Posted on by Jon Voisey

Thankful Astronomer

The Milky Way from Earth. Image Credit: Kerry-Ann Lecky Hepburn (Weather and Sky Photography)

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Typically, I’ve been known as the Angry Astronomer. But since it’s Thanksgiving here in the US, I figured I should take a break and remind everyone that there’s a lot to be thankful for.

I’m thankful for our galaxy. Aside from being quite nice to look at, its collective (but weak) magnetic field, and the pressure from all the stars within it, protect us from the shock of plowing through the intergalactic medium as well as intergalactic cosmic rays.

I’m thankful for quantum mechanics. While it wasn’t the most fun course I’ve ever taken the fact that particles often behave as waves, giving rise to atomic orbitals, is what makes up the discreet absorption an emission spectra. Without this astronomers wouldn’t be able to determine the composition of stars from great distances.

I’m thankful for Newton’s third law; that one about equal and opposite forces and all that. It’s what lets the moon create tides. This may have had important effects in stabilizing our axial tilt and making life feasible on the planet in the first place. It’s also what allows us to detect planets around other stars through the “wobble method” and exoplanets are just cool.

I’m thankful for the immensely pristine vacuum that exists just beyond our atmosphere. Its existence allows astronomers to test theories at some of the lowest densities imaginable.

I’m thankful for neutron stars and black holes which allow astronomers to test theories at the highest densities imaginable.

I’m thankful for the supernovae which produce these objects and seed the universe with the heavy elements necessary to make planets, people, pineapples, and platypi.

I’m thankful that we’ve had the relatively close supernova (SN 1987a) to study. While I’d love to have another one in our own galaxy, I’m thankful we haven’t had one too close, or that directed a Gamma Ray Burst our way. With all the other issues we face from the universe, another Ordovician extinction just doesn’t sound too fun.

I’m thankful for dark matter. It may be a huge headache for astronomers trying to figure out what it is, but even if we can’t see it, it’s still like the Force: It binds the galaxies together.

I’m thankful for the Sun. Its nearly 1400 watts per square meter pours energy onto our planet, making all life possible, Creationist claims and ignorance aside.

I’m thankful for our atmosphere. It’s generally pretty breathable and it does a great job of blocking out that cancer causing UV. If only it would lighten up and let some more IR through so we didn’t have to send telescopes to space to study this region of the spectrum.

I’m thankful for this lump of rock, third from the Sun, we’re all riding on. It the grand scheme of things, it’s just a pale blue dot, but that’s home. And it’s not so bad.

So what is everyone else thankful for?

Posted on by Jon Voisey

Exploring the Atmosphere of Exoplanet WASP-14b

Conceptual orbit of WASP 14b system. Credit: SuperWASP team
Conceptual orbit of WASP 14b system. Credit: SuperWASP team

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First discovered in 2008, WASP 14b is an interesting exoplanet. It is roughly seven times as massive as Jupiter, but only 30% larger, making it among the densest known exoplanets. Recently, it was the target of observations from the Spitzer space telescope which was able to pick out the infrared radiation emitted by the planet and is giving astronomers new clues to how the atmospheres of Hot Jupiters function, contradicting expectations based on observations of other exoplanet atmospheres.

Images of the system were taken by a team of astronomers led by Jasmina Blecic and Joseph Harrington at the University of Central Florida. The team took images using three filters which allowed them to analyze the light at specific wavelengths. The brightness in each one was then compared to predictions made by models of atmospheres which included molecules such as H2O, CO, CH4, TiO, and VO as well as more typical atmospheric gasses like hydrogen, oxygen, and nitrogen.

While not having a large number of filters wouldn’t allow the team to conclusively match a specific model, they were able to confidently rule out some possible characteristics. In particular, the team rules out the presence of a layer of atmosphere that changes sharply in temperature from the regions directly around it, known as a “thermal inversion layer”. This comes as quite a surprise since observations of other hot Jupiters have consistently shown evidence of just such a layer. It was believed that all hot Jupiter type exoplanets should feature them if their atmospheres contained TiO or VO, molecules which filter out visible light. If they were present at a specific altitude, then that sudden layer of absorption would create a sudden shift in the temperature. The lack of this layer supports a 2009 study which suggested that such heavy molecules should settle out of the atmosphere and not be responsible for the thermal inversion layers. But this leaves astronomers with a fresh puzzle: If those molecules don’t cause them, then what does?

