What Are Multiple Star Systems?

What Are Multiple Star Systems?

When we do finally learn the full truth about our place in the galaxy, and we’re invited to join the Galactic Federation of Planets, I’m sure we’ll always be seen as a quaint backwater world orbiting a boring single star.

The terrifying tentacle monsters from the nightmare tentacle world will gurgle horrifying, but clearly condescending comments about how we’ve only got a single star in the Solar System.

The beings of pure energy will remark how only truly enlightened civilizations can come from systems with at least 6 stars, insulting not only humanity, but also the horrifying tentacle monsters, leading to another galaxy spanning conflict.

Yes, we’ll always be making up for our stellar deficit in the eyes of aliens, or whatever those creepy blobs use for eyes.

What we lack in sophistication, however, we make up in volume. In our Milky Way, fully 2/3rds of star systems only have a single star. The last 1/3rd is made up of multiple star systems.

The Milky Way as seen from Devil's Tower, Wyoming. Image Credit: Wally Pacholka
The Milky Way as seen from Devil’s Tower, Wyoming. Image Credit: Wally Pacholka

We’re taking binary stars, triple star systems, even exotic 7 star systems. When you mix and match different types of stars in various Odd Couple stellar apartments, the results get interesting.

Consider our own Solar System, where the Sun and planets formed together out a cloud of gas and dust. Gravity collected material into the center of the Solar System, becoming the Sun, while the rest of the disk spun up faster and faster. Eventually our star ignited its fusion furnace, blasting out the rest of the stellar nebula.

But different stellar nebulae can lead to the formation of multiple stars instead. What you get depends on the mass of the cloud, and how fast it’s rotating.

Check out this amazing photograph of a multiple star system forming right now.

ALMA image of the L1448 IRS3B system, with two young stars at the center and a third distant from them. Spiral structure in the dusty disk surrounding them indicates instability in the disk, astronomers said. Credit: Bill Saxton, ALMA (ESO/NAOJ/NRAO), NRAO/AUI/NSF
ALMA image of the L1448 IRS3B system, with two young stars at the center and a third distant from them. Spiral structure in the dusty disk surrounding them indicates instability in the disk, astronomers said. Credit: Bill Saxton, ALMA (ESO/NAOJ/NRAO), NRAO/AUI/NSF

In this image, you can see three stars forming together, two at the center, about 60 astronomical units away from each other (60 times the distance from the Earth to the Sun), and then a third orbiting 183 AU away.

It’s estimated these stars are only 10,000 to 20,000 years old. This is one of the most amazing astronomy pictures I ever seen.

When you have two stars, that’s a binary system. If the stars are similar in mass to each other, then they orbit a common point of mass, known as the barycenter. If the stars are different masses, then it can appear that one star is orbiting the other, like a planet going around a star.

When you look up in the sky, many of the single stars you see are actually binary stars, and can be resolved with a pair of binoculars or a small telescope. For example, in a good telescope, Alpha Centauri can be resolved into two equally bright stars, with the much dimmer Proxima Centauri hanging out nearby.

The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Credit: Skatebiker at English Wikipedia (CC BY-SA 3.0)
The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Credit: Skatebiker at English Wikipedia (CC BY-SA 3.0)

You have to be careful, though, sometimes stars just happen to be beside each other in the sky, but they’re not actually orbiting one another – this is known as an optical binary. It’s a trap.

Astronomers find that you can then get binary stars with a third companion orbiting around them. As long as the third star is far enough away, the whole system can be stable. This is a triple star system.

You can get two sets of binary stars orbiting each other, for a quadruple star system.

In fact, you can build up these combinations of stars up. For example, the star system Nu Scorpii has 7 stars in a single system. All happily orbiting one another for eons.

If stars remained unchanging forever, then this would be the end of our story. However, as we’ve discussed in other articles, stars change over time, bloating up as red giants, detonating as supernovae and turning into bizarre objects, like white dwarfs, neutron stars and even black holes. And when these occur in multiple star systems, well, watch the sparks fly.

There are a nearly infinite combinations you can have here: main sequence, red giant, white dwarf, neutron star, and even black holes. I don’t have time to go through all the combinations, but here are some highlights.

This artist’s impression shows VFTS 352 — the hottest and most massive double star system to date where the two components are in contact and sharing material. The two stars in this extreme system lie about 160 000 light-years from Earth in the Large Magellanic Cloud. This intriguing system could be heading for a dramatic end, either with the formation of a single giant star or as a future binary black hole. ESO/L. Calçada
VFTS 352 is the hottest and most massive double star system to date where the two components are in contact and sharing material. ESO/L. Calçada

For starters, binary stars can get so close they actually touch each other. This is known as a contact binary, where the two stars actually share material back and forth. But it gets even stranger.

When a main sequence star like our Sun runs out of hydrogen fuel in its core, it expands as a red giant, before cooling and becoming a white dwarf.

When a red giant is in a binary system, the distance and evolution of its stellar companion makes all the difference.

If the two stars are close enough, the red giant can pass material over to the other star. And if the red giant is large enough, it can actually engulf its companion. Imagine our Sun, orbiting within the atmosphere of a red giant star. Needless to say, that’s not healthy for any planets.

An even stranger contact binary happens when a red giant consumes a binary neutron star. This is known as a Thorne-Zytkow object. The neutron star spirals inward through the atmosphere of the red giant. When it reaches the core, it either becomes a black hole, gobbling up the red giant from within, or an even more massive neutron star. This is exceedingly rare, and only one candidate object has ever been observed.

