Two thousand light years from Earth is a star known as Epsilon Aurigae. It’s a third magnitude star most of the time, but about every 27 years it dims to about half its brightness for nearly two years. The cause of the dimming is a bit of a mystery. 

It’s long been thought that the dimming is the result of Epsilon Aurigae being a binary system. With a companion star in a large orbit, it could pass in front of the primary star, making it appear to dim. The star is indeed a binary star (if not a multiple star) but the details of the dimming mechanism have been difficult to pin down.

Historically there have been two main ideas. The first is that Epsilon Aurigae is yellow supergiant about 15 times the mass of the Sun, with a companion of similar mass obscured somewhat by dust. This idea is supported by the fact that the spectrum of Epsilon Aurigae has many of the signatures common to yellow supergiants. However the companion star has a spectral signature more similar to a B-type main sequence star.

The other idea is that Epsilon Aurigae is much smaller, with a mass of 2 to 4 solar masses. This would make it smaller than the B-type companion with a mass of about 6 solar masses. In order for the companion to be much dimmer than Epsilon Aurigae, it would have to be surrounded by a thick disk of dust, and that disk would have to be aligned edge on when seen from Earth. It would be odd for a main sequence star to have a thick dusty disk, since they are more commonly seen around young stars that are still forming.

When the most recent dimming occurred in 2009 – 20011, both amateur astronomy groups and the Spitzer infrared telescope made observations of the transit. It now seems that both models were at least partly right. The model that now seems to best fit the data assumes Epsilon Aurigae is only about 10 solar masses, but it moving toward the end of its life. This means it is much brighter than a main sequence star of similar mass. The B-type companion is therefore much dimmer by comparison. With smaller masses, the two stars would be close enough that the companion would capture gas and dust pushed away from Epsilon Aurigae, thus explaining the companion’s dusty disk.

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

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

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

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

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

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A contact binary star occurs when two stars are close enough that their outer layers are in contact with each other. Often this can occur when one member of a close binary system enters a red giant stage and swells to the point where it is in contact with the companion star. Such systems can create novae, and perhaps supernovae. But recently we’ve discovered a contact binary consisting of two O-type stars.

Currently the two stars orbit each other about once a day, as seen in the artist video above. But their eventual fate is still unknown. Stars of this size typically have a cosmically short lifetime of about 5 million years. It’s possible that they merge into a single, fast-rotating star. Such a supermassive could end its life with a long duration gamma ray burst. But it’s also possible that the stars remain a stable binary throughout their lifetime. If that’s the case, then each would end their lives as supernovae, and produce a black hole. This could create a close black hole binary.

Such a black hole binary would be a boon for astronomers trying to detect gravitational waves, since close binary black holes would create the strongest consistent signal.

Paper: Almeida, L. A., et al. Discovery of the massive overcontact binary VFTS 352: Evidence for enhanced internal mixing. The Astrophysical Journal 812 (2) (2015)

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In August of last year a star briefly brightened by a factor of 5 in a single day. Known as Gaia 14aae is a cataclysmic variable. It is a binary where the two stars orbit closely enough for the outer layers of one star to be captured by the other. Not only do these stars orbit each other every 50 minutes, their orbits are aligned so that the larger star totally eclipses the smaller one  with each orbit. 

The light curve of the binary system. Credit: Campbell et al.

The smaller star actually has the greater mass. It is a white dwarf with a mass about 80% that of our Sun. The larger star only has a mass 15% of our Sun, but has expanded to 125 times the diameter of our Sun as it approaches the end of its life. One of the striking features about this binary is that it doesn’t contain much hydrogen. This is exactly what you’d expect if the white dwarf has already stripped off much of the outer layer from its companion star. As the white dwarf continues to capture material from the other star, we can expect more flare ups similar to the brightening of last year.

But what makes this system particularly interesting is that it could provide clues as to just how type Ia supernovae might form. These “standard candle” supernovae are used to determine the distances of the earliest galaxies, but we still aren’t entirely sure what causes them. One possibility is the stellar dance of close binaries like Gaia 14aae.

Paper: H. C. Campbell et al. Total eclipse of the heart: the AM CVn Gaia14aae/ASSASN-14cn. MNRAS 452 (1): 1060-1067 (2015)

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The pulsar J1906+0746 has gone silent, and that’s good news for general relativity.

