Among the most contentious unsolved mysteries in astronomy is the question of how, exactly, a white dwarf star explodes. Now, as described at the American Astronomical Society’s winter meeting, a team of scientists has come up with an idea that just might solve part of the problem.
Nearly two decades ago, scientists used these exploding stars to measure cosmic distances and came up with a surprising result: The rate at which the universe is flying apart is increasing. Calling that discovery profound is no overstatement – it transformed our understanding of the cosmos and pointed toward the existence of an enigmatic force called dark energy.
But scientists still don’t thoroughly understand how to blow up a white dwarf. It’s kind of like understanding the point of a story without being fluent in the language. The plot is discernible, but important subtleties are lost. Learning how those subtleties shape the final story is crucial if scientists want to understand how dark energy behaves over the lifetime of the universe.
“Say, for example, an exploding white dwarf happens differently in the early universe than it does now. Or it happens differently in different types of galaxies,” says Rosanne Di Stefano, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. “If we don’t understand the differences, then we can’t correct for them and make a more precise ruler.”
The mile-marking explosions are called type 1a supernovas. They’re the result of a runaway thermonuclear reaction that rips through a white dwarf star and blows its guts into space, transforming the dwarf from a boring point of light into a beacon that shines brighter than entire galaxies. Because those beacons blaze with a predictable brightness that diminishes with distance, scientists can figure out how far away they are.
But it’s the part before the explosion that has everyone so perplexed. A lonely white dwarf won’t spontaneously combust. However, when a dwarf lives with a companion star, its enormous gravity means that it sometimes steals material from its companion. Like an out-of-control shoplifter, the dwarf star will continue stealing until its mass exceeds a threshold beyond which physics can’t keep the star in one piece. Then, boom.
What scientists have been arguing about is the identity of the dwarf’s stellar companion. Opinions have shifted dramatically over the last few decades, with some favoring a large, gassy companion star like a red giant, and others favoring something smaller and denser, perhaps another white dwarf. In the last five years, observational evidence has made it clear that both of those scenarios can happen.
The trouble is, the two recipes don’t account for the frequency of type 1a supernovas. “For some reason, we’re underpredicting them,” Di Stefano says.
So, she and her colleagues began to look for other ways to cook a type 1a.
What if, Di Stefano asks, some white dwarfs don’t need starry companions? Could a passing rocky body – like a large asteroid or a planet – smash into and detonate a dwarf?
The answer, she says, is potentially yes. There’s already a pile of observational evidence suggesting that collisions between white dwarfs and rocky bodies do happen. While it might sound strange, those observations aren’t necessarily unexpected.
White dwarfs are the corpses of collapsed stars that were once very much like the sun. Like the sun, many of them probably have asteroids and other rocky debris lying around that never quite made it to planethood. In our solar system, we see this debris in the asteroid and Kuiper belts, and suspect it’s also in the Oort Cloud (though it hasn’t been directly observed). Many of these bits and pieces are far enough out that they will survive the collapse of their stars unscathed, unlike any planets that happen to be close to their stars.
So, Di Stefano reasons, more than a few white dwarfs are probably flying through the galaxy surrounded by clouds of rocky debris. She calls these clouds “balls of planetoidal gas,” and notes that they can extend up to 100,000 times farther from a star than Earth is from the sun. Every now and then, and especially in crowded places like the galactic bulge, a dwarf will come close enough to another star for its gravity to perturb the rocky bits and pieces.
“These balls, every once in a while, they’ll graze each other. They’ll overlap with each other, and that leads to consequences,” Di Stefano explains. “Some of the planetoids, particularly the asteroids and comets, are thrown out into interstellar space, and some of them are placed on orbits where they’re going to end up coming close to the white dwarf.”
Simulations suggest that maybe 1 percent of those disruptions will lead to collisions with one of the stars.
If the conditions are just right – the collisions energetic enough, the rocky hammer big enough, the dwarf’s history of similar encounters optimal – it’s possible the impact could explode the star. Calculations suggest you’d need something at least as big as Earth to trigger an explosion, but this doesn’t have to happen particularly often to help solve the type 1a supernova frequency problem.
“Even if there’s an explosion one of out every 10 million times, it would actually be a significant contribution to the rate of type 1a supernovas,” Di Stefano says. “It’s worth exploring. And even if they don’t make the white dwarf explode, they’ll do something interesting that we should be able to see.”
Foley likes the idea of exploding a dwarf with a rocky body, and calls it a creative way to address an abiding problem. “The first part seems to be true — it should happen,” he says. “And then the second part just needs to be nailed down. It’s not crazy.”
A strange and sneaky type of supernova has exploded in a galaxy not so far away.