The team also found that the planet was brighter than expected when it was near the full phase which suggested that it is not as capable of redistributing its heat as some other exoplanets have been found to be. The team also confirmed that the planet has a notably elliptical orbit, despite being close to the star which should circularize the orbit. The astronomers that originally made the discovery of this planet postulated that this may be due to the presence of another planet which had a recent interaction that placed WASP 14b into its present orbit.

Posted on by Jon Voisey

Seeing the Phases of Exoplanets

Phases of Venus. Image credit: ESO

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Everyone is familiar with the fact that the moon changes phases. But what many don’t know is that planets also go through phases. Shown above are the phases for Venus. We look inwards on Venus from a more distant vantage point in our solar system, but in principle, planets in other solar systems would also go through phases as they orbited. While we are far too distant to resolve these phases any time soon, the percentage of reflected light may give clues about the size, composition, and atmosphere of a potential planet.

A new study by astronomers at the University of Bordeaux in France, analyzes differences in the way light would be reflected from various exoplanet configurations.

In a previous paper by the same team, they had analyzed how much light planets at different phases should reflect in different wavelengths of light in the infrared. Planets with atmospheres showed significant lack of emission at some wavelengths while rocky planets with no atmosphere reflected most strongly at one wavelength and faded smoothly off. The heavier the atmosphere, the more pronounced this effect was. As such, the team concluded that simply by looking at the reflected light in a few wavelengths, they could quickly determine whether the planet were likely to have an atmosphere.

The new paper adds to this by exploring what the effects of properties such as stellar type, orbital distance, radius of the planet, and inclination would have on these observations. They found that the presence of an atmosphere made determining many of these properties more difficult since it would be able to retain heat and reradiate it different manners instead of simply reflecting.

Rocky, airless planets were simpler and the light curves could be used more directly to determine the radius of the planet with an accuracy of about 10% with an instrument such as the James Webb Space Telescope. The orbital inclination could be narrowed down to within 10°. Currently, the only way astronomers can determine this property is if the planet is in the narrow ranges of inclination that allow it to transit the star, so while observing the phases to determine this property leaves large uncertainties, it is a start at the very least. These observations could also be used to determine the albedo, or reflectivity of the planet. This property could be used to help constrain the possible chemicals on the surface or in the atmosphere.

Posted on by Jon Voisey

Different Supernovae; Different Neutron Stars

Artist concept of a neutron star. Credit: NASA
Artist concept of a neutron star. Credit: NASA

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Astronomers have recognized various ways that stars can collapse to undergo a supernova. In one situation, an iron core collapses. The second involves a lower mass star with oxygen, neon, and magnesium in the core which suddenly captures electrons when the conditions are just right, removing them as a support mechanism and causing the star to collapse. While these two mechanisms make good physical sense, there has never been any observational support showing that both types occur. Until now that is. Astronomers led yb Christian Knigge and Malcolm Coe at the University of Southampton in the UK announced that they have detected two distinct sub populations in the neutron stars that result from these supernova.

To make the discovery, the team studied a large number of a specific sub-class of neutron stars known as Be X-ray binaries (BeXs). These objects are a pair of stars formed by a hot B spectral class stars with hydrogen emission in their spectrum in a binary orbit with a neutron star. The neutron star orbits the more massive B star in an elliptical orbit, siphoning off material as it makes close approaches. As the accreted material strikes the neutron star’s surface it glows brightly in the X-rays, becoming, for a time, an X-ray pulsar allowing astronomers to measure the spin period of the neutron star.