A pulsar is a rapidly spinning neutron star. Neutron stars have incredibly strong magnetic fields. As a charged particles are trapped near the magnetic poles, they give off intense beams of radio waves. The rotation of the neutron star (and thus the poles) sweeps the beam around, much like the light from a lighthouse. When the beam is facing us, we see a pulse.

This particular pulsar is a close binary, and orbits another star of similar mass. The two stars are separated by about the width of our Sun, and orbit each other ever four hours. They are so close to each other that their orbits are affected by general relativity, including the effect of gravitational waves.

Over the past five years, a team has monitored the pulsar continuously, capturing about a billion pulses from J1906+0746. The goal was to compare the orbital predictions of general relativity with the observed orbital behavior of the pulsar. Not only did the observations match, the team also observed the pulsar fade over time, which is another prediction from general relativity.

The pulsar hasn’t actually stopped emitting energy, but its beam no long sweeps in our direction. That’s because the axis of rotation has shifted, through a process known as precession. We see this effect with Earth, where due to the gravitational pull of the Sun and Moon, our north pole drifts relative to the stars. This is why our current north star hasn’t always been the north star. In general relativity precession also occurs, but it happens at a slightly different rate. In the case of this pulsar, that is about two degrees of drift every year.

The pulsar hasn’t disappeared forever, though. We should start seeing it pulse again around 2170.

Paper: J. van Leeuwen, et al. The Binary Companion of Young, Relativistic Pulsar J1906+0746. 2015 ApJ 798 118 (2015)

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In the constellation of Camelopardalis (also known as the Giraffe) is a faint star known as MY Cam. It appears faint because it is about 13,000 light years away, but its spectrum showed that it was actually a blue supergiant. Then recently, detailed measurements indicated that it was a spectroscopic binary consisting of two blue supergiants. Now a new paper in Astronomy & Astrophysics has revealed that the two stars orbit very, very closely.

Brightness curve of the binary. Credit: J. Lorenzo, et al.

In analyzing the binary, the team found it had an orbital period of a bit more than a day. They also found that the masses of the two stars were about 38 and 32 solar masses. Given their masses and orbital periods, the two stars must be close enough to share a common envelope. In other words, the two stars are basically touching, and their outer layers are connected.

What makes this discovery particularly interesting is that these stars are also quite young (about two million years old). This means they must have formed very close to each other. Binary stars such as this have been proposed as a way to form hypergiant stars, and this discovery gives support to this idea.

Of course the big question is what will happen when they merge? Will it produce a supernova-like explosion, or will the resulting hypergiant star stabilize quickly. We’ll just have to wait and see.

Paper: J. Lorenzo, et al. MY Camelopardalis, a very massive merger progenitor. A&A 572, A110 (2014)

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With the growing number of discovered exoplanets, we’ve come to realize that planets and solar systems are very diverse. For ages it was suspected that planets orbited other stars. It was also generally thought that they would be similar to our own, with small, rocky, inner planets and large, gaseous outer worlds. Now we know that Jupiter-type planets are often quite close to their sun. We know that planets don’t just form around stars like our Sun, but also around red dwarfs, and sometimes even alone without a sun. We’ve also found planets that orbit two stars.

Although we’ve found planets in binary star systems, it isn’t clear how they could have formed. Before computer simulations, it was thought that planetary orbits in a binary star system would be too unstable to exist for long. We now know that there can be regions around a star in a binary system that where planets can have quite stable orbits. But the other question has been whether such planets could form in the first place. In a binary system, the two stars would tend to capture most of the material as the solar system formed, which would leave little left over for planets to form.

Images of the debris ring of GG Tau-A. Credit: Anne Dutrey, et al.

But a paper this week in Nature seems to show that planets can, in fact, form in a binary system. The work is based on data gathered at the ALMA radio telescope array, and shows a binary star system in the process of forming planets. The system is known as GG Tau-A, and images show it has two debris regions. The first is a large debris ring around the binary system itself, while the second is a debris region around the primary star. This by itself isn’t too surprising. The outer ring is the remnant of the binary system’s formation, and the inner region has yet to be swept clean by the two stars.

But the team also found evidence of material from the outer ring being swept to the inner region. Thus, while inner region material might be captured by the stars, it is replenished by material from the outer ring. This means the inner region could contain material long enough for planets to form.

The question remains as to whether this transfer-effect is common in binary systems, or if this is just an unusual fluke. But if it is common we could have an explanation for binary solar systems. Either way, it is one more example of just how diverse planetary systems can be.