A Type Ia supernova occurs when a white dwarf accretes material from a companion star until it exceeds the Chandrasekhar limit and explodes. By studying these exploding stars, astronomers can measure dark energy and the expansion of the universe. CfA scientists have found a way to correct for small variations in the appearance of these supernovae, so that they become even better standard candles. The key is to sort the supernovae based on their color. Credit: NASA/CXC/M. Weiss
A white dwarf accreting material from a companion star. Credit: NASA/CXC/M. Weiss

When a binary pair is a white dwarf, the dead remnant of a star like our Sun, then material can transfer to the surface of the white dwarf, causing novae explosions. And if enough material is transferred, the white dwarf explodes as a Type 1A supernova.

If you’re a star that was unlucky enough to be born beside a very massive star, you can actually kicked off into space when it explodes as a supernova. In fact, there are rogue stars which such a kick, they’re on an escape trajectory from the entire galaxy, never to return.

If you have two neutron stars in a binary pair, they release energy in the form of gravitational waves, which causes them to lose momentum and spiral inward. Eventually they collide, becoming a black hole, and detonating with so much energy we can see the explosions billions of light-years away – a short-period gamma ray burst.

The combinations are endless.

How Earth could look with two suns. Credit: NASA/JPL-Caltech/Univ. of Ariz.
How Earth could look with two suns. Credit: NASA/JPL-Caltech/Univ. of Ariz.

It’s amazing to think what the night sky would look like if we were born into a multiple star system. Sometimes there would be several stars in the sky, other times just one. And rarely, there would be an actual night.

How would life be different in a multiple star system? Let me know your thoughts in the comments.

In our next episode, we try to untangle this bizarre paradox. If the Universe is infinite, how did it start out as a singularity? That doesn’t make any sense.

We glossed over it in this episode, but one of the most interesting effects of multiple star systems are novae, explosions of stolen material on the surface of a white dwarf star. Learn more about it in this video.

A Pulsar and White Dwarf Dance Together In A Surprising Orbit

Searching the Universe for strange new star systems can lead to some pretty interesting finds. And sometimes, it can turn up phenomena that contradict everything we think we know about the formation and evolution of stars. Such finds are not only fascinating and exciting, they allow us the chance to expand and refine our models of how the Universe came to be.

For instance, a recent study conducted by an international team of scientists has shown how the recent discovery of binary system – a millisecond pulsar and a low-mass white dwarf (LMWD) – has defied conventional ideas of stellar evolution. Whereas such systems were believed to have circular orbits in the past, the white dwarf in this particular binary orbits the pulsar with extreme eccentricity!

To break it down, conventional wisdom states that LMWDs are the product of binary evolution. The reason for this is because that under normal circumstances, such a star – with low mass but incredible density – would only form after it has exhausted all its nuclear fuel and lost its outer layers as a planetary nebula. Given the mass of this star, this would take about 100 billion years to happen on its own – i.e. longer than the age of the Universe.

An artist's impression of an accreting X-ray millisecond pulsar. The flowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit: NASA / Goddard Space Flight Center / Dana Berry
An artist’s impression of an accreting X-ray millisecond pulsar. The flowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit: NASA/Goddard/Dana Berry

As such, they are generally believed to be the result of pairing with other stars – specifically, millisecond radio pulsars (MSPs). These are a distinct population of neutron stars that have fast spin periods and magnetic fields that are several orders of magnitude weaker than that of “normal” pulsars. These properties are thought to be the result of mass transfer with a companion star.

Basically, MSPs that are orbited by a star will slowly strip them of their mass, sucking off their outer layers and turning them into a white dwarf. The addition of this mass to the pulsar causes it to spin faster and buries its magnetic field, and also strips the companion star down to a white dwarf. In this scenario, the eccentricity of orbit of the LMWD around the pulsar is expected to be negligible.

However, when looking to the binary star system PSR J2234+0511, the international team noticed something entirely different. Here, they found a low-mass white dwarf paired with a millisecond pulsar which the white dwarf orbited with a period of 32 days and an extreme eccentricity (0.13).  Since this defies current models of white dwarf stars, the team began looking for explanations.

As Dr. John Antoniadis – a researcher from the Dunlap Institute at University of Toronto and the lead author of the study – told Universe Today via email:

“Millisecond pulsar-LMWD binaries are very common. According to the established formation scenario, these systems evolve from low-mass X-ray binaries in which a neutron star accretes matter from a giant star. Eventually, this star evolves into a white dwarf and the neutron star becomes a millisecond pulsar. Because of the strong tidal forces during the mass-transfer episode, the orbits of these systems are extremely circular, with eccentricities of ~0.000001 or so.”
 An artist's impression of a millisecond pulsar and its companion. The pulsar (seen in blue with two radiation beams) is accreting material from its bloated red companion star and increasing its rotation rate. Astronomers have measured the orbital parameters of four millisecond pulsars in the globular cluster 47 Tuc and modeled their possible formation and evolution paths. Credit: European Space Agency & Francesco Ferraro (Bologna Astronomical Observatory)
An artist’s impression of a millisecond pulsar and its companion. The pulsar (blue) is accreting material from its bloated red companion star and increasing its rotation rate. Credit: ESA/Francesco Ferraro (Bologna Astronomical Observatory)

For the sake of their study, which appeared recently in The Astrophysical Journal – titled “An Eccentric Binary Millisecond Pulsar with a Helium White Dwarf Companion in the Galactic Field” – the team relied on newly obtained optical photometry of the system provided by the Sloan Digital Sky Survey (SDSS), and spectroscopy from the Very Large Telescope from the Paranal Observatory in Chile.