Such systems are common in the Small Magellanic Cloud which appears to have a burst of star forming activity about 60 million years ago, allowing for the massive B stars to be in the prime of their stellar lives. It is estimated that the Small Magellanic Cloud alone has as many BeXs as the entire Milky Way galaxy, despite being 100 times smaller. By studying these systems as well the Large Magellanic Cloud and Milky Way, the team found that there are two overlapping but distinct populations of BeX neutron stars. The first had a short period, averaging around 10 seconds. A second group had an average of around 5 minutes. The team surmises that the two populations are a result of the different supernova formation mechanisms.

The two different formation mechanisms should also lead to another difference. The explosion is expected to give the star a “kick” that can change the orbital characteristics. The electron-captured supernovae are expected to give a kick velocity of less than 50 km/sec whereas the iron core collapse supernovae should be over 200 km/sec. This would mean the iron core collapse stars should have preferentially longer and more eccentric orbits. The team attempted to discern whether this too was supported by their evidence, but only a small fraction of the stars they examined had determined eccentricities. Although there was a small difference, it is too early to determine whether or not it was due to chance.

According to Knigge, “These findings take us back to the most fundamental processes of stellar evolution and lead us to question how supernovae actually work. This opens up numerous new research areas, both on the observational and theoretical fronts.

Posted on by Jon Voisey

Rare Pallasite Meteorite Found in Missouri

Dr. Randy Korotev at Washington University with the Conception Junction meteorite.
Dr. Randy Korotev at Washington University with the Conception Junction meteorite.

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Meteorite hunter Karl Aston finds meteorites not by digging in the ground, but by placing ads in local newspapers. He asked people who found unusual rocks to contact him. Most responses were bum leads, but in 2009 Aston heard from a farmer in the northwestern Missouri of Conception Junction, who found something interesting: An unusually heavy stone buried in a hillside. The overall size was similar to that of a basketball and had a mass of 17 kilograms (37 pounds). Its rusty exterior hid its true nature. When the farmer had sawed off one end, olive-green crystals embedded in a shining metal shone forth. It was one of the rarest types of meteorites, a pallasite, of which only 61 samples are currently known. Recently, scientists at Washington University in St. Louis have gotten involved in an attempt to discover the meteorite’s history.

Pallasites and other meteorites are relics of the formation of the solar system. The most commonly accepted story for their formation is that they represent a boundary region inside larger asteroids where the heat from formation melted the iron and nickel metal which sunk to the core. The lighter crystals would float, and near this transition, there would be some mixing which, when broken apart due to later impacts, would form the pallasites. These asteroids formed in the asteroid belt between Mars and Jupiter and similar layers would likely be found in larger asteroids still present as well as in planets like the Earth. An alternative theory is that the materials formed independently and were mixed more recently due to large impacts.

The rusting fusion crust on the outside of the Conception Junction meteorite disguises it as just another rock, but one glimpse of the interior gives the game away. The olive-green crystals set in lustrous metal are unique to pallasites. Image credit: Dave Gheesling
The rusting fusion crust on the outside of the Conception Junction meteorite disguises it as just another rock, but one glimpse of the interior gives the game away. The olive-green crystals set in lustrous metal are unique to pallasites. Image credit: Dave Gheesling

Within the United States, 20 pallasite meteorites have been discovered. The majority of them belong to a single family of “main group” pallasites due to a similar chemical composition of their olivine crystals. When compared to other samples, the Conception Junction meteorite was unique. Because of this, the sample was given a unique designation this past August, named after the location of discovery. Before the Meteoritical Society recognizes a designation, it is required that a museum or other institutional collection houses a “type specimen” which will make the material available for scholarly research. As such a portion of the sample will be housed at UCLA where the chemical analysis on the metal was performed (the olivine was examined at Washington University).

The rarity of pallasite meteorites makes them uncommonly valuable. Some slices of the Conception Junction meteorite are still available for sale or trade, but don’t expect it to be an impulse buy. While more common stony meteorites sell for a few dollars per gram, pallasite meteorites sell for a few hundred dollars per gram. The overall price is also determined by the condition (some are unstable in Earth’s atmosphere) and whether or not it has a unique history. Meteorites for which the fall was observed are especially prized.