Called a type Iax, the supernova belongs to a class of explosions astronomers affectionately call “weirdos,” because they look a lot like standard supernovae but come with a twist. It likely exploded earlier this month, and was spotted on Oct.29 by amateur astronomer Koichi Itagaki.
“We’re interested in the weirdos in part because they’re fun, and in part because they’re extreme cases of physics going on that are easily noticeable,” UC-Berkeley astronomer Alexei Filippenko said, when we spoke about these explosions a bit ago.
The new supernova doesn’t have an official name yet. It’s located about 55 million light-years from Earth in a beautiful spiral galaxy known as M61, and could be the nearest type Iax supernova spotted so far (though two explosions in 2010 are close contenders).
Type Iax explosions are a recent discovery, and often masquerade as the more common — and much more well-known — cosmic firecrackers known as a type Ia supernovae. It’s only by looking closely at the light produced by the explosions that astronomers can distinguish between the two.
“The spectra of supernovae Ia and supernovae Iax are very similar. In fact, people often mistakenly classify supernovae Iax as supernovae Ia, and we have identified really old supernovae — all the way back to 1991 — as Iax by looking carefully,” says astronomer Ryan Foley of the University of Illinois at Urbana-Champaign.
Type Iax supernovae are kind of like mini-Ias. A normal type Ia occurs when a white dwarf steals a bit of material from a companion star, eventually growing massive enough to blow itself to bits. Because of their predictable brightness, type Ia supernovae are used to measure cosmic distances, and played a crucial role in the late 1990’s discovery that the expansion of the universe is speeding up.
Type Iax explosions, however, are a less energetic form of Ia that might not completely obliterate the white dwarf star. Their explosions are weaker, don’t shine as brightly, and produce ever so slightly different ratios of chemical elements than a normal Ia. Foley and his colleagues suspect that a Iax ignites when a white dwarf steals a bit of material from a companion star that’s rich in helium; earlier this year, he and others successfully identified the stars involved in a 2012 Iax explosion as being a white dwarf star and a luminous blue helium star.
Now, astronomers are wondering whether the white dwarfs might survive these explosions. Foley says the close proximity of the Iax in M61 should help answer that question. “We’ll be able to monitor this supernova for at least a couple of years,” he says. “This will help us determine if the star was disrupted.”
Scientists are still struggling to understand the mechanics of type Ia supernovae and their weirdo cousins. In particular, the issue of which stars are involved has been the source of much recent controversy. The good news with this explosion is not only that it’s nearby, but that there are pre-supernova Hubble Space Telescope images of the galaxy. That means it should be possible to get a good look at the pair of stars that contributed to the explosion, before they got all roughed up.
By studying the weird supernovas, astronomers can gain a better understanding of what the normal type Ias are — and what they are not. “We have no idea what a normal Ia is,” says astrophysicist Brad Tucker of both UC-Berkeley and Australian National University. “It will be quite ironic if the first two supernova Ia progenitors we detect are of this small weirdo class, while with the other thousands of normal Ias, we detected nothing!”
When I joined Phenomena, Carl Zimmer asked: What obsesses you? Among my obsessions, I answered, are type 1a supernovae. Here we go.
How can an astronomical object of such crucial cosmological importance remain so fundamentally mysterious?
When a runaway thermonuclear explosion rips through a white dwarf star and blows the star to bits, it’s called a type 1a supernova. These explosions are incredibly violent and incredibly bright, sometimes outshining entire galaxies. Thought to occur about once every two centuries in a galaxy like the Milky Way, these stellar cataclysms are relatively frequent events.
The star doing the exploding is a white dwarf with a fairly standard mass, so the supernova’s brightness is predictable. And because luminosity decreases with distance, scientists can use the difference between an explosion’s observed and predicted brightness to determine how far away the blazing starstuff is. That characteristic has led to type 1a supernovae being called “cosmic mile markers” and “standard candles.”
In the late 1990s, distance measurements based on type 1a supernovae revealed that the expanding universe is accelerating. In other words, it’s flying apart more quickly now than it was billions of years ago. Scientists still don’t know exactly what’s going on, but they attribute the phenomenon to an enigmatic thing called dark energy. The discovery represented a fundamental shift in cosmology and earned the Nobel Prize in physics in 2011.
But here’s the thing: Despite their crucial cosmological importance, type 1a supernovae are still very much a mystery. As astronomers study more and more of them, it’s becoming increasingly clear just how non-standard these explosions actually are – and how little we really know about them.
“They’re standardizable candles, not standard candles,” astrophysicist Brad Tucker told me a bit ago, while I was working on a feature describing type 1a supernovae for the Proceedings of the National Academy of Sciences. Tucker splits his time between UC Berkeley and the Australian National University.
