mology The second approach is rather like the “bet- ting form” of a horse when entering a new race. Consideration of the photographic plate by the much more efficient Here, we wish to know the position it obtained charge-coupled device, and by the introduc- in the previous races that it entered. And not Breakthroughs tion of adaptive optics. entering a race does not count. In this method, all the points allocated to a breakthrough are 1. The Milky Way is not the only galaxy in In 1900, with the exception of a small incursion added up, but this number is then divided by the Universe. Many of the fuzzy nebular blobs into the infrared using blackened thermometers the number of times that that breakthrough has that Charles Messier (1730-1817) charted in and bolometers, all astronomy was restricted been chosen, and the results are then listed in the mid-18th century are actually distant star to the limited visual wavelength range. By order, giving List 2. systems just like our own. The breakthrough 2000, the surface of the Earth was dotted with occurred in 1923, when Edwin Hubble (1889- radio telescopes and a legion of gamma-ray, Both lists indicate that galaxies win clearly. The 1953) used the 100 inch Hooker reflector and X-ray, UV and IR telescopes had been placed top two places in both lists go to the discov- discovered a Cepheid variable in M31 (later above the atmosphere in low Earth orbit. ery that the Universe actually contains a huge published in Hubble, 1929a). By 1924 he had number of galaxies, as opposed to just the discovered twelve more. Using the calibrated In 1900, if we wanted to travel, we caught a single one (ours!), and the discovery that the Magellanic Cloud Cepheid data obtained by railway train or a ship. By 2000, everyone was galactic distribution was not static, but ever Henrietta Leavitt (1868-1921), see Leavitt & flying and twelve men had walked on the Moon expanding. The galaxy/cosmology party then Pickering (1912), he realised that M31 was (albeit in the 1969-72 time period). The space try to dominate List 1 by having the cosmic mi- 900,000 light years away, nine times further age had also seen craft flying by all the planets crowave background in third place, the empha- than the outer edge of our Milky Way galaxy. except for Pluto (although NASA’s New Hori- sis here being on the Big Bang theory and its Soon it was realized that the Universe con- zon mission is expected to fly by Pluto and its conclusions as to the age of the Universe. The tained over 1011 galaxies and not just the one. satellite Charon in July 2015), going into orbit galactic bias is further underlined by the high This is a marvellous example of an astronomical around Venus, Mars Jupiter and Saturn, and position of the astronomical “exotics”. Much is breakthrough and paradigm shift. Astronomers actually landing and roving about on Mars. made of quasars, active galactic nuclei, galac- did not just double the number of galaxies, or tic accretion discs and galactic central black change it by a factor of ten. A single unique In 1900, the world boasted around 2000 active holes, all of which are powered by a range of entity, our Galaxy, suddenly, in the late 1920s astronomers, working in around 100 observato- highly energetic physical processes, these be- found itself to be merely one among over 125 introduction of mass-energy equivalence, this perature]) for stars in general. Both Hertzprung billion. Some change! being exemplified by the iconic equation E = and Russell found that there were two main mc2. Clearly, mass is not converted into energy types of stars. By far the commonest were 2. The Universe is expanding. This entry re- under normal physical conditions. Before E = the “dwarfs”— approximately Sun-sized stars lies somewhat on the previous one. The stars mc2 could be embraced, astrophysicists like occupying a “main sequence” along which in the Galaxy are clearly orbiting the centre of Sir Arthur Stanley Eddington (1882-1944) had luminosity was proportional to temperature mass, and astronomers envisaged a stellar to show just how extraordinary the centre of a to the power of approximately 6.7. Less com- system of a specific size, with a nuclear bulge star was. Eddington was one of the first to re- mon were the “giants”. Here, we had stars with at the centre and an edge beyond which there alize that stars were gaseous throughout, and absolute magnitudes of around zero. (As time were very few stars. It was a great leap to in- that stars owed their stability to the balance be- went by more stellar classes were added. One troduce another 1011 or so galaxies. And Ein- tween the force exerted by gravity and the op- class was the faint Earth-sized white dwarfs, stein’s