# How to form Copper from Calcium in a supernova explosion?

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What are the reasonable chain of reactions to form copper (63, 29) from ca(40, 20) during supernova explosion? And how it will happen? I do not understand the theory behind it. I thought that the reaction will end at Iron and now it does not make sense to me.

Copper is not thought to be primarily made in a supernova. It is thought to be mainly produced by the s-process of slow neutron capture onto iron-peak nuclei that already exist inside a star. These reactions are endothermic.

The source of the neutrons is still somewhat debated, it could either be from the decay of $^{13}$C in relatively low-mass asymptotic giant branch stars or is more likely from the decay of $^{22}$Ne in more massive evolved stars (e.g. Pignatari et al. 2010).

Below you can see a typical route for producing copper from $^{56}$Fe. The axes of the plot are neutron number on the x-axis and proton (atomic) number on the y-axis. Three neutron captures are followed by a beta decay, a neutron capture, a beta decay, then 3 more neutron captures followed by another beta decay to form $^{63}$Cu.

The net process is $$^{56}{ m Fe} + 7n ightarrow ^{63}{ m Cu} + 3e + 3ar{ u}_e$$

The copper is then distributed into the interstellar medium by a later supernova explosion in the same star.

If there are no "seed" iron-peak nuclei (e.g. in the first/second generation of stars), then copper can be inefficiently produced by explosive r-process neutron capture during supernovae explosions. However, this would not contribute very much to the copper we see on the Earth.

## Hubble observes calcium-rich supernovae

Hubble Space Telescope imaging (blue: F435W, green: average(F435W, F606W), red: F606W) of the host, NGC 5714, and immediate environment of SN 2003dr. The host image is 200 arcsec on a side, and the sizes of the individual filter images are indicated by the magenta square. The location of SN 2003dr is shown at the center of each zoomed image, with the dashed circle having a radius of 0.2 arcsec. Kron apertures of SExtractor detected sources are marked on their respective images, which have been slightly smoothed to aid visual identification of sources. At the distance of NGC 5714 one arcsec is ∼ 160 pc. Credit: Joseph Lyman et al., 2016.

The NASA/ESA Hubble Space Telescope offers a multitude of spectacular images of celestial objects and a huge amount of scientific data helpful for astronomers. A team of scientists from UK and Sweden has recently made use of Hubble to study the host galaxies and environments of five calcium-rich supernovae that could provide new insights on the evolution of stellar systems. Their research was published online on Feb. 25 in the arXiv journal.

Calcium-rich supernovae, also called calcium-rich transients, are a type of supernova that eject a preponderance of calcium into space, less luminous than other supernova types and evolving more rapidly. According to previous studies, a large fraction of them are found at significant distances from the nearest galaxy, well outside the bulk of the stellar light. It is believed that this type of transient may be a major producer of calcium in the universe.

The researchers, led by Joseph Lyman of the University of Warwick, UK, employed Hubble's Advanced Camera for Surveys/Wide Field Channel (ACS/WFC) and Wide Field Camera 3 (WFC3) to obtain images of five calcium-rich supernovae, among which three exhibit large offsets and two are coincident with the disk of their hosts.

"Our sample consists of five examples of the calcium-rich supernova class, which were targeted with Hubble for two orbits each," the scientists wrote in a paper published on arXiv.

The supernovae, lying well outside their hosts, are designated SN 2003dr, SN 2005E and SN 2007ke, and were observed using ACS. The scientists found no detected sources underlying the locations of these supernovae, ruling out the presence of massive stars, dwarf galaxies and globular clusters at these locations.

SN 2003dr is the most interesting of this group as it is quite complex. It lies offset along the minor axis of the galaxy and thus off the disk light, although it is relatively close in linear distance. Hubble images also show a strong tidal feature that passes through the location of this supernova along the southern and western sides of the galaxy.

The two calcium-rich transients appearing to be in line-of-sight with the disks of late type galaxies, named SN 2001co and SN 2003dg, were imaged by Hubble's WFC3. The team discovered that they both display strong patchy star formation and significant dust lanes – typical for their morphological types.