“These are very powerful tools in cosmology,” he said. “But we really don’t know what’s going on with them.”
It’s true. The uncertainties swirling around these fascinating explosions are kind of astonishing. Here are a few.
1. Until now, there was no proof that white dwarfs were doing the exploding.
For starters, we didn’t have solid observational evidence pointing to white dwarfs as the culprits behind type 1a supernovae until earlier this year, as reported yesterday in the journal Nature. Decades of solid theoretical work (and circumstantial evidence) suggested as much, but the observations weren’t there to back it up.
But in January, a star exploded in the Cigar Galaxy. Essentially next door at only 11.5 million light-years away, it was the closest type 1a supernova to Earth in four centuries. Chemical signatures in the billowing debris cloud revealed that supernova 2014J, as it’s called, is a type 1a supernova. Because the explosion was so nearby, astronomers were able to detect gamma-rays coming from the debris, a type of radiation that hasn’t been observable in other type 1a supernovae.
Simulations of type 1a supernovae predicted that exploding white dwarfs would produce gamma-rays, but the particles don’t normally make it to Earth.
Yet earlier this year, a team of scientists observed gamma-rays coming from 2014J with the European Space Agency’s INTEGRAL satellite.
Using pathways that explain how gamma-rays are formed from the synthesis of iron, cobalt, and nickel, the team worked backward from the detection and determined what kind of star had exploded. It was a white dwarf star, scientists said, with about 1.4 solar masses of material stuffed into it.
This is the most solid observation to date that implicates detonating white dwarfs in the production of type 1a supernovae, and one that astronomers have been eagerly awaiting, for a long time.
“The importance of this discovery is not because something new/unknown was discovered, but we had an observation of a long-standing theory that had no real evidence,” Tucker wrote to me a few days ago. “Knowing that our fundamental physics is correct is an important thing, especially given how all our other ideas (progenitors systems, mass, donor stars) don’t seem to be working!”
(It’s worth mentioning that another recent nearby supernova – 2011fe, which exploded in the Pinwheel Galaxy in 2011 – provided fairly good evidence for a white dwarf being the progenitor star. Those constraints came from the PIRATE telescope in Mallorca, which serendipitously observed the supernova several hours after it exploded. Because the billowing debris cloud wasn’t yet visible, astronomers concluded the explosion must have been the work of a white dwarf. But there was still wiggle room. “It’s the most compelling, by a reasonable margin,” astronomer Alexei Filippenko of UC Berkeley said of the evidence late last year, when were discussing this point for the PNAS feature. That was before 2014J exploded.)
2. The Chandrasekhar Limit sometimes isn’t.
Secondly, there’s a well-known fact that commonly appears when people write about type 1a supernovae: That white dwarfs explode when they get to be about 1.4 times as massive as the sun – a mass known as the Chandrasekhar Limit. All of this is true, but it’s not the whole story.
White dwarfs are incredibly dense, dead stars, formed from the collapse of stars that were once very much like the sun (and yes, our sun will become a white dwarf). They’re about the size of Earth, but with a sun’s mass of material squeezed in. Most of the time, white dwarfs can happily exist in this state for billions of years.
That’s because the white dwarf’s intense, crushing gravity is counteracted by a quantum mechanical thing called electron degeneracy pressure, which basically prohibits electrons from being shoved any closer together. In other words, degeneracy pressure prevents the star from collapsing further. (Degeneracy pressure is the reason why white dwarfs are called degenerate stars.)
These two competing forces can keep a white dwarf stable forever, as long as it doesn’t get too massive. But if the star gains enough material and exceeds about 1.4 solar masses, gravity normally wins and degeneracy pressure fails and all hell breaks loose.
That’s the Chandrasekhar limit. And it’s important, but it’s not the only thing that lights up a supernova.
Something else happens at around 1.4 solar masses, and it turns out that this is a crucial part of exploding the star: At this point, the star is massive enough to begin fusing carbon – and that is what ignites the runaway thermonuclear reaction.
“Fusion happens in a flash,” astronomer Robert Kirshner of the Harvard Smithsonian Center for Astrophysics writes this week in Nature. “A thermonuclear flame rips through the white dwarf, fusing carbon into heavier elements with a sudden release of energy that tears the star apart.”
Many white dwarfs are made from a ton of carbon and a lot of oxygen. So when carbon ignites, it just keeps going and the star is blown to bits. That this happens around the Chandrasekhar limit is kind of a coincidence, says astronomer Ryan Foley of the University of Illinois, Urbana-Champaign. “The Chandrasekhar limit is a red herring. It’s a physical thing that’s very, very important, but for type 1a supernovae, it’s not the most important thing,” he told me, when we were talking about supernova 1as for the PNAS feature. “You have to have the carbon ignition, somewhere near the center of the star.”
