Calibrating Cosmic Candles

Two properties make Type Ia supernovae powerful tools to map the expansion history of the universe. These supernovae are incredibly bright, making them visible from great distances across the universe. They exhibit the same intrinsic brightness so that astronomers, after making straightforward corrections, can use the measured brightness to determine a supernova’s distance. Detailed measurements of a Type Ia supernova (SN 2011fe) in our galactic neighborhood confirm how reliably these objects can be used for these purposes.

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Just about a week ago, in the spirit of Independence Day, I watched an impressive set of fireworks from one-and-a-half miles away. I know the distance because sound travels one mile in just over five seconds and it took about eight seconds for the sound from the fireworks blasts to reach my ears. I later verified the distance using a map.

When astronomers try to measure distances to objects in the universe, they can’t simply look at a map to double check their calibrations. Instead, they rely on a number of techniques, such as measuring the brightness of Type Ia supernovae, to ensure accurate calculations. Recently, a nearby supernova explosion confirmed the reliability of this technique, which helps astronomers measure the expansion rate and age of the universe, both key points in the discussion over the origin and history of the cosmos.

In 2011, scientists observed the light from SN 2011fe exploding from the nearby (astronomically speaking) Pinwheel Galaxy (or Messier 101). At a distance of just 21 million light-years, this was the closest and brightest Type Ia supernova observed with intensive follow-up observations. Astronomers began monitoring SN 2011fe just 12 hours after the explosion and observed it for more than 110 days.

How to measure the universe Image credit: NASA/JPL-Caltech

How to measure the universe
Image credit: NASA/JPL-Caltech

The fact that all Type Ia supernovae appear to exhibit the same brightness level makes them convenient “standard candles” (though astronomers must compensate for a few known differences). While estimating distances to fireworks requires seeing and hearing the explosion, Type Ia supernovae’s similar brightness means astronomers need only the measured brightness to determine the distance to other galaxies. This calculation plus the speed at which these galaxies are moving away from us allow astronomers to then calculate the universe’s expansion history.

Yet all these calculations depend on the accuracy of Type Ia supernovae measurements. The extensive data on such a close supernova as SN 2011fe allows researchers to test Type Ia supernovae’s reliability in two ways.

First, the measurements of SN 2011fe confirm its unusually normal nature.1 In astrophysics, it often happens that opportunities for closer and more detailed observation of phenomena reveal mistakes in researchers’ previous assumptions and understanding. Thus, in SN 2011fe’s case, it was almost expected to exhibit differences in brightness, duration, metal content, or numerous other measureable parameters, thereby undermining the Type Ia supernovae’s reliability as a standard candle—but it didn’t.

SN 2011fe’s remarkably normal nature means that the incredible detail, both spectral and timing, observed from this supernova allows astronomers to compare future Type Ia supernovae with SN 2011fe and look for any relevant differences. Any differences would test whether the supernovae occur under the same conditions or not and also provide additional calibration tools to compensate for those differences.

Second, astronomers continually seek to reconstruct the processes leading to Type Ia supernova explosions in computer models. The details that SN 2011fe’s closeness enabled astronomers to measure will provide helpful constraints for the computer models. Such constraints will lead to more accurate models, which in turn, will lead to better distance calculations from observed supernovae.

Although astronomers cannot travel to distant locations or consult maps of the universe, they have developed a useful set of tools that allow them to construct their own “maps.” These studies reveal the spectacular and fascinating universe in which we live and, in doing so, confirm the remarkable fine-tuning required for life.

Endnotes:

1. R. Pereira et al., “Spectrophotometric Time Series of SN 2011fe from the Nearby Supernova Factory,” Astronomy and Astrophysics 554 (June 2013): A27.

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