In addition, they consulted recent studies that looked at other binary star systems that show this same kind of eccentric relationship. “We now know [of] 5 systems which deviate from this picture in that they have eccentricities of ~0.1 i.e. several orders of magnitude larger that what is expected in the standard scenario,” said Antoniadis. “Interestingly, they all appear to have similar eccentricities and orbital periods.”

From this, they were able to infer the temperature (8600 ± 190 K) and velocity ( km/s) of the white dwarf companion in the binary star system. Combined with constraints placed on the two body’s masses – 0.28 Solar Masses for the white dwarf and 1.4 for the pulsar – as well as their radii and surface gravity, they then tested three possible explanations for how this system came to be.

These included the possibility that neutrons stars (such as the millsecond pulsar being observed here) form through an accretion-induced collapse of a massive white dwarf. Similarly, they considered whether neutron stars undergo a transformation as they accrete material, which results in them becoming quark stars. During this process, the release of gravitational energy would be responsible for inducing the observed eccentricity.

Artist's illustration of a rotating neutron star, the remnants of a super nova explosion. Credit: NASA, Caltech-JPL
Artist’s illustration of a rotating neutron star, the remnants of a super nova explosion. Credit: NASA, Caltech-JPL

Second, they considered the possibility – consistent with current models of stellar evolution – that LMWDs within a certain mass range have strong stellar winds when they are very young (due to unstable hydrogen fusion). The team therefore looked at whether or not these strong stellar winds could have been what disrupted the orbit of the pulsar earlier in the system’s history.

Last, they considered the possibility that some of the material released from the white dwarf in the past (due to this same stellar wind) could have formed a short-lived circumbinary disk. This disk would then act like a third body, disturbing the system and increasing the eccentricity of the white dwarf’s orbit. In the end, they deemed that the first two scenarios were unlikely, since the mass inferred for the pulsar progenitor was not consistent with either model.

However, the third scenario, in which interaction with a circumbinary disk was responsible for the eccentricity, was consistent with their inferred parameters. What’s more, the third scenario predicts how (within a certain mass range) that there should be no circular binaries with similar orbital periods – which is consistent with all known examples of such systems. As Dr. Antoniadis explained:

“These observations show that the companion star in this system is indeed a low-mass white dwarf. In addition, the mass of the pulsar seems to be too low for #2 and a bit too high for #1. We also study the orbit of the binary in the Milky way, and it looks very similar to what we find for low-mass X-ray binaries. These pieces of evidence together favor the disk hypothesis.”

Cross-section of a neutron star. Credit: Wikipedia Commons/Robert Schulze
Cross-section of a neutron star. Credit: Wikipedia Commons/Robert Schulz

Of course, Dr. Antoniadis and his colleagues admit that more information is needed before their hypothesis can be deemed correct. However, should their results be borne out by future research, then they anticipate that it will be a valuable tool for future astronomers and astrophysicists looking to study the interaction between binary star systems and circumbinary disks.

In addition, the discovery of this high eccentricity binary system will make it easier to measure the masses of Low-Mass White Dwarfs with extreme precision in the coming years. This in turn should help astronomers to better understand the properties of these stars and what leads to their formation.

As history has taught us, understanding the Universe requires a serious commitment to the process of continuous discovery. And the more we discover, the stranger it seems to become, forcing us to reconsider what we think we know about it.

Further Reading: The Astrophysical Journal

How Do Supernovae Fail?

We’ve written quite a few articles on what happens when massive stars fail as supernovae. Here’s a quick recap.

A star with more than 8 times the mass of the Sun runs out of usable fuel in its core and collapses in on itself. The enormous amount of matter falling inward creates a dense remnant, like a neutron star or a black hole. Oh, and an insanely powerful explosion, visible billions of light-years away.

There are a few other classes of supernovae, but that’s the main way they go out.

But it turns out some supernovae just don’t bring their A-game. Instead hitting the ball out of the park, they choke up at the last minute.

They’re failures. They’ll never amount to anything. They’re a complete and utter disappointment to me and your mother. Oh wait, we were talking about stars, right.

So, how does a supernova fail?

New research shows that some old stars known as white dwarfs might be held up by their rapid spins, and when they slow down, they explode as Type Ia supernovae. Thousands of these "time bombs" could be scattered throughout our Galaxy. In this artist's conception, a supernova explosion is about to obliterate an orbiting Saturn-like planet.   Credit: David A. Aguilar (CfA)
In this artist’s conception, a supernova explosion is about to obliterate an orbiting Saturn-like planet. Credit: David A. Aguilar (CfA)

In a regular core collapse supernova, the infalling material pushes the star denser and denser until it reaches the density of 5 billion tons per teaspoon of matter. The black hole forms, and a shockwave ripples outward creating the supernova.

It turns out that the density and energy of the shockwave on its own isn’t enough to actually generate the supernova, and overcome the gravitational force pulling it inward. Instead, it’s believed that neutrinos created at the core pile up behind the shockwave, and give it the push it needs to blast outward into space.

In some cases, though, it’s believed that this additional energy doesn’t show up. Instead of rebounding from the core of the star, the black hole just gobbles it all up. In a fraction of a second, the star is just… gone.

According to astronomers, it might be the case that 1/3rd of all core collapse supernovae die this way, which means that a third of the supergiant stars are just disappearing from the sky. They’re there, and then a moment later, they’re not there.

