From the December 2022 issue

Type Ia supernovae: Inside the universe’s biggest blasts

These supernovae serve as vital distance indicators that show how our cosmos is expanding. But the events are still shrouded in mystery.
By | Published: December 8, 2022
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G299.2–2.9 is the result of a white dwarf tearing itself apart roughly 4,500 years ago. Today, astronomers have imaged the structure at different wavelengths to better understand the event that caused it. This composite, made with data from the Chandra X-ray Observatory, colors low-, medium-, and high-energy X-rays as red, green, and blue, respectively. Iron atoms are responsible for at least some of the emission at medium energies, while silicon and sulfur emit some of the highest-energy light.
NASA/CXC/U. Texas

Supernovae are the most spectacular fireworks in the universe. Just one such explosion can briefly outshine the rest of the stars in a galaxy combined.
Astronomers have become adept at spotting these events across the cosmos, with particular attention to a specific class called type Ia supernovae.

Much of modern cosmology rests on these events. That’s because thanks to their intrinsic physics, these stellar blasts should always have the same intrinsic brightness. This property makes them excellent distance indicators, which allows astronomers to obtain their distance simply by observing how dim their light appears upon reaching Earth. Ultimately, the information gleaned by mapping type Ia supernovae across the cosmos has allowed astronomers to measure the expansion of the universe itself.

And yet, perhaps the most incredible thing about these particular supernovae is that despite their reliability as distance markers, we don’t know all the details of what exactly triggers their progenitors to explode in the first place. This gap in our understanding could affect the accuracy of our estimates of the universe’s expansion — and it leaves astronomers thirsting for answers.

“It’s just wild to me that we don’t know what causes a type Ia explosion when they’re so important to science,” says Tyler Holland-Ashford, an astronomer at the Harvard-Smithsonian Center for Astrophysics. “It’s still a mystery, and one I would like to know the answer to.”

He’s not alone. Numerous researchers are striving to find that answer, using clues from the supernovae themselves and the remnants they leave scattered across the galaxy.

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Type Ia supernovae have revealed much about the nature of our universe — including the fact that not only is it expanding, but it is doing so faster now than in the past. Astronomers have determined this by measuring the brightness of distant supernovae and plotting them against their redshift, a proxy for distance. The blue line shows the best fit to the current observations, which match the predicted model for a universe whose expansion is accelerating, rather than decelerating — i.e., a universe where dark energy contributes. The white line indicates how the brightness of supernovae would look with distance in a universe with no dark energy. It does not fit the observations, as it predicts that distant supernovae should appear brighter than they actually do.
Astronomy: Roen Kelly, after Carroll, Bradley W. and Ostlie, Dale A., An Introduction to Modern Astrophysics, 2nd Ed., Pearson Education, Inc., 2007.

Classes of blasts

Supernovae are separated into two broad categories: type I and type II. Type I supernovae have little to no hydrogen in their spectra, meaning none of this element is present in their progenitor. Type II supernovae, which are created by massive stars exploding at the end of their lives, do contain hydrogen because these suns still have ample reserves of unfused hydrogen near their surface. This second class of blasts occurs when a massive star collapses into itself — leaving behind a neutron star or black hole.

A type Ia (pronounced “one-A”) supernova is generated through an entirely different process. It starts with a stellar remnant called a white dwarf. When a star that is not large enough to undergo a type II supernova can no longer keep fusion going at its core, it will shed its outer layers and leave behind a white dwarf. (This is the fate that awaits our Sun one day.) A white dwarf is not massive enough for additional usion; instead, it glows due to residual heat, which it very slowly loses as it cools.

However, there is an exception to this no-more-fusion rule: when a type Ia supernova happens. Stellar fusion occurs because a star is massive enough to squeeze the elements within it into new ones. Stars like our Sun convert hydrogen into helium, releasing energy in the form of light, and die once this fuel is exhausted. The more massive a star, the heavier the elements it can fuse.

