The most powerful way a star can die is through a supernova explosion. Through nuclear physics and the pull of gravity, a star can be ripped apart, shining brighter than an entire galaxy for a brief moment of cosmic time. There are several kinds of supernovae, but one particular kind known as Type Ia is particularly interesting to astronomers. And we still aren’t entirely sure how they occur. 

Type Ia supernovae are useful to astronomers because they always explode with about the same intensity. By measuring the apparent brightness of distant Type Ia supernovae, we can calculate the size of our universe, and how rapidly it’s expanding. But what makes a Type Ia supernova so consistent? Other supernovae can vary in brightness, so why not these?

The most popular theory is that they are caused by particular kind of binary star, where a white dwarf orbits a red giant. White dwarf stars typically have a mass about equal to our Sun. That’s because if they get much larger they reach what is known as the Chandrasekhar limit, and collapse under their own weight. If a white dwarf happens to orbit closely to a red giant, outer layer of the red giant can be captured by the gravitational pull of the white dwarf. This increases the mass of the white dwarf. And if its mass reaches the limit, then boom!

Evidence of an early helium flash before a supernova is triggered. Credit: Institute of Astronomy, the University of Tokyo

At least that’s the idea. The challenge is to work out the details. Just how does the gravitational collapse of a white dwarf trigger the type of nuclear reactions necessary to cause the star to explode? But new research shines more light on that question.

Using data from the Subaru telescope and the Gemini-North telescope, a team of astronomers captured the earliest moments of a Type Ia supernova. They found that a small initial flash occurred before the much larger supernova explosion. The spectrum of the initial explosion seems to indicate that it was caused by the rapid fusion of helium on the star’s surface. This effect has also been seen in computer simulations.

In this model, a build up of helium on the surface of the white dwarf causes it to start fusing rapidly. The explosive detonation of helium creates shock waves that propagate to the center of the star, where they trigger a chain reaction of fusion in its core. In this way, the explosion is not caused directly by reaching the Chandrasekhar limit, but by a chain reaction of rapid nuclear fusion. This would explain why there is some small variation in the brightness of Type Ia supernovae. It could also help us better refine how these supernovae are used to measure galactic distances.

We’ll need to observe more of these supernovae to learn whether this is the primary trigger mechanism for Type Ia supernovae, or if there are other ways these stars can be detonated. But for now it seems clear that with a bit of helium we can detonate a star.

Paper: Ji-an Jiang, et al. A hybrid type Ia supernova with an early flash triggered by helium-shell detonation. Nature 550, 80–83 (2017)

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Supernovae are the bright, but short lived explosions of a dying star. At their brightest they can outshine an entire galaxy. The brightest ones, known as superluminous supernovae, can be more than 10 times brighter than type Ia supernovae used to measure the distances of far galaxies. But there’s a limit to how bright a supernova can be, so when we observed a supernova last year that seemed to exceed that limit, it raised an interesting question. Is our model of superluminous supernovae wrong, or is something else going on? 

After the supernovae ASASSN-15lh reached its peak brightness, a team of astronomers observed the source for the next 10 months. They found that the way in which the supernova dimmed over time (known as its light curve) didn’t agree well with the light curves of other supernovae. This would imply the event was not caused by the explosion of an old star. Add too this the fact that supernovae generally occur where there are plenty of bright blue stars (since they tend to die as supernovae) and ASASSN-15lh occurred in a galaxy that mostly consists of smaller, redder stars.

One thing the team did see was that the supernovae got much brighter in the ultraviolet a while after the initial event. This is likely due to a reheating of the stellar material. That isn’t expected to happen in a supernovae event, but it can happen when a star is ripped apart by a black hole. So the team compared the data to models of star-eating black holes. They found that the best math for the data is a rapidly rotating black hole that ripped apart a small star. This doesn’t guarantee that the event really was a black hole’s lunch, but it points to the idea that some supernovae might not be stellar explosions after all.

We will still need more observations of superluminous supernovae to confirm this model, but it’s certainly likely given the gravitational power of black holes.

Paper: M. Fraser, et al. The superluminous transient ASASSN-15lh as a tidal disruption event from a Kerr black hole. Nature Astronomy 1 (2016) DOI:10.1038/s41550-016-0002

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The first elements to appear in the Universe were hydrogen and helium, created soon after the big bang. Other elements on the periodic table are produced through nuclear interactions within stars. Lighter elements such as carbon, nitrogen and oxygen are formed through nuclear fusion in a star’s core, but heavier elements such as gold are formed through catastrophic events such as a supernova explosion or the collision of neutron stars. It’s known as r-process nucleosynthesis (due to the rapid neutron interactions) and is still a bit of a mystery. 