Paper: Anne Dutrey, et al. Possible planet formation in the young, low-mass, multiple stellar system GG Tau A. Nature 514, 600–602 (2014)

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An occultation is where one object passes in front of another from your vantage point. It is similar to an eclipse or transit, but in this case the occulting object completely blocks the more distant object. Typically occultations occur when the moon occults planets, or when solar system objects occult stars.

Usually occultations hit the news when the moon occults a planet or particularly bright star, which are fairly rare. But lunar occultations of faint stars occurs all the time, and it can be a useful tool for astronomers.

The Moon has two properties that are particularly useful during occultations. The first is that it has a large apparent size (about half a degree of arc), so it can occult a fairly wide strip of the sky. The second is that it has no atmosphere. This means that as the Moon passes in front of a planet or star, there is no atmospheric distortion. The Moon acts as a hard edge that sweeps across the object to block it.

This means we can observe a star as it is gradually blocked or revealed by the Moon to get slice by slice data. By analyzing this data we can gain information about the star. For example, recently lunar occultations of faint stars were analyzed to discover 25 new binary stars.

Credit: F. C. Fekel, et al

These new binary stars are too close together to easily resolve as separate stars, and using the spectroscopic method to distinguish them as binaries would require long term observations. But a lunar occultation can reveal their binary nature. You can see how this was done in the figure above. The brightness of a star is measured as the Moon begins to pass in front of it. As you can see, the brightness level drops in two steps. The first drop occurs as one star of the binary is occulted by the Moon, and the second drop occurs when the other star is occulted. If it were a single star, there would be only a single drop as the star is blocked.

By observing faint stars occulted by the moon, the team was able to identify the ones with a two-step change in brightness, thus revealing them as binary stars. Further analysis of the brightness levels can tell us about the relative brightness of each star, which helps us determine the size of each star in the binary pair.

Sometimes a chance celestial alignment can reveal more about our stellar neighborhood.

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In the constellation of Ursa Major (of which the Big Dipper is a part), in the crook of the tail (or bend in the handle) is a double star known as Mizar and Alcor, seen above. These stars are visually separated just enough to be distinguishable to the naked eye. Distinguishing them requires good vision, and they were sometimes used as a test of visual acuity.

Mizar and Alcor are a double star, since they appear close together from our point of view. Some double stars are binary stars, meaning that they orbit each other under mutual gravitational attraction, but others are simply aligned in the same direction by chance, and can be light years away from each other. With Mizar and Alcor it is unclear whether they are also a binary system. They are similar distances from us (about 80 light years), but they are about a light year from each other, which would be quite distant for a binary system. On the other hand, they have similar proper motions (their apparent motion through the sky) which would imply they are gravitationally connected.

Viewed under a good amateur telescope, Mizar is itself seen to be a double star. It was the first telescopic double star to be discovered, in 1617 by Benedetto Castelli. The two stars, known as Mizar A and B were later confirmed by Galileo. Together they form a binary system, being gravitationally bound to each other.

In 1889 Edward Pickering discovered that Mizar A was also a binary star. Pickering couldn’t resolve Mizar A as a double star, but instead looked at its spectrum. The spectral lines could be observed to shift gradually back and forth toward the red then toward the blue. This shifting is due to the Doppler effect of light, where light from an object moving toward you is blue shifted slightly, and light from an object moving away from you is red shifted slightly.

The oscillation of Mizar A’s spectrum from red to blue meant that it is oscillating toward and away from us over time. This is because it is in orbit with a companion star. So even though Mizar A could not be distinguished visually as a binary star, it could be observed by its spectrum. It thus marks the first spectroscopic binary to be discovered. In 1996 this system was directly observed using the Navy Precision Optical Interferometer.

In 1909, Mizar B was also discovered to be a spectroscopic binary. Finally in 2009 Alcor was determined to be a spectroscopic binary as well. This means that what appears to be a double star with the naked eye actually consists of 6 stars. At least four of them form a double binary system.

What makes this system particularly interesting is that it contains the history of astronomy. The Big Dipper is an asterism dating to early astronomy (and given several names), with a naked-eye double known for thousands of years. With the rise of telescopes Mizar is seen as a double, then with the rise of spectroscopy they are seen as doubles, and only within the last few years has a high precision array of telescopes been able to discover the third binary of the system.