view was that the Universe was static. posing pressure exerted by gas and radiation. with absolute visual magnitudes between 10 The realization that, on average, the galaxies This led to the mass-luminosity relationship, and 14 and spectral types around B and A, and seemed to be moving away from us was a which was vital for the understanding of stellar the other, the rarer supergiants with absolute major paradigm shift. And this was bolstered evolution. For example, the main sequence lu- visual magnitudes in the -5 to -8 range.) by the discovery that the recessional velocity minosity of a star is proportional to mass3.5 and increased with distance. Again, the 100 inch the main sequence lifetime of a star is propor- 5. We now understand the composition Hooker telescope was responsible. This huge tional to mass-2.5. These relationships enabled of the baryonic matter in the Universe. In instrument had been used to take spectra of astronomers to estimate such important char- 1900, the general consensus was that stars galactic radiation. Vesto Slipher (1875-1969) acteristics as stellar cluster masses and ages. were made of “earth”. Since 1925 astronomers measured redshifts, as did Edwin Hubble. started to realize that stars are predominantly These Doppler velocities were reasonably ac- Eddington (1926) intimated that the density of made of hydrogen and helium, this clearly curate. Hubble estimated galactic distances the gas at the centre of the Sun was well over a being a major paradigm shift. Cecelia Payne using Cepheids, for the close ones, and then hundred times that of water, and that the tem- led the way, in her famous Harvard PhD the- magnitude and size comparisons for the more perature of this region was higher than 107 K. sis Stellar Atmospheres, A Contribution to distant. Needless to say, the assumption that Stellar interiors were certainly hot enough for the Observational Study of High Temperature galaxies of a specific type all had similar ab- the nuclear reaction rate to be non-negligible. in the Reversing Layer of Stars, a thesis that solute magnitudes and diameters led to errors But what was the form of the mass that was be- led to her 1925 Radcliffe College (Cambridge, in the estimated distances. By 1929, however, ing destroyed? Luckily, at about the same time Massachusetts) doctorate. She used the 1920 Hubble had obtained 46 values of both velocity (1920), Francis William Aston (1877-1945) was equation developed by Meghnad Saha (1894- and distance. using a mass spectrometer (an instrument that 1956) to convert spectroscopic line strengths he invented) to measure the masses of certain into atomic number counts and eventually A graph indicated that velocity was propor- atoms and isotopes. He realized that four hy- stellar photospheric compositions. A second tional to distance (see Hubble, 1929b). The drogen atoms were heavier than one helium important breakthrough in this field was the gradient was 500 km s-1 Mpc-1, this positive atom. Others at the time (see later) were hinting realization that stars come in two main compo- value indicating that the Universe was smaller that hydrogen and helium were the major com- sitional sorts; metal rich Population I and metal in the past. It was noted at the time that the ponents of stellar composition. These factors poor Population II. This was discovered by inverse of the gradient (assuming no retarda- combined to solve the stellar energy genera- Walter Baade (1893-1960) in 1943 (see Baade tion) gave the time since the expansion started. tion problem. But one had to show exactly how 1944), using photographic plates that he had Astronomers could thus measure the age of it worked. Hans A. Bethe (1906-2005) did this taken of the M31, The Andromeda Galaxy, with the Universe, or at least the time since it was in 1939, when he proposed the carbon-nitrogen the Hooker, under the conditions of the wartime all “squashed” into a primeval “atom”. Initially -oxygen (CNO) cycle. Later on he introduced blackout. A third breakthrough was the expla- this worryingly revealed that the Universe was the proton-proton cycle. Interestingly, these nation of why the stars actually had the compo- younger than the Earth, but cosmologists processes were extremely slow, so stars spent sitions that they did, and how that composition speedily reassessed the “Hubble constant”, long periods of time on the main sequence, varies with time. There were two components whose present value, combining WMAP with gently converting hydrogen into helium. During to this breakthrough: first the explanation of the other cosmological data, is around 71±4 km this period, their luminosity changed very little. initial 75%:25% hydrogen helium mix