Wondering if the discovered meteorite was part of a larger body, Aston and other meteorite collectors including Robert Ward and Dave Gheesling conducted an extensive search of the region. They looked for 16 months in concentric circles centered on the original discovery location, but did not find any other specimens.

Posted on by Jon Voisey

Forget Exoplanets. Let’s Talk Exomoons

An artist impression of an exomoon orbiting an exoplanet, could the exoplanet's wobble help astronomers? (Andy McLatchie)

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It wasn’t that long ago that astronomers began discovering the first planets around other stars. But as the field of exoplanetary astronomy explodes, astronomers have begun looking to the future and considering the possibility of detecting moons around these planets. Surprisingly, the potential for doing so may not be that far off.

Before exploring how we might detect satellites of distant planets, astronomers must first attempt to get an understanding of what they may be looking for. Fortunately, this question ties in well with the rapidly developing understanding of how solar systems form.

In general, there are three mechanisms by which planets may obtain satellites. The simplest is for them to simply form together from a single accretion disk. Another is that a massive impact may knock material off of a planet which forms into a satellite as astronomers believe happened with our own Moon. Some estimates have indicated that such impacts should be frequent and as many as 1 in 12 Earth like planets may have formed moons in this way. Lastly, a satellite may be a captured asteroid or comet as is likely for many of the moons of Jupiter and Saturn.

Each of these cases produces a different range of masses. Captured bodies are likely to be the smallest and therefor are unlikely to be detectable in the near future. Impact generated moons are expected to only be able to form bodies with 4% of the total mass of the planet and as such, are rather limited as well. The largest moons are thought to form in the disks around forming Jupiter like planets. These are the most likely to be detectable.

The first method by which astronomers may detect such moons is by the changes they would make in the wobble of the star that has been used to detect many extrasolar planets to date. Astronomers have already studied how a pair of binary stars may affect a binary star system may have on a third star it orbits. If the binary star is swapped out for a planet and a moon it turns out that the easiest systems to detect are massive moons that are distant from the planet, but close to the parent star. However, except in extreme cases, the amount of wobble that the pair could induce in the star is so small that it would be swamped by the convective motion of the star’s surface, making detection through this method nearly impossible.

Astronomers have begun detecting large numbers of exoplanets by transits, where the planet causes minor eclipses. Could astronomers also detect the presence of moons this way? In this case, the limit on detection would again be based on the size of the moon. Currently, the Kepler satellite is expected to detect planets similar in mass to Earth. If moons exist around a super-Jovian planet that are also similar in size to Earth, they too should be detected. However, forming moons this large is difficult. The largest moon in the solar system in Ganymede which is 40% of the diameter of Earth, putting it modestly below current detection thresholds, but potentially in reach of future exoplanet missions.

However, direct detection of the eclipses caused by transits isn’t the only way transits could be used to discover exomoons. In the past few years, astronomers have begun using the wobble of other planets on the ones they had already discovered to infer the existence of other planets in the system in the same way the gravitational tug of Neptune on Uranus allowed astronomers to predict Neptune’s existence before it was discovered. A sufficiently massive moon could cause detectable variations in when the transit of the planet would begin and end. Astronomers have already used this technique to place limits on the mass of potential moons around exoplanets HD 209458 and OGLE-TR-113b at 3 and 7 Earth masses respectively.

The first discovered exoplanet was discovered around a pulsar. The tug of this planet caused variation of the regular pulsation of the pulsar’s beat. Pulsars often beat hundreds to thousands of times per second and as such, are extremely sensitive indicators of the presence of planets. The pulsar PSR B1257+12 is known to harbor one planet that is a mere 0.04% the mass of Earth, which is well below the mass threshold of many moons. As such, variations in these systems, caused by moons would be potentially detectable with current technology. Astronomers have already used it to search for moons around the planet orbiting PSR B1620-26 and ruled out moons more than 12% the mass of Jupiter within half an Astronomical Unit (the distance between the Earth and Sun or 93 million miles) of the planet.

