Bose-Einstein Basics

Bose-Einstein Basics

The Bose-Einstein state of matter was on Earth in1995. Two scientists, Eric A. Cornell of the National Institute of Standards and Technology and Carl E. Wieman of the University of Colorado at Boulder, finally managed to produce condensate in the lab. For this achievement, they received the 2001 Nobel Prize in physics.

When you think condensate, think about and the way gas molecules come together and condense and to a liquid. The molecules get denser or packed closer together.

Much earlier, two scientists, Satyendra Bose and Albert Einstein, predicted BCE in the 1920s, but no laboratory had the equipment and facilities to achieve it at that time. Now we do. If plasmas are super hot and super excited atoms, the atoms in a Bose-Einstein condensate (BEC) are total opposites. They are super unexcited and super cold atoms.

About Condensation

Consider condensation first. Condensation happens when several gas molecules come together and form. It all happens because of a loss of energy. Gases are really excited atoms. When they lose energy, they slow down and begin to collect. They can collect into one drop. Water (H2O) vapor in the form of a liquid condenses on the lid of your pot when you boil water. It cools on the metal and becomes a liquid again. You would then have a condensate.

The BEC happens at super low temperatures. We have talked about temperature scales and Kelvin. At zero Kelvin (0® K– absolute zero) all molecular motion stops. Scientists have been able to achieve a temperature only a few billionths of a degree above absolute zero. When temperatures get that low, you can create a BEC with a few special elements. Cornell and Weiman did it with rubidium (Rb).

Let the Clumping Begin

So, it’s cold. A cold ice cube is still a solid. When you get to a temperature near absolute zero, something special happens. Atoms begin to clump. The whole process happens at temperatures within a few billionths of a degree, so you won’t see this at home. When the temperature becomes that low, the atomic parts can’t move at all. They lose almost all of their energy.

Since there is no more energy to transfer (as in solids or liquids), all of the atoms have exactly the same levels, like twins. The result of this clumping is the BEC. The group of rubidium atoms sits in the same place, creating a “super atom.” There are no longer thousands of separate atoms. They all take on the same qualities and, for our purposes, become one blob.

Bose-Einstein-Condensates form the core of neutron stars. These embers of a dying star are about the same size as Manhattan Island yet more massive than the Sun. A teaspoonful of one would weigh about a billion tons. On the outside, neutron stars are brittle. They are covered by an iron-rich crust. On the inside, they are fluid. Each one harbors a sea of neutrons — the debris from atoms crushed by a supernova explosion. The whole ensemble rotates hundreds of times each second, and so spawns powerful quantum tornadoes within the star.

Above: Neutron stars are formed in supernova explosions. This supernova remnant (known as the Crab Nebula) harbors one that spins 30 times every second.

You probably wouldn’t want a neutron star on your desktop. That is, unless you’re an experimental physicist.

Neutron stars and their cousins, white dwarfs and black holes, are extreme forms of matter that many scientists would love to tinker with– if only they could get one in their lab. But how? Researchers experimenting with the new form of matter, Bose-Einstein condensates, may have found a way.

Bose-Einstein condensates (BECs) are matter waves formed when very cold atoms merge to become a single “quantum mechanical blob.” They contain about ten million atoms in a droplet 0.1 mm across. In most ways, BECs and neutron stars are dissimilar. BECs are 100,000 times less dense than air, and they are colder than interstellar space. Neutron stars, on the other hand, weigh about 100 million tons per cubic centimeter, and their insides are 100 times hotter than the core of the Sun. So what do they have in common? Both are superfluids — that is, liquids that flow without friction or viscosity.

Perhaps the best-known example of a superfluid is helium-4 cooled to temperatures less than 2.2⁰ K (-271⁰ C). If you held in your hand a well-insulated cup of such helium and slowly rotated the cup, the slippery helium inside wouldn’t rotate with it.

Yet superfluids can rotate. And when they do, weird things happen. “Superfluids cannot turn as a rigid body — in order to rotate, they must swirl,” explains Ketterle. Among physicists he would say that “the curl of the velocity field must be zero.” This basic physics holds for BECs and neutron stars alike.

In 2001, following similar experiments in 1999 by researchers in Colorado and France, Ketterle and his colleagues at MIT decided to spin a BEC and see what would happen. Ketterle says he didn’t have neutron stars in mind when he did the experiment: “BECs are a new form of matter, and we wanted to learn more about them. By rotating BECs, we force them to reveal their properties.” Simulating the inside of a weird star was to be an unintended spin-off.

