# How long does an over contact binary star system last?

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I read recently about VFTS 352, an overcontact binary star system where both stars have roughly equal mass. All of the reports I've read (in mass-media type publications) have said that the system has one of two fates: either the two stars will merge, or they'll supernova. But when will this happen?

The wikipedia page for contact binaries says that they have a lifespan of millions to billions of years, but doesn't say if that's different for overcontact binaries. It also says that they're often confused with common envelopes, which have a lifespan of months to years, and I'm not sure where in that spectrum an overcontact lies (or really what the distinction is, since the page for contact binaries says they share an envelope, which sounds the definition of a common envelope). I'm also not sure whether the fact that both stars have roughly equal mass affects the lifespan.

The mass-media articles I've read have implied that the merger-or-supernova is happening soon, but I don't know if this is on a human scale (months) or galactic scale (millions of years).

Short answer: $$t lesssim 10^5 mathrm{years}$$ (maybe)

An "overcontact binary" is just another way of saying "common envelope binary". The two phrases are exactly the same and it's frustrating that the authors on the VFTS 352 paper decided to create their own convention - as if astrophysical classifications weren't confusing enough!

A contact binary exists on timescales predominantly dependent on stellar evolution, so figuring out how long a contact binary will exist is heavily dependent on the mass, metallicity, and rotation of the primary star among other things.

Deriving the timescale:

Let's keep the scope to systems like VFTS 352, where the primary is massive and the binary has an orbital period less than 4 years (2.5 AU separation). In order to have a common envelope event, the stars must have overflown their Roche lobes. The radius for the Roche lobe of two point masses is $$egin{equation*} r_L = frac{0.49 q^{frac{2}{3}}}{0.6q^{frac{2}{3}}+mathrm{ln}(1+q^{frac{1}{3}})}a end{equation*}$$ where $$a$$ is the separation. For close binaries, the general observed trend is a high mass ratio $$q=M_2/M_1$$. So, if we assume $$q=1$$, then $$r_L = 0.38a$$. Hence, for a binary with $$a<2.5$$ AU, egin{align*} r_L &lesssim 1 mathrm{AU} r_L &lesssim 215 R_{odot} end{align*} since $$q=1$$ is an upper bound on the Roche lobe radius. Now, performing some trivial rearrangement of the blackbody luminosity equation $$L=4pisigma_{SB}R^2T^4$$, we find that $$egin{equation*} R approx 3.31 imes10^{7} igg(frac{L}{L_{odot}}igg)^{frac{1}{2}}igg(frac{1 mathrm{K}}{T}igg)^2 R_{odot}. end{equation*}$$ Massive stars typically have roughly constant luminosity, so we will choose $$Lapprox10^5 L_{odot}$$. Hence, $$egin{equation*} Rapprox 1 imes10^{10}igg(frac{1 mathrm{K}}{T}igg)^2 R_{odot} end{equation*}$$

The massive star needs to evolve until its radius is equal to that of the Roche lobe radius, so we find that the star reaches the common envelope phase for $$egin{equation*} T gtrsim 7000 mathrm{K} end{equation*}$$ Taking a peek at an HR diagram, this star varies from about $$30000 mathrm{K}$$ to $$4000 mathrm{K}$$ from ZAMS to end of main-sequence. Thus the primary spends roughly 3/4 of its time on the main-sequence not in the common envelope phase. Hence, this binary's common envelope phase lasts for, at most, 1/4 the primary's total lifetime, which is on the order of $$10^6$$ years. Thus, the upper bound for the timescale of a common envelope event with massive stars with negligible rotation is $$sim10^5$$ years.

Please note that this derivation does not take into account the bulging effect that occurs as the separation decreases. This will certainly lower this upper bound, but by how much I'm not sure. It could lower it by 1 year, or $$10^5 mathrm{years}$$.

Lower bounds to this timescale are entirely ambiguous and not particularly helpful in any physical context. The stars could be spinning really fast, have high or low metallicity, the binary could have a different mass ratio, there could be another binary close by, and there may be magnetic interaction (?). The list goes on! I'm sure there's something I left out.

The articles indicate one of two possible outcomes: merger, followed ultimately by gamma ray burst, or permanent separation, separate supernovae leading to binary black holes.

In the second case the supernovae will be in a few million years, the typical life span for massive stars.

In the first case the merger could happen sooner, perhaps hundreds of thousands of years, so "soon" in astronomical terms, but long in comparison to a human life.

## New Light from Binary Stars

After 29 years of research, a creation astronomer has observable evidence that stars can’t be billions of years old.