produced s-1 Mpc-1 (see http://map.gsfc.nasa.gov/m_uni/ just after the Big Bang, and second the 1957 uni_101expand.html). So the Universe is about The recognition of the source of stellar energy breakthrough due to the work of Margaret Bur- (13.8±0.8) x 109 yr old, about three times older led eventually to the general solution of the stel- bidge, Geoffrey Burbidge, William Fowler and than planet Earth. lar evolution problem, an endeavour that took Fred Hoyle. This takes the nuclear e-process about 35 years. that converts hydrogen into helium and ex- Another breakthrough discovery associated tends the sequence on to the production of with Hubble’s early work was the realization 4. There are only two common types of carbon and oxygen, silicon, sulphur, argon and that the Universe looked very similar in all direc- stars. Slightly before our understanding of how calcium, and ending with the iron peak. These tions. This led to the suggestion that the Uni- stellar energy was gained came the realization four scientists then showed how the r-process verse would look similar from the vast majority that the vast majority of stars are essentially of takes over in supernova explosions and moves of places inside it, and thus that the formative just two types, the so-called “dwarfs” and “gi- the composition on towards gold, platinum and Big Bang must have been amazingly homoge- ants”. This is rather surprising nomenclature for uranium. neous and isotropic. objects that typically have diameters of around 106 and around 20 x 107 km respectively. The 6. Exotics. In 1900, the “Universe” consisted 3. The generation of stellar energy. The year 1910 saw certain astronomers draw- of planets, other minor members of the Solar next three breakthroughs indicate just how lit- ing up lists of stellar luminosities and surface System, stars and a single Galaxy. The objects tle we knew about stars in 1900 and how the temperatures (as time went by these lists were known at that time were relatively mundane. first few decades of theth 20 century led to a extended to included radii and masses). Hertz- But there is a special class of astronomer who major transformation of our views. In 1900, prung (1911) plotted graphs showing the ap- yearns for the exotic, and the last century has astronomers realized that stars were old, well parent magnitude as a function of spectral type provided such celestial bodies in abundance. over a billion years old, and that they were very for stars in specific open clusters (i.e. nearby The exotics, by their very nature, stretch cos- luminous for much the greater part of their life. “moving groups” of closely related stars), such mic physics to extremes, and it is this that But astronomers did not know how the huge as the Pleiades and the Hyades. Russell (1914) leads to the breakthrough. First one has the amounts of stellar energy were produced. took full advantage of recent parallax work and stellar exotics. Typical examples are found at The breakthrough was triggered by Albert Ein- plotted absolute magnitude (i.e. luminosity) as the end points of stellar evolution. Low-mass stein’s 1905 paper on special relativity and the a function of spectral type (i.e. log[surface tem- stars evolve into Earth-sized white dwarfs, bod-

The Top Ten Astronomical “Breakthroughs” of the 20th Century CAP Vol. 1, No. 1, October 2007 Page 14 The Top Ten Astronomical “Breakthroughs” of the 20th Century — Continued

ies governed by the laws of degenerate mat- ments have indicated that this radiation, in the The observations of a host of other planetary ter. Neutron stars were predicted by Subrah- rest-frame of the Universe, is isotropic down to systems were expected to provide clues as to manyan Chandrasekhar (1910-1995) in 1930 1 part in 105. Huge amounts of money have the origin of our own system. They have not. to be the evolutionary endpoint of stars more been expended in launching satellites such as massive than 1.4 solar masses. Many of these COBE (1992) and WMAP (2001) to investigate A side issue to the breakthrough discoveries of are produced by supernova explosions, as the isotropy on ever smaller scales. many exoplanetary systems is the realization suggested by Walter Baade and Fritz Zwicky that we are still alone. Life seems to be rare; (1898-1974) in their joint 1933 paper. The radio 8. Dark Matter. Most of the Universe seems and intelligent, inquisitive, communicating life, wave observations of of Jocelyn Bell (now Bell- to consist of material that we cannot see. The rarer still. Look though we may, we have found Burnell) and Anthony Hewish in 1967, showed “luminous”, radiating, bodies in our Universe absolutely no evidence of life having broken out that pulsars