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.

CANDELS Finds the Most Distant Type Ia Supernova Yet Observed

Image credit: NASA

A couple weeks ago, the supernova science team from CANDELS was pleased to announce the discovery of the most distant Type Ia supernova ever seen at more than 10 billion light years away, a time when the universe was only about 3.5 billion years old. There wasn't ever much doubt that supernovae existed more than 10 billion years ago -- but it's still an exciting moment for us when we're able to photograph one with the Hubble Space Telescope. The image on the right shows the supernova's location in one of the five fields that CANDELS is searching. Almost every single dot in the upper panel is a distant galaxy, filled with billions of stars. Our team searches through these images every couple of months, hoping that one of these stars has exploded.

But ironically, the most exciting thing about this supernova for many of us is that we haven't seen more. In terms of gaining an understanding of how these objects are created, its really the lack of supernovae in the early universe that is the most telling observation. The Hubble has the power to observe one of these objects from more than 11 billion light years away, and we've been staring at the sky in search of them for nearly 3 years. So far, we've found only a single confirmed Type Ia supernova from 10-11 billion years ago -- a billion years of cosmic history.

In astronomy, our uncertainty about the universe is somewhat striking. 70% of the Universe is the mysterious "dark energy", completely unknown until about 15 years ago. Another 25% is dark matter -- also a mystery, since we only observe its gravitational effects. This leaves 5% of the universe that is composed of matter more familiar to us. In the context of this uncertainty, the supernova progenitor mystery might not seem so fundamental. And yet, every second, about 5-10 supernovae are going off somewhere in the observable universe with 10 billion times the brightness of our sun. These supernovae produce about half of the iron in the universe, some of the raw material for creating planets like the Earth. And the ways that these explosions happen -- both the stellar evolutionary steps that lead to the explosion and the physics of the detonation itself -- remain mysterious.

The composition of the universe.  Only 5% of the universe is normal matter, and only .03% of the universe consists of the heavier elements we're most familiar with on Earth.
Image source: http://www.lsst.org/lsst/public/dark_energy

This dearth of supernovae must be telling us something fundamental about the nature of these objects -- so what is it? Finding the earliest supernovae sheds light on this puzzle in a couple of different ways because fundamentally, the early universe was a very different place than the universe we live in today. There were fewer heavy elements, stars were younger, and galaxies were producing stars at a faster rate than they are today. The way in which supernovae form in this environment tells us something about their nature.

First of all, if supernovae are formed from younger stars, do they explode in the same way? Some theorists think that the answer is no -- it's predicted that only more massive stars have time to become supernovae within the first 3-4 billion years of the Universe, and nuclear fusion in more massive stars will result in a different blend of heavy elements in the core.  When the star's remnant (called a white dwarf) gains matter from a binary companion and explodes, its brightness is powered by the radioactive decay of elements fused in the explosion; a different chemical composition results in a different brightness. We're looking for such a change in brightness -- it can tell us a bit more about how supernovae are created.

Secondly, if a burst of star formation occurs in a galaxy, how long will it be before the Type Ia supernovae start exploding? How quickly stars evolve and form supernovae can tell us a lot. If most Type Ia supernovae occur when two white dwarfs form in a binary system and then slowly merge together, the time it takes them to explode will be based on the distribution of initial separations that binary stars are born with. If, on the other hand, they tend to occur when a normal or giant star is slowly pouring mass onto a white dwarf at a given speed, one might expect something closer to a single characteristic time from formation to explosion. By knowing when most of the stars formed in the universe, and observing how quickly supernovae are exploding at different ages of the universe, we can determine which model for supernovae is correct. This knowledge may also make it easier to understand the physics of a white dwarf's detonation.

All of this work can be tied back to the fact that Type Ia supernovae are not just interesting in and of themselves; they are cosmological tools with a characteristic brightness that can be used to set a distance scale for the Universe. Understanding better how supernovae explode can be used to answer questions like: why are supernovae dimmer in smaller galaxies? What process creates unpredictable and unusual supernovae like Type Iax? How can we better calibrate these tools to learn more about dark energy?

These are some of the questions that our team hopes to answer in the coming years. In the meantime, we're searching the night sky for exploding stars -- and each time we find one, it tells us a bit more about the universe we live in.