Twinkle, twinkle, little . . . stars? Unless youʼve been living in a cave for the past decade, you’ve heard about the popular television competition Dancing with the Stars. Everyone loves a star. But the real stars are in the nighttime sky, and most of them have a dancing partner, too.

When you stare up at a twinkling point of light in the night sky, you might actually be looking at a system of stars, not just a single one. Systems of two stars, binary stars, orbit one another on a continual basis. More than 60% of the single points of light we observe at night are multiple star systems. Some are three or more stars, but such groups are usually unstable. As a professional stellar astronomer, my focus for the past 30 years has been the observation and analysis of binary stars that regularly eclipse each other (one star periodically passes in front of the other).

The variation in a binary star’s apparent brightness during an eclipse reveals helpful details about both stars, including their temperature, atmosphere, geometry, mass, and much more. Without binary stars, we could only guess what the nature of stars is! We find that they are suns similar to our own, burning in the heavens!

As a creationist who believes God created the universe only a few thousand years ago, I have discovered that these fascinating two-in-one stars shed light on another aspect of our vast, mysterious universe. These stars must be young . . . a finding that undermines deep time theories of binary star evolution!

## Yo dawg, I heard you like eclipsing binaries, so I put an eclipsing binary around your eclipsing binary

When you look at the stars in the sky, they appear alone, each separate from the other. But that’s an illusion brought by distance in fact as many as half of all the stars in the sky are members of multiple star systems. Binaries, trinaries, even some with five or six stars, all orbiting each other in complicated manners.

There’s a special class of binary stars called eclipsing binaries: These are stars whose orbit we see edge-on (or nearly so), so that from our point of view we see one star physically block the other, then half an orbit later we the second star block the first.

In general, one star is brighter (called the primary) and the other is dimmer (the secondary). During primary eclipse, when the brighter star is blocked by the dimmer one, the light from the system drops a lot. During secondary eclipse, when the primary blocks the secondary, the light doesn’t drop as much. So when you make a graph of the system’s light over time (what astronomers call a light curve), there’s a big dip, then a smaller dip, then a big dip again, and on and on.

If you want to see this in action, there’s an online applet that shows two stars orbiting and the amount of light you see from them. Enter A for the first star and F for the second, and make the distance 4 solar radii (don’t forget to hit the “Enter Values” button). You’ll see how this works pretty clearly (note: in the sim the bigger star is brighter the values for temperature and size are displayed).

Stars like this are astrophysically important! We can determine the stars’ masses from their orbital period, for example, and because they’re at essentially the same distance from Earth, the brightness difference between the stars represents a real difference in luminosity. From this lots of other physical characteristics fall out. Many astronomers dedicate a lot of time to observing them.

An astronomer, Pavel Cagaš, used a private 30-centimeter telescope in Zlín, a city in the Czech Republic to look at stars in the constellation of Auriga. And oh my did he find an interesting one! It’s called CzeV1640 (from the Czech Variable star catalog it’s also called UCAC4 591-028146 from a different catalog), and it shows what looks at first glance to be an obvious eclipsing binary light curve. There’s a big dip, then a smaller one, and the pattern repeats.

But. When you look at the curve carefully, it’s not so smooth. There are a bunch of deviations to it, added smaller dips, most noticeable in the second half of the light curve.

A graph of brightness versus time (a 'light curve') showing the mutual eclipses of CzeV1640. The overall brightness is shown on the top panel (displayed twice for ease of analysis). The curve just for CzeV1640A is on the bottom left, and for CzeV1640B on the bottom right. Credit: Pavel Cagas

What’s going on here? What Cagaš proposed is that we’re not seeing two stars here, we’re seeing four. There are actually two eclipsing binary pairs here, two pairs of binary stars which in turn orbit each other. This is called a hierarchical multiple system.

So there’s one binary, called CzeV1640A (composed of two stars which would then be CzeV1640 AA and AB), and another called CzeV1640B (with the two stars CzeV1640 Ba and BB). CzeV1640A has brighter stars, and creates the primary double-dip seen in the graph, while CzeV1640B has fainter stars, which create the added smaller dips. We see the eclipses of both pairs at the same time, creating the weird, confusing light curve.

The two stars making up CzeV1640A orbit each other every 0.55 days, while the stars making up CzeV1640B have an 0.84 day orbit. Those are tight orbits! Sometimes when stars get to close their mutual gravity pulls their material together: They wind up sharing their outer layers and form a peanut-shaped star called a contact binary. In this case, though, the stars are separated enough in both binaries to be called detached.