were just rotating neutron stars. Fi- only make up about 4% of the total mass. on other planets in our system. Even though we nally, one has the black holes, these being the This strange and still unexplained phenom- listen diligently, we have intercepted no incom- endpoints of the evolution of stars more mas- enon was first discovered by Fritz Zwicky ing radio signals from “extraterrestrials”. sive than three solar masses. (1937). The application of the virial theorem to the Coma cluster of galaxies indicated that 10. Solar neutrinos and helioseismology. Another addition to the tally of exotic break- it contained 400 times more mass than that We cannot “see” inside a star. Our vision of the throughs was the1963 discovery of the quasi- indicated by the visible parts of the galaxies. solar photosphere extends to a depth of about stellar object 3C 273 by Maarten Schmidt. Here, 500 km, but, in comparison with the solar ra- we were confronted by a strong radio source, Galaxies are more massive than they look. We dius of around 700,000 km, this still leaves a at a redshift of 0.158, which visually looked just can count all the stars and add up their mass- very long way to go. Until recently, the stellar like a 13th magnitude star moving away from es, and then include the gas and the dust. But interior was the realm of the theoretical astro- Earth at 16.6% of the velocity of light. Subse- it is still not enough. Vera Rubin showed that physicist. Two breakthroughs have occurred in quently, radio-quiet quasars were found, as the velocity curve of a typical galaxy indicated the last 50 years. The first was the detection well as quasars that varied in brightness over that the velocity of rotation did not decrease and monitoring of solar neutrinos, these be- timescales of a few weeks. Soon, quasars were significantly as a function of distance from the ing produced by the host of nuclear reactions being equated to accreting discs around 107 galactic spin axis (see Rubin, 1978, 1983). that convert hydrogen into helium. Raymond to 108 solar mass black holes, these being the Everyone was expecting most of the galactic Davis Jr and his huge tank of 37Cl in the mine at very active nuclei at the centre of distant (and mass to be in the nucleus. If this were the case, Homestake, South Dakota, measured at least a thus young) galaxies. Seyfert galaxies (first the rotation velocity would decrease as the in- few of the 6.5 x 1014 neutrinos m-2 s-1 that pass described in 1943) are thought to be a spe- verse square root of distance from the massive through the Earth. This experiment started cific class of quasars with rather low luminosity. central body (as happens in the Solar System). in 1968. Detectors using gallium started op- They are near-normal spiral galaxies with rea- The typical spiral galaxy actually has a massive eration in 1991 (see, for example, Stix, 2002). sonably active nuclei Quasars/AGN are perfect halo, which has a density that decreases as a examples of late 20th century exotics, lending function of the inverse square of the distance The second breakthrough was the observa- themselves to multi-wavelength investigation. from the spin axis. The composition, or form, tion of seismic waves on the solar surface. As of the “missing mass” in this halo is not known. waves of different frequency penetrate to differ- 7. The Microwave Background Radiation. It Some of our contributors to the breakthrough ent depths, they can be used to estimate spin is one thing to suggest that the Universe started listings suggested that the discovery of “dark rates in the solar interior as well as the position with a “big bang” (a derogatory term coined by matter” should only achieve breakthrough sta- of the region where radiative energy transport Fred Hoyle (1915-2001) in a BBC broadcast, tus when the actual physical form of the dark changes to convective energy transport. Helio- see Hoyle, 1950), but it is another to prove it. matter has been identified. This is somewhat seismic oscillations were discovered in 1960 It is one thing to measure an expanding Uni- unfair. One of the great joys of modern astron- and reported by Leighton et al. (1962). The de- verse, but it is another to work out what made it omy and astrophysics is the host of mysteries tailed structure of the five-minute evanescent expand. Robert Henry Dicke et al. (1965) real- that abound. oscillations were reported in 1975 (Deubner, ized that a Big Bang (the term quickly became 1975) and the lowest wavelength modes were capitalized) would not only accelerate matter 9. Exoplanetary systems. In 1900, there observed in 1979 (see Claverie et al. 1979). away from a singularity but would also pro- was one known planetary