Ketterle’s team shined a rotating laser beam on the condensate, which was held in place by magnets. He compares the process to “stroking a ping-pong ball with a feather until it starts spinning.” Suddenly, a regular array of whirlpools appeared.

“It was a breathtaking experience when we saw those vortices,” recalls Ketterle. Researchers had seen such whirlpools before (in liquid helium and in BECs) but never so many at once. The array of quantum tornadoes was just the sort of storm astronomers had long-suspected might swirl within neutron stars.

No one has ever seen superfluid vortices inside a neutron star, but we have good reason to believe they exist: Many neutron stars are pulsars — that is, they emit a beam of radiation as they spin. The effect is much like a light house: we see a flash of light or radio waves each time the beam sweeps by. The pulses arrive at intervals so impressively regular that they rival atomic clocks. In fact, when Jocelyn Bell Burnell and Tony Hewish discovered pulsars in 1967, they wondered if they were receiving intelligent signals from aliens! Sometimes, though, pulsars “glitch” like a cheap wristwatch that suddenly begins to run too fast. The glitches are likely due to superfluid vortices forming or decaying within the star, or perhaps vortices brushing against the star’s crust.

The possibilities don’t end there: “If the condensed atoms [in a BEC were to] attract each other, then the whole condensate can collapse,” Ketterle adds. “People have actually predicated that the physics is the same as that of a collapsing neutron star. So it’s one way, maybe, to realize a tiny neutron star in a small vacuum chamber.”

Small, confined and tame — a pet neutron star? It sounds far-fetched, yet researchers are learning to make BECs collapse in real-life experiments.

BECs are formed with the aid of magnetic traps. Carl Wieman and colleagues at NIST have discovered that atoms inside a BEC can be made to attract or repel one another by “tuning” the magnetic field to which the condensate is exposed. Last They tried both: First, they made a self-repelling BEC. It expanded gently, as expected. Then, they made a mildly self-attracting BEC. It began to shrink — again as expected — but then it did something wholly unexpected.

It exploded!

Many of the atoms in the BEC flew outward, some in spherical shells, others in narrow jets. Some of the ejecta completely disappeared — a lingering mystery. Some remained as a smaller core at the position of the original condensate.

To an astrophysicist, this sounds remarkably like a supernova explosion. Indeed, Wieman et al dubbed it a “Bosenova” (pronounced “bose-a-nova”). In fact, the explosion liberated only enough energy to raise the temperature of the condensate 200 billionths of a degree. A real supernova would have been 1075 times more powerful. But you have to start somewhere.

If researchers eventually do craft miniature neutron stars, they might learn to make white dwarfs and black holes as well. Such micro-stars pose no danger to Earthlings. They are simply too small and their gravity too feeble to gobble objects around them. But such pets would no doubt be popular among physicists and astronomers.

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Neutron Star Interior

Neutron Star Interior[1]

It is a rare opportunity when we can use astronomical observations to push the frontiers of physics in a way that is not possible in any laboratory on Earth. Yet neutron stars, the dead, dense remnants of massive stars, provide us with just that opportunity.

Crushed under the inward pull of gravity once a massive star’s fuel is exhausted, the stellar core that becomes a neutron star reaches matter densities that are not naturally encountered anywhere else in the universe. (Black holes don’t count: Although they may infinite energy densities, they hide behind horizons and are inaccessible). In fact, the matter that makes up neutron stars has fundamentally changed character, taking us into regimes that are still poorly understood in physics. As a result, a lot of recent efforts have focused on probing these stars’ interiors with astronomical instruments. To be sure, there are some aspects of that unusual matter that we can confidently predict, based on theoretical calculations and laboratory experiments. For example, during the implosion of the star, the electrons that normally surround he nucleus of an atom in the core get pushed into the atomic nuclei. There, they combine with the protons through weak interactions, one of the four types of interactions that take place between particles in the universe. (The other three are strong, gravitational, and electromagnetic.) That combination produces a neutron—thus giving this new star its name—as well as nearly massless, ghostly particles called neutrinos that rapidly escape from the star, carrying with them a large amount of energy the first observational confirmation of this process happened with Supernova 1987 observed in 1987 in the Large Magellanic Cloud (see S&T Feb 2017, p. 36), when two neutrino observatories detected a burst of particles around the same time as the supernova’s visible light appeared in the sky.