Artist's rendering of a black hole. Image Credit: NASA
And this is all that remains. Image Credit: NASA

Seriously, imagine the forces and energy it must take to swallow an entire red supergiant star whole. Black holes are scary.

Astronomers have gone looking for these things, and they’ve actually been pretty tricky to find. It’s like one of those puzzles where you try to figure out what’s missing from a picture. They studied images of galaxies taken by the Hubble Space Telescope, looking for bright supergiant stars which disappeared. In one survey, studying a large group of galaxies, they only turned up a single candidate.

But they only surveyed a handful of galaxies. To really get serious about searching for them, they’ll need better tools, like the Large Synoptic Survey Telescope due for first light in just a few years. This amazing instrument will survey the entire sky every few nights, searching for anything that changes. It’ll find asteroids, comets, variable stars, supernovae, and now, supergiant stars that just disappeared.

We’ve talked about failed supernovae. Now let’s take a few moments and talk about the complete opposite: super successful supernovae.

When a star with more than 8 times the mass of the Sun explodes as a supernova, it leaves behind a remnant. For the lower mass star explosions, they leave behind a neutron star. If it’s a higher mass star, they leave behind a black hole.

But for the largest explosions, where the star had more than 130 times the mass of the Sun, the supernova is so powerful, so complete, there’s no remnant behind. There’s an enormous explosion, and the star is just gone.

No black hole ever forms.

Artist's impression of a Type II supernova explosion which involves the destruction of a massive supergiant star. Credit: ESO
Artist’s impression of a supernova explosion which involves the destruction of a massive supergiant star. Credit: ESO

Astronomers call them pair instability supernovae. In a regular core collapse supernova, the layers of the star collapse inward, producing the highly dense remnant. But in these monster stars, the core is pumping out such energetic gamma radiation that it generates antimatter in the core. The star explodes so quickly, with so much energy, it totally overpowers the gravity pulling it inward.

In a moment, the star is completely and utterly gone, just expanding waves of energy and particles.

Only a few of these supernovae have ever been observed, and they might explain some hypernovae and gamma ray bursts, the most powerful explosions in the Universe.

Beyond 250 times the mass of the Sun, however, gravity takes over again, and you get enormous black holes.

As always, the Universe behaves more strangely than we ever thought possible. Some supernova fail, completely imploding as a black hole. And others detonate entirely, leaving no remnant behind. Trust the Universe to keep mixing it up on us.

What Are Magnetars?

In a previous article, we crushed that idea that the Universe is perfect for life. It’s not. Almost the entire Universe is a horrible and hostile place, apart from a fraction of a mostly harmless planet in a backwater corner of the Milky Way.

While living here on Earth takes about 80 years to kill you, there are other places in the Universe at the very other end of the spectrum. Places that would kill you in a fraction of a fraction of a second. And nothing is more lethal than supernovae and remnants they leave behind: neutron stars.

We’ve done a few articles about neutron stars and their different flavours, so there should be some familiar terrain here.

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

As you know, neutron stars are formed when stars more massive than our Sun explode as supernovae. When these stars die, they no longer have the light pressure pushing outward to counteract the massive gravity pulling inward.

This enormous inward force is so strong that it overcomes the repulsive force that keeps atoms from collapsing. Protons and electrons are forced into the same space, becoming neutrons. The whole thing is just made of neutrons. Did the star have hydrogen, helium, carbon and iron before? That’s too bad, because now it’s all neutrons.

You get pulsars when neutron stars first form. When all that former star is compressed into a teeny tiny package. The conservation of angular motion spins the star up to tremendous velocities, sometimes hundreds of times a second.

But when neutron stars form, about one in ten does something really really strange, becoming one of the most mysterious and terrifying objects in the Universe. They become magnetars. You’ve probably heard the name, but what are they?

As I said, magnetars are neutron stars, formed from supernovae. But something unusual happens as they form, spinning up their magnetic field to an intense level. In fact, astronomers aren’t exactly sure what happens to make them so strong.

This artist’s impression shows the magnetar in the very rich and young star cluster Westerlund 1. This remarkable cluster contains hundreds of very massive stars, some shining with a brilliance of almost one million suns. European astronomers have for the first time demonstrated that this magnetar — an unusual type of neutron star with an extremely strong magnetic field — probably was formed as part of a binary star system. The discovery of the magnetar’s former companion elsewhere in the cluster helps solve the mystery of how a star that started off so massive could become a magnetar, rather than collapse into a black hole. Credit: ESO/L. Calçada
This artist’s impression shows the magnetar in the very rich and young star cluster Westerlund 1. Credit: ESO/L. Calçada

One idea is that if you get the spin, temperature and magnetic field of a neutron star into a perfect sweet spot, it sets off a dynamo mechanism that amplifies the magnetic field by a factor of a thousand.

But a more recent discovery gives a tantalizing clue for how they form. Astronomers discovered a rogue magnetar on an escape trajectory out of the Milky Way. We’ve seen stars like this, and they’re ejected when one star in a binary system detonates as a supernova. In other words, this magnetar used to be part of a binary pair.

And while they were partners, the two stars orbited one another closer than the Earth orbits the Sun. This close, they could transfer material back and forth. The larger star began to die first, puffing out and transferring material to the smaller star. This increased mass spun the smaller star up to the point that it grew larger and spewed material back at the first star.