The last method by which astronomers have detected planets that could potentially be used for exomoons is direct observation. Since direct imaging of exoplanets has only become realized in the past few years, this option is likely still a ways off, but future missions like the Terrestrial Planet Finder Coronagraph may put it into the realm of possibility. Even if the moon is not fully resolved, the offset of the center of the dot of the pair may be detectable with current instruments.

Overall, if the explosion of knowledge on planetary systems continues, astronomers should be capable of detecting exomoons within the near future. The possibility already exists for some cases, like pulsar planets, but due to their rarity, the statistical likelihood of finding a planet with a sufficiently large moon is low. But as equipment continues to improve, making detection thresholds lower for various methods, the first exomoons should come into view. Undoubtedly, the first ones will be large. This will beg the question of what sorts of surfaces and potentially atmospheres they may have. In turn, this would inspire more questions about what life may exist.

Source:
The Detectability of Moons of Extra-Solar Planets – Karen M. Lewis

Posted on by Jon Voisey

Borexino Collaboration Detects pep Neutrinos

View from inside the Borexino neutrino detector. Image Credit: Borexino Collaboration
View from inside the Borexino neutrino detector. Image Credit: Borexino Collaboration

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Solar neutrino physics has quieted down over the past decade. In the past, it had been a source of major excitement and puzzlement for scientists as they struggled to detect these elusive particles emitted from the fusion reactions in the center of the Sun. Although difficult to detect, they provide the most direct probe of the Solar core. Once astronomers learned to detect them and solved the Solar neutrino problem, they were able to confirm their understanding of the main nuclear reaction that powers the sun, the proton-proton (pp) reaction. But now, astronomers have for the first time, detected the neutrinos of another, far rarer nuclear reaction, the proton-electron-proton (pep) reaction.

At any given time, several separate fusion processes are converting the Sun’s hydrogen into helium, creating energy as a byproduct. The main reaction requires the formation of deuterium (hydrogen with an extra neutron in the nucleus) as the first step in a series of events that leads to the creation of stable helium. This typically takes place by the fusion of two protons which ejects a positron, a neutrino, and a photon. However, nuclear physicists predicted an alternative method of creating the necessary deuterium. In it, a proton and electron fuse first, forming a neutron and a neutrino, and then they join with a second proton. Based on solar models, they predicted that only 0.23% of all Deuterium would be created by this process. Given the already elusive nature of neutrinos, the diminished production rate has made these pep neutrinos even more difficult to detect.

While they may be hard to detect, pep neutrinos are readily distinguishable from ones created by the pp reaction. The key difference is the energy they carry. Neutrinos from the pp reaction have a range of energy up to a maximum of 0.42 MeV, while pep neutrinos carry a very select 1.44 MeV.

However, to pick out these neutrinos, the team had to carefully clean the data of signals from cosmic ray strikes which create muons that could then interact with carbon inside the detector to generate a neutrino with similar energy that might create a false positive. In addition, this process would also create a free neutron. To eliminate these, the team rejected all signals of neutrinos that occurred within a short amount of time from a detection of a free neutron. Overall, this indicated that the detector received 4,300 muons passing through it per day, which would generate 27 neutrons per 100tons of detector liquid, and similarly, 27 false positives.

Removing these detections, the team still found a signal of neutrinos with the appropriate energy and used this to estimate the total amount of pep neutrinos flowing through every square centimeter to be about 1.6 billion, per second, which they note is in agreement with predictions made by the standard model used to describe the interior workings of the Sun.

Aside from further confirming astronomers understanding of the processes that power the Sun, this finding also places constraints on another fusion process, the CNO Cycle. While this process is expected to be minor in the Sun (making only ~2% of all helium produced), it is expected to be more efficient in hotter, more massive stars and dominate in stars with 50% more mass than the Sun. Better understanding the limits of this process would help astronomers to clarify how those stars work as well.

Posted on by Jon Voisey

Is M85 Missing a Black Hole?