White dwarfs aren’t stars anymore — they are simply the dead, leftover cores. Without the heat of fusion to hold it up, the white dwarf shrinks under its own gravity until it becomes not a ball made of atoms, but something called degenerate matter instead. Degenerate matter consists of atomic nuclei swimming in a sea of electrons. Now the force holding the white dwarf up against gravity is electron degeneracy pressure, which arises from the fact that the electrons cannot get any closer to each other without breaking the laws of physics dictating how many can occupy the same space at once.

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Type Ia supernovae are readily visible even in distant galaxies. The Hubble Space Telescope captured this stunning image of one such explosion (lower left), cataloged as SN 1994D, flaring to life in the lenticular galaxy NGC 4526, some 55 million light-years away in Virgo.
NASA, ESA, The Hubble Key Project Team, and The High-Z Supernova Search Team

Unless the white dwarf exceeds 1.44 times the mass of the Sun (an amount called the Chandrashekar mass), that is. As the white dwarf gains more mass, its temperature increases. And once it hits this critical mass, it will reach an ignition temperature to fuse heavier elements — notably carbon, which the star was not massive enough to fuse earlier. At this so-called critical mass, the white dwarf will reignite and tear itself apart within a matter of seconds. This is what gives rise to a type Ia supernova.

The usefulness of type Ia supernovae comes from two facts. First, they are more luminous than almost all other supernovae — about 5 billion times brighter than the Sun. This allows us to easily see them even billions of light-years away.

And second, because each type Ia arises from the same type of progenitor and thus has roughly the same amount and type of combustible material, their light curves — the way their brightness evolves over time — are incredibly uniform. After the peak, the rate at which their light fades follows a predictable pattern over a few months, as heavy elements at the core of the white dwarf radioactively decay.

This is why type Ia supernovae can be used to measure distance — even when it’s vast. Galaxies themselves can vary greatly in size and brightness, making it hard to tell whether, for instance, a galaxy that appears bright in the sky is intrinsically luminous or simply nearby. But type Ia supernovae are standard candles: The dimmer one appears, the farther away it lies. This property allowed astronomers to discover that the expansion of the universe is accelerating, thanks to mysterious dark energy — a find that was awarded the 2011 Nobel Prize in Physics.

You might think that such a fundamental cosmic distance marker must be perfectly understood. But astronomers still debate what actually triggers a gigantic type Ia explosion in the first place — a detail that could have repercussions for our understanding of the role of the dark energy these blasts have revealed.

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Modern-day astronomers have been using X-rays to study the composition of ejecta left behind by Kepler’s supernova, which appeared in the sky more than four centuries ago. They hope to learn more about the progenitor of the explosion by looking at the specific mix of elements it left behind.

X-ray: NASA/CXC/NCSU/M.Burkey et al; Optical: DSS

Degenerate details

The trouble is this: Supernovae are rare, with just one every century in a galaxy like our Milky Way. Type Ia supernovae are rarer still because of the unusual circumstances that produce them. This means we have not been able to actually see what the system around the white dwarf is like when it blows — we only spot a bright new point of light from millions of light-years away.

And in astrophysics, the devil is in the details. “It doesn’t require extreme precision to know the universe’s acceleration is expanding,” explains Ken Shen, an astronomer at the University of California, Berkeley, who specializes in type Ia supernova theory. “But there are appropriate concerns that there might be systematic problems in observations.”

That’s because while most type Ias appear to reach a standard maximum luminosity before their predictable decay, a smaller subset appear too bright or too dim compared to what is typical. This discrepancy isn’t big enough to change the observation that the universe is expanding and accelerating, but it is enough to affect how precisely we can measure this expansion. And Shen is convinced that understanding the progenitors of type Ias is a key to doing that.

“Knowing what the progenitors of type Ia are is an important thing,” says Shen. “How did these supernovae happen? The physics there is what drives me.”

