A Blog by

A Tale of Four Supernovas

Explosions are always messy, and this is especially true when combustion occurs on a cosmic scale.

In space, small, dead stars called white dwarfs occasionally blow themselves to smithereens, producing enough light to drown out entire galaxies. Astronomers use these explosions – called type 1a supernovas – to calculate cosmic distances. In the late 1990s, observations based on type 1a supernovas revealed that the universe is flying apart faster and faster as time goes on. The force behind this acceleration, called dark energy, is still enigmatic, but the discovery earned a Nobel Prize in 2011 and is considered one of the most fundamental in cosmology.

Yet despite their role as the workhorses of the cosmological distance ladder, type 1a supernovas are still poorly understood. Blowing up a white dwarf, both in terms of the detonation physics and the necessary stellar ingredients, is still a scientific conundrum. Now, two studies published this week in Nature are helping solve that ingredients problem, even though at first the observations appear to disagree with one another.

Together, the studies tell “a rich and interesting story about the entire system that leads to supernova of type 1a,” says UC Berkeley astronomer Joshua Bloom. “It’s remarkable how important type 1a supernova were – and still are – for cosmology, given how relatively little we know about the diversity of the progenitor channels.”

Type 1a supernova 1994D in the galaxy NGC 4526, as seen by Hubble. (NASA/ESA/more)
Type 1a supernova 1994D in the galaxy NGC 4526, as seen by Hubble. (NASA/ESA/more)

Doomed Dwarfs

White dwarfs are the extinguished corpses of stars that were once very much like the sun. They’re incredibly dense, with a sun’s mass of material stuffed into something the size of the Earth. Left alone, a white dwarf will simply fade to black over billions or trillions of years.

But dwarfs with a starry companion can suffer a different fate. Sometimes, the two stars are close enough for the dwarf’s incredible gravity to begin stealing material from the companion. If the dwarf gets too greedy and gains enough mass, a runaway thermonuclear explosion ignites and blasts the star to pieces.

For a long time, scientists thought white dwarfs must be burgling a large companion star – something like a red giant that’s big and gassy and easy to steal from. Yet pre-explosion observations of nearby type 1a supernovas have failed to identify any red giant companions. Nearby supernova remnants show similarly scarce evidence for giant companions, which would not be destroyed during the explosion, just banged up and tossed from the system.

Other observations have suggested the doomed dwarfs could be dancing with a more or less normal star – a main sequence star, like the sun.

Scientists couldn't find evidence for a large, red giant companion star in  supernova remnant SNR0509-67.5, in the Large Magellanic Cloud. (NASA/CXC/SAO/J.Hughes/ESA/Hubble Heritage Team)
Scientists couldn’t find evidence for a large, red giant companion star in supernova remnant SNR0509-67.5, in the Large Magellanic Cloud. (NASA/CXC/SAO/J.Hughes/ESA/Hubble Heritage Team)

And another possibility, supported by multiple observations but thought to be extremely unlikely up until the last decade or so, is that type 1a supernovas could be the nuclear death spasms produced when two white dwarfs merge. These two-dwarf pairs are known as “double-degenerate” systems, because white dwarfs are stabilized by electron degeneracy pressure.

(There are some even crazier ideas being batted around.)

Different pairs of stars producing type 1a explosions is troubling for some scientists, who wonder how such different starting ingredients can produce such similar explosions. For now, the data suggest that as long as a white dwarf is involved, it’s possible that “type 1as can come from essentially anything,” says Brad Tucker, who splits his time between UC Berkeley and the Australian National University.

A Tale of Four Supernovas

This week’s Nature papers make that even more clear.

One of studies reports that when a type 1a supernova exploded in May 2014, the blast debris crashed into the former companion star. The collision produced a pulse of ultraviolet light that scientists could detect using NASA’s Swift satellite, which swiveled to watch the explosion four days after it started. It’s the first time this type of shock-induced brightening has been seen with a type 1a explosion, says lead study author Yi Cao of Caltech, and indicates that the dwarf’s friend is not another white dwarf. Instead, that companion was probably a bigger, normal star.

A computer simulation showing debris produced by an exploding white dwarf slamming into a companion star, in blue. (UC Berkeley/Daniel Kasen)
A computer simulation showing debris produced by an exploding white dwarf slamming into a companion star, in blue. (UC Berkeley/Daniel Kasen)

The second study used NASA’s exoplanet-hunting Kepler telescope to observe three explosions almost from the moments they ignited in 2011 and 2012.

Those three supernovas, described by the University of Maryland’s Rob Olling, look like type 1a supernovas and are all very far from Earth, between 600 million and 2 billion light-years away. But unlike Cao’s team, Olling and his colleagues saw no brief bump in brightness produced by debris colliding with a companion star. “The data support the idea that the companions are quite compact, with the most likely explanation being double-degenerate systems, two white dwarfs merging,” says Saurabh Jha of Rutgers University, who was not involved in the study.

Apples to Apples

While these results may seem contradictory in that different ingredients are involved, they actually mostly fit together pretty well – and that’s because there’s one more piece of the puzzle to consider.

Over the years, it’s become clear that type 1a supernovas don’t come in just one flavor. Put very simply, some are brighter than normal and others are dimmer. “It’s not just apples,” Olling explains. “There are Granny Smiths and Jonagolds and Braeburns, and we don’t know exactly how many varieties there are.”