Scientists can’t create such neutron-rich matter. Simply smashing together and squeezing them to high densities in heavy ion colliders, such as the RHIC experiment in Brookhaven National Laboratory, doesn’t work. First, collisions in accelerators create very hot matter, which behaves differently than cold matter inside a neutron star. (It’s “cold” not because the temperature is low, but because the thermal energy is so small compared to the energies of other internal interactions that it’s unimportant.) Second, the weak interaction acts over a relatively long time-scale, much longer than the time that particles have to interact with one another when crushed against each other in a collider. Imagine that a plane were flying nearby, in the opposite direction to yours. You might have time to catch a glimpse of a passenger on that flight, but you certainly wouldn’t have time for a hearty handshake—even the if it were physically possible.

Still, if the creation of neutrons during the implosion were all that could happen to the core’s atomic nuclei, astronomers would by now consider the question of what lies inside a neutron star a solved problem. But transforming run-of-the-mill atoms into a super-dense soup of (almost) entirely neutrons turns out to be only the beginning of the particles’ journey.

The Fate of Collapsing Matter

As neutrons become squeezed further together in neutron star cores, they reach densities that are difficult to fathom While the star’s crust may look more or less like normal mater, the core can reach densities that are a hundred trillion (! that is 1014) times higher than the densest natural elements on Earth.

Under those conditions, neutrons not only start vehemently repelling one another but also interacting in new ways. This is because neutrons are examples of particles called fermions, which require increasingly higher energy to be confined closer and closer together. To counter this rising energy, neutrons may find it “energetically favorable”–basically less hassle—to dissolve into their even smaller constituents, called quarks, creating a quark soup. Alternatively, they may form different combinations of quarks than what normally make up a neutron or proton. Such hyper-nuclei, called hyperons, can be created in laboratories but survive only or a short time. In neutron stars, they might be stable.

Yet another possibility is that fermions pair up with one another to form a type of particle called a boson. These particles, which behave differently from fermions, can transition into an unusual superfluid state of matter (the same state that is observed in low-temperature helium fluids, superconductors, and some other metals, called a Bose-Einstein condensate). This sate will have strange properties, such as flowing without friction. If this transition indeed occurs inside the core—and we have good reason to think that it does—it will relieve some of the pressure built up by the high densities that mater experiences there.

But which of these possibilities actually take place in a neutron star’s interior? One of the things that complicate this puzzle is that, while we’ve observed hyperons and many types of bosons as standalone particles in laboratories, a quark has never been observed by itself, in what is called an “unconfined” state. Therefore, predicting and testing the behavior of quark matter becomes very difficult.

But if one could see into the cores of neutron stars and prove that they contain quark matter, it would constitute a major advance in our understanding of these smallest constituents of matter.

Probing the Invisible

How would astronomers go about probing the deep interiors of not only the densest but also the smallest stellar objects in the universe? At roughly 20 km across, neutron stars are smaller than some of the Solar System’s asteroids. Yet they pack into that tiny volume up to two times the mass of the Sun. And unlike asteroids, they are hundreds to many thousands of light-years away. It turns out that measuring the exact sizes of neutron stars, which can be done from a distance, provides the best possible tool for getting a complete picture of their interiors. Our calculations tell us that, if only neutrons remain in the interior, the pressure building up from the repulsive interactions will support a star of a particular size. If any constituents other than neutrons form, their interactions would cause a different amount of repulsion, creating a star of a different size. Thus, astronomers can discover the possibilities in the realm of physics simply by measuring exact diameters of these stars.

Astronomers are used to measuring the sizes of far-away objects by collecting and analyzing the light they emit. Indeed, nearly all our knowledge about the sizes of normal stars comes from measuring both the total light emitted by the star, referred to as its luminosity, as well as the breakdown of that emission into different wavelengths of light, known as its spectrum. The spectrum of the star allows us to determine its temperature; the hotter the stat, the higher the energy of the light that it emits. It’s this temperature that regulates exactly the amount of radiation that emerges from each patch of the surface, allowing us to determine the star’s intrinsic brightness. Comparing that to the total observed luminosity give us an exact measurement of the star’s surface area.