There are two leading theories on how to prompt a white dwarf to create a type Ia supernova, and both require the white dwarf to have a companion star that donates the extra mass to tip it over the limit and cause the explosion. The first is a single-degenerate (SD) system, where the white dwarf is in a close binary system with a fusion-burning star. (The “degenerate” here is the white dwarf, which, remember, is made of degenerate matter.) Over time, the white dwarf siphons material away from the star and, if it accretes enough material, it may reach the Chandrashekar mass and explode.

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Astronomy: Roen Kelly

But in recent years, the SD scenario has run into some troubles — namely, it doesn’t appear to explain what we see in some type Ia supernovae. One notable example was supernova SN 2011fe, a quintessential type Ia explosion that occurred in the Pinwheel Galaxy (M101) in 2011, some 23 million light-years away. By type Ia supernova standards, it was right next door, and it was detected just hours after the explosion — the earliest a type Ia had ever been discovered. This allowed astronomers to search for evidence of a pre-existing companion star as well as any material in the system that was falling onto the white dwarf that exploded.

They found neither. According to a 2013 review of the supernova published in Publications of the Astronomical Society of Australia, written by Laura Chomiuk at Michigan State University, SN 2011fe’s environment was low density. So, there wasn’t considerable material available to collect onto the white dwarf and tip it over the weight limit into a supernova.

Instead, Chomiuk writes, “the data imply that SN 2011fe may have been the merger of two white dwarfs.” This is the second popular theory explaining type Ia supernovae: a so-called double-degenerate (DD) system, where there are two white dwarfs, presumably remnants from a stellar binary. Over many thousands of years — long enough for any gas and dust for the original system to disperse — the two are drawn together by gravity and eventually merge, creating the type Ia supernova.

The exact details are still debated — for one, it’s not clear whether a full merger into a single object is needed, or whether a contact touch between the two white dwarfs is enough. Some researchers even think such a system might not need to reach the Chandrasekhar mass — even if the two white dwarfs add up to less than 1.4 solar masses, the force of the collision could still trigger a runaway nuclear reaction and cause them to explode. Either way, it’s different enough from the SD scenario that a DD explosion would have a different signature, provided astronomers could see sufficient detail.

And this is where a twist of astronomical history comes in: While no one alive has seen a type Ia supernova with the level of detail required to crack their code, who’s to say astronomers haven’t in the past? What if we can use clues from the historic astronomical record to solve a modern astrophysical mystery?

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RCW 86 was left by a type Ia supernova. High-energy X-ray data from NASA’s Chandra X-ray Observatory and the ESA’s XMM-Newton Observatory are colored blue and green, and show how interstellar gas has been heated to extreme temperatures as the ever-expanding shock wave from the explosion passes through. Yellow and red colors are lower-energy infrared light gathered by NASA’s Spitzer Space Telescope and Wide-Field Infrared Survey Explorer, showing warm dust.
X-ray: NASA/CXC/SAO & ESA; Infrared: NASA/JPL-Caltech/B. Williams (NCSU)

Turning back the clock

Humans have been studying supernovae for thousands of years, though of course it was only recently that we understood what they are. If it’s close enough to Earth and with minimal dust along the line of sight, a supernova can be visible all over the world as a bright new naked-eye star for several months. And you can bet that people noticed — some with fear, some with wonder, some with confusion — which often led early astronomers to write down what they saw. Ancient Chinese astronomers were particularly careful record-keepers, detailing many bright “guest stars” over the centuries, along with their locations. The earliest such supernova record dates to A.D. 185 and was visible for eight months; in modern times, astronomers found the remnant from the explosion, RCW 86, and determined it was created by a type Ia supernova.