So when you find the Big Dipper in the night sky, look for the double star in its handle, and think upon the hidden variables it contains.

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Recently popular-science websites have been buzzing with news of a new pulsar putting Einstein’s theory of gravity to its greatest test yet. In particular, some tout it as a test of alternatives to general relativity. While the attention this work has gotten in the press implies this is a new breakthrough, that’s not quite the case. So what’s the real deal on these latest findings?

The results have been recently published in Science. In the paper the team presents observations of a binary system known as PSR J0348+0432. You can see this system in the lower right of the figure below, compared with PSR J0737-3039A/B, which consists of two pulsars, and PSR B1913+16, also known as Hulse-Taylor or H-T.

Orbits of various known binary pulsar systems.
Credit: Norbert Wex/MPIfR

The Hulse-Taylor system is perhaps the most famous pulsar with a binary companion, since it provided the first observational evidence of gravitational waves. In Newtonian gravity, objects orbiting each other should continue to do so basically forever. This is because in Newton’s gravity stable binary orbits have no way to gain or lose energy. But Einstein’s theory of general relativity treats gravity as a curvature of space. This means that orbiting masses should create ripples in space known as gravitational waves (similar to the way stirring your coffee creates ripples). These gravitational waves carry energy, so over time orbits should lose energy by radiating gravitational waves. As a result, members of a binary system will move closer and closer to each other until they collide.

For most objects, such as binary star systems, this effect is far too small to matter. But if the members of a binary system are massive and close, then it can have a measurable effect. As you can see in the figure these binary systems are not that much bigger than the Sun, but the masses of these objects are as large or larger than the Sun. So they provide a good test of general relativity.

The Hulse-Taylor system consists of two neutron stars, one of which is a pulsar. Since pulsars give off radio pulses at a very precise rate, we can measure the signal to know just how these two objects are orbiting. Hulse and Taylor observed this system over many years, and showed that they were slowly moving closer together just as predicted by general relativity and gravity waves. This won them the Nobel prize in physics in 1993.

This new system (PSR J0348+0432) is hitting the press because it’s a little bit different. Most close binary systems like Hulse-Taylor consist of two neutron stars. Neutron stars typically have about the same mass (about 1 – 2 solar masses), so that means they tend to be rather symmetric. This new system consists of a pulsar (neutron star) of about 2 solar masses, and a white dwarf companion about a fifth the mass of our Sun. So this system is very non-symmetric. That means it might be able to distinguish between general relativity and alternative theories of gravity.

General relativity has passed every experimental test so far, but there are alternative theories such as scalar-tensor-vector gravity, or tensor-scalar-vector gravity (yes, they are two different models) that also make similar predictions for gravity waves. These alternative models are more complex than general relativity, and they are typically proposed as a modified gravity to explain things like the galaxy rotation curves that are commonly attributed to dark matter. They aren’t popular models among astronomers, but without a direct observation of dark matter, we can’t entirely rule them out.

In addition to explaining away dark matter, these alternative gravitational models make different predictions about gravitational waves. This means the orbital decay predicted by general relativity and these alternative models differ in a measurable way. But to really see these differences we would want to observe a close binary system where the two members differ significantly in mass. Ideally they would also be in very elliptical orbits. This latest system has the non-symmetric aspect we want, but their orbits are very circular.

This initial paper mainly just verifies the masses and orbits of this pulsar and its companion. It confirms that the two masses are very different, and it demonstrates that (so far) general relativity is still confirmed. That’s not really a big deal, so the attention it has gotten in the press is a bit overhyped. But the work also places some loose constraints on alternative models of gravity. It’s not nearly enough to verify or disprove them yet, but over the next decade or two it could provide a real test of these alternative models. That’s another reason for some of the hype. It is a way for the team to claim this particular pulsar system as theirs.

So it is an interesting system, and it may provide an interesting test of gravity in the future. But it isn’t yet the “greatest test of general relativity” as many popular articles claim.

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Last time I talked about how large stars can become a supernova through a collapse of their core.  But this only occurs in stars much larger than our Sun.  So how can a solar mass star become a supernova?  For that, it has to dance with another star.
If you recall from last time, when a Sun-sized star begins to run out of hydrogen to fuse in its core, it fuses helium for a while, causing it to swell into a red giant.  After this stage the star can’t fuse any higher elements, and what remains of the star compresses down to a white dwarf.  All the mass of the star is squeezed to about the size of the Earth.  In a white dwarf it isn’t the heat and pressure of fusion that balances against the weight of gravity, but the pressure of the electrons pushing against each other.  For a single star such as our Sun, that’s the end of the story.  The white dwarf simply cools over time.  But for a binary star system, something more interesting can occur.