Mind you, this is a preliminary paper, so there are still lots of questions. What are the actual stellar types of the stars making up this system, (like, are they massive and blue, or low mass and red, or in between like the Sun?), what are their orbits actually like, and how long does it take the two binaries to orbit each other?

Artwork depicting the birth of a quadruple star system similar to CzeV1640, with two binary systems in turn orbiting each other. Credit: Institute of Astronomy / University of Hawaii

I wonder. It seems likely that these stars all formed together form the same gas cloud, since we see both binary pairs with their orbits edge-on. Does this mean we see the orbit of the two binary pairs around each other edge on? That means that we’d see them eclipse each other when they line up, and that would create an ungodly mess of a light curve. I’d love to see that!

The area of the sky around CzeV1640 (center), showing there are several stars apparently very close to it. Credit: Skyview/DSS

I have to note that when I looked for an image of this in the Digitized Sky Survey, the sky is part of a relatively tight knot of other stars. Given this, it’s possible that we’re seeing two unrelated binary pairs that happen to lie close to together. I can’t discount that. However, Cagaš notes that the ratio of the two orbital periods is close to 3:2 (in other words, 0.84 / 0.55 = 1.52). Sometimes in gravitational systems, where objects are in orbit, you see simple fractions like this. It’s called a resonance, and it pops up naturally as all the objects interact with each other. I’m not sure how this would work in a hierarchical binary, unless the two pairs of stars interact with each other somehow, but that seems unlikely to me. It could be coincidence, but it may point to a deeper connection.

As usual, the discovery of an unusual object just leads to more questions. I’d be fascinated to see a follow-up observation of this interesting pair, and a more thorough analysis of their orbits and physical characteristics. Stars like this may be rare, but when you have a galaxy with a few hundred billion stars in it, you get a lot of chances for rare stuff to happen.

## Classifications

### By methods of observation

Binary stars are classified into four types according to their observable properties. Η] Any binary star can belong to several of these classes for example, several spectroscopic binaries are also eclipsing binaries.

#### Visual binaries

A visual binary star is a binary star for which the angular separation between the two components is great enough to permit them to be observed as a double star in a telescope. The resolving power of the telescope is an important factor in the detection of visual binaries, and as telescopes become larger and more powerful an increasing number of visual binaries will be detected. The brightness of the two stars is also an important factor, as brighter stars are harder to separate due to their glare than dimmer ones are.

The brighter star of a visual binary is the primary star, and the dimmer is considered the secondary. In some publications (especially older ones), a faint secondary is called the comes if the stars are the same brightness, the discoverer "chooses" the primary. ⎖] The position angle of the secondary with respect to the primary is measured, together with the angular distance between the two stars. The time of observation is also recorded. After a sufficient number of observations are recorded over a period of time, they are plotted in polar coordinates with the primary star at the origin, and the most probable ellipse is drawn through these points such that the Keplerian law of areas is satisfied. This ellipse is known as the apparent ellipse, and is the projection of the actual elliptical orbit of the secondary with respect to the primary on the plane of the sky. From this projected ellipse the complete elements of the orbit may be computed, with the semi-major axis being expressed in angular units unless the stellar parallax, and hence the distance, of the system is known. ⎗]

#### Spectroscopic binaries

A spectroscopic binary star is a binary star in which the separation between the stars is usually very small, and the orbital velocity very high. Unless the plane of the orbit happens to be perpendicular to the line of sight, the orbital velocities will have components in the line of sight and the observed radial velocity of the system will vary periodically. Since radial velocity can be measured with a spectrometer by observing the Doppler shift of the stars' spectral lines, the binaries detected in this manner are known as spectroscopic binaries. Most of these cannot be resolved as a visual binary, even with telescopes of the highest existing resolving power.

In some spectroscopic binaries the spectra of both stars are visible and the lines are alternately double and single. Such stars are known as double-line binaries. In others, the spectrum of only one of the stars is seen and the lines in the spectrum shift periodically towards the blue, then towards red and back again. Such stars are known as single-line spectroscopic binaries.

The orbit of a spectroscopic binary is determined by making a long series of observations of the radial velocity of one or more component of the binary. The observations are plotted against time, and from the resulting curve a period is determined. If the orbit is circular, then the curve will be a sine curve. If the orbit is elliptical, the shape of the curve will depend on the eccentricity of the ellipse and the orientation of the major axis with reference to the line of sight.