For neutron stars, the approach we take isn’t too different. So far, much of the information we’ve obtained about their sizes has come from nearly the same methods we apply to normal stars, but with a few complications. First off, it is difficult to see down to the surface of many neutron stars. This is because the majority of neutron stars that have ever been observed are enshrouded by strong magnetic fields (called magnetospheres), as well as energetic charged particles that swarm around in these fields like a cloud. These types of neutron stars are exciting in their own right, observable as the beautiful radio and gamma-ray pulsars that spin like lighthouses with exquisite regularity. However, it is difficult to see down to the star’s surface without being overwhelmed by the light emitted from the surface of the neutron star itself, we focus instead on those stars that have very weak magnetic fields, which allows the surface’s x-ray glow to shine through. Some of these stars are not pulsars at all, while others have very weak pulsar emission.

The second difficulty astronomers encounter in measuring sizes comes from the extremely strong gravitational fields neutron stars possess. Thanks to the vast amount of matter packed into such a small volume, a neutron star strongly bends space-time around itself and the path of the light that the star emits becomes significantly distorted. Indeed, the gravitational field is so strong that a neutron star hovers just this side of catastrophe—any denser, and it would collapse into a black hole.

What this means for measuring the size of a neutron star is that an extra step is required to map the observed light through its distorted path back to its origin. This procedure allows us to accurately account for every piece of the surface that contributes to the observed light and obtain a measurement of the entire surface area.

The results we have obtained to date with this method have been pretty striking: Neutron stars turn out than what we would predict if they were made up only of neutrons. More precisely, if none of the possible interactions that give rise to new particles or unconfined quarks take place in their cores, we would expect neutron stars to be 25-26 km across. The measurements point to 20-22 km instead. That may seem like a small difference, but it is in fact large: The central density of two such stars differs by a factor of two. This is enough to have a profound effect on the amount of repulsion the particles experience.

As with all scientific experiments, these results need to be confirmed with independent methods. So far, though, the measurements indicate that neutrons are taking at least one of the possibilities available to them to partially release the pressure valve. Which one, we’re still unsure. We need both additional, independent observations and theoretical investigations to find out.

Gravity to the Rescue

One of the powerful techniques that can reveal the sizes of a neutron star relies on the general relativistic effects that arise due to the star’s extreme gravity—the exact same effects that cause complications in surface area measurements. This time though, they come to our help We apply a technique known as pulse profile modeling to a special class of pulsars. In these sources, the magnetic fields that are anchored on the surface are weak enough that the swarm of particles around the star doesn’t overwhelm the light from its surface. Nevertheless, these magnetic fields are strong enough to guide charged particles toward the star’s magnetic poles, producing a hotspot where the poles meet the crust. As the star spins on its axis, the hotspots come in and out of sight, generating a characteristic pulse in the x rays.

Measuring the pulses’ shapes allows us to determine the size of the star that emitted them. This is because the amount that the light path bends as it leaves the surface of a neutron star depends on how large the star is. In other words, two neutron stars of the same mass but with different sizes, say 20 and 25 km, would create a different pattern of modulation in the light they emit. These patterns can be calculated very precisely and compared to the observed pulses, revealing the sizes of these pulsars.

We are poised to conduct this experiment with an instrument called the Neutron Star Interior Composition Explorer (NICER), which is scheduled for launch to the International Space Station (ISS) this year (2017). NICER is approximately a meter across and consists of carefully designated optical elements that focus the incoming x rays onto 56 silicon detectors. After its journey on a SpaceX resupply mission, it will be unpacked and mounted onto its home on the ISS platform. A star-tracker-based pointing system will then allow the high-precision x-ray timing instrument to point to and track pulsar targets over nearly half of the sky.

What makes NICER unique is its unprecedented capability to record the arrival times of incoming photons with 100-nanosecond precision. This capability will enable the highly faithful reconstruction of the pulse waveforms for a number of pulsars. The detectors will also capture the pulsars’ spectra. Coupled with the precisely determined pulse shape, these measurements will provide all the information necessary for a precise size measurement within a year after its launch.