"In each case, the transient's location appears close to regions of star formation. (…) Furthermore, the lack of distinguishable underlying sources at their locations is in agreement with the findings for the remote sample, although in these cases there is clearly an underlying stellar population from the galactic disk," the paper reads.

Thanks to the new research, almost every calcium-rich supernova located within 300 million light years has been observed in detail. The study also confirmed that the majority of host galaxies of these transients are disturbed or merging systems.

However, how these calcium-rich supernovae form is still debated. It could be due to white dwarf mergers with neutron stars due to the collapse of massive stars. According to Lyman, the mechanism of the supernova explosion could cause the neutron star to be 'kicked' to very high velocities.

"This high-velocity system can then escape its galaxy, and if the binary system survives the kick, the white dwarf and neutron star will merge causing the explosive transient," Lyman said.

The scientists concluded that as new members of the class emerge, it will be prudent to further test this apparent bias of calcium-rich supernovae production in disturbed and merging systems.

## DEPARTMENT OF PHYSICS AND ASTRONOMY

August 6, 2020

Image above: Artist’s interpretation of the calcium-rich supernova 2019ehk. Shown in orange is the calcium-rich material created in the explosion. Purple coloring represents gas shedded by the star right before the explosion, which then produced bright X-ray emission when the material collided with the supernova shockwave. Image Credit: Aaron M. Geller, Northwestern University

Most of the calcium in the universe — including the very calcium in our teeth and bones — was created in the last gasp of dying stars.

Called “calcium-rich supernovae,” these stellar explosions are so rare that astrophysicists have struggled to find and subsequently study them. The nature of these supernovae and their mechanism for creating calcium, therefore, have remained elusive.

Now a Northwestern University-led team has potentially uncovered the true nature of these rare, mysterious events. For the first time ever, the researchers examined a calcium-rich supernova with X-ray imaging, which provided an unprecedented glimpse into the star during the last month of its life and ultimate explosion.

## Show us the moon rocks

Figure 2. Predicted values of the distribution factor for Lunar samples. The left panel is for material arriving from 20 degrees south latitude (relative to the rotational axis of the moon), which is towards the Sco-Cen association. The right panel is for material arriving at 65 degrees south latitude, corresponding the Tuc-Hor region (which is the region that the authors predicted for the supernova’s origin). The yellow circles represent the landing sights of the Apollo missions that acquired lunar samples. Figure 2 from today’s paper.

There happens to be such a place only 239,000 miles away! Iron-60 deposited into the lunar regolith by such a supernova would not have suffered the effects caused by an atmosphere or ocean, meaning their initial trajectory would be preserved within about 1 degree (this 1 degree was found to be the approximate deflection due to interplanetary effects). If you have really good memory, you might remember that we went to the moon over 43 years ago and brought back over 800 pounds of lunar samples! Though the Apollo landings were at similar lunar latitudes (Figure 2), comparing the iron-60 content of these samples could better constrain the location of the supernova in the sky, and the authors predict that these samples will reveal distinct differences in the abundance of iron-60 at different lunar locations. Collecting more lunar samples from differing latitudes and time periods could help pinpoint more nearby supernova that exploded in the past, though until then we’re stuck with the annoyance of oceans and an atmosphere.

## Exploding stars created the calcium in our bones and teeth, study says

The calcium in our bones and teeth likely came from stars exploding in supernovas and scattering this mineral across the universe in massive quantities, according to a new study.

We truly are made of star stuff, as famed astronomer Carl Sagan once said.

In fact, half of the calcium in the universe likely came from calcium-rich supernovae. But these explosions have turned out to be incredibly rare events that scientists have had difficulty observing and analyzing, so they weren’t sure how the calcium was created.

Explosions and mergers of stars are also known to create other heavy elements, like gold and platinum. But the calcium has presented more of a mystery.

That changed when a global team of almost 70 scientists from around the world collaborated after receiving a tip from an amateur astronomer. The study published Wednesday in The Astrophysical Journal.