It is impossible to determine individually the semi-major axis a and the inclination of the orbit plane i. However, the product of the semi-major axis and the sine of the inclination (i.e. a sin i) may be determined directly in linear units (e.g. kilometres). If either a or i can be determined by other means, as in the case of eclipsing binaries, a complete solution for the orbit can be found. ⎘]

#### Eclipsing binaries

An eclipsing binary, with an indication of the variation in intensity. ⎙] ⎚]

An eclipsing binary star is a binary star in which the orbit plane of the two stars lies so nearly in the line of sight of the observer that the components undergo mutual eclipses. In the case where the binary is also a spectroscopic binary and the parallax of the system is known, the binary is quite valuable for stellar analysis. ⎛]

In the last decade, measurement of eclipsing binaries' fundamental parameters has become possible with 8 meter class telescopes. This makes it feasible to use them as standard candles. Recently, they have been used to give direct distance estimates to the LMC, SMC, Andromeda Galaxy and Triangulum Galaxy. Eclipsing binaries offer a direct method to gauge the distance to galaxies to a new improved 5% level of accuracy. ⎜]

Eclipsing binaries are variable stars, not because the light of the individual components vary but because of the eclipses. The light curve of an eclipsing binary is characterized by periods of practically constant light, with periodic drops in intensity. If one of the stars is larger than the other, one will be obscured by a total eclipse while the other will be obscured by an annular eclipse.

The period of the orbit of an eclipsing binary may be determined from a study of the light curve, and the relative sizes of the individual stars can be determined in terms of the radius of the orbit by observing how quickly the brightness changes as the disc of the near star slides over the disc of the distant star. If it is also a spectroscopic binary the orbital elements can also be determined, and the mass of the stars can be determined relatively easily, which means that the relative densities of the stars can be determined in this case. ⎝]

#### Astrometric binaries

An astrometric binary star is a binary star for which only one of the component stars can be visually observed. The visible star's position is carefully measured and detected to have a wobble, due to the gravitational influence from its counterpart. The position of the star is repeatedly measured relative to more distant stars, and then checked for periodic shifts in position. Typically this type of measurement can only be performed on nearby stars, such as those within 10 parsecs. Nearby stars often have a relatively high proper motion, so astrometric binaries will appear to follow a sinusoidal path across the sky.

If the companion is sufficiently massive to cause an observable shift in position of the star, then its presence can be deduced. From precise astrometric measurements of the movement of the visible star over a sufficiently long period of time, information about the mass of the companion and its orbital period can be determined. ⎞] Even though the companion is not visible, the characteristics of the system can be determined from the observations using Kepler's laws. ⎟]

This method of detecting binaries is also used to locate extrasolar planets orbiting a star. However, the requirements to perform this measurement are very exacting, due to the great difference in the mass ratio, and the typically long period of the planet's orbit. Detection of position shifts of a star is a very exacting science, and it is difficult to achieve the necessary precision. Space telescopes can avoid the blurring effect of the Earth's atmosphere, resulting in more precise resolution.

### By configuration of the system

Another classification is based on the distance of the stars, relative to their sizes: ⎠]

Detached binaries are a kind of binary stars where each component is within its Roche lobe, i.e. the area where the gravitational pull of the star itself is larger than that of the other component. The stars have no major effect on each other, and essentially evolve separately. Most binaries belong to this class.

Semidetached binary stars are binary stars where one of the components fills the binary star's Roche lobe and the other does not. Gas from the surface of the Roche lobe filling component (donor) is transferred to the other star (accretor). The mass transfer dominates the evolution of the system. In many cases, the inflowing gas forms an accretion disc around the accretor. Examples of this type are X-ray binaries and Cataclysmic variable stars.

A contact binary is a type of binary star in which both components of the binary fill their Roche lobes. The uppermost part of the stellar atmospheres forms a common envelope that surrounds both stars. As the friction of the envelope brakes the orbital motion, the stars may eventually merge. ⎡]

## Talk:Binary system

Imagine you just stumbled on this article from the outside. It gives maybe an impression of a democratic debate but what about people who are looking for a source of information? Is Wikipedia just another chat room? Why not to leave this discussion to the talk page and avoid these pathetic tags?

In fact, they do not orbit each other, they rotate around their common "centre of weight" <- how do you say that in English?

All bodies in orbit do so around a common center of weight. That is what an orbit is.Derek Balsam 15:15, 25 August 2006 (UTC)

That table of known binary systems seems to indicate that the only known systems are the two listed there. Furthermore, it is too short to be of any use. There are many interesting points that could be discussed when it comes to binary systems, such as mass transfer between the stars and common envelope orbits. It has been too long since I read about this for me to write about this, but it should definitly be a part of the article. Amaurea 01:57, 13 December 2005 (UTC)

The result of the proposal was no merge.