system — the one duce extremely hot radiation that would cool we inhabit. As the century progressed certain as the Universe expanded. If the Universe was astronomers, such as Peter van de Kamp Discussion and 11 13,000,000,000 yr old it should have a radius (1975), hinted that the slight astrometric wob- Conclusions 10 of= 13,000,000,000300,000 light years, and the radia- ble of the celestial paths of certain nearby tion should now have a temperature of only a stars indicated that they had planetary com- The timing of the breakthroughs is rather in- few K. This corresponds to an energy emis- panions. By the end of the 1900-2000 period, formative. Those relating to stars occurred sion maximum at a wavelength of a millimetre the planetary floodgates had opened. The rather early on in the 20th century. The stellar or so. Dicke planned to search for this micro- Doppler shift of a planet’s parent star could energy problem was well on the way towards a wave maximum, but was pipped at the post by now be monitored accurately. A profusion of solution in 1905; stellar diversity was indicated a serendipitous discovery. Arno Penzias and planetary discoveries were reported (see, for by the 1911-14 Hertzprung-Russell diagrams; Robert Wilson of Bell Telescope Laboratories example, Mayor & Queloz, 1995, who used and stellar composition was reasonably well un- detected the 3.1±1 K background radiation the telescope at the Haute-Provence Observa- derstood by 1925. The two huge extragalactic tory in France, and Butler & Marcy, 1996, who breakthroughs, the discovery of galactic multi- (λmax = 0.93 mm) in 1965 when trying to elimi- nate static that was interfering with their satel- confirmed the discovery using the telescope at plicity and the expansion of the Universe, both lite communication system. Their 4080 MHz the Lick Observatory in California, USA). Later occurred at the end of the 1920s. The year 1937 horn antenna was about as big as a house. on, some of these discoveries were confirmed saw the discovery of dark matter. So six out of by the observation of stellar transits. This was ten of our breakthroughs occurred in the first 37 This breakthrough immediately converted cos- th mology from a vibrant exciting subject with a fascinating breakthrough. Our Solar System years of the 20 century. Three more occurred two flourishing and competing theories, into was proved not to be the only one in the Gal- in the 1960s: the discovery of quasars in 1963, a boring dirge where everyone sings from the axy. Rather unexpectedly, however, the vast the cosmic microwave background in 1965 and same “Big Bang” hymn sheet and the “steady majority of these newly discovered planetary the detection of solar neutrinos in 1968. The state” theorists are cast into outer darkness. systems are nothing like the system that we mid-1990s saw the discovery of exoplanets. Interestingly, in whichever direction one looked live in. Instead of having Jupiter-like planets from Earth, the radiation was very close to the orbiting the central star every decade or so, With the exception of quasars and the mi- same temperature of 3.1 K. More recent refine- their “hot Jupiters” are in Mercury-like orbits. crowave background, the visual portion of the electromagnetic spectrum dominates the used it to show that the Moon has mountains, Another serious “yet-to-come” breakthrough breakthrough scene. It is also rather interest- Venus goes round the Sun, and Jupiter has concerns cosmogony. It is fair to say that we ing to note that the 100 inch Hooker telescope satellites . . .’ have a very tenuous understanding of how our provided two of the breakthroughs, and larger planetary system formed, and why there are only telescopes have not helped a great deal in • ‘I have invented a prismatic instrument that eight planets in it, and why it essentially ends providing the remainder. Perhaps there are a splits light into its different colours, and when at Neptune. The discovery of planets around host of future breakthroughs awaiting the next I look at Sun-light I see lots of dark lines, at other stars simply has not helped. The major- generation of large telescopes, but this is rath- specific wavelengths, just the job to help my ity of these systems have Jupiter-sized planets er unlikely. Computers seem to have led to no physicists measure the refractive index varia- in Mercury-like orbits. In fact, many of the new top-ten breakthroughs at all. Neither has space tions of glass. Blast, an astronomer has devel- systems are nothing like the system that we live exploration. The later is rather unexpected. oped the instrument, fitted it to a telescope, in and were probably formed in different ways. Maybe the discoveries of the planetary flyby and measured the chemical composition of probes, orbiters and landers, discoveries such the Universe, stellar surface temperatures, And then we have the problem of the origin as magnetic fields around Mercury, impact cra- the radial velocities of stars and planetary of the Universe. Many astronomers are rather ters on Venus, thick crusts on the non-Earth surfaces . . .’ uncomfortable about the creatio ex nihilo as- facing hemisphere of the Moon, great canyons pects of the Big Bang. And the addition of the on Mars, smooth sandblasted asteroids, kilom- • ‘I am using a new-fangled millimetre wave spice of inflation, dark energy and dark- mat etric dirty snowball nuclei at the centre of com- radio horn antewweiver to pick up messages ter does little damp down their suspicions that ets, active volcanoes on Io, huge subsurface from submarines and am trying to reduce we might not yet be on exactly the right track. water oceans on Europa, lakes of liquid meth- the background noise. But I am not going ane on Titan, large blue spots on Neptune, etc. to pass this interesting noise data on to an We also worry that angular momentum still might have crept into the top thirty, but not the astronomer. I shall publish the results myself seems to be rather too difficult a topic for as- top ten. There again, maybe the bodies in the and thus prove that the Universe started with tronomers. As university lecturers we have Solar System turned out to be very much as we a Big Bang.’ always been somewhat embarrassed by be- expected, and there were few major surprises. ing unable to explain to our students why, for example, the Sun and Venus are spinning so Breakthroughs come in two main categories; (i) Those breakthroughs associated with ‘the solu- slowly and the Universe is not thought to be the completely unexpected, and (ii) the solution tion to a long-standing problem’ usually arose spinning at all. to a longstanding problem. Considering lists 1 from a combination of instrumental advance, and 2 it is clear that nobody predicted the exist- prolific data collection or theoretical enlighten- One of the great joys of astronomy is the simple ence of quasars before they were found, or had ment. The “chemical composition of the cos- fact that, even though breakthroughs abound, suggested that the vast majority of the material mos” is a perfect example, relying, as it did, on and occur at a fairly regular rate, there is a vast in the Universe was “dark”. Also, the expecta- the invention of the spectrometer, the analysis of amount of evidence indicating that there are still tion was that the space between the stars and spectral lines, the discovery of the electron and a huge number of breakthroughs yet to come. galaxies was well behaved, empty and flat. the theoretical work of Menghnad N. Saha. The The discovery of interstellar dust and gas by Hertzprung-Russell diagram is another. Here Finally, let us mention some general points. Robert Julius Trumpler in 1930, and the con- we have a “discovery” whose time had arrived. Before starting this exercise we thought that sequent light absorption, together with the dis- If Ejnar Hertzprung (1873-1967) and Henry Nor- different types of astronomers might come covery of 21 cm radio waves emitted by neutral ris Russell (1877-1957) had not reached for the up with completely different lists of break- atomic hydrogen in the Universe, put paid to graph paper, others would have done the job throughs. Surprisingly, this was not the case. the second of these assumptions. The gravita- in the next year or so. A similar situation arose There was considerable agreement between tional flatness disappeared with the introduc- with a mini-breakthrough around the same such diverse groups as, for example, the tion of General Relativity by Albert Einstein in time, this being the discovery of the Cepheid cosmologists, planetary astronomers, stel- 1916. The “proof” of space curvature came period-luminosity relationship. Henrietta Swan lar theoreticians and astro-historians. Many with Eddington’s observations of the starlight Leavitt’s work in 1912 was ground-breaking, alluded to the temporal nature of our quest. from the Hyades cluster during the totality of as was the calibration and use of the relation- What we today (in 2007) regard as being the the 29 May 1919 solar eclipse, followed, more ship by Ejnar Hertzprung and Harlow Shapley great breakthroughs of the 1900-2000 pe- importantly, by the detection of the gravitational (1885-1972) to measure the 94,000 light year riod might differ somewhat from what astrono- lensing introduced by super-massive galaxies distance to the Small Magellanic Cloud. But mers in 2107 would regard as the significant