In April 2019, Joel Shepherd observed a bright burst as he observed the spiral galaxy called Messier 100, which is 55 million light-years away, through his telescope. He also spied a dot that was bright orange. Shepherd shared his observation with the astronomy community through a survey.

The news spread like wildfire among the community and telescopes from across the world were aimed at the galaxy and its anomaly. The event was named SN2019ehk.

NASA’s orbiting Neil Gehrels Swift Observatory, the Lick Observatory in California and the W.M. Keck Observatory in Hawaii, which researchers at Northwestern University can access remotely, all were turned toward the anomaly.

They requested that Keck observe the event in optical light, while University of California Santa Barbara graduate student Daichi Hiramatsu used the Swift observatory to observe it in X-ray and ultraviolet light.

These follow-up observations of the event occurred about 10 hours after the supernova was detected, and X-ray emissions from the explosion were only visible for about five days before they disappeared from view.

“Observing supernovae within hours of explosion is the new ‘it’ thing in our field right now,” said Wynn Jacobson-Galan, study author, first-year Northwestern graduate student and National Science Foundation Graduate Research Fellow, via email.

“As this particular supernova revealed, when you discover something so young, you now have access to the final moments of the star’s life right before explosion.”

### The ultimate calcium-rich supernova

In fact, scientists had observed a calcium-rich supernova. The X-rays revealed intriguing new information about the explosion and the star itself before it exploded.

“The stars responsible for calcium-rich supernovae shed layers of material in the last months before explosion,” Jacobson-Galan said. “The X-rays are the result of the explosion violently colliding with this ejected material and stimulating a brilliant burst of high energy photons.”

The heat and pressure of the explosion actually drives the chemical reaction that creates calcium, the researchers said.

Usually, only a small amount of calcium is produced by each star as it burns through its supply of helium. However, when a calcium-rich supernova occurs, massive amounts of calcium are created and released in a matter of seconds.

“The explosion is trying to cool down,” said Raffaella Margutti, senior study author and assistant professor of physics and astronomy in Northwestern’s Weinberg College of Arts and Sciences. “It wants to give away its energy, and calcium emission is an efficient way to do that.”

This occurs because the hot ball of material created by the explosion is trying to reach an equilibrium with its environment, Jacobson-Galan said.

“Calcium-rich supernovae produce just enough additional calcium in the explosion to provide an efficient means of emitting photons that in turn release heat,” he said. “Nature chooses the path of least resistance and calcium provides that path when enough of it is present in a supernova.”

And SN 2019ehk emitted the most calcium ever observed a single event, the researchers said.

“It wasn’t just calcium rich,” Margutti said. “It was the richest of the rich.”

The Hubble Space Telescope has observed this galaxy for the last 25 years, but never actually registered the particular star that caused this calcium-rich explosion. That’s likely because it was very faint, either a white dwarf, or a dead exploded core of a star, or a very low-mass star, Jacobson-Galan said.

“Without this explosion, you wouldn’t know that anything was ever there,” Margutti added. “Not even Hubble could see it.”

That faint nature is also possibly true of calcium-rich supernovae as well, when compared to common supernovae that occur when massive stars die.

“This makes finding new calcium-rich supernovae difficult to see large distances from the Earth,” Jacobson-Galan said. “Future telescope surveys that can scan the sky every night and see further into space will detect many more of these supernovae.”

This includes the upcoming Vera Rubin Observatory.

The scientitsts’ finding suggested that the stars responsible for these explosions must be undergoing some sort of instability, which then causes the ejection of the star’s outer layers right before explosion, he said.

The researchers are working on a follow-up study that includes how the supernova is evolving after the explosion.

They are also refocusing their search to include X-ray emissions from calcium-rich supernovae, which was entirely unexpected until this observation.

“We are designing observing strategies that would allow us to find a supernova when they are very young (and faint) and immediately repoint the X-ray spacecraft to catch the bright but short lived X-ray emission,” Margutti said.