We can distinguish r-process elements not only by their presence in stars, gas and dust, but also by their relative abundances. The r-process abundances are distinctly different from other nucleosynthesis methods such as the s-process (slow neutron) that occurs in the late stage fusion of large stars. So we know that heavier elements can are produced through r-process events, but one of the big debates has been over which type of events create the most heavy elements.

There’s basically been two schools of thought. One is that core-collapse supernova are the main factor. These are fairly common on a cosmic scale, but the amount of heavy elements released in a particular supernova is relatively low. In this model a galaxy would be seeded with a low but steady flow of heavy elements. The other idea is that stellar collisions create most heavy elements. The collision of two neutron stars, for example, is fairly rare, but the amount of heavy elements released from such an explosion would be quite high. In this model heavy elements are seeded into a galaxy in bursts every now and then. The challenge is to determine which model is right.

Recently astronomers found evidence that the collision model seems to be the right one. They looked at the abundance of elements in a dwarf galaxy known as Reticulum II. They found that the 9 brightest stars in this galaxy have heavy element abundances 100 to 1,000 times greater than seen in other similar galaxies. This would imply that the abundance of r-process elements was unusually high during their formation, which is what you would expect if they are produced at high quantities in rare events. It seems clear, then that stellar collisions play a major role in the production of heavy elements.

Since gold is one of those heavy elements, you could say that the collision of neutron stars causes a galaxy to enter a gilded age.

Paper: Alexander P. Ji, et al. R-process enrichment from a single event in an ancient dwarf galaxy. Nature 531, 610–613 (2016) doi:10.1038/nature17425

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In the spring 0f 1006 an object appeared in the night sky in the constellation Lupus. Its appearance was noted by astronomers across the world, and it was said to be extremely bright. By some accounts it could be seen during the day, and estimates put it at being 16 times brighter than Venus. It is thought to have been the brightest supernova to appear in recorded human history. 

Despite its clear record as a supernova, a remnant of the stellar explosion wasn’t easy to find. It wasn’t until 1965 that a radio source was observed showing a circular shell of radio emissions characteristic of a remnant. It has since been confirmed as originating with SN 1006. We now know that the supernova occurred about 7,000 light years away. Detailed searches of the region find no evidence of a remaining stellar companion, so it was probably caused by the merger of two white dwarfs, making it a type Ia supernova. It is this kind of supernova that we use as a standard candle to determine the distance of galaxies.

Despite the closeness and brightness of this supernova, there were no serious effects on Earth at the time, which goes to show that much of the “Betelgeuse will kill us” hype is largely nonsense. That isn’t to say there wasn’t any effect. The supernova did cause a flux of gamma rays that left a record in Antarctic ice. This is true for other close supernova events as well, where spikes in nitrate concentrations have been found to correlate with historical supernovae.

Most recently, a new historical observation of the supernova has come to light, made by the Persian astronomer Ibn Sina. From historical reports such as these we can try to get a better understanding of things such as its apparent color and light curve. Historical observations are all we have, since there hasn’t been a close supernova in the history of modern astronomy.

Paper: Yuko Motizuki, et al. An Antarctic ice core recording both supernovae and solar cycles. arXiv:0902.3446 [astro-ph.HE]

Paper: Ralph Neuhaeuser, et al. An Arabic report about supernova SN 1006 by Ibn Sina (Avicenna).  arXiv:1604.03798 [astro-ph.SR]

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Virtually all the elements beyond hydrogen and helium come not from the big bang, but from the astrophysical processes of stars. The Earth and Moon are largely made of stardust, as the saying goes. We generally think of this process as having occurred long before our solar system formed, but it is an ongoing process we can see occurring more recently as well.

Evidence of cosmologically young elements on Earth can be seen in ocean floor sediment. In layers about 2 million years old there are concentrations of iron-60. This particular isotope of iron is interesting because it is seen in supernova remnants, and is is mainly supernova explosions that produces it. It also has a half life of about 2.6 million years. Any iron-60 that was around during the formation of our solar system billions of years ago would have long since decayed into cobalt-60. So the spike of iron-60 in ocean sediment would seem to be due to a supernova that occurred a couple million years ago. To be present on Earth, it would also need to have occurred fairly close to our solar system.