So there are mechanisms that allow white dwarfs to bypass the Chandrasekhar limit (astronomers theorize that something like a very rapid spin rate might help the star avoid catastrophic collapse) and produce explosions that are anything but standard.
Conversely, there are type 1a supernovae that are ridiculously, anomalously dim. These mini-supernovae, discovered in 2013, are called type 1ax explosions. Astronomers have spotted about 30 of them.
Two weeks ago, Foley and his colleagues reported in Nature that they’d found the progenitor system for a type 1ax called SN 2012Z: A white dwarf, paired with a bright blue helium star companion. The dwarf snagged some material from its gassy blue friend, the team says, but didn’t gain enough mass to completely explode. So instead of a bang, the star let out a whimper.
Scientists sorted this out by staring at old Hubble Space Telescope images of the supernova’s home and determining which stars were involved in the eventual conflagration. It’s the first time anyone has been able to so precisely see a progenitor system before it blew up.
Astronomers affectionately refer to the collection of anomalous supernovae as “weirdos.” By studying the weirdos, they hope to better understand the normal type 1as, the explosions that are useful for things like cosmology.
“By identifying and studying the extreme cases, we can hope to learn not only about these interesting weirdos, but the weirdos might end up teaching us something about the more normal ones,” Filippenko said.
3. We don’t know who’s helping to kill the dwarf.
But is there a type of normal supernova 1a? Well…yes. Kind of.
For a long time, astronomers figured that since 1as were all so similar, there must be just one way to cook them up. A key step in the process of making a 1a is that the white dwarf has to somehow grab enough material to ignite carbon fusion and explode.
This means that the white dwarf can’t live on its own. It needs to live in close proximity to a stellar friend it can steal from. As the two stars orbit one another, the dwarf’s intense gravity will siphon material from its companion, and it will eventually gain enough mass to detonate.
For decades, astronomers thought that companion star was something like a red giant, something big and gassy that’s easy to steal from. Many textbooks and diagrams still depict this kind of supernova precursor, called a single-degenerate system (because there’s only one degenerate star, the white dwarf).
And there is some recent observational evidence implicating this lethal pairing in type 1a supernovae.
But as with everything we’re learning about type 1a supernovae, those single-degenerate systems are not the only game in town. Now, many astronomers think those systems produce a minority of type 1a supernovae and that most of the explosions we see come from a different deadly combination.
“Less than 1 percent of all 1a supernovae could be from a companion red giant star,” Tucker said. “It’s a dramatic shift in what we’re thinking. It’s gone from the most popular scenario to one no one wants to touch.”
Recently, evidence has been amassing that suggests type 1as result from a pair of two white dwarf stars (a double-degenerate system). So, instead of being locked in a deadly dance with a red giant, the white dwarf is dancing with another white dwarf.
This kind of progenitor system was discounted years ago because explosion stimulations couldn’t quite make it work – scientists had trouble bringing the white dwarfs close together quickly enough, or couldn’t get the variety of elements produced by the explosion to come out right.
But in the last five or 10 years, that’s changed. And a variety of new observations point toward double degenerate systems as producing type 1a supernovae.
Now, it seems, instead of arguing about which of the two progenitors channels is The One, astronomers are debating how many different type 1a progenitor systems exist, and how common each of them is. This bothers some scientists, who are having a tough time reconciling the observed similarities in normal type 1a explosions with the fact that so many different ingredients could be involved.
“Somehow, these completely different stellar systems that have evolved in completely different ways and are completely different at the end of their lives – when the star explodes – make things that are almost identical,” Foley said. “That troubles me a bit.”
Ultimately, it will be really important to sort this out because the more astronomers know about type 1a supernovae, the more accurately they’ll be able to measure cosmic distances. Even the “normal” type 1as – the ones used for cosmology – explode with varying brightness. Their explosions vary depending on what kind of galaxy they’re in, where in the galaxy they are, and, potentially, which progenitor system is involved.
So that whole predictable brightness thing? It’s not quite that simple.
4. Uh-oh. Does this mean dark energy is going away?
Nope. Dark energy and the accelerating universe are here to stay. But this is an era of precision cosmology, and tiny inaccuracies can have a big effect on what scientists understand about the repulsive force that is dark energy.
This is especially true for the supernovae scientists are really hoping to detect – the really, really ancient ones that can help nail down the behavior of dark energy in the very early universe.
“The question is, are those supernovae really the same as the ones nearby? This issue of progenitors really affects that,” said Saurabh Jha, an astrophysicist at Rutgers University.
Ten billion years ago, the universe was filled with a different population of stars than we see today. The matter making up those stars had different chemical compositions. And if those ancient supernova 1as are behaving differently than the more recent ones, scientists need to know about it. Otherwise, imprecise distance measurements will yield an inaccurate understanding of what was happening during these earlier time periods.
