The Top Ten Astronomical “Breakthroughs” of the 20Th Century CAPCAP Vol
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The Top Ten Astronomical “Breakthroughs” of the 20th Century Research & Applications David W. Hughes Richard de Grijs Department of Physics and Astronomy, Department of Physics and Astronomy University of Sheffield University of Sheffield E-mail: [email protected] E-mail: [email protected] Summary Key Words Astronomy was revolutionized in the 20th century. The electron was discovered in 1897 and this transformed spectroscopy and introduced plasma and Breakthroughs magnetohydrodynamic physics and astro-chemistry. Einstein’s E = mc2, Research solved the problem of stellar energy generation and spawned the study of 20th Century astronomy Advancement elemental nuclear synthesis. Large telescopes led to a boom in astronomical spectroscopic and photometric data collection, leading to such cornerstones as the Hertzprung-Russell diagram and the mass-luminosity relationship, and to the realization that the Universe contained a multitude of galaxies and was expanding. Radio astronomy was introduced and the advent of the space age saw the astronomical wavelength range expand into the ultraviolet, X-ray and gamma-ray regions, as well as the infrared and millimetre. We also started wandering around roaming the Solar System instead of merely glimpsing its members from the bottom of our warm, turbulent atmosphere. Astronomical “breakthroughs” abounded. We have asked astronomers to select their “top ten” and these are listed and discussed in this paper. was demoted to being a mere planet. The Sun our precise knowledge of the cosmic distance Introduction then became the centre of the Universe, but scale at the time. To quote John Michell (1767) The progress of astronomy leapt forward when even this view did not last long. astrophysics was added to its sub-disciplines. “[T]he want of a sensible parallax in the fixed The science of astrophysics essentially started In this paper we aimed to recognize the ma- stars, is owing to their immense distance.” in the early 19th century and has advanced at a jor astronomical breakthroughs that occurred great pace, especially so in the last century. In in the 20th century. These stand out as land- An understanding of the relationship between fact we might suggest that the 20th century was marks in the progress of astronomical history. stellar brightness and apparent magnitude, an epoch of enlightenment, in which our under- Our subtext is the implicit suggestion that the coupled with an understanding that the flux standing of the Universe was revolutionized. As breakthroughs of the twentieth century might from a specific star decreased as a function with many of today’s sciences, we might won- have been better and more numerous than of the inverse square of the distance from that der whether this rate of progress will continue. the breakthroughs of previous centuries. We star, would provide a clue as to typical inter- are also asking the reader to consider whether stellar spacings. The fact that the Sun is about Science advances in two ways. On the one it is it possible that a similar number of major 1011 times brighter than the next ten brightest hand we have the gradual accumulation of changes and impressive breakthroughs might stars in the sky, coupled with a guess that all knowledge and data. There are many exam- also occur in the next century. Perhaps the rate stars might have luminosities similar to the ples of this in astronomy. Just think of the slow of astronomical advance is slowing down. Sun’s (a rather optimistic11 assumption, given and painstaking accumulation of accurate stel- that the median absolute10 = magnitude300,00 0of the fifty lar distances, masses, luminosities, tempera- Let us start by being pedantic, and define the closest stars to the Sun is 11.85, indicating a tures and spectra. On the other hand, we have word “breakthrough”. In the context of astron- median luminosity of L /640), leads us to the “breakthroughs”. These are major paradigm omy this can be thought of in terms of param- suggestion that typical interstellar distances in shifts, the realization that we have actually been eters, processes, or objects. To illustrate this the galactic disc are around 1011 = 300,000 au ‘barking up the wrong tree.’ Here, our concept we will provide examples in each category. = 1.5 pc. This means that when we are trying of the astronomical Universe changes dramati- to measure the “sensible” stellar parallax of the cally over a relatively short period of time. The (i) Parameters. A typical astronomical param- nearest stars we are attempting to measure an- Earth’s cosmic position is a good example. In eter would be “the distance between the Earth gles that are at best about 1/1.5 arcsecond in the 15th century the vast majority of thinkers and nearby stars”. Here, we stray away from size. These parallax angles had been hunted placed the Earth at the centre of the Universe. the 20th century. In the early part of the 19th for since the days of Nicolaus Copernicus and By the 17th century our understanding of the century we knew the Earth-Sun distance, some his promotion of the heliocentric Solar System cosmos had changed dramatically and Earth 150 x 106 km (1 au), but that was the extent of in 1543. Only by the 1830s had telescopes im- 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.