The initially smaller star detonated as a supernova first, ejecting the other star into this escape trajectory, and then the second went off, but instead of forming a regular neutron star, all these binary interactions turned it into a magnetar.  There you go, mystery maybe solved?

The strength of the magnetic field around a magnetar completely boggles the imagination. The magnetic field of the Earth’s core is about 25 gauss, and here on the surface, we experience less than half a gauss. A regular bar magnet is about 100 gauss. Just a regular neutron star has a magnetic field of a trillion gauss.  Magnetars are 1,000 times more powerful than that, with a magnetic field of a quadrillion gauss.

What if you could get close to a magnetar? Well, within about 1,000 kilometers of a magnetar, the magnetic field is so strong it messes with the electrons in your atoms. You would literally be torn apart at an atomic level. Even the atoms themselves are deformed into rod-like shapes, no longer usable by your precious life’s chemistry.

But you wouldn’t notice because you’d already be dead from the intense radiation streaming from the magnetar, and all the lethal particles orbiting the star and trapped in its magnetic field.

Artist's conception of a starquake cracking the surface of a neutron star. Credit: Darlene McElroy of LANL
Artist’s conception of a starquake cracking the surface of a neutron star. Credit: Darlene McElroy of LANL

One of the most fascinating aspects of magnetars is how they can have starquakes. You know, earthquakes, but on stars… starquakes. When neutron stars form, they can have a delicious murder crust on the outside, surrounding the degenerate death matter inside. This crust of neutrons can crack, like the tectonic plates on Earth. As this happens, the magnetar releases a blast of radiation that we can see clear across the Milky Way.

In fact, the most powerful starquake ever recorded came from a magnetar called SGR 1806-20, located about 50,000 light years away. In a tenth of a second, one of these starquakes released more energy than the Sun gives off in 100,000 years. And this wasn’t even a supernova, it was merely a crack on the magnetar’s surface.

Magnetars are awesome, and provide the absolute opposite end of the spectrum for a safe and habitable Universe. Fortunately, they’re really far away and you won’t have to worry about them ever getting close.

What are Quark Stars?

We’ve covered the full range of exotic star-type objects in the Universe. Like Pokemon Go, we’ve collected them all. Okay fine, I’m still looking for a Tauros, and so I’ll continue to wander the streets, like a zombie staring at his phone.

Now, according to my attorney, I’ve fulfilled the requirements for shamelessly jumping on a viral bandwagon by mentioning Pokemon Go and loosely connecting it to whatever completely unrelated topic I was working on.

Any further Pokemon Go references would just be shameless attempts to coopt traffic to my channel, and I’m better than that.

It was pretty convenient, though, and it was easy enough to edit out the references to Quark on Deep Space 9 and replace them with Pokemon Go. Of course, there is a new Star Trek movie out, so maybe I miscalculated.

Anyway, now that we got that out of the way. Back to rare and exotic stellar objects.

The white dwarf G29-38 (NASA)
The white dwarf G29-38. Credit: NASA

There are the white dwarfs, the remnants of stars like our Sun which have passed through the main sequence phase, and now they’re cooling down.

There are the neutron stars and pulsars formed in a moment when stars much more massive than our Sun die in a supernova explosion. Their gravity and density is so great that all the protons and electrons from all the atoms are mashed together. A single teaspoon of neutron star weighs 10 million tons.

And there are the black holes. These form from even more massive supernova explosions, and the gravity and density is so strong they overcome the forces holding atoms themselves together.

White dwarfs, neutron stars and black holes. These were all theorized by physicists, and have all been discovered by observational astronomers. We know they’re out there.

Is that it? Is that all the exotic forms that stars can take?  That we know of, yes, however, there are a few even more exotic objects which are still just theoretical. These are the quark stars. But what are they?

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

Let’s go back to the concept of a neutron star. According to the theories, neutron stars have such intense gravity they crush protons and electrons together into neutrons. The whole star is made of neutrons, inside and out. If you add more mass to the neutron star, you cross this line where it’s too much mass to hold even the neutrons together, and the whole thing collapses into a black hole.

A star like our Sun has layers. The outer convective zone, then the radiative zone, and then the core down in the center, where all the fusion takes place.

Could a neutron star have layers? What’s at the core of the neutron star, compared to the surface?

The idea is that a quark star is an intermediate stage in between neutron stars and black holes. It has too much mass at its core for the neutrons to hold their atomness. But not enough to fully collapse into a black hole.

The difference between a neutron star and a quark star (Chandra)
The difference between a neutron star and a quark star. Credit: Chandra

In these objects, the underlying quarks that form the neutrons are further compressed. “Up” and “down” quarks are squeezed together into “strange” quarks. Since it’s made up of “strange” quarks, physicists call this “strange matter”. Neutron stars are plenty strange, so don’t give it any additional emotional weight just because it’s called strange matter. If they happened to merge into “charm” quarks, then it would be called “charm matter”, and I’d be making Alyssa Milano references.

And like I said, these are still theoretical, but there is some evidence that they might be out there. Astronomers have discovered a class of supernova that give off about 100 times the energy of a regular supernova explosion. Although they could just be mega supernovae, there’s another intriguing possibility.

They might be heavy, unstable neutron stars that exploded a second time, perhaps feeding from a binary companion star. As they hit some limit, they converting from a regular neutron star to one made of strange quarks.

But if quark stars are real, they’re very small. While a regular neutron star is 25 km across, a quark star would only be 16 km across, and this is right at the edge of becoming a black hole.

