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accumulating gas slowly from its companion, or through themerger of two white dwarfs. If the total mass of the two dwarfsexceeded 1.4 solar masses, then collapse and detonation wouldalso occur. Therein lies the problem for cosmology. Type Ias areuseful because of the 1.4 solar-mass limit, but if they arise fromtwo merging white dwarfs it may not be possible to correct forthe varying masses in a way that’s truly believable. Evidence fora “super-Chandrasekhar” explosion was reported in 2006 (2006July 18 issue of Nature), and there is now growing belief thatthere are indeed two types of type Ia supernovae, increasing thepressure to find a progenitor.Voss and Nelemans have been searching the archives ofthe Chandra X-ray Observatory and the Hubble Space Telescopeto see if they have earlier observations of the regions aroundrecent type Ia supernovae. They hit the jackpot with SN 2007on,which went off in NGC 1404. There was an X-ray source at thelocation of the supernova in 2003, but nothing in the opticalwavelengths. This led them to conclude that the X-ray sourcewas the signature of gas from a giant falling onto a white dwarf— the first known progenitor of a type Ia.There is a model wherein the lower mass white dwarf in apair could break up, form a disk, and then bleed matter downonto the more massive dwarf, also producing X-rays, but thebest calculations seem to show that the X-ray luminosity ofsuch a disk would be at least a factor of ten less than what wasobserved.Now astronomers have a new way to track down supernovaprogenitors. Perhaps the various orbiting telescopes shouldfactor this into their “legacy” programmes, to ensure that wehave high-resolution images of as many galaxies as possible,providing astronomers with a good database to search after asupernova goes off.Leslie J. Sage is Senior Editor, Physical Sciences, for NatureMagazine and a Research Associate in the Astronomy Departmentat the University of Maryland. He grew up in Burlington,Ontario, where even the bright lights of Toronto did not dim hisenthusiasm for astronomy. Currently he studies molecular gasand star formation in galaxies, particularly interacting ones, butis not above looking at a humble planetary object.Deep-Sky ContemplationsWhat You Don’t Seeby Doug Hube (jdhube@telus.net) and Warren Finlay (warren.finlay@interbahn.com), Edmonton CentreNothing personal, but you are a baryonic beast. As ithappens, so too is your telescope, your toothbrush,and the paper — or computer terminal — on whichthese words appear. Baryons are the massive particles ofordinary matter, specifically protons and neutrons. Baryonsare the principal constituents of people, planets, and stars.Surprisingly, baryons are not the principal constituents ofgalaxies nor, indeed, of the Universe.Approximately 70 years ago, Fritz Zwicky found the firstobservational evidence for the existence of “dark matter,”that still-mysterious, non-luminous, gravitating “stuff” thatis responsible for the dynamical behaviour of matter withingalaxies and of galaxies within clusters of galaxies. Morethan 20 years ago, Vera Rubin and her collaborators beganto accumulate data on the rotational velocity curves of spiralgalaxies. Many others have contributed in subsequent years.Classical Newtonian gravitational theory tells us that in asystem with structural and dynamical symmetry such as is foundin a spiral galaxy, the rotational velocity at a distance r from thecentre is governed by the gravitational mass, M, interior to raccording to v rot2= GM/r, where G is the gravitational constant.(The same expression applies to planets revolving in the Sun’sgravitational field). Leaving out many details (!), within aFigure 1 — Position of NGC 3198 in the night sky.galaxy v rotcan be measured using the Doppler effect, and r canbe determined if we know the distance to the galaxy. Hence Mcan be calculated. We can also combine apparent brightnessand distance to compute the luminosity, L, of a galaxy, or of aportion of a galaxy. The mass-luminosity ratio, M/L, is a usefulparameter in describing the nature of matter. Using solar units,Continued on page 68April / Avril 2008JRASCBuilding for the International Year of Astronomy (IYA2009)65