But if that’s the case, then we should see similar evidence on other solar system bodies. A recent paper in Physical Review Letters finds just such evidence on the surface of the Moon. It comes from lunar samples gathered by the Apollo missions. Measuring the isotope levels of lunar material, a team found that it had concentrations of iron-60 about 10 times higher than you would expect normally, similar to the spike in layers of ocean sediment on Earth. The team also found elevated levels of manganese-56 which is produced when cosmic rays strike iron atoms. This is important because iron-60 can also be produced by cosmic rays. By comparing the manganese and iron isotopes, it became clear that cosmic rays couldn’t account for such a high level of iron-60. Thus it must be due to a local supernova.

From the lunar levels, this supernova likely occurred roughly 2 million years ago about 300 light years away from Earth, which is surprisingly close on a cosmic scale.

Paper: L. Fimiani et al. 2016. Interstellar Fe⁶⁰ on the Surface of the Moon. Phys. Rev. Lett. 116, 151104; doi: 10.1103/PhysRevLett.116.151104

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When we observe a supernova, we can identify its type by looking at its spectrum. Supernovae that have strong hydrogen lines are known as type II, and are caused by the core collapse of a star at the end of its life. Stars without a strong hydrogen line are known as type I, and there’s a variation of type I supernovae with a strong silicon line known as type Ia. This last variety is particularly useful for astronomers, because they always have about the same absolute magnitude. This means they can be used as “standard candles” to determine the distance of a galaxy. While we know type Ia supernovae are not caused by the core collapse of a large star, we aren’t entirely sure what triggers them.  

There are two main models for type Ia supernovae. The first is that they are caused when a white dwarf star closely orbits a companion star. The outer layer of the companion star can be gravitationally captured by the white dwarf, which causes a runaway fusion process that results in a supernova. The other is that they are triggered by the collision of two white dwarfs. Understanding the origin of these supernovae is important, particularly in light of the fact that there is some evidence they might not be as standard as we thought.

One of the challenges with studying these supernovae is that they typically occur in other galaxies. There hasn’t been a clear supernova in our own galaxy for several hundred years, even though there’s evidence that one occurs in our galaxy about every 50 years. This is likely due to the fact that much of our galaxy is blocked by gas and dust, making them difficult to observe.  But recently observations from the Chandra x-ray observatory have found the remnant of a supernova in our galaxy that occurred about 110 years ago. Since it’s in a region of the galaxy rich in dust, the visible light from the supernova was obscured, which is why astronomers at the time didn’t see it. Looking through archive data from Chandra, a team found the remnant has been increasing in brightness at x-ray and radio wavelengths as the remnant interacts with interstellar gas and dust. Such an increase is what you’d expect from the collision model of type Ia supernovae, but not the binary model.

While this supports the idea that type Ia supernovae can occur from white dwarf mergers, it doesn’t necessarily mean that they can’t occur through a dance of binary stars as well. It’s possible that both mechanisms can produce supernovae of this type. It will take more observations to determine whether mergers are the only cause.

Paper: Sayan Chakraborti, et al. Young Remnants of Type Ia Supernovae and Their Progenitors: A Study of SNR G1.9+0.3. The Astrophysical Journal, Volume 819, Number 1 (2016)  arXiv:1510.08851 [astro-ph.HE]

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What do you do when you see the flash? In the case of a dying star, you know a supernova is coming. 

A core-collapse supernova occurs when a large star runs out of hydrogen and other elements to fuse. The fusion of elements in a star’s core creates the heat and pressure necessary to prevent a star from collapsing under its own weight. When a star stops fusing elements, its core cools rapidly and collapses. The heavy elements in the core slam into each other and recoil, creating a shockwave that rips the star apart. This causes a dramatic brightening of the star over a period of days, which is what we observe as a supernova. But before the star explodes the shockwave ripples through the layers of the star. It first causes instabilities in the star’s surface, such as plasma jets. When the full shockwave reaches the surface, it liberates a tremendous amount of photons from the star’s interior, causing an initial flash of light before the star begins to brighten.

At least that was the theory. The problem is the initial flash only lasts a few minutes, and it occurs before the star swells into a supernova. Usually we don’t notice a supernova until the star has already brightened a bit. To see the initial shockwave flash, you need to be watching a star before it goes supernova. Since we have no way of predicting when a particular star will begin to explode, we haven’t been able to catch the initial flash. That is, until now, when the Kepler space observatory happen to observe a supernova in its earliest moments.

The Kepler space observatory was designed to find exoplanets. It does this by observing stars for long periods of time, measuring their brightnesses to look for small dips in brightness. Such dips can indicate that a planet is passing in front of the star. It just so happened that a star in its field of view began to go supernova, and so Kepler caught the initial flash of the shockwave. It was really just blind luck, but it confirms the shockwave of a core-collapse supernova.