Suppose there was a binary star system, consisting of two stars similar to the Sun, and reasonably close together.  These two stars will spend most of their lives as main sequence stars, but they will eventually run out of hydrogen and become red giants.

Of the two stars, one is likely to be a bit larger than the other.  This means throughout it’s main sequence period the larger star will burn just a little bit hotter, and use up its hydrogen a bit faster.  As a result, the larger star will enter its red giant stage while the smaller star is still a main sequence star.

A red giant is fairly large.  For a Sun-like star the red giant can be as large as the Earth’s orbit.  In a binary system this means the outer layers of the red giant can can be captured by the companion star.  It can even engulf the two stars in what is called a common envelope.  The result is that some of the mass of the larger star is transferred to the smaller star during this stage.  The larger star then collapses into a white dwarf.

Some time later, the smaller star enters its red giant stage, and a similar process occurs.  The smaller star swells into a large red giant, and some of the outer material is captured by the first star, which is now a white dwarf.  You can see an artist’s rendering of this process in the image above.

A white dwarf is formed because the electron pressure within the white dwarf is able to balance its gravitational weight.  But this electron pressure has a limit (about 40% greater than the mass of the Sun) known as the Chandrasekhar limit.  As the white dwarf continues to capture mass from the other star, its mass continues to rise, approaching that ultimate limit.

As the this limit is reached, the star begins to collapse into a neutron star.  This causes a cascade of rapid fusion within its core, which rips the white dwarf apart.  The white dwarf explodes as a supernova.  This stellar dance ends with a bang.

What makes this type of supernova particularly interesting is that it always happens with a white dwarf of about 1.4 solar masses, and it always occurs through the same general process, destroying the white dwarf completely.  This means a supernova created in this way always has about the same brightness.  They are called type Ia supernovae, and they are very useful to astronomers.

There are various ways to measure the distances of galaxies.  One way is to observe a particular kind of star that varies in brightness, known as a Cepheid variable star.  We can use these stars to measure the distance of closer galaxies, but only as far as about 90 million light years.  For more distant galaxies these stars are just too dim to observe accurately.  But we’ve also observed type Ia supernovae in galaxies for which we have measured their distance.  We can observe how bright the supernova appear, and knowing their distance we can determine how bright they actually are.  What we find is that type Ia supernova always have the same absolute brightness.

This means we can use them as a “standard candle”.  If we observe a type Ia supernova in a distant galaxy, we can observe how bright it appears.  Since we know how bright it actually is, we can calculate the distance to the galaxy, since the more distant a light source is, the dimmer it appears.  We can therefore use this type of supernova to measure the distance to its galaxy.

Since these supernovae are very bright, we can measure the distance of galaxies billions of light years away.  Without them, the size and structure of the universe would still be a great mystery.

Two stars, having a last dance, and helping us to discover the secrets of the cosmos.

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A new paper in Astronomy and Astrophysics heralds the discovery of a yellow hypergiant star known as HR 5171.  Such stars are exceedingly rare, with only 12 such stars known to exist within our galaxy.

An image of HR 5171. Credit: ESO.

A yellow hypergiant star is a very massive, very luminous star.  They typically have a mass between 20 – 50 times that of the Sun, but their temperature is roughly similar to that of the Sun.  What makes them particularly rare is that they are unstable.  They are so bright that their light pushes away the outer layers of the star.  Because of this, they are stars in transition.

What makes this particular yellow hypergiant unusual is that it is also a binary star.  The team analyzed observations of the star and found that it is an eclipsing binary with a period of about 1300 days.  This is surprising because the star itself has a diameter about 6 times that of Earth’s orbit.  Given its size and mass, the companion star must be so close that they touch, known as an interacting binary.

Because the two stars are in contact, they each have an impact on the other’s evolution.  The companion can strip material from the primary, and in turn will increase in mass.  As the primary hypergiant continues to expand, the two stars will interact more strongly.

Needless to say, things could get very interesting for this system.

Paper: O. Chesneau, et al. The yellow hypergiant HR 5171 A: Resolving a massive interacting binary in the common envelope phase. A&A 563, A71 (2014).

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