Another exciting avenue into the neutron star interior will become possible through the detection of gravitational waves with LIGO. Even though the first two events detected by LIGO were coalescing black hole binaries, LIGO is also sensitive to signals from merging neutron stars (S&T Dec, 2015, p. 26). Shortly before the expected coalescence, the pair of inspiraling neutron stars start distorting and pulling each other apart through tidal interactions, obeying the same principles as the Moon’s effect on Earth’s oceans, but far more severe. How severe it is depends on how deformable the stars are, which in turn depends on their size, density, and interior composition. Remarkably, the distortions caused by these tidal interactions are then encoded into the gravitational wave signals that are emitted throughout the inspiral, offering one more penetrating glimpse into the neutron star interior.

If NICER and LIGO experiments confirm the existing measurements of small sizes, the results would point to new physics that emerges when matter becomes ultra-dense. Or the experiments may offer other surprises—that remains to be seen.

But no matter how small and impenetrable they may seem, neutron stars will not be able to hold onto their innermost secrets for much longer

[1] See Feryal Őzel, “The Inside Story of Neutron Stars,” Sky and Telescope (134, 1, 2017, pp. 16-21). Feryal Őzel is a professor of astronomy and physics at the University of Arizona and a current Guggenheim Fellow. She studies neutron stars and black holes and is a member of the NICER team.

Neutron-Core Stars

Neutron-Core Stars[1]

In an infinite universe, even the most bizarre thought experiments by astronomers—perhaps conceived late at night, perhaps proposed simply to see how weird stars can get—can come to pass. Imagine a massive star, near the end of its life and puffed up to the red supergiant phase, with a tiny neutron star, the skeletal remnant of an even more massive star, at its core. No one knows quite how this Frankenstar might form or how long It would live, and the fusion process would be anything but normal, yet the physics checks out. This mysterious star, called a Thorne-Żytkow object (TZO), could exist. But does it? Amazingly, 40 years after its conception, astronomers think they might have found one of these stars, and it has the potential to upend our understanding of stellar evolution.

A working theory

TZOs are named after Kip Thorne and Anna Żytkow, two astronomers who worked out detailed calculations of what this strange system would look like in 1977 at the California Institute of Technology. They proposed a completely new class of star with a novel, functional model for a stellar interior. Scientists had explored the idea of stars with neutron star cores when neutron stars were first thought of in the 1930s, but their work lacked a detailed analysis or any firm conclusions.

The origin of a TZO goes like this: For reasons not yet clear, the majority of the massive stars we observe in the universe are in binary systems. These stars are several times more massive than our Sun (at least eight times bigger, though stars as large as hundreds of solar masses have been observed) and spend their fuel much more quickly. The largest stars in the universe burn all their fuel in just a few million years, while a star the size of our Sun burns for several billion. In a binary system where the two stars’ masses are unequal, then, the larger of the two runs out of fuel and dies before its partner. The massive component explodes in a fiery supernova as bright as an entire galaxy. When the fireworks are over, this future TZO system is already exotic—the normal, lower-mass star is now paired with a rapidly rotating neutron star with a radius as tiny as 6 miles (10 kilometers), composed entirely of neutrons packed so tightly that they test the extremes of quantum mechanics,

Astronomers already have observed many such neutron star/normal star systems. As the two orbit each other, gas from the normal star can flow onto the outer layers of the neutron star, causing bright x-ray flares. These flares are millions of times more luminous than the x rays emitted by normal stars and are in fact some of the brightest sources of x rays in our galaxy.

But such systems raise a question: What ultimately happens to a system where a neutron star and a regular star orbit each other, but their orbits are unstable? This could occur for a variety of reasons, such as the supergiant’s puffed off gas layers dragging down the neutron star and causing it to spiral in or as a result of the super¬nova explosion that tore apart the first star. In many cases, the neutron star will get a gravitational “kick” that ejects it from the system. But for others, the binary system may reach a final stage of evolution wherein the neutron star orbits closer and closer to its companion, which by this stage is nearing the end of its own life and is a red supergiant star. Eventually, the two stars merge, the red supergiant swallowing the neutron star, and a TZO is born.

In a galaxy the size of our Milky Way, containing hundreds of billions of stars, such mergers should be happening routinely. In fact, scientists have proposed that as many as 1 percent of all red supergiants might actually be TZOs in disguise. “Mergers between a neutron star and a star are common,” confirms Selma de Mink, an astronomer at the University of Amsterdam whose research focuses on stellar evolution. “The question is, what does that look like? For me, that is the big excitement—this happens all the time, but we have no clue.” She explains that some sort of transient and observable event should occur at the moment of the merger—perhaps there is a flare of energy in the x ray or a nova explosion in visible light. Theorists are working on various models, but as yet there is no consensus on what scientists would see at the birth of a TZO.