If you had X-ray vision — real X-ray vision, meaning you could actually see this type of high-energy radiation — your view of the universe would be vastly different than it is now. Especially if you could sail above the Earth’s atmosphere and stare at the sky. There, all kinds of objects that are normally hidden would suddenly appear.
Alas, humans have not evolved the ability to detect X-rays (or fly); luckily, we’ve built instruments that can. One of the greatest of these is the orbiting Chandra X-Ray Observatory, a space telescope launched 15 years ago today. Carried into Earth orbit by the space shuttle Columbia, Chandra has peered deep into some of the universe’s most mysterious realms — like exploding stars and the hearts of galaxies, where supermassive black holes churn away.
X-rays are produced in some of the most hot and energetic environments the cosmos can cook up. They can illuminate a pulsar’s jet (see this image of the Crab Pulsar), betray the gassy guts of supernova remnants, and reveal the turbulence of newborn star factories. X-rays even help astronomers find black holes, which by definition are invisible. Nothing, not even light, escapes a black hole’s massive gravity. But the area around a black hole is a roiling, chaotic environment that emits X-rays like crazy and points astronomers to the spot.
I’ve compiled some of my favorite Chandra images in the gallery above, but there are many, many more on Chandra’s website. It’s great fun to click through the images and flip back and forth between the X-ray, optical, and infrared data that are often layered on various composites. You’ll be able to see how observations in each wavelength add up to the total — and even uncover some features you probably never would have known were there.
About 900 light-years away, an ancient white dwarf star has cooled into a crystallized chunk of carbon — a diamond. But this isn’t just any old diamond hiding in space: It’s the size of Earth, and it’s 11 billion years old.
The diamond-star, described in a study published in The Astrophysical Journal, is among the coldest white dwarfs astronomers have found. In fact, it’s so cool and dim that it can’t even be seen — its feeble light isn’t nearly powerful enough to pierce the darkness of the cosmos, even from relatively nearby.
Instead, teams inferred the presence of the crystallized dwarf based on the way its gravity perturbs the normally steady radio pulses coming from a spinning companion star.
If you were to look in the sky in the direction of the constellation Aquarius, you’d be looking in roughly the right direction to see the system, which is actually a pair of dead stars: The spinning neutron star is the extremely dense remnant of a formerly huge star that ended its life in a supernova. A white dwarf is all that remains of a formerly Sun-like star, contracted into a clump the size of Earth. Left on their own, dwarfs will slowly cool and fade to black over billions of years (but sometimes, with the help of a stellar companion, they can detonate and create dazzling supernovas the outshine entire galaxies).
Anyway, astronomers first spotted the pulsar in 2007. Then-graduate student Jason Boyles began studying the spinner, using the Green Bank Telescope at the National Radio Astronomy Observatory in West Virginia. Normally, pulsars spin so steadily that they rival the best atomic clocks. Boyles measured this pulsar’s spin rate at a relatively pokey 30 times per second — some can spin around themselves in only a few thousandths of a second.
But something was different. The pulses arriving at Earth were periodically delayed, as if some unseen companion were causing the pulsar’s radio emission to take a somewhat circuitous route to Earth. This can happen when an orbiting, massive companion’s gravity messes with the fabric of space, causing things like light and radio waves to travel along twisted pathways.
Boyles and his colleagues suspected a dense, hidden object was paired up with the pulsar; more observations suggested the unseen body orbited the pulsar every 2.4 days, and that it was roughly as massive as the Sun. The team guessed they were dealing with either another neutron star (which would be a very rare pairing) or a dense white dwarf star.
To identify the missing object, astronomers needed to know more accurately how far from Earth the pulsar and its friend were. So, Adam Deller at the Netherlands Institute for Radio Astronomy led the charge to determine that distance, using the Very Long Baseline Array telescopes.
With that information in hand, and with the delay time measurements, astronomers could now begin to un-mask the pulsar’s friend. They calculated that the companion must be about 1.05 times as massive as the Sun (within the range of both a white dwarf and a neutron star), and that the pulsar was slightly more massive, at 1.2 Suns. But scientists also determined that the pulsar and its friend were in a roughly circular, rather than elliptical (or eccentric) orbit. That suggested the system hadn’t been walloped by something like a second, neutron star-forming supernova.
“If there were two neutron stars that means two supernova explosions. And a supernova explosion should make the orbit pretty eccentric,” says study coauthor David Kaplan of the University of Wisconsin, Madison. “We see that the orbit is very circular.”