The most recent type Ia supernova seen with the naked eye (and the last supernova observed within our Milky Way) was first spotted in October 1604 and named Kepler’s Supernova, after astronomer Johannes Kepler. Kepler was not the first one to discover the supernova, but he took meticulous records of its position and its light curve for over a year and compiled his measurements with those of other astronomers for a book, De Stella Nova. The work is so meticulous that not only have modern astronomers identified the location of Kepler’s supernova remnant centuries later (some 20,000 light-years from Earth), they have even reconstructed the light curve to confirm it’s consistent with a type Ia supernova. Such historical records are so vital because they have guided modern astronomers to the remnants and allowed them to verify their ages — and such still-fresh remains are our best chance of distinguishing between the SD and DD scenarios.
Four hundred years may sound like a long time, but that’s a blink of an eye, cosmically speaking. “This is still the time where we’re probing what the actual explosion itself made,” explains Holland-Ashford, who is studying the remnant using data from the Japanese Suzaku X-ray telescope. The X-rays we see are still from the material ejected by the explosion itself, known as ejecta — some of which is speeding outward at a whopping 23 million mph (37 million km/h), even centuries later. Holland-Ashford is studying the elemental composition of this ejecta. Different types of explosions “would have different elements,” he says. So, by conducting the most detailed study of these elements to date, Holland-Ashford aims to find what event led to the “stella nova” that Kepler saw in the sky more than four centuries ago.

Supernova remnants are a promising way to unlock the clues of their progenitors, but they’re not the only potential clue hiding in our galaxy. Shen has proposed a DD scenario where both stars don’t get shredded apart: Instead, back-to-back explosions first end one white dwarf as a type Ia supernova and then fling outward the second white dwarf at a fantastic speed. The surviving white dwarf would travel at thousands of miles a second; such “hypervelocity white dwarfs” would theoretically be all over the galaxy. According to Shen’s idea, if the majority of type Ia supernovae are produced this way, there should be about 30 such hypervelocity white dwarfs within 3,000 light-years of Earth. But do such stars exist?

“We didn’t really know if they’d survive,” recalls Shen, but he and his team have used data from the European Space Agency (ESA) observatory Gaia to find proof that some do. Gaia has obtained precise positional data on approximately 1 billion astronomical objects, and Shen and his team led a search for local hypervelocity white dwarfs. After follow-up observations, they found three hypervelocity white dwarfs that fit the bill, each speeding along at a whopping 2.2 million to 6.7 million mph (3.5 million to 10.7 million km/h). What’s more, the team traced the path each white dwarf has traveled in the past. Two of the candidates show no sign that they originated in a nearby supernova remnant, which is perhaps not surprising, as the remnants could be faint or have dissipated over time. But one traced back to the location of a large, faint supernova remnant called G70.0–21.5, estimated to be from a supernova explosion approximately 90,000 years ago.
It’s not quite a smoking gun — for one thing, Shen’s study fell a bit short on finding the right number of hypervelocity white dwarfs. But there are many reasons Gaia might not have spotted them, Shen says. The white dwarfs the team did see were bright, but because these remnants cool over time, they also fade. Some may have dimmed below Gaia’s ability to see them, Shen says, though future surveys may pick them up.

Going to gravitational waves

The true origin of type Ia supernovae is unlikely to hide forever. One of the ESA’s primary future research missions is a gravitational-wave detector called the Laser Interferometer Space Antenna (LISA), a space-based observatory that will look for ripples in space-time itself. Gravitational-wave studies are still in their infancy — the first detection by the Laser Interferometer Gravitational-wave Observatory (LIGO) happened in 2016, and LIGO is not sensitive enough to study white dwarf binary pairs.

However, when it launches in 2037, LISA will be able to detect binary white dwarf pairs in our galaxy with very short periods and glean details such as how long it will take for them to merge and the rate of such events. Perhaps, if we are very lucky, LISA might detect a signal just before a type Ia supernova lights up the sky as a new guest star. Using LISA, astronomers will finally know whether such mergers explain all type Ia explosions or if more than one scenario is at play — and perhaps uncover a bit more about fundamental physics along the way. What’s clear is that in a universe filled with cosmic explosions as exotic as type Ia supernovae, there is still much to uncover.