As we continue to make large scale sky surveys, the chances of observing the early stages of a supernova such as this become more likely. That’s important because it’s only by studying the early stages of a supernova that we will gain a better understanding of their triggering mechanisms.

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A superluminous supernova is an immense supernova more than ten times that of the type Ia supernovae used to measure cosmic distances. They are so intense that they challenge our understanding of just how they occur. Two possible mechanisms include the idea that they may be caused by magnetic heating as the core collapses into a magnetar, or that it’s intensity is strengthened by pair-instability reactions in its core. The evidence leaned toward the magnetar model, but observations of a new supernova challenge that idea. 

The new supernova is known as ASASSN-15lh, and it was more luminous than any supernova ever seen. About 20 times more luminous than the entire Milky Way. It’s light has traveled for about 2.7 billion years, so it’s apparent brightness in our sky wasn’t particularly bright, but in terms of absolute magnitude it was about three times as bright as other known superluminous supernovae. It is also unusual in that it occurred in a bright galaxy where there is not much new star formation. Other superluminous supernovae occur in active dwarf galaxies.

The team observing the spectra of this supernova found that it was low in hydrogen. This is indicative of a star that has cast off its hydrogen-rich outer later, and would seem to support the magnetar model. But the extreme energy of ASASSN-15lh puts it at the upper limit of the model. If this was indeed a magnetar supernova, then it was at the upper limit of the hypothetical energy range. That seems a bit unusual, and it raises the question of whether the magnetar model might be flawed.

The key to solving this mystery will be the discovery of similar superluminous supernovae. This particular supernova was discovered by the All Sky Automated Survey for SuperNovae (ASASSN) which is a collection of small (14 centimeter) telescopes in Chile and Hawaii. It’s a relatively low cost project that lays the groundwork for larger projects such as LSST. So over time we’re bound to find similar supernovae.

Paper: Subo Dong, et al. ASASSN-15lh: A highly super-luminous supernova. Science Vol. 351, Issue 6270, pp. 257-260 (2016)

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Eta Carinae is one of the brightest and most massive stars in our galaxy. It is a dying and unstable star, having changed in brightness significantly over the past few centuries. Because of a large eruption of surface material in the 1800s, it is a star surrounded by a veil of gas to this day. This makes it rather unique in the Milky Way. 

Stars similar to Eta Carinae as seen in other galaxies. Credit: NASA, ESA and R. Khan (GSFC and ORAU)

As far as we know, there isn’t another star like Eta Carinae in our galaxy, but new observations have found five similar stars in other galaxies. As outlined in a recent paper, the team that made this discovery developed a kind of infrared fingerprint making it easier to identify these kinds of stars. By comparing data from the Spitzer infrared telescope and visible observations from the Hubble space telescope they were able to locate these five stars.

Since large stars like Eta Carinae are short lived on a cosmic scale, they are quite rare. Now that we have several examples, we can study how such stars end their lives, and possibly how they behave leading up to a supernova explosion.

Paper: Rubab Khan, et al. Discovery of Five Candidate Analogs for ηCarinae in Nearby Galaxies. The Astrophysical Journal, 2015; 815 (2): L18 (2015)

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A supernova is a dying star. In a last moment of brilliance a supernova can shine brighter than an entire galaxy. There is still much we don’t understand about supernovae, in part because of how difficult they are to observe. Since we can’t predict when a star will explode, we typically discover supernovae after they have brightened significantly. Ideally we’d like to know where and when a supernova will occur before it happens so we can observe one in its entirety. Thanks to the effects of general relativity, we can. 

In general relativity, spacetime is warped by the presence of mass. Because of this warping, light can be deflected from a straight path, an effect known as gravitational lensing. The effect was first observed by Arthur Eddington in 1919, but is most commonly observed when light from a distant galaxy is deflected by a closer galaxy on its way to reaching us. This lensing effect often produces multiple images of a distant galaxy as its light reaches us along different paths.

Light from a distant galaxy is lensed to produce multiple images of the galaxy. Credit: NASA & ESA.

The speed of light is finite, so it takes time for light from a distant galaxy to reach us, often billions of years. Since the images of a galaxy come to us along different paths, each having a slightly different distance, the time it takes light to reach us along each path is different. This means each image of the galaxy we see is from a slightly different time. Normally this is insignificant, since galaxies change very slowly. But a supernova occurs over a short period of time, so it can appear in one image of the galaxy first, only to reappear in another image years later.