## Abstract

We present observations and modeling of SN 2016hnk, a Ca-rich supernova (SN) that is consistent with being the result of a He-shell double-detonation explosion of a C/O white dwarf. We find that SN 2016hnk is intrinsically red relative to typical thermonuclear SNe and has a relatively low peak luminosity ( M B = -15.4 mag), setting it apart from low-luminosity SNe Ia. SN 2016hnk has a fast-rising light curve that is consistent with other Ca-rich transients (t r = 15 days). We determine that SN 2016hnk produced 0.03 ± 0.01 M o˙ of 56Ni and 0.9 ± 0.3 M o˙ of ejecta. The photospheric spectra show strong, high-velocity Ca ii absorption and significant line blanketing at λ < 5000 Å, making it distinct from typical (SN 2005E-like) Ca-rich SNe. SN 2016hnk is remarkably similar to SN 2018byg, which was modeled as a He-shell double-detonation explosion. We demonstrate that the spectra and light curves of SN 2016hnk are well modeled by the detonation of a 0.02 M⊙ helium shell on the surface of a 0.85 M⊙ C/O white dwarf. This analysis highlights the second observed case of a He-shell double-detonation and suggests a specific thermonuclear explosion that is physically distinct from SNe that are defined simply by their low luminosities and strong [Ca ii] emission.

## Calcium-rich supernova examined with X-rays for first time

Artist's interpretation of the calcium-rich supernova 2019ehk. Shown in orange is the calcium-rich material created in the explosion. Purple coloring represents gas shedded by the star right before the explosion, which then produced bright X-ray emission when the material collided with the supernova shockwave. Credit: Aaron M. Geller/Northwestern University

Half of all the calcium in the universe—including the very calcium in our teeth and bones—was created in the last gasp of dying stars.

Called "calcium-rich supernovae," these stellar explosions are so rare that astrophysicists have struggled to find and subsequently study them. The nature of these supernovae and their mechanism for creating calcium, therefore, have remained elusive.

Now a Northwestern University-led team has potentially uncovered the true nature of these rare, mysterious events. For the first time ever, the researchers examined a calcium-rich supernova with X-ray imaging, which provided an unprecedented glimpse into the star during the last month of its life and ultimate explosion.

The new findings revealed that a calcium-rich supernova is a compact star that sheds an outer layer of gas during the final stages of its life. When the star explodes, its matter collides with the loose material in that outer shell, emitting bright X-rays. The overall explosion causes intensely hot temperatures and high pressure, driving a chemical reaction that produces calcium.

"These events are so few in number that we have never known what produced calcium-rich supernova," said Wynn Jacobson-Galan, a first-year Northwestern graduate student who led the study. "By observing what this star did in its final month before it reached its critical, tumultuous end, we peered into a place previously unexplored, opening new avenues of study within transient science."

"Before this event, we had indirect information about what calcium-rich supernovae might or might not be," said Northwestern's Raffaella Margutti, a senior author of the study. "Now, we can confidently rule out several possibilities."

The research will be published on August 5 in The Astrophysical Journal. Nearly 70 co-authors from more than 15 countries contributed to the paper.

Margutti is an assistant professor of physics and astronomy in Northwestern's Weinberg College of Arts and Sciences and a member of CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics). Jacobson-Galan is an NSF Graduate Research Fellow in Margutti's transients research group.

Hubble Space Telescope image of SN 2019ehk in its spiral host galaxy, Messier 100. The image is a composite made of pre- and post-explosion images. Credit: CTIO/SOAR/NOIRLab/NSF/AURA/Northwestern University/C. Kilpatrick/University of California Santa Cruz/NASA-ESA Hubble Space Telescope

'A global collaboration was ignited'

Amateur astronomer Joel Shepherd first spotted the bright burst, dubbed SN2019ehk, while stargazing in Seattle. On April 28, 2019, Shepherd used his new telescope to view Messier 100 (M100), a spiral galaxy located 55 million light years from Earth. The next day, a bright orange dot appeared in the frame. Shepherd reported the anomaly to a community astronomical survey.