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The conventional wisdom of galaxies is that they should have a central massive black hole (CMBH). The presence of such objects has been confirmed in our own galaxy as well as numerous other galaxies, including the Andromeda galaxy (M31) and even some dwarf galaxies. The mass of these objects, several million times the mass of the Sun, has been found to be related to many properties of galaxies as a whole, indicating that their presence may be critical in the formation and evolution of galaxies as a whole. As such, finding a massive galaxy without a central black hole would be quite surprising. Yet a recent study by astronomers from the University of Michigan Ann Arbor seems to have found an exception: The well known M85.

To determine the mass of the CMBH, the team used the spectrograph on board the Hubble Space Telescope to examine the pull the central object had on stars in the nearby vicinity. The higher this mass is, the more quickly the stars should orbit. This orbital velocity is detected as a shift in the color of the light, blue as the stars move towards us, red as they move away. The amount the light is shifted is dependent on just how fast they move.

Doppler shift of gas and dust caused by M84's supermassive black hole. Image Credit: Gary Bower, Richard Green (NOAO), the STIS Instrument Definition Team, and NASA
Doppler shift of gas and dust caused by M84's supermassive black hole. Image Credit: Gary Bower, Richard Green (NOAO), the STIS Instrument Definition Team, and NASA
This technique has been used previously in other galaxies, including another large elliptical of similar brightness in the Messier catalog, M84. This galaxy had its CMBH probed by Hubble in 1997 and was determined to have a mass of 300 million solar masses.

When this method was applied to M85 the team did not discover a shift that would be indicative of a black hole with a mass expected for a galaxy of such size. Using another, indirect method of determining the CMBH mass by looking at the the amount of overall light from the galaxy, which is generally correlated with black hole mass, would indicate that M85 should contain a black hole of 300 million to 2 billion solar masses. Yet this study indicates that, if M85 contains a central black hole at all, the upper limit for the black hole would be around 65 million solar masses.

This study is not the first to report a non-detection for the galaxy, a 2009 study led by Alessandro Capetti from Osservatorio Astronoimco di Torino in Italy, searched M85 for signs of radio emission from the black hole region. Their study was unable to detect any significant radio waves from the core which, if M85 had a significant black hole, should be present, even with a small amount of gas feeding into the core.

Overall, these studies demonstrate a significant shortcoming in secondary methods of black hole mass estimation. Such indirect methods have been previously used with confidence and have even been the basis for studies drawing the connection between galaxy evolution and black hole mass. If cases like M85 are more common that previously thought, it may prompt astronomers to rethink just how connected black holes and a galaxies properties really are.

Posted on by Jon Voisey

Was the “First Photographed UFO” a Comet?

First photograph of a UFO sighting, taken 12 August 1883 by Jose Bonilla.
First photograph of a UFO sighting, taken 12 August 1883 by Jose Bonilla.

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On August 12th, 1883, Mexican astronomer José Bonilla was preparing to study the Sun at the recently opened Zacatecas Observatory. However, the Sun’s surface was marred by numerous objects quickly travelling across its disk. Over the course of the day and the next, Bonilla exposed several wet plates to take images of the 447 objects he would observe. They weren’t released publicly until January 1st, 1886 when they were published in the magazine L’Astronomie. Since then, UFOlogists have crowed these photographs as the first photographic evidence of UFOs. The chief editor of L’Astronomie passed the observations off as migrating animals, but a new study proposes the observation was due to the breakup of a comet that nearly hit us.

The only piece of evidence the authors, led by Hector Manterola at the Universidad Nacional Autónoma de México, use to suggest that this was a comet in the process of breaking up, was the descriptions of the objects as being “fuzzy” in nature and leaving dark trails behind them. Assuming this were the case, the authors consider how close the object would have been. Since astronomers at observatories in Mexico City, or Puebla had not reported the objects, this would imply that they did not cross the disc of the Sun from these locations due to parallax. As such, the maximum distance the object could have been is roughly 80,000 km, roughly 1/5th the distance to the moon.

But the team suggests the fragments may have passed even closer. By the time comets reach the inner solar system, they have a significant velocity of some tens of kilometers per second. In such a case, to transverse the disc of the Sun in the time reported by Bonilla (a third to a full second), the object would have been, at most, at a distance of ~8,000km.