as observed by Dennis Walsh et al. (1979). again, if these astronomers had not done the breakthroughs. And clearly the breakthroughs job some one else would have, soon after. of 1900-2000 bear scant relationship to the Likewise, if one combs through the research breakthroughs of 1800-1900 and 1700-1800. papers of the 19th century the possibility of Let us conclude by hinting at some of the there being a multitude of galaxies was hardly breakthroughs that we are still waiting for. Some It was also interesting to compare the speed with mentioned, and when this multitude was dis- of these concern astronomical bodies that are which certain breakthroughs became recog- covered, again, the expectation was that they embarrassingly close to planet Earth. Consid- nized. One can well imagine that the discovery would be orbiting their centre of mass as op- er the second brightest object in the sky, our of the cosmic background radiation was real- posed to rushing away from the Big Bang. Moon. Do we know where it came from? The ized to be a breakthrough in about half an after- short answer is, no. Some contemporary re- noon. The elevation of the HR diagram to break- These “completely unexpected” breakthroughs searchers hint that a Mars-sized asteroid sim- through status clearly took a couple of decades. sometimes depended on the invention of a ply knocked a chunk off the Earth’s mantle and completely new type of scientific instrument. that this ejected material subsequently con- One also feels sorry for the topics that did not Often, the “new instrument” started life having densed and accumulated to form our Moon. quite make it. The 20th century was the era of very little to do with astronomy. Just consider But it would be most unusual if there was just astronomical ages. At the beginning, we did these possible statements and consequences. the one large impact in the history of our planet. not like to talk about such a delicate topic as In those times there were many asteroids, and age, such was our uncertainty. At the end, plan- many big ones, so similar impacts should have • ‘I have invented a two-lens telescope that ets, meteorites, stars, stellar clusters, galaxies, occurred quite a few times. If our Moon were brings distant things closer, and reveals and even the Universe itself, had well known the result of an impact it is rather surprising that bodies too faint for the eye to see, marvel- ages. It was also the century of interiors. Stellar we do not have quite a few moons, as opposed lous for army and navy use and for spotting and planetary interiors were mysterious places to just the one. And Mars, Venus and Mercury in 1900. By 2000, these had been successfully your enemies when a long way away. Blast, should be blessed with satellite families too. an astronomer has usurped the device and

The Top Ten Astronomical “Breakthroughs” of the 20th Century CAPCAP Vol. Vol. 1, 1, No. No. 1, 1, October October 2007 2007 Page 16 The Top Ten Astronomical ‘breakthroughs’of“Breakthroughs” of the the 20th 20th century Century — — Continued Continued

modelled and we had a detailed understanding • Brush, Stephen G. 1996, Transmuted past: The • Rubin, V. C. 1983, Dark matter in spiral galaxies, of the variability of pressure, density, tempera- age of the Earth and the evolution of the elements Scientific American, June ture and composition, and the origin of such from Lyell to Patterson (Cambridge: Cambridge University Press) • Rubin, V. C., Ford W. K. Jn. & Thonnard, N. 1978, characteristics as heat and magnetism. Tem- Extended rotation curves of high-luminosity spiral perature ranges also expanded hugely during • Butler R. P. & Marcy, G. W. 1996, A planet orbiting galaxies, Astrophys.J. Lett., 225: L107 the century. The expansion of the observed 47 Ursae Majoris, Astrophys. J., 464: L153 • Russell, H. N. 1914, Relations between the spectra wavelength bandwidth enabled us to investi- • Chandrasekhar, S. 1931, The maximum mass of an and other characteristics of the stars, Nature , 93: gate the high temperatures of such places as ideal white dwarf, Astrophys.J., 74: 81 227-230; 252-258; 281-286 the solar corona and the surfaces of neutron • Claverie, A., Isaak, G. R., McLeod, C. P. van der stars, and such freezing spots as the centres • Schmidt, M. 1963, A Star-like object with large red Raay, H. B., & Roca Cortes, T. 1979, Solar struc- shift, Nature, 197: 1040 of giant molecular clouds and the midnight ture from global studies of the 5-minute oscillation, regions of Pluto. The century has also been a Nature, 282: 591 • Seyfert, C. K. 1943, ‘Nuclear emission in spiral nebulae’, Astrophys.J., 97: 28-40 period when the isolation of the Earth was less- • Croswell. Ken 1997, Planet Quest: The Epic Dis- ened. In 1900 the only magnetic field that we covery of Alien Solar Systems (Oxford: Oxford Uni- • Stix, Michael 2002, The Sun: An Introduc- could measure was the field at the surface of versity Press) tion, 2nd Edn (Berlin: Springer), p. 139. our planet. By 2000, we had measured mag- • de Grijs, R., & Hughes, D. W. 2006, A&G, 47: 6.8 netism in such diverse places as the centres • Trumpler, R. J. 1930, Preliminary results on the dis- of sunspots and the surfaces of white dwarfs. • Deubner, F.