A neutron star (~25km across) next to a quark star (~16km across). Original Image Credit: NASA's Goddard Space Flight Center
A neutron star (~25km across) next to a quark star (~16km across). Original Image Credit: NASA’s Goddard Space Flight Center

If quark stars do exist, they probably don’t last long. It’s an intermediate step between a neutron star, and the final black hole configuration. A last gasp of a star as its event horizon forms.

It’s intriguing to think there are other exotic objects out there, formed as matter is compressed into tighter and tighter configurations, as the different limits of physics are reached and then crossed. Astronomers will keep searching for quark stars, and I’ll let you know if they find them.

How Fast Can Stars Spin?

Everything in the Universe is spinning. Spinning planets and their spinning moons orbit around spinning stars, which orbit spinning galaxies. It’s spinning all the way down.

Consider that fiery ball in the sky, the Sun. Like all stars, our Sun rotates on its axis. You can’t tell because staring at the Sun long enough will permanently damage your eyeballs. Instead you can use a special purpose solar telescope to observe sunspots and other features on the surface of the Sun. And if you track their movements, you’ll see that the Sun’s equator takes 24.47 days to turn once on its axis. Unlike its slower poles which take 26.24 days to turn.

The Sun isn’t a solid ball of rock, it’s a sphere of hot plasma, so the different regions can complete their rotation at different rates. But it rotates so slowly that it’s an almost perfect sphere.

If you were standing on the surface of the Sun, which you can’t, of course, you would be whipping around at 7,000 km/h. That sounds fast, but just you wait.

How does that compare to other stars, and what’s the fastest that a star can spin?

Achenar is located at the lower right of the constellation Eridanus.
Achenar rotates much faster than our Sun. It is located at the lower right of the constellation Eridanus.

A much faster spinning star is Achenar, the tenth brightest star in the sky, located 139 light-years away in the constellation of Eridanus. It has about 7 times the mass of the Sun, but it spins once on its axis every 2 days. If you could see Achenar up close, it would look like a flattened ball. If you measured it from pole to pole, it would be 7.6 Suns across, but if you measured across the equator, it would be 11.6 Suns across.

If you were standing on the surface of Achenar, you’d be hurtling through space at 900,000 km/h.

The very fastest spinning star we know of is the 25 solar mass VFTS 102, located about 160,000 light-years away in the Large Magellanic Cloud’s Tarantula Nebula – a factory for massive stars.

If you were standing on the surface of VFTS 102, you’d be moving at 2 million km/h.

In fact, VFTS 102 is spinning so quickly, it can just barely keep itself together. Any faster, and the outward centripetal force would overcome the gravity holding its guts in, and it would tear itself apart. Perhaps that’s why we don’t see any spinning faster; because they couldn’t handle the speed. It appears that this is the fastest that stars can spin.

This is an artist's concept of the fastest rotating star found to date. The massive, bright young star, called VFTS 102, rotates at a million miles per hour, or 100 times faster than our Sun does. Centrifugal forces from this dizzying spin rate have flattened the star into an oblate shape and spun off a disk of hot plasma, seen edge on in this view from a hypothetical planet. The star may have "spun up" by accreting material from a binary companion star. The rapidly evolving companion later exploded as a supernova. The whirling star lies 160,000 light-years away in the Large Magellanic Cloud, a satellite galaxy of our Milky Way.  Credit: NASA, ESA, and G. Bacon (STScI)
This is an artist’s concept of VFTS 102, the fastest rotating star found to date. Credit: NASA, ESA, and G. Bacon (STScI)

One other interesting note about VFTS 102 is that it’s also hurtling through space much faster than the stars around it. Astronomers think it was once in a binary system with a partner that detonated as a supernova, releasing it into space like a catapult.

Not only stars can spin. Dead stars can spin too, and they take this to a whole other level.

Neutron stars are what you get when a star with much more mass than the Sun detonates as a supernova. Suddenly you’ve got a stellar remnant with twice the mass of the Sun compressed down into a tiny ball about 20 km across. All that angular momentum of the star is retained, and so the neutron star spins at an enormous speed.

The fastest neutron star ever recorded spins around 700 times a second. We know it’s turning this quickly because it’s blasting out beams of radiation that sweep towards us like an insane lighthouse. This, of course, is a pulsar, and we did a whole episode on them.

A regular star would be torn apart, but neutron stars have such intense gravity, they can rotate this quickly. Over time, the radiation streaming from the neutron star strips away its angular momentum, and it slows down.

A black hole with an accretion disk. Credit: (NASA/Dana Berry/SkyWorks Digital)

Black holes can spin even faster than that. In fact, when a black hole is actively feeding from a binary companion, or a supermassive black hole is gobbling up stars, it can rotate at nearly the speed of light. The laws of physics prevent anything in the Universe spinning faster than the speed of light, and black holes go right up to the edge of the law without breaking it.

Astronomers recently found a supermassive black hole spinning up to 87% the maximum speed permitted by relativity.

If you were hoping there are antimatter lurking out there, hoarding all that precious future energy, I’m sorry to say, but astronomers have looked and they haven’t found it. Just like the socks in your dryer, we may never discover where it all went.

Are There Antimatter Galaxies?

One of the biggest mysteries in astronomy is the question, where did all the antimatter go? Shortly after the Big Bang, there were almost equal amounts of matter and antimatter. I say almost, because there was a tiny bit more matter, really. And after the matter and antimatter crashed into each other and annihilated, we were left with all the matter we see in the Universe.