Made of star stuff

TZOs are important because they have the potential to tell astronomers where some of the more exotic elements in the universe come from. Hydrogen, helium, and trace amounts of lithium were created immediately after the Big Bang. All the heavier elements in the universe, though, formed not at the dawn of the cosmos, but within the heart of a star. Some of these elements we know and love from our daily lives—carbon, oxygen, and iron, to name a few—are produced inside stars through regular processes that are fairly well understood. But the origin of some particularly heavy elements, such as molybdenum, yttrium, ruthenium, and rubidium, is less clear. “These elements are not household names, but still you might want to know where the atoms that make up our universe came from,” jokes Philip Massey, an astronomer at Lowell Observatory in Arizona whose research includes the evolution of massive stars,

Theory suggests that these elements might be created in TZOs. A neutron star inside a red supergiant leads to an unusual method for energy production: The object’s burning is dominated not by the standard nuclear fusion that occurs in other stars, but instead by thermonuclear reactions where the extremely hot edge of the neutron star touches the puffy supergiant’s gas layers. These reactions power the star and also create those heavy elements. Convection that circulates hot gas in the star’s outer layers transports these new elements throughout the star and ultimately even to its surface, where a keen-eyed observer with the right telescope might just spy them.

Hunting for TZOs

But tracking these mysterious objects down is not an easy task. “To an outside observer, TZOs look very much like extremely cool and luminous red supergiants,” explains Żytkow, now at the Institute of Astronomy at the University of Cambridge in England. This means they are nearly indistinguishable from the thousands of other normal, bright supergiant stars that many surveys observe. “However, they are somewhat redder and brighter than stars such as Betelgeuse in the constellation Orion,” she says, naming the famous red supergiant familiar to stargazers.

The only way to distinguish a TZO from a regular bright super-giant is to look at high, resolution spectra—patterns of light astronomers use as stellar fingerprints—to find the specific lines caused by the unusual elements more abundant in TZOs than in typical stars. Such work is severely complicated by the massive number of complex spectral lines from other elements and molecules in the star, which easily number in the thousands. “It is a needle in a haystack kind of problem,” says de Mink.

Despite this, a team of astronomers thinks they might have found the first needle. Nearly four decades and several unsuccessful searches have passed since Żytkow initially worked on the theory behind TZOs. When she saw new research on some unusually behaving bright red supergiants, however, she was intrigued. Emily Levesque, an astronomer at the University of Colorado at Boulder, spearheaded the work with Massey, whom she has been researching red supergiants with ever since an undergraduate summer internship in 2004. Two years later, they discovered several red supergiant stars in the Magellanic Clouds—satellite galaxies of our own—that were unusually cool and variable in brightness. This avenue of research eventually attracted Żytkow’s attention, so she asked whether the team had considered the possibility that these stars might be TZOs.

The potential to find the first TZO was exciting, but identifying a candidate from within the sample of red supergiants would require higher-resolution spectra than ever taken before. Levesque, along with her former mentor Massey and additional collaborator Nidia Morrell of the Carnegie Observatories in La Serena, Chile, secured time to observe a sample of several dozen red supergiants both in the Milky Way and in the Magellanic Clouds using the 3.5-meter telescope at Apache Point Observatory, New Mexico, and the 6.5-meter telescope at Las Campanas Observatory, Chile, respectively. They observed each of the stars with some of the most powerful spectrographs available and then began the meticulous task of identifying the various emission lines in the data and working out the relative elemental abundances in each star.

“It wasn’t immediately obvious at a glance if we had a TZO,” Levesque recalls, “but there was one star that jumped out at us.” A star called HV 2112 in the Small Magellanic Cloud had a particularly bright hydrogen emission line astronomers saw even in the raw data they glanced at as it came in. In fact, it was so unusual that it prompted Morrell to joke at first look, “I don’t know what it is, but I like it!”

It turns out there was much more to like about HV 2112—it had unusually high concentrations of the elements lithium, molybdenum, and rubidium, which are predicted TZO signatures. While finding a star with an unusual abundance of one key element can happen for a variety of reasons, this was the first time astronomers saw all the critical elements in the same star; the team published their results identifying HV 2112 as a TZO candidate in the summer of 2014. “It could still turn out not to be a TZO in the long run,” explains Levesque, “but even if not, it’s definitely a very weird star.”