-L., 1975, Observations of low wave tances, dimensions and space distribution of open We were also beginning to appreciate and number non-radial eigenmodes in the Sun, Astron. star clusters, Lick Obs. Bull., XIV: No. 420, p. 154 Astrophys., 44: 371 understand the influence that solar magnetic • van de Kamp, P. 1975, Unseen astrometric com- variation had on terrestrial characteristics. The • Dicke R. H., Peebles, P. J. E., Roll, P. G. & Wilkin- panions of stars, Ann.Rev. Astron.Astrophys, 13: century, which started only three years after J. son, D. T. 1965, Cosmic background radiation, 295 Astrophys.J., 142: 414 J. Thomson discovered the electron, was also • Walsh, D., Carswell, R. F., & Weymann, R. J. 1979, a period when the significance of plasma was • Eddington, A. S. 1919, The total eclipse of 1919 0957 + 561 A,B — twin quasi-stellar objects or first appreciated. May 29 and the influence of gravitation on light, gravitational lens, Nature, 279: 381-384 Observatory, 42: 119-122 • Zwicky, F. 1937, On the masses of Nebulae and of The expression ‘the textbooks will have to be • Eddington, A. S. 1926, The Internal Constitution Clusters of Nebulae, Astrophys.J., 86: 217 rewritten’ is often overused in modern media of the Stars (Cambridge: Cambridge University discussions of scientific progress. But in the Press) case of “breakthroughs” it often turns out to be • Einstein, A. 1905, Ist die Trägheit eines Körpers true. van seinem Energienhalt abhängig, Annalen der Bios Physik, 18: 639 • Einstein, A., 1916, Die Grundlage der allgemeinen Professor David W. Hughes has spent the Acknowledgements Relativitätstheorie, Annalen der Physik, 49: 769- last forty years researching the origin and 822. decay of comets, asteroids, meteorites and We would like to express our heartfelt thanks Solar System dust and the history of early, to all respondents to our initial request for • Faul, Henry 1996, Ages of Rocks, Planets, and renaissance and recent astronomy. He was breakthrough top tens. In particular, at the Stars (New York: McGraw-Hill) a co-investigator on the camera and dust University of Sheffield: Susan Cartwright, Paul detector of the ESA GIOTTO mission to Comet • Goldsmith, Donald 1997, Worlds Unnumbered: Crowther, Vik Dhillon, James Furness, Simon Halley. The Search for Extrasolar Planets (Sausalito, Cali- Goodwin, Katherine Inskip, and Chris Watson; fornia: University Science Books) Richard de Grijs is a Senior Lecturer in to readers of Astronomy & Geophysics: Pedro • Harwit, Martin 1981, Cosmic Discovery: The Astrophysics at the University of Sheffield, Augusto, John Hearnshaw, Daniele Malesani, Search, Scope and Heritage of Astronomy (Bright- where he has been a permanent staff member Naomi Pasachoff, Stephen Smartt, George on: Harvester Press) since 2003. He has held research posts at the Sudbury, Patrick Sudbury, Virginia Trimble, and Universities of Virginia and Cambridge (UK). He obtained his Ph.D. from the University of Keith Tritton; and to contributors to the PSCI- • Hertzsprung, E. 1911, Uber die Verwendung pho- tographischer effecttiver Wellenlängen zur Bestim- Groningen (Netherlands) in October 1997. COM public communication of science mail- mung von Farbäquivalenten, Potsdam Astrophys. ing list: David DeVorkin, David England, Elin Obs. Pub., XXII (1): No 63, p. 29 Roberts, Louise Thorn, and Michael White. • Hoyle, Fred 1950, The Nature of the Universe: A Series of Broadcast Lectures (Oxford: Basil Black- We also acknowledge a great debt to Martin wood), p. 102 Harwit and his classic book Cosmic Discovery (1981). • Hubble E. P. 1929a, A spiral nebula as a stellar system, Astrophys.J. 69: 103 • Hubble E. P. 1929b, Relationship between distance and radial velocity among extra-galactic nebulae, References Proc. Nat. Acad. Sci. USA, 15: 168-173 • Baade, W. 1944, The resolution of Messier 32, NGC • Leavitt, H. S. & Pickering, E. C. 1912, Periods of 25 205 and the central region of the Andromeda Neb- variable stars in the Small Magellanic Cloud, Har- ula, Astrophys. J., 100: 137-146 vard Coll. Obs. Circ., 173: 1-3. • Baade, W. & Zwicky F. 1933, On super-novae: cos- • Leighton, R. 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