In 2014, astronomers observed a supernova in a galaxy known as MACS J1149.5+2223. Because of gravitational lensing, at least three images of this galaxy can be seen. Using computer models they calculated the path distances of these multiple images, and determined that the supernova should appear in another galaxy image sometime between 2015 and 2020. They’ve been keeping an eye on the galaxy, and this month they were rewarded with a new appearance of the same supernova. The supernova has been nicknamed Refsdal after the Norwegian astronomer Sjur Refsdal, who first proposed the idea of time delayed supernovae in 1964.

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The Crab Nebula is a pulsar that’s only about 1,600 light years away. It is the remnant of a supernova that occurred in 1054, and was recorded by Chinese astronomers. Pulsars are rotating neutron stars that produce bursts of energy we observe as pulses. Most pulsars are observed at radio wavelengths, but the Crab Nebula pulsar can also be observed in the visible. Because of its relative proximity, it’s also one of the few neutron stars we can observe directly. You can see it as the bright central dot in the x-ray image above.

When the supernova occurred in 1054, it had a maximum magnitude of about -6, which is much brighter than the brightest stars in the sky, and even brighter than Venus at its maximum. It should have been easily visible across the globe, and yet there is limited confirmed recording of the event in historical records. There are some hints of recordings, but the Chinese observation is the only one with sufficient accuracy to confirm. It’s an interesting example of how transient events in the sky didn’t always gain attention.

One of the mysteries about the Crab Nebula is that the calculated mass is about 3 solar masses. Combined with the mass of the neutron star itself (about 2 solar masses), the estimated mass of the original star would be about 5 solar masses. However to create a supernova of this size and chemical composition, the original star should have been about 9 – 11 solar masses. We’re still not sure where the missing mass went, though a good possibility is that the outer layers of the star were pushed away by the star before the supernova occurred. We see this happen with Wolf-Rayet stars.

The Crab Nebula itself can be seen with the naked eye, and some of its structure can be observed with binoculars or a small telescope. So if you get the chance, it’s worth checking out.

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The evidence for dark energy lies in our ability to relate the redshift of a galaxy with it’s distance. While we often talk about how the observed redshift of a galaxy allows us to determine its distance, that assumes our understanding of dark energy is correct. To prove dark energy is real we have to measure redshift and distance independently, and that takes a bit of doing.

Measuring redshift is fairly straightforward. By comparing the spectrum of a distant galaxy with the known spectra of atoms and molecules here on Earth, we can determine the amount of redshift expressed in a quantity known as z. To measure distance, however, we need to use observations of a kind of supernova known as type Ia. These are often described as “standard candles” that always explode with the same brightness, but that isn’t actually the case. Some type Ia supernovae are brighter than others, so you can’t simply use their observed brightness as a measure.

Raw light curves (top) vs. calibrated light curves (bottom) for type Ia supernovae.

Type Ia supernovae are identified by their emission spectrum. Their spectrum lacks hydrogen lines, and has a distinct silicon emission line when it is near maximum brightness. From this we can clearly distinguish type Ia from other supernovae. What we know from observing type Ia supernovae in nearby galaxies is that there is a specific relation between their peak brightness and the time it takes for them to decay. Bright supernovae shine longer than dim supernovae. From the ratio of peak to width of their light curve, we can calibrate these supernovae to determine their absolute magnitude. Comparing that with their observed magnitude we can determine their distance.

Calculating distance is based upon two assumptions. The first is that our view of the supernovae is relatively unobscured. We calculate distance using the inverse-square relation for light, but that only works if there isn’t gas or dust absorbing some of the light. While there can be gas and dust between us and a supernova, it wouldn’t absorb all frequencies of light by the same amount. Blue wavelengths are absorbed much more than red wavelengths (creating an effect known as reddening) and infrared wavelengths aren’t absorbed much at all. Since distant galaxies are deeply reddened, gas and dust have little effect on their observed brightness. So we know our first assumption is valid.

The second assumption is that nearby type Ia supernovae are the same as distant ones. Interestingly, in recent years there’s been some evidence that might not be the case. Recent observations of a large number of supernovae seem to show two classes of type Ia supernovae, with slightly different ratios. If this is true, then it could readjust the amount of dark energy the universe has. However this would be a minor adjustment to our understanding of cosmology, not a revolutionary change. While supernovae are a great way to observe the effects of dark energy, they aren’t the only way. We can also look at things such as the clustering of galaxies on large scales, and the fluctuations within the cosmic microwave background to determine the amount of dark energy in the universe. What we find is that they all agree reasonably well.

So while type Ia supernovae aren’t standard candles, they are standardizable candles, and they tell us a great deal about the cosmos.

Paper: Peter A. Milne et al. The Changing Fractions of Type Ia Supernova NUV–Optical Subclasses with Redshift. ApJ 803 20. (2015)

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