This discovery was also satisfying for Żytkow, who was instrumental in pushing for telescope time and analysis of the spectral lines. “Work on the discovery of a candidate object which Kip Thorne and I first predicted many years ago is great fun,” Żytkow says. “Since we proposed our models of stars with neutron cores, people were not able to disprove our work. If theory is sound, experimental confirmation shows up sooner or later.”

Revisiting stellar evolution

While finding a “star within a star” sounds intriguing in itself, the discovery of a TZO is particularly interesting to astronomers for what its existence can tell them about stellar evolution. Major research advances in recent years in areas such as stellar convection allow astronomers to update their models for TZOs. These changes may yield new elemental abundances for observers to watch for. Astronomers also want to know whether TZOs can explain where some of the heavy elements come from: Rough estimates so far suggest there could be enough TZOs to explain their formation, but the numbers are highly uncertain.

With only one observed TZO in their stable, how do astronomers estimate how many TZOs are still in the wild, waiting to be discovered? This is not easy to answer: For one thing, no one is sure how long TZOs can be stable. Some models predict that they would be very short-lived objects—lasting only a few thousand years—either due to being torn apart by extremely strong stellar winds or collapsing into a black hole. “Computationally, this is one of the hardest things out there to model,” says de Mink, “so we aren’t sure.”

Research also has focused on finding the remnant of a TZO after it has died. Recently, an international team of astronomers examined the abstrusely named x-ray source 1E161348-5055, which has perplexed scientists since its discovery several years ago. Initial results suggested its power comes from a neutron star—1E161348¬5055 is in fact located in a supernova remnant estimated to be just 2,000 years old—but its rotation period is 6.67 hours. Such a young neutron star should be rotating thousands of times faster; this slow period is more indicative of a neutron star that is several million years old. Several theories have been suggested over the years—perhaps the neutron star has a stellar companion, or perhaps it has an unusually high magnetic field—but no one has explained this mysterious x-ray source to everyone’s satisfaction.

A TZO ghost may fit the bill. As a TZO, it might have burned for up to a million years. But a TZO’s outer layers are not as dense as a normal star’s, meaning this envelope of material is prone to dissipating over reasonably short time scales. The strong stellar wind common in larger stars could be all that’s needed to blow the outer envelope away. This would leave behind a shell similar to a supernova remnant and a neutron star that is far older than its environment suggests—exactly what astronomers see in 1E161348-5055.

Looking deeper

Astronomers also are considering whether some parts of our galactic neighborhood might be easier hunting grounds for TZOs. Globular clusters present a particularly appealing target. Stars in a globular cluster all formed around the same time, are densely packed, and are old, meaning they have few of the heavy elements that enrich newer stars. A crowded globular cluster hosts the ideal circumstances to give a neutron star the needed “kick” to merge with a red supergiant star, and the unusual spectroscopic lines would stand out more easily in the metal-poor population.

As spectrographs and telescopes improve and surveys probe ever deeper into our celestial surroundings, TZO-hunters will keep trying to learn more about these weird stars, how they form and how they die, and how many others are waiting to be discovered. As Levesque explains, “It is very exciting to see what’s out there.”

How to make a TZO

A Thorne-Żytkow object starts its life as a normal binary star. One partner is close in mass to the Sun while the other is significantly hotter and more massive (the images to be shown are not at all to scale.) The heavier star buns through its fuel quickly and explodes as a supernova.

After the supernova, the massive partner leaves behind a tiny neutron star (even less to scale!). The Sun-like star consumes its hydrogen fuel more slowly and expands into a red supergiant. At some point , the stars’ orbits become unstable, and they begin to spiral toward each other.

 

 

 

The stars circle each other on decreasing orbits until they merge. The moment of the merger should be observable, but astronomers aren’t sure exactly what to look for. From most perspectives, the newly formed TZO now appears as a normal, if bright, red supergiant.

[1] See Yvette Cendes, “The Weirdest Star in the Universe,” Astronomy (43, 9, 2016, pp. 50-55). At the time of writing, Yvette Cendes was a Ph.D. candidate in radio astronomy at the University of Amsterdam.