That suggested the team was looking for a white dwarf. At that measured distance, astronomers reasoned, such a star should be visible from Earth. So, the team tried to get a visual on the object, using telescopes in Chile and Hawaii. They searched the region in multiple wavelengths, in the infrared and visible.
The trouble was, no matter how hard they tried, scientists just couldn’t coax the dwarf to reveal itself.
The only way that’s possible, the astronomers write, is if the dwarf is cooler than 3,000 Kelvin — which would make it among the chilliest white dwarfs ever discovered. And the only way the star could have cooled to that temperature and NOT be older than the Milky Way galaxy is if it were already crystallized into diamond.
Kaplan says that such diamond stars are probably sprinkled throughout the galaxy — they’re just too cold and dim for us to see them. But, up above the world so high, a sky full of ancient, glittering diamonds is an astonishingly beautiful thing to imagine.
On Earth, echoes are produced when sound waves bounce around like pinballs. In space, echoes are produced when light does the bouncing.
Just as sounds can echo, so, too, can cosmic light. But instead of ricocheting off damp cavern walls, light traveling through the universe bounces off soft, dusty clouds.
Sometimes, this happens after an explosive event such a supernova. On Earth, most of the light we’d see from one of these exploding stars would have come directly here. But supernovas explode in three dimensions, sending light in all directions. Not all of that light is aimed toward Earth. If the geometry is right, some of the light zooming away from Earth might run into a cloud and end up being redirected toward us – kind of like a billiard ball bouncing off the cushion and rolling cleanly into the pocket. Because it took a more circuitous route through the cosmos, this rerouted light arrives at Earth after the light from the original explosion. Often, it’s delayed by centuries, arriving long after the supernova’s embers have been extinguished.
Called light echoes, these ancient reverberations bear the original signatures written into light by the explosion.
In other words, light echoes act as astronomical time machines or portals to the past. In some echoes, astronomers have deciphered details that reveal which kinds of stars exploded in the 16th and 17th centuries – information that would otherwise be lost to the cosmos, since telescopes of the time didn’t have the spectrometers needed to read the chemical inscriptions in the light.
Other echoes are being used to solve the mystery of Eta Carinae, one of the weirdest and most tempestuous binary star systems in the galaxy.
Located 7,500 light-years away, Eta Carinae lives in the Carina nebula, in the southern sky.
One of its two stars is, as described in last week’s episode of Cosmos: A Spacetime Odyssey, “pushing the upper limit of what a star can be.” The star is a bulging, nuclear behemoth more than 100 times as massive than the sun and more than a million times as bright. Circling that crazy huge star is a giant companion, but it wasn’t until 1996 that a Brazilian astronomer named Augusto Damineli realized it was there.
For much of recorded astronomical history, Eta Carinae looked like just another star.
But in the early 1800’s, it began to erupt. The outburst lasted for two decades, from 1837 to 1858. For a turn, Eta Carinae became the second-brightest star in the sky; astronomer John Herschel (William’s son), observed the first brightening in late 1837 from South Africa, and kept track of the flare-up.
“What origin can we ascribe to these sudden flashes and relapses? What conclusions are we to draw as to the comfort or habitability of a system depending for its supply of light and heat on so uncertain a source?” Herschel would write in 1847.
Pulses of light produced by the stellar eruptions wafted off into space, passing through the burgeoning clouds of dust and gas that were gathering around the spasming star.
Light curves recorded by Herschel and others at the time showed peaks in 1837, 1843, and 1845. After the outbursts subsided, the star had changed. It lost more than 10 solar masses of material, and was shrouded by a double-lobed, organic-looking structure called the Homunculus Nebula – the cosmic consequence of the giant star’s spectacular tantrums, and the chaotic shroud that hid its companion until recently.
For years, astronomers considered Eta Carinae the prototype for a class of explosions known as “supernova impostors.” Rather than destroying the star, as any self-respecting supernova would, these impostor novae shine as brightly as their counterparts yet leave their stars intact.
But light echoes from Eta Carinae’s 19th-century outbursts are challenging its status as the impostor nova poster child. Now, these echoes are bouncing off dust clouds near the star system. Since 2011, a team of astronomers has been monitoring and measuring these light echoes, the first of which was accidentally captured in an image from 2003.
“Eta Car’s great eruption is so long,” says astronomer Armin Rest of the Space Telescope Science Institute in Baltimore, MD. “It seems now to be a much more complicated thing, that we have found in the light echoes.”
The echoes reveal that the temperature of Eta Carinae’s Great Eruption was much cooler than expected – about 5,000 Kelvin, or 2,000 degrees cooler than anticipated. That relatively frigid temperature suggests the mechanism of the eruption is nothing like what scientists had surmised; instead of being the result of an energetic stellar wind, the explosion could have been triggered by a blast from the star’s surface or by an interaction with its binary companion.