"As soon as the world knew that there was a potential supernova in M100, a global collaboration was ignited," Jacobson-Galan said. "Every single country with a prominent telescope turned to look at this object."

This included leading observatories in the United States such as NASA's Swift Satellite, W.M. Keck Observatory in Hawaii and the Lick Observatory in California. The Northwestern team, which has remote access to Keck, was one of the many teams worldwide who triggered its telescopes to examine SN2019ehk in optical wavelengths. University of California Santa Barbara graduate student Daichi Hiramatsu was the first to trigger Swift to study SN2019ehk in the X-ray and ultraviolet. Hiramatsu also is a staff scientist at Las Cumbres Observatory, which played a crucial role in monitoring the long-term evolution of this supernova with its global telescope network.

The worldwide follow-up operation moved so quickly that the supernova was observed just 10 hours after explosion. The X-ray emission detected with Swift only lingered for five days and then completely disappeared.

"In the world of transients, we have to discover things very, very fast before they fade," Margutti said. "Initially, no one was looking for X-rays. Daichi noticed something and alerted us to the strange appearance of what looked like X-rays. We looked at the images and realized something was there. It was much more luminous than anybody would have ever thought. There were no preexisting theories that predicted calcium-rich transients would be so luminous in X-ray wavelengths."

'The richest of the rich'

While all calcium comes from stars, calcium-rich supernovae pack the most powerful punch. Typical stars create small amounts of calcium slowly through burning helium throughout their lives. Calcium-rich supernovae, on the other hand, produce massive amounts of calcium within seconds.

"The explosion is trying to cool down," Margutti explained. "It wants to give away its energy, and calcium emission is an efficient way to do that."

Using Keck, the Northwestern team discovered that SN 2019ehk emitted the most calcium ever observed in a singular astrophysical event.

"It wasn't just calcium rich," Margutti said. "It was the richest of the rich."

SN2019ehk's brief luminosity told another a story about its nature. The Northwestern researchers believe that the star shed an outer layer of gas in its final days. When the star exploded, its material collided with this outer layer to produce a bright, energetic burst of X-rays.

"The luminosity tells us how much material the star shed and how close that material was to the star," Jacobson-Galan said. "In this case, the star lost a very small amount of material right before it exploded. That material was still nearby."

Although the Hubble Space Telescope had been observing M100 for the past 25 years, the powerful device never registered the star—which was experiencing its final evolution—responsible for SN2019ehk. The researchers used the Hubble images to examine the supernova site before the explosion occurred and say this is yet another clue to the star's true nature.

"It was likely a white dwarf or very low-mass massive star," Jacobson-Galan said. "Both of those would be very faint."

"Without this explosion, you wouldn't know that anything was ever there," Margutti added. "Not even Hubble could see it."

## A Field Guide to Supernova Spectra

By: Leif J. Robinson July 21, 2006 0

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Supernova types are characterized by their chemistry and light curves.

S&T illustration source: Astronomy and Astrophysics Encyclopedia (1994).

"The Revival of Amateur Spectroscopy", low-resolution spectra of objects as faint as magnitude 13 or thereabouts are accessible to modest amateur equipment. (A few superposed 20-minute exposures with a 12-inch telescope or so should produce an adequate image.) But what will supernovae spectra look like — especially shortly after the outburst begins — as captured by small telescopes and low-resolution spectrographs?

Here's your field guide. To prepare it, we started with high-resolution, calibrated spectra supplied by Alexei Filippenko (University of California, Berkeley). Then, to simulate Gavin's CCD results, we degraded the spectra to a resolution of 50 angstroms per pixel. Finally, and with dramatic results, we changed the intensity along each spectrum to reflect variations in the unfiltered sensitivity of popular CCD chips — the KAF-0400 from Kodak and the ICX055BL from Sony. Thus, what you see here is what you will get! (Astrophotographers using panchromatic emulsions will record spectra that look much like the originals.)

Below are suites of spectra for different supernovae to see all five sets of spectra, click here. The following descriptions are largely taken from notes supplied by Filippenko. One should keep in mind that the progenitors of Type II supernovae are heterogeneous, so it's not surprising that their spectra are heterogeneous too.

