Monday, January 27, 2014

A New Type Ia Supernova in M82

Just a couple of days ago, a dim, but quickly brightening, supernova was discovered in M82, the beautiful "cigar galaxy." At "only" 12 million light years away, this is the nearest supernova to Earth since 1987 and the nearest Type Ia supernova since 1972. With the enormous changes in our imaging technology since then (including the launch and subsequent improvements to the Hubble Space Telescope), this is a fantastic opportunity for precision measurements of one of the brightest and most mysterious explosions in the universe.

The new supernova in M82, discovered by students at the University College London
Observatory.  Photo by Adam Block/Mount Lemmon SkyCenter/University of Arizona

Discovering more about the nature of Type Ia supernovae has been one of the primary goals of the CANDELS project. These supernovae begin as stars like our sun, which have shed their outer layers at the end of their lives and become white dwarfs. White dwarfs are the extremely dense cores of a burned-out star, and although they're only the size of our earth, they have the mass of our entire sun.  The detonation happens when a nearby star adds even more mass onto this dwarf -- when the weight becomes too much, nuclear fusion ignites it and a supernova occurs.

In CANDELS, we study the most distant Type Ia supernovae that we can find, the farthest of which stands at over 10 billion light years away. Our supernovae tell us about the early expansion of the universe (and its Dark Energy), the chemical evolution of the universe, and how quickly supernovae form and explode around 8-10 billion years ago -- at the peak of star formation in the universe.

This nearby galaxy offers a completely different, and rarer, perspective. In 1972, when the last Type Ia supernova this close to Earth exploded, it was still a year before anyone proposed the idea that these supernovae were formed in binary star systems. It was 12 years before someone realized that both stars could be white dwarfs, and 18 years before supernovae could be studied from space with the Hubble Space Telescope. It was over 25 years before such supernovae were used to discover that Dark Energy was accelerating the expansion of our universe.

Motivated by the knowledge and technology gained since the last close Type Ia supernova went off, scientists will be asking an entirely different set of questions this time around. First, we'll be looking for a giant companion star that could have fed mass onto the white dwarf. If a companion star is visible, this would be the first direct evidence that a system with one white dwarf can lead to a supernova; if a companion star is not found, the theory that two white dwarfs can make a Type Ia supernova will gain credibility.

Artist's conception of the single-degenerate (one white dwarf)
theory of Type Ia supernova explosions, wherein
a white dwarf accretes mass from its companion
star.  (original) © ESA and Justyn Maund (Queens Univ. Belfast)
Artist's conception of the double-degenerate theory
of Type Ia supernova explosions, in which two white dwarfs merge
together as they emit gravitational waves. (original) © NASA,
Tod Strohmayer
(GSFC),  and Dana Berry (Chandra X-ray Observatory)

Second, scientists will be studying the geometry of the supernova from the fraction of polarized light emitted. Polarization, the orientation of a light ray's electric field, is entirely random when it originates from a spherically symmetric star. However, if one side becomes longer than the other, the light's polarization will have a preferential direction that can be measured on Earth. As the outer layers of the M82 supernova expand, they will become transparent and expose the inner material. Over the next month, scientists will be able to measure the shape of different layers and examine the three-dimensional explosion. With this structural information, we'll learn more about how supernova detonation occurs; specifically, how nuclear fusion begins and spreads through the layers of the white dwarf.

The location of M82 on the night sky from Sky and Telescope.
A more detailed chart is available here
Lastly, Type Ia supernovae are nearly uniform in brightness, serving as excellent distance indicators for most of the visible universe. CANDELS supernova principal investigator Adam Riess -- among others -- will be measuring the distance and doppler shift velocity (the reddening of its light) of this supernova to determine how fast the local universe is expanding -- and infer the amount of the mysterious Dark Energy that surrounds us.

This supernova is particularly rare in that it offers opportunities not only to scientists, but for anyone with access to a dark night sky. It will brighten for approximately a week and a half, and at its peak it will be visible near Ursa Major (the Big Dipper) to anyone with a set of binoculars. Although it's impossible to predict when the next close supernova will be, I'm looking forward to seeing an exploding star with my own eyes - it may be 40 years before there's another opportunity.

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