You, and everything you know is just a mathematical remainder, left over from the great division of the Universe’s first day.

We did a whole article on this mystery, so I won’t get into it too deeply.

But is it possible that the antimatter didn’t actually go anywhere? That it’s all still there in the Universe, floating in galaxies of antimatter, made up of antimatter stars, surrounded by antimatter planets, filled with antimatter aliens?

Aliens who are friendly and wonderful in every way, except if we hugged, we’d annihilate and detonate with the energy of gigatons of TNT. It’s sort of tragic, really.

If those antimatter galaxies are out there, could we detect them and communicate with those aliens?

First, a quick recap on antimatter.

Antimatter is just like matter in almost every way. Atoms have same atomic mass and the exact same properties, it’s just that all the charges are reversed. Antielectrons have a positive charge, antihydrogen is made up of an antiproton and a positron (instead of a proton and an electron).

It turns out this reversal of charge causes regular matter and antimatter to annihilate when they make contact, converting all their mass into pure energy when they come together.

We can make antimatter in the laboratory with particle accelerators, and there are natural sources of the stuff. For example, when a neutron star or black hole consumes a star, it can spew out particles of antimatter.

In fact, astronomers have detected vast clouds of antimatter in our own Milky Way, generated largely by black holes and neutron stars grinding up their binary companions.

Wyoming Milky Way set. Credit and copyright: Randy Halverson.
Wyoming Milky Way set. Credit and copyright: Randy Halverson.

But our galaxy is mostly made up of regular matter. This antimatter is detectable because it’s constantly crashing into the gas, dust, planets and stars that make up the Milky Way. This stuff can’t get very far without hitting anything and detonating.

Now, back to the original question, could you have an entire galaxy made up of antimatter? In theory, yes, it would behave just like a regular galaxy. As long as there wasn’t any matter to interact with.

And that’s the problem. If these galaxies were out there, we’d see them interacting with the regular matter surrounding them. They would be blasting out radiation from all the annihilations from all the regular matter gas, dust, stars and planets wandering into an antimatter minefield.

Astronomers don’t see this as far as they look, just the regular, quiet and calm matter out to the edge of the observable Universe.

That doesn’t make it completely impossible, though, there could be galaxies of antimatter as long as they’re completely cut off from regular matter.

But even those would be detectable by the supernova explosions within them. A normally matter supernova generates fast moving neutrinos, while an antimatter supernova would generate a different collection of particles. This would be a dead giveaway.

There’s one open question about antimatter that might make this a deeper mystery. Scientists think that antimatter, like regular matter, has regular gravity. Matter and antimatter galaxies would be attracted to each other, encouraging annihilation.

But scientists don’t actually know this definitively yet. It’s possible that antimatter has antigravity. An atom of antihydrogen might actually fall upwards, accelerating away from the center of the Earth.

The ALPHA experiment, one of five experiments that are studying antimatter at CERN Credit: Maximilien Brice/CERN

Physicists at CERN have been generating antimatter particles, and trying to detect if they’re falling downward or up.

If that was the case, then antimatter galaxies might be able to repel particles of regular matter, preventing the annihilation, and the detection.

If you were hoping there are antimatter lurking out there, hoarding all that precious future energy, I’m sorry to say, but astronomers have looked and they haven’t found it. Just like the socks in your dryer, we may never discover where it all went.

Can We Now Predict When A Neutron Star Will Give Birth To A Black Hole?

A neutron star is perhaps one of the most awe-inspiring and mysterious things in the Universe. Composed almost entirely of neutrons with no net electrical charge, they are the final phase in the life-cycle of a giant star, born of the fiery explosions known as supernovae. They are also the densest known objects in the universe, a fact which often results in them becoming a black hole if they undergo a change in mass.

For some time, astronomers have been confounded by this process, never knowing where or when a neutron star might make this final transformation. But thanks to a recent study by a team of researchers from Goethe University in Frankfurt, Germany, it may now be possible to determine the absolute maximum mass that is required for a neutron star to collapse, giving birth to a new black hole.

Continue reading “Can We Now Predict When A Neutron Star Will Give Birth To A Black Hole?”

Andromeda’s First Spinning Neutron Star Found

Andromeda's spinning neutron star. Though astronomers think there are over 100 million of these objects in the Milky Way, this is the first one found in Andromeda. Image: ESA/XMM Newton.

On a clear night, away from the bright lights of a city, you can see the smudge of the Andromeda galaxy with the naked eye. With a backyard telescope, you can take a good look at the Milky Way’s sister galaxy. With powerful observatories, it’s possible to see deep inside Andromeda, which is what astronomers have been doing for decades.

Now, astronomers combing through data from the ESA’s XMM Newton space telescope have found something rare, at least for Andromeda; a spinning neutron star. Though these objects are common in the Milky Way, (astronomers think there are over 100 million of them) this is the first one discovered in Andromeda.

A neutron star is the remnant of a massive star that went supernova. They are the smallest and most dense stellar objects known. Neutron stars are made entirely of neutrons, and have no electrical charge. They spin rapidly, and can emit electromagnetic energy.

If the neutron star is oriented toward Earth in just the right way, we can detect their emitted energy as pulses. Think of them as lighthouses, with their beam sweeping across Earth. The pulses of energy were first detected in 1967, and given the name pulsar.” We actually discovered pulsars before we knew that neutron stars existed.

Many neutron stars, including this one, exist in binary systems, which makes them easier to detect. They cannibalize their companion star, drawing gas from the companion into their magnetic fields. As they do so, they emit high energy pulses of X-ray energy.