These examples of supernova spectra are depicted both as intensity tracings and as simulated CCD images black dots mark conspicuous features identified in the 'Original' spectrum.

## Binary Star’s Rare Supernova Explosion Polluted Its Stellar Friend With Calcium

Astronomers have discovered a rare star full of calcium that could teach us more about the stellar explosions called supernovas.

It’s a story of love and death, with two stars orbiting the same point — a binary star system — until one day the larger star explodes and floods its mate with material. A study in Nature Astronomy found the scientists detected a star in the remnants of the supernova comparable in size to our sun, but its atmosphere is “strongly polluted with calcium and other elements” to have them in concentrations many times greater than the sun. That could mean the supernova is part of a “class of calcium-rich supernovae … the origin of which is strongly debated.”

Those supernovas are rare, so discovering another could help scientists understand how they work.

Lomonosov Moscow State University, which had an astrophysicist leading the research, explained that in the supernova remnant there is a neutron star, which would be what is left of the exploded giant, which can be detected from X-rays. But there is also a sun-like star that can be seen in the optical wavelengths — the ones that include ultraviolet, visible light and infrared.

Astrophysicists have found a binary star system polluted with calcium at the heart of a supernova remnant. Photo: Vasilii Gvaramadze

A supernova is the biggest possible explosion in outer space and occurs when a star takes on too much mass. That can happen because it is an old star or, in the case of some binary stars, because it consumed its companion.

“It would look beautiful up close, right up until the energy intensity vaporized you,” astrophysicist Neil deGrasse Tyson explained. “One of the greatest events in the universe. … happens maybe only once per century per galaxy.”

If you’re looking to get a front row seat to one, you are out of luck — NASA said the sun is just not big enough to cause a supernova once it’s ready to die.

Vasilii Gvaramadze, the lead scientist from Lomonosov Moscow State University, said his team will continue studying the new binary system in hope of understanding how the stars orbit, what the neutron star was like before it exploded and the levels of other elements in the sun-like star’s atmosphere.

Previously, scientists generally thought calcium-rich supernovas formed with old stars that have used up all their fuel when there is a “detonation” in a shell of helium around the star, the university said. But the new findings “imply that under certain circumstances a large amount of calcium could also be synthesized by explosion of massive stars in binary systems.”

Learning about these space explosions gives us clues about how our universe works. “One kind of supernova has shown scientists that we live in an expanding universe, one that is growing at an ever increasing rate,” NASA said. “Scientists also have determined that supernovas play a key role in distributing elements throughout the universe. When the star explodes, it shoots elements and debris into space. Many of the elements we find here on Earth are made in the core of stars. These elements travel on to form new stars, planets and everything else in the universe.”

## Possible new type of Supernovae puts Calcium in your Bones

KAMUELA, HI — New data from several telescopes, including the W. M. Keck Observatory, suggest astronomers may have identified a new type of supernovae. The stellar death is thought to have originated in a star that was a low-mass white dwarf accumulating helium from a companion star. When the white dwarf exploded, about half of the mass ejected from the supernova was in the form of calcium. The finding suggests that a couple of supernovae like this exploding every 100 years would produce the high abundance of calcium observed in galaxies like the Milky Way, and the calcium present in life on Earth.

The supernova, SN 2005E, was discovered five years ago by the University of California, Berkeley’s Katzman Automatic Imaging Telescope (KAIT), and is one of only eight known “calcium-rich supernovae” that appear to be distinct from the two main classes of supernovae: the Type Ia supernovae, thought to be old, white dwarf stars that accrete matter from a companion until they undergo a thermonuclear explosion that blows them apart entirely and Type Ib/c or Type II supernovae, thought to be hot, massive and short-lived stars that explode and leave behind black holes or neutron stars.