The supernova described by Cao, which likely resulted from a white dwarf stealing material from a normal star, is one of those dimmer subtypes. It’s similar to a flavor known as a type 1ax, which is less luminous than your classical kaboom (kind of like a “peculiar cousin,” to borrow Jha’s description, or a “weirdo” to use Tucker’s).

Conversely, the supernovas described by Olling are “normal” type 1a explosions – the type with a predictable brightness that can be used to measure cosmic distances. These likely result from two white dwarfs merging and turning into a giant nuclear bomb.

“Cosmological 1a supernovas do tend to show that they might be doing the same thing, which is great,” Tucker says. “But then there are the weirdoes. I like weirdoes. We all like weirdoes. But cosmology doesn’t like weirdoes.”

So, could it be that two merging dwarfs produce normal type 1a supernovas, while systems with only one white dwarf lead to peculiar explosions?

If yes, then what’s happening is that these observations neatly fall into line. Regardless, not only is it becoming clear that type 1a explosions come from different ingredients, it’s becoming clear that those ingredients don’t all cook the same supernova. And maybe, just maybe, scientists are getting closer to determining exactly how that all works.

Supernova 2014 exploded in the Cigar Galaxy, affording astronomers their closest look at a supernova in three decades. (NASA/ESA/Hubble Heritage Team)
Supernova 2014 exploded in the Cigar Galaxy, affording astronomers their closest look at a supernova in three decades. (NASA/ESA/Hubble Heritage Team)

7 thoughts on “A Tale of Four Supernovas

  1. If you read the good multi-disciplinary research (which most specialists don’t), you would conclude that the last nearby type IA in our galaxy blasted a hole in the milky Way that is still visible as an empty, dark spot. Native Americans saw it as the path to afterlife. They have many legends of massive fires. The repeated disruption of our solar system by stuff traveling at a variety of speeds, pelted the Earth with material from disrupted orbits, and ended the last Ice Age, quite abruptly. It is also associated with the extinction of the Megafauna, which could not find shelter from radiation and flying fire.
    You can still see many of the impact craters in The Southeast U.S. and elsewhere in the Northern Hemisphere, as circular or oval lakes, with unusual raised berms as borders.
    References and citations on request.

  2. So does this mean that those original calculations will need to be reconsidered? Is there a sufficiently significant difference between Type 1a and Type 1ax output that might affect how much dark matter/energy we think is there?

  3. Apologies for any troll-like tendencies, but … surely it is ‘supernovae’ and not ‘supernovas’…? This is the home of science, no?

  4. What will happen with our Sun when the end begins? Let first remind again the supernova event evolution:
    The heavier elements which aren’t form solid stuff of the star will come to disintegration when the pressure and temperature within its volume reach the following values: 2.〖10〗^(27 ) atm.[kg/〖cm〗^2 ];3.〖10〗^11°K, see USM http://www.kanevuniverse.com The same is corresponds to the evolution of the neutron’s stars. When the fuel of the some star comes to its end, which means that not only the thermonuclear synthesis of hydrogen atoms already is spend, but the thermo nuclear synthesis of more heavier atoms also is on its end, then the star begins to collapse (to shrinking) and when the above values of the pressure and temperature are reached, then these atoms come to disintegration finally again into hydrogen atoms, but old ones see USM http://www.kanevuniverse.com After that again according to part I of the theory the pressure and temperature begin to decreases and again is ignites the thermonuclear synthesis, the star begin to expand reaching the pressure and temperature above which this reaction is possible: 2.〖10〗^9 atm.;〖10〗^7°K Below these parameters the thermonuclear reaction stops and the star disperse itself. Why the gravitation field cannot stop this process? Because the star is already on the periphery of its galaxy where it hasn’t necessary resonance radius to spring up again, see page 80, 81, 82, 83 USM http://www.kanevuniverse.com where is calculated the radius of birth of the Sun and why the stars are birth on the center of their own galaxies. So there is one exception if the observed star is very massive one, then if the resonance radius where the star is situated during its collapsing is enough short then there can be born some little star which its own centripetal acceleration correspond to their current parameters: velocity of bird and radius of bird. However, most probably the star springs up like supernova and after that die. What will happen with dispersed from such star old protons and old neutrons…they also come to disintegration into hypothetically substars and subplanets from the subspace see USM part I, part II and superconductivity http://www.kanevuniverse.com
    Let remind also: That is about newly found out very distant two super nova 10 bln light years away……… Explanation it isn’t correct, because there must to take into account the thickening of the space, which is huge in such large distance 10 bln light years (then the UV will be with a lot longer length, not only by the expansion of the universe), that the big bang never happen (this events particularly is proving that), the super magnetic nucleus of super nova is a consequence by the formation of so called common magnetic moment of nuclear chain in super huge pressures and temperatures (that is why there we can’t observe hydrogen atoms after the super nova exploding, but not because of the big bang)…for all these see joromachine USM Q&A http://www.kanevuniverse.com
    And also this: About pulsars, this explanation is correct with one exception, namely remainder after the star explosion in fact is nuclear chain material with common magnetic moment (see joromachine http://www.kanevuniverse.com ) which can’t radiate anything because such material has full (not approximately) but full reflection….. that is why such material shine so brightly but not because it is in high energy state eventually! G.Kanev

  5. I do not see four of anything in this article. Did the author create the title for this article or did some one else that did not read the article?

  6. Nice post. I have a problem with your use of the word “combustion”, however — in scientific jargon, it’s only used for _chemical_ reactions.

Leave a Reply

Your email address will not be published. Required fields are marked *