Most recently, in a paper uploaded to the arXiv on April 15, Rest and his colleagues report the presence of nitrogen-rich molecules in the light echoes – cyanide, in fact, which contains a carbon and a nitrogen atom. “You need cool temperatures for that,” Rest says. “If you have hotter temperatures, the molecules break apart.”
Over time, Rest says, the characteristics of the light echoes are changing, indicating the expulsion of material and dense amounts of dust. Essentially, “what we are seeing is the formation of the Homunculus Nebula,” he says.
But the team is no longer sure which of the Great Eruption peaks they’re seeing. Initially, scientists suspected they were watching the star’s 1843 outburst. The shape of the light curve from the echoes fit the shape of the historical light curve, which rose over a few hundred days, plateaued, and then dropped. The only trouble is, the team isn’t seeing the next peak in the eruption, which happened in 1845. “It has stayed down for more than two years,” Rest says, of the light echo curve. He thinks they could be seeing echoes from the earlier, 1837 outburst that Herschel noticed, or perhaps the reverberations from a previous explosion that went unnoticed.
“It’s truly a mystery,” Rest says. “I would’ve bet $1000 bucks that we were seeing the ’43 peak, but I was wrong.”
So, astronomers aren’t yet sure what’s going on with this star and its giant eruptions. But Eta Carinae’s massive size and repeated outbursts portend the death of the system, which promises to be a spectacular cataclysm – a hypernova, or superdupernova – well worth the price of a ticket to the southern hemisphere to check it out when it does blow.
Eta Carinae isn’t the only complex stellar object whose secrets hide in these aging glimmers. Light from supernovas that shined in Earth’s skies centuries ago is also bouncing off dust clouds. These fading echoes are like time capsules that preserve information about the temperature and chemical spectrum of the original explosion; by reading the clues inscribed into the echoes, astronomers have been able to identify exactly what kind of stellar explosion produced long burnt-out remnants.
Rest and his colleagues used spectral lines to classify one of the remnants, in the Large Magellanic Cloud, as not only the result of a type Ia supernova (produced by an exploding white dwarf star), but a subtype of that class of explosion. Known as an overluminous type Ia, the explosion that produced remnant SNR 0509-67.5 likely resulted from the merger of two white dwarfs.
Light echoes can be fleeting and illusive, as demonstrated by the series of images below from another famous nova, known as V838 Monocerotis (watch a video here). What looks like an expanding cloud is actually the light from the explosion traveling outward and being reflected by the already existing cloud’s shells and cavities.
Twenty-four years and two days ago, on a Tuesday morning, the space shuttle Discovery hitched a ride to low Earth orbit from Cape Canaveral, Florida. Aboard the shuttle? NASA’s newest eye in the sky, the Hubble Space Telescope, an instrument capable of peering deep into the cosmos and capturing the universe’s inhabitants in exquisite detail. It had taken decades of design and planning to get the telescope ready for work. The next day, on April 25, astronauts delivered the telescope to space.
Then, scientists eagerly waited for Hubble to start revealing cosmic secrets.
But a flaw in the telescope’s primary mirror meant the images weren’t sharp. Observing incredibly faint objects, such as very distant galaxies, wasn’t possible. It would be three years before the first of five servicing missions let astronauts correct the defect and upgrade Hubble’s vision to what it should have been.
Since then, though, the Hubble space telescope has continually delighted Earthlings with its breathtaking views of stars, galaxies, and our planetary neighbors. Its impact on science has been no less important. Among other discoveries, Hubble helped scientists determine that the universe is expanding at an accelerating rate. This discovery, which happened in the late 1990s, is something we still can’t fully explain.
Every day, tales of life and death in the universe are told through faraway supernovas, galactic collisions and clusters, and violent stellar nurseries. These stories are often accompanied by profoundly beautiful images. Some of these, like the million-second-long exposure that produced the Hubble Ultra-Deep Field, need to be viewed full-size for the appropriate amounts of cosmic oomph. Others, like the Pillars of Creation, have become extremely well-known — looking at these photos can be like seeing the smiling face of an old friend.
Here, in honor of Hubble’s 24th launchiversary, are 25 images that might be slightly less familiar…and I’ve added one to grow on, just for good measure.
Ancient astronomers chronicled the shifting heavens, diligently charting the movements of our star-studded canopy. The moon’s face morphed nightly, our planetary neighbors came and went, and occasionally a brilliant, icy vagrant would sweep by.
But the stars? Those stayed pretty much in the same place, relative to one another. So when new, starry points of light briefly appeared and then faded away, Earth’s sky gazers noticed.