At such distances, the overall size of the fragments would be in rough agreement of sizes of other fragmented comets such as 73P/Schwassmann-Wachmann 3, which gave off several fragments in 2006. Based on the number of fragments, estimated sizes, and density of an average comet, the authors estimate that the mass may be anywhere between 2 x 1012 and 8 x 1015 kg. While this is a very large range (three orders of magnitude), it roughly brackets the range of known comets, again making it plausible. The upper range of this mass estimate is on par with Mars’ moon Deimos, which is generally held to be similar in mass to the progenitor of the impact that killed the dinosaurs.

One oddity is that one would likely expect such a close breakup to result in a meteor storm. The timing of these events is just before the annual Perseid meteor shower, but reports for that year, such as this one, do not depict it as being exceptional, or having a different radiant than should be expected. Instead, it notes that 157 of the 186 meteors observed on the 11th were definitively Perseids, and that the “year’s display cannot be reckoned as a fine one by any means.” Meanwhile, the Leonid meteor shower (peaking in November), was exceptional that year, generating an estimated 1,000 meteors an hour, but again, no records seem to indicate an unusual origin.

In total, I find the characterization of Bonilla’s observation as a comet plausible, but generally unconvincing. However, if it were a fragmented comet, we’re very lucky it wasn’t any closer.

Posted on by Jon Voisey

Missing Black Holes

Artists concept of a black hole.

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As astronomers began working out how stars die, they expected that the mass of remnants, whether white dwarfs, neutron stars, or black holes, should be essentially continuous. In other words, there should be a smooth distribution of remnant masses from a fraction of a solar mass, up to nearly 100 times the mass of the sun. Yet observations have shown a distinct lack of objects at the borderline of neutron stars and black holes weighing 2-5 solar masses. So where have they all gone and what might this imply about the explosions that create such objects?

The gap was first noted in 1998 and was originally attributed to a lack of observations of black holes at the time. But in the past 13 years, the gap has held up.

In an attempt to explain this, a new study has been conducted by a team of astronomers led by Krzystof Belczynski at Warsaw University. Following the recent observations, the team assumed the paucity was not caused by a lack of observations or selection effect, but rather, there simply weren’t many objects in this mass range.

Instead, the team looked at the engines of supernovae that would create such objects. Stars less than ~20 solar masses are expected to explode into supernovae, leaving behind neutron stars, while ones greater than 40 solar masses should collapse directly into black holes with little to no fanfare. Stars between these ranges were expected to fill this gap of 2-5 solar mass remnants.

The new study proposes that the gap is created by a fickle switch in the supernova explosion process. In general, supernovae occur when the cores are filled with iron which can no longer create energy through fusion. When this happens, the pressure supporting the star’s mass disappears and the outer layers collapse onto the immensely dense core. This creates a shockwave which is reflected by the core and rushes outwards, slamming into more collapsing material and creates a stalemate, where the outwards pressure balances the infalling material. For the supernova to proceed, that outwards shockwave needs an extra boost.

While astronomers disagree on exactly what might cause this revitalization, some suggest that it is generated as the core, superheated to hundreds of billions of degrees, emits neutrinos. Under normal densities, these particles travel right past most matter, but in the superdense regions inside the supernova, many are captured, reheating the material and driving the shockwave back out to create the event we observe as a supernova.

Regardless of what causes it, the team suggests that this point is critical for the final mass of the object. If it explodes, much of the mass of the progenitor will be lost, pushing it towards a neutron star. If it fails to push outwards, the material collapses and enters the event horizon, piling on mass and driving the final mass upwards. It’s an all or nothing moment.

And moment is a good description of how fast this occurs. At most, astronomers suggest that this interplay between the outwards shock and the inwards collapse takes a single second. Other models place the timescale at a tenth of a second. The new study notes that the more quickly the decision takes place, the more pronounced the gap is in the resulting objects. As such, the fact that the gap exists may be taken as evidence for this being a split second decision.