The star in question, which astronomers, with their characteristic flair for language, have named 3XMM J004301.4+413017, spins rapidly: once every 1.2 seconds. It’s neighbouring star orbits it once every 1.3 days. While these facts are known, a more detailed understanding of the star will have to wait for more analysis. But 3XMM J004301.4+413017 does appear to be an exotic object.

“It could be what we call a ‘peculiar low-mass X-ray binary pulsar’ – in which the companion star is less massive than our Sun – or alternatively an intermediate-mass binary system, with a companion of about two solar masses,” says Paolo Esposito of INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, Milan, Italy. “We need to acquire more observations of the pulsar and its companion to help determine which scenario is more likely.”

“We’re in a better position now to uncover more objects like this in Andromeda, both with XMM-Newton and with future missions such as ESA’s next-generation high-energy observatory, Athena,” added Norbert Schartel, ESA’s XMM-Newton project scientist.

This discovery is a result of EXTraS, a European Project that combs through XMM Newton data. “EXTraS discovery of an 1.2-s X-ray pulsar in M31” by P. Esposito et al, is published in the Monthly Notices of the Royal Astronomical Society, Volume 457, pp L5-L9, Issue 1 March 21, 2016.

What’s the Big Deal About the Pentaquark?

“Three quarks for Muster Mark!,” wrote James Joyce in his labyrinthine fable, Finnegan’s Wake. By now, you may have heard this quote – the short, nonsensical sentence that eventually gave the name “quark” to the Universe’s (as-yet-unsurpassed) most fundamental building blocks. Today’s physicists believe that they understand the basics of how quarks combine; three join up to form baryons (everyday particles like the proton and neutron), while two – a quark and an antiquark – stick together to form more exotic, less stable varieties called mesons. Rare four-quark partnerships are called tetraquarks. And five quarks bound in a delicate dance? Naturally, that would be a pentaquark. And the pentaquark, until recently a mere figment of physics lore, has now been detected at the LHC!

So what’s the big deal? Far from just being a fun word to say five-times-fast, the pentaquark may unlock vital new information about the strong nuclear force. These revelations could ultimately change the way we think about our superbly dense friend, the neutron star – and, indeed, the nature of familiar matter itself.

Physicists know of six types of quarks, which are ordered by weight. The lightest of the six are the up and down quarks, which make up the most familiar everyday baryons (two ups and a down in the proton, and two downs and an up in the neutron). The next heaviest are the charm and strange quarks, followed by the top and bottom quarks. And why stop there? In addition, each of the six quarks has a corresponding anti-particle, or antiquark.

Six types of quark, arranged from left to right by way of their mass, depicted along with the other elementary particles of the Standard Model. The Higgs boson was added to the right side of the menagerie in 2012. (Image Credit: Fermilab)

An important attribute of both quarks and their anti-particle counterparts is something called “color.” Of course, quarks do not have color in the same way that you might call an apple “red” or the ocean “blue”; rather, this property is a metaphorical way of communicating one of the essential laws of subatomic physics – that quark-containing particles (called hadrons) always carry a neutral color charge.

For instance, the three components of a proton must include one red quark, one green quark, and one blue quark. These three “colors” add up to a neutral particle in the same way that red, green, and blue light combine to create a white glow. Similar laws are in place for the quark and antiquark that make up a meson: their respective colors must be exactly opposite. A red quark will only combine with an anti-red (or cyan) antiquark, and so on.

The pentaquark, too, must have a neutral color charge. Imagine a proton and a meson (specifically, a type called a J/psi meson) bound together – a red, a blue, and a green quark in one corner, and a color-neutral quark-antiquark pair in the other – for a grand total of four quarks and one antiquark, all colors of which neatly cancel each other out.

Physicists are not sure whether the pentaquark is created by this type of segregated arrangement or whether all five quarks are bound together directly; either way, like all hadrons, the pentaquark is kept in check by that titan of fundamental dynamics, the strong nuclear force.

The strong nuclear force, as its name implies, is the unspeakably robust force that glues together the components of every atomic nucleus: protons and neutrons and, more crucially, their own constituent quarks. The strong force is so tenacious that “free quarks” have never been observed; they are all confined far too tightly within their parent baryons.

But there is one place in the Universe where quarks may exist in and of themselves, in a kind of meta-nuclear state: in an extraordinarily dense type of neutron star. In a typical neutron star, the gravitational pressure is so tremendous that protons and electrons cease to be. Their energies and charges melt together, leaving nothing but a snug mass of neutrons.

Physicists have conjectured that, at extreme densities, in the most compact of stars, adjacent neutrons within the core may even themselves disintegrate into a jumble of constituent parts.

The neutron star… would become a quark star.

The difference between a neutron star and a quark star (Chandra)
The difference between a neutron star and a quark star. (Image Credit: Chandra)

Scientists believe that understanding the physics of the pentaquark may shed light on the way the strong nuclear force operates under such extreme conditions – not only in such overly dense neutron stars, but perhaps even in the first fractions of a second following the Big Bang. Further analysis should also help physicists refine their understanding of the ways that quarks can and cannot combine.

The data that gave rise to this discovery – a whopping 9-sigma result! – came out of the LHC’s first run (2010-2013). With the supercollider now operating at double its original energy capacity, physicists should have no problem unraveling the mysteries of the pentaquark even further.

A preprint of the pentaquark discovery, which has been submitted to the journal Physical Review Letters, can be found here.