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Standard Candles

April 4, 2014

Science is performed with the tools at hand. At one time, voltage was measured with middling accuracy using gold leaf electroscopes, later replaced with moving coil galvanometers. Today, most laboratories have digital voltmeters of high sensitivity and high resolution. In my graduate student days, there were no computers for automated data acquisition. There was just me, staring at the needle of a voltmeter or ammeter, recording data in my laboratory notebook, or the occasional pasted snip from a chart recorder.

Applied optics has been with us at least as long as Archimedes reportedly used a heat ray on Roman ships during the Siege of Syracuse (c. 214-212 BC). It's supposed that the "Archimedes heat ray" was formed by focusing mirrors directing sunlight to the easily ignited tarred timbers.

Scientists can only measure things with respect to standards. Some standards are physical objects, such as those maintained as the international prototype kilogram and the international prototype meter at the International Bureau of Weights and Measures (Bureau international des poids et mesures) near Paris. Although the meter is now defined as a multiple of the wavelength of an energy level transition in krypton-86, the prototype kilogram is still an important standard.

Photometry and radiometry are areas of physics for which standardization has always proven difficult. The candela is the unit of luminous intensity. One candela is roughly the luminous intensity of a candle simply because a standard candle was used as a reference.

Candles, however, vary in their light output, so the composition and structure of a standard candle is specified. The wax is pure spermaceti, a wax found in the head cavities of sperm whales, and there are specifications for flame height and burn rate, since these are factors in the candle's light output. The German Association of Gas and Water Industries adopted a different standard in 1868 for which the candle material is paraffin wax.[1]

Photometry in the nineteenth century was done by comparison of the light of such a candle to the light source under study. The figure shows one such device in which the light of the standard candle was compared by eye to other light sources on a split screen. The human eye is good at seeing differences in light intensity this way, so the distance to the standard candle or the light source under study were changed to equalize the light intensity. Simple mathematics gave you the source intensity in candelas.

A Cornu Photometer

A photometer invented by French physicist, Marie Alfred Cornu (1841-1902).

The distance to the standard candle (l) and light source (E) were varied to equalize light on both sides of the split screen (C-C').

(Fig. 26, page 59, of
Ref. 1.)[1)]

Astronomers can measure the distances to nearby stars using parallax, whence the distance unit, parsec, from "parallax arcsecond." Present day satellite measurements have allowed distance determination out to about a thousand light years (9,460,730,472,580 km). This seems like a long distance, but our Milky Way Galaxy is about 100,000 light-years in diameter, so another measurement technique is required.

Although there are other methods, one straightforward technique is to measure the brightness of stars and compute distances according to the iinverse-square law with reference to stars measured by the parallax method. Not all stars are the same, but we can group them according to their spectral class to get good distance estimates. however, the nearby Andromeda Galaxy is still 2.5 million light years away, which was too far for the optical telescopes of the last century to image their individual stars.

There are some stars, type Ia supernovae, that are individually visible, even at galactic distances. These supernovae, which are exploding stars, will briefly outshine their host galaxy, and it's been found that the peak brightness of all type Ia supernovae is the same. It's thus possible to calculate their distance. These are among the better "standard candles" of the universe.

Discovery image of supernova 2010dn in NGC3184

X marks the spot. Well, part of an X.

Discovery image of supernova 2010dn in NGC3184.

NGC3184 is about 40 million light years distant.

(Image by Kevin Heider, via Wikimedia Commons.)

Every new supernova is an opportunity to refine the statistics for this standard candle. Astronomers at the University of California, Berkeley, and the Itagaki Astronomical Observatory (Yamagata, Japan) have just published their analysis of the recent supernova, SN 2014HJ, discovered in January, 2014.[2-4] This supernova is important, since it's the nearest known type Ia supernova since SN 1986G, discovered on May 3, 1986, in the galaxy, Centaurus A, just 15 million lightyears distant.

Supernova SN 2014HJ is located in the galaxy, M82, also called NGC 3034 (or the "Cigar Galaxy"). This galaxy is 11.4 million light years distant. Although this supernova was discovered about seven days after its explosion, prediscovery images were obtained by the Katzman Automatic Imaging Telescope at Lick Observatory, and by Koichi Itagaki, a Japanese amateur astronomer. These data were sufficient to estimate the time of initial explosion as January 14, 2014, at about 18:00 UT, or 0.82 ± 0.21 days before the first possible detection.[2-3]

The data were unusual, since they showed a varying power law in early-time light curves. Supernova SN 2013dy had the same, so now there are two examples of this effect, which is a faster brightening than other type-Ia supernovae.[2-3] The response is different from that of a third type-Ia supernova, SN 2011fe, discovered three years ago.[4] Says study author and University of California, Berkeley, astronomer Alexei Filippenko,
"Now, two of the three most recent and best-observed Type Ia supernovae are weird, giving us new clues to how stars explode... This may be teaching us something general about Type Ia supernovae that theorists need to understand. Maybe what we think of as 'normal' behavior for these supernovae is actually unusual, and this weird behavior is the new normal."[4]

Supernova SN2014HJ

Supernova SN2014HJ, located in M82, 11.4 million light years distant.

This is a composite of different images, with the supernova marked with an arrow.

(UC Berkeley image by W. Zheng and A. Filippenko.)

Such discoveries as this are only possible because of automated searches that scan selected patches of sky on a continual basis.[4] The Katzman Automatic Imaging Telescope, operated by postdoctoral scholar, WeiKang Zheng, takes images in five different spectral bands. In 2012, it captured images of supernova SN 2012cg just three minutes after its discovery.[4] Says Filippenko,
"Very, very early observations give us the most stringent constraints on what the star's behavior really is in the first stages of the explosion, rather than just relying on theoretical speculation or extrapolating back from observations at later times, which is like observing adolescents to understand early childhood."[4]

This research was funded by the National Science Foundation and various private foundations.[4]


  1. Adrien Palaz, George Washington Patterson and Merib Rowley Patterson, "A treatise on industrial photometry with special application to electric lighting," D. Van Nostrand company, 1896, 324 pages (Google Books).
  2. WeiKang Zheng, Isaac Shivvers, Alexei V. Filippenko, Koichi Itagaki, Kelsey I. Clubb, Ori D. Fox, Melissa L. Graham, Patrick L. Kelly and Jon C. Mauerhan, "Estimating the First-Light Time of the Type Ia Supernova 2014J in M82," The Astrophysical Journal Letters, vol. 783, no. 1, Document No. L24, doi:10.1088/2041-8205/783/1/L24.
  3. WeiKang Zheng, Isaac Shivvers, Alexei V. Filippenko, Koichi Itagaki, Kelsey I. Clubb, Ori D. Fox, Melissa L. Graham, Patrick L. Kelly and Jon C. Mauerhan, "Estimating the First-Light Time of the Type Ia Supernova 2014J in M82," arXiv Preprint Server, February 10, 2014.
  4. By Robert Sanders, "Closest, brightest supernova in decades is also a little weird," University of California, Berkeley, Press Release, February 27, 2014.
  5. Web Site of the Katzman Automatic Imaging Telescope at Lick Observatory.

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