Powers of Ten: Astronomy's Greatest Hits

Compiled by Virginia Trimble

Editor's Note: Owing to the huge response generated by prior "Top Ten" lists published in APS News, we offer the following list of great discoveries in astronomy. As always, readers are welcome - nay, encouraged - to take issue with the rankings, omissions, erroneous inclusions, and the like. We also encourage those in other fields to compile their own "Powers of Ten" lists for possible future publication, modeled on the format below.

Top Ten Astronomical Triumphs of the Last Millennium

1. Celestial objects are:

• NOT immutable (Tycho's supernova of 1572 and comet of 1577 not in atmosphere)
• NOT perfect (mountains on the moon, Galileo 1610; sunspots, Harriot, 1610)
• NOT marching to a different drummer (apples and the moon, Newton 1687)
• NOT fully inventoried (W. Herschel discovery of Uranus, 1781)
• NOT made of anything funny (Bunsen & Kirchhof, spectroscopy, 1858).

2. Eclipses and some periodic comets are predictable. (Halley 1695/1758, eclipses, 1715; Neptune was also predicted and discovered in 1844).

3. We are not at the center of

• the solar system (Copernicus 1500/1543)
• the galaxy (Shapley, 1920)
• the universe (Digges, 1500 for infinite universe; Einstein 1915 for finite universe).

4. There exist

• other galaxies (Hubble, 1924)
• other solar systems (Mayor/Queloz/Marcy/Butler, 1995+)
• other universes (Steinhardt, Linde, Guth, Hawking, Hartle, Rees, etc., 1990s).

5. The universe is expanding (Hubble 1929 and eventual elimination of tired light alternative) and went through a hot dense phase 10-20 Gyr ago. (Gamov, Alpher, Herman, 1948; Ryle & Scheuer 1955; Penzias & Wilson, 1965).

6. Light moves at finite speed. (Roemer, 1675, eclipses of the moons of Jupiter; Bradley, 1729, aberration of starlight. Also requires Earth to move and stellar distances at least 105 times the solar distance).

7. Continued expansion of our ideas about the size of the universe.

• Sun at 1079 Earth radii; stars ten times as far (Greeks to Kepler)
• Parallax of Mars; Sun more than 50,000,000 miles away (Cassini, Flamsteed, 1672)
• Stars at least 104 times further (Huygens, Gregory, Newton, 1650-1700)
• Measurement of stellar parallaxes; all stars at least 2 x 105 x solar distance (Struve, Bessell, Henderson, 1838)
• Distances to other galaxies from 105 to 106 parsecs (Hubble, using Shapley scale)
• Extragalactic distance scale expands by a factor of between 5 and 10 (Walter Baade, Allan Sandage, and others, 1952-1999).

8. Celestial objects are born and die, and must have energy sources.

• Conservation of energy (Kelvin, Helmholtz and others, 1850s)
• Giant-and-dwarf theory of stellar evolution (HN Russell, 1910)
• Stars run on nuclear energy (Eddington to Bethe)
• Star formation is real, ongoing process (Spitzer, Schwarzschild et al., 1940s)
• Galaxy evolution can be calculated (Tinsley 1967)
• Galaxy formation can be observed (everybody, 1990s).

9. Leadership in science can move from one place to another. Chinese astronomy, Mayan astronomy, and Arabic astronomy all once outranked Europe. Recent history has seen the gradual migration of the largest population of astronomers and journals from Germany and England to the United States, and from there, who knows?

10. Multiplication of wavebands and tools beyond wildest dreams. (And the universe does not look the same through every window!)

Top Ten Drivers of Discoveries in Astronomy

1. Dry photographic plates (and onward to CCDs).

2. Left-over World War II radar dishes, radar operators, and the beginning of radio astronomy.

3. Captured V-2 rockets and the beginning of ultraviolet and X-ray astronomy.

4. "Invasions" by people from particle physics (1980s-1990s), nuclear physics (1930s), chemistry and other disciplines. Also, spectroscopy in the 19th century.

5. Massive relocation of skilled scientists before and after World War II.

6. Admission of women to curiosity-driven research. (Payne and the composition of stars; Tinsley and quantitative evolution of galaxies).

7. Fruitful, although wrong, ideas from people approaching problems for the first time: the classic example is steady state cosmology; another is Russell's giant and dwarf theory of stellar evolution.

8. Revolutions in computation, Monroes to Microscoft.

9. International collaborations become the norm, from Carte du Ciel to the International Astronomical Union and beyond.

10. Gradual erosion of the gentlemanly agreement not to observe someone else's star.

Top Ten Predicted Hits of the 21st Century

1. Neutrino astronomy will find a third source and become routine.

2. Extremely high energy cosmic rays will reveal new kinds of physics or new particles.

3. Gravitational radiation astronomy; backgrounds and sources.

4. What came before the Big Bang? How did large scale structure form? What is the dark matter? It is also predicted that these will turn out to have the same answer.

5. Theory of star formation. (Initial mass function, binary populations, etc).

6. Chemical and dynamical evolution of galaxies will be freed from the Curse of the Variable Parameter.

7. Spectroscopy of extra-solar, earth-mass planets reveal non-equilibrium chemistry (or not).

8. Additional insights into processes under extreme conditions, including astrophysical masers, two-photon processes, induced Raman scattering, Landau levels, and astronomical dynamos.

9. Something I haven't thought of.

10. Something even you haven't thought of.

Virginia Trimble is a professor of physics/astronomy at University of California, Irvine, and visiting professor of Astronomy at University of Maryland, College Park. The above lists were compiled for a presentation at the APS April Meeting in Long Beach, California.