In the past decade, robotic telescopes have turned astronomers’ attention to scads of strange exploding stars, one-offs that may or may not point to new and unusual physics. “With the sheer numbers of supernovae we’re detecting, we’re discovering weird ones that may represent different physical mechanisms compared with the two well-known types, or may just be variations on the standard themes,” said Alex Filippenko, KAIT director and UC Berkeley professor of astronomy. “But SN 2005E was a different kind of ‘bang.’ It and the other calcium-rich supernovae may be a true suborder, not just one of a kind.”

Filippenko is coauthor of a paper appearing in the May 20 issue of the journal Nature describing SN 2005E and presenting evidence that the original star was a low-mass white dwarf stealing helium from a binary companion until the temperature and pressure ignited a thermonuclear explosion – a massive fusion bomb – that blew off at least the outer layers of the star and perhaps blew the entire star to smithereens. The team of astronomers was led by Hagai Perets, now at the Harvard-Smithsonian Center for Astrophysics, and Avishay Gal-Yam of the Weizmann Institute of Science in Rehovot, Israel.

In November 2009, Filippenko and former UC Berkeley post-doctoral fellow Dovi Poznanski, currently at Lawrence Berkeley National Laboratory and also coauthor on the Nature paper, reported another supernova, SN 2002bj, that they believe explodes by a similar mechanism: ignition of a helium layer on a white dwarf.

“SN 2002bj is arguably similar to SN 2005E, but has some clear observational differences as well,” Filippenko said. “It was likely a white dwarf accreting helium from a companion star, though the details of the explosion seem to have been different because the spectra and light curves differ.” Astronomers have so far found only one example of this supernova.

Filippenko and UC Berkeley research astronomer Weidong Li first reported an unusual calcium-rich supernova in 2003, and since then, KAIT has discovered several more, including SN 2005E on Jan. 13, 2005. Because these supernovae, like Type Ib, show evidence for helium in their spectra shortly after they explode, and because in the later stages they show strong calcium emission lines, the UC Berkeley astronomers were the first to refer to them as “calcium-rich Type Ib supernovae.”

It was SN 2005E, which went off about 110 million years ago in the spiral galaxy NGC 1032 in the constellation Cetus, that initially drew the attention of Perets, Gal-Yam and their colleagues. Using data provided by Filippenko and Li, and taken by the W. M. Keck Observatory in Hawaii, the Palomar Observatory in California and the Liverpool Observatory in the United Kingdom, they created a detailed picture of the explosion. The small amount of mass ejected in the explosion, estimated at 30 percent the mass of the Sun, and the fact that the galaxy in which the explosion occurred was old with few hot, giant stars, led them to the conclusion that a low-mass white dwarf was involved.

The newly discovered supernova threw off unusually high levels of the elements calcium and radioactive titanium, which are the products of a nuclear reaction involving helium rather than carbon and oxygen that are involved in Type Ia supernovae.

“We know that SN 2005E came from the explosion of an old, low-mass star because of its specific location in the outskirts of a galaxy devoid of recent star formation,” Filippenko said. “And the presence of so much calcium in the ejected gases tells us that helium must have exploded in a nuclear runaway.”

The paper’s authors note that, if these eight calcium-rich supernovae are the first examples of a common, new type of supernovae, they could explain two puzzling observations: the abundance of calcium in galaxies and in life on Earth, and the concentration of positrons – the anti-matter counterpart of the electron – in the center of galaxies. The latter could be the result of the decay of radioactive titanium-44, produced abundantly in this type of supernova, to scandium-44 and a positron, prior to scandium’s decay to calcium-44. The most popular explanation for this positron presence is the decay of putative dark matter at the core of galaxies.

“Dark matter may or may not exist,” says Gal-Yam, “but these positrons are perhaps just as easily accounted for by the third type of supernova.”

Filippenko and Li hope that KAIT and other robotic telescopes scanning distant galaxies every night in search of new supernovae will turn up more examples of calcium-rich or even stranger supernovae, which can then be observed with larger telescopes such as Keck.

“The research field of supernovae is exploding right now, if you’ll pardon the pun,” Filippenko said. “Many supernovae with peculiar new properties have been found, pointing to a greater richness in the physical mechanisms by which nature chooses to explode stars.”