Nearly 1,000 years ago, one of these new stars began shining brightly in the northern sky. It was July 4, 1054, and the people of Earth – from North America to China – turned their attention skyward. Glimmering near the star Zeta Tauri, the new star was much more than a distant, pale point of light: For almost a month, it even shone during the day. Chinese astronomers, who politely referred to the newcomer as a “guest star,” kept detailed records of the stellar visitor. Those records show that the star stuck around for more than two years before slowly fading from the sky like a guest saying good night for the evening.
Seven centuries years later, French astronomer Charles Messier was peering through his telescope in Paris, looking for comets. One night in 1758, Messier saw a strange, fuzzy object in the constellation Taurus. He briefly thought it might be the comet Edmund Halley had predicted would return that year. But the object wasn’t moving; it was fixed in the sky, near where Chinese stargazers had marked the appearance of their guest star almost exactly 700 years earlier. That fuzzy dot, which Messier came to realize was a gassy nebula, became known as M1 – it was the first entry in his new catalogue of astronomical objects.
By the mid-1800s, the nebula had another name: The Crab Nebula, a result of Irish astronomer William Parsons sketching the object and thinking it looked vaguely crab-like.
It wasn’t until the early 20th century that a series of observations finally revealed what the Chinese guest star was: In 1054, a massive star had exploded and died. At 6,500 light-years away, the supernova, as these stellar explosions are called, was so close by that its light pierced the heavens and arrived on Earth with no difficulty at all. The explosion produced a bright, expanding shell of gas — the nebula Messier, Parsons, and others had seen. When astronomers in the 1920s measured how fast the nebula was growing, they realized they were looking at an object that began ballooning outward nearly 900 years earlier.
By 1942, there no doubt the nebula was linked to the observations from 1054. But the story isn’t quite over yet.
For most of its lifetime in Earth’s skies, the Crab Nebula has only been observed in optical wavelengths – that small sliver of the electromagnetic spectrum that humans have evolved to perceive as colors. In addition to visible light, there are also such things as X-rays, gamma rays, infrared, ultraviolet and radio waves. They’re all part of the same spectrum, but vary in wavelength and energy. It’s only been in the last 100 years that astronomers have finished working out how to view the skies through all these different lenses.
“These are not just different ways of seeing the same thing,” says Neil DeGrasse Tyson in this week’s episode of Cosmos: A Spacetime Odyssey. “These other kinds of light reveal different objects and phenomena in the cosmos.”
And the Crab Nebula – it’s got a racing pulse that astronomers wouldn’t be able to take until the late 1960’s.
The big image above shows what the Crab looks like to an eye like the Hubble Space Telescope. Here, in visible wavelengths, it shines brightly blue in the center, surrounded on its fringes by tendrils of red. Because of the spectral signatures carried by visible light – those lines that Tyson describes in Cosmos – astronomers know what kinds of chemical elements lived in the Crab Nebula. That doesn’t mean there aren’t still surprises: Late last year, observations in the infrared revealed the presence of argon hydride, a molecule scientists didn’t expect to see.
As the Cosmos script says, we perceive infrared wavelengths as thermal radiation. We can’t sense them with our eyes, but we can feel their heat with our skin – unless of course, their source is far away. Infrared light, unlike optical wavelengths, is very good at traveling through cosmic dust and clouds – so viewing a skyscape in these wavelengths can reveal objects and structures that are too cold or obscured to see in other ways.
Now, how about looking at the Crab in X-rays? There, in the center, amidst all that blue is something curious: It looks like a jet, spewing from a disk encircling a central object. X-rays are produced by the most energetic events in the cosmos – things like black holes tearing things apart and neutron stars spinning like crazed street performers. To observe an object in X-rays is to know that something enormously energetic is happening.
Here, that feature in the X-ray image is the product of a neutron star in the Crab’s center – the remnant of a star that was much more massive than our sun. When that star collapsed and died, it flung its guts outward, creating the gassy part of the Crab Nebula. Its core, though, contracted into a dense, spinning neutron star – an object with 1.4 solar masses squeezed into a diameter of only 10 miles.
When neutron stars spin, they’re called pulsars. In 1967, the first pulsars were discovered by Jocelyn Bell Burnell, who’d been looking at the sky through a radio telescope. Radio waves, like infrared, travel really well through dust and clouds; among other things, they’re produced when high-energy electrons go spiraling around magnetic field lines, like those surrounding a neutron star. When you look at the Crab in radio, the pulsar is in the middle of the image is so bright that it’s just colored white.
Pulsars, it turns out, are among the brightest astronomical objects in the radio. And the Crab Pulsar? That thing is among the brightest of all, as seen from Earth. Nearly 1,000 years after it formed, the pulsar is still spinning wickedly fast, sending a beam of detectable radiation toward Earth 30 times a second.