I propose a mergefrom Double planet. Double planet is the informal term. --Md84419 07:59, 25 August 2006 (UTC)

### Survey Edit

• Oppose double planet is a sizable article, it would completely swamp this. 132.205.44.134 03:08, 4 September 2006 (UTC)
• Oppose - The proposed merge would combine multiple topics inappropriately, and most of the articles in question should remain separate articles. George J. Bendo 11:14, 6 September 2006 (UTC)
• Oppose — Agree with above. — RJH (talk) 17:47, 6 September 2006 (UTC)
• Oppose per all of the above. Nick Mks 18:13, 8 September 2006 (UTC)

### Consensus Conclusion Edit

The two weeks have passed, consensus is oppose. Nick Mks 09:33, 9 September 2006 (UTC)

The above discussion is preserved as an archive of the proposal. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

The result of the proposal was no merge.

I propose a mergefrom binary asteroid. Binary system is the more generic term and the asteroid page is a stub. --Md84419 07:59, 25 August 2006 (UTC)

### Survey Edit

• Oppose - The proposed merge would combine multiple topics inappropriately, and most of the articles in question should remain separate articles. George J. Bendo 11:14, 6 September 2006 (UTC)
• Oppose — Agree with above. — RJH (talk) 17:47, 6 September 2006 (UTC)
• Oppose per all of the above. Nick Mks 18:13, 8 September 2006 (UTC)

### Discussion Edit

What about merging Binary star too, or at least give a brief description of it in a section? Patrickov 02:21, 2 September 2006 (UTC)

• Actually, I don't really see much point to this article (that is, Binary system (astronomy)) except as a disambiguation page. Instead of trying to merge everything into an oversized mess, leave the separate topics separate and link them from here. Chaos syndrome 19:53, 4 September 2006 (UTC)

### Consensus Conclusion Edit

The two weeks have passed, consensus is oppose. Nick Mks 09:35, 9 September 2006 (UTC)

The above discussion is preserved as an archive of the proposal. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

The result of the proposal was no merge.

This is the only article that logically should be merged into this article. But should it even be merged? 132.205.44.134 03:11, 4 September 2006 (UTC)

### Survey Edit

• Oppose - The proposed merge would combine multiple topics inappropriately, and most of the articles in question should remain separate articles. George J. Bendo 11:14, 6 September 2006 (UTC)
• Oppose — Agree with above. — RJH (talk) 17:47, 6 September 2006 (UTC)
• Oppose per all of the above. Nick Mks 18:13, 8 September 2006 (UTC)

### Disucssion Edit

• Instead of merging, I'd suggest turning Contact binary into a disambiguation page and creating Contact binary (asteroid) to deal with the topic of contact binary asteroids. The topic deserves a far more expanded treatment than it is currently given. Chaos syndrome 19:48, 4 September 2006 (UTC)

### Consensus Conclusion Edit

The two weeks have passed, consensus is oppose. I'll be incorporating the suggestion in the Discussion though. Nick Mks 19:55, 19 September 2006 (UTC)

The above discussion is preserved as an archive of the proposal. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

The result of the proposal was no merge.

### Survey Edit

• Oppose Binary star is a long article. A section here should contain binary star information though. 132.205.44.134 03:07, 4 September 2006 (UTC)
• Strongly oppose. Binary star is a major topic deserving a full treatment. Chaos syndrome 19:40, 4 September 2006 (UTC)
• Oppose - The proposed merge would combine multiple topics inappropriately, and most of the articles in question should remain separate articles. George J. Bendo 11:14, 6 September 2006 (UTC)
• Oppose — Binary star is a rich and interesting topic unto itself. It would not benefit from a merge with this page. My preference would be to have a brief paragraph with a main article link to the binary star page. — RJH (talk) 14:19, 6 September 2006 (UTC)
• Strongly oppose per all of the above. Please oh please don't let this article fall victim to the Pluto frenzy as well. Nick Mks 18:13, 8 September 2006 (UTC)

### Discussion Edit

Binary star was a featured article, which means that it is already in a "satisfactory" state. The proposed merge would severely and negatively impact the article, so it should not be carried out. George J. Bendo 11:19, 6 September 2006 (UTC)

### Consensus Conclusion Edit

The two weeks have passed, consensus is oppose. Nick Mks 19:54, 19 September 2006 (UTC)

The above discussion is preserved as an archive of the proposal. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

Can someone please provide a rationale to explain why we can use this image in the article. I don't see how its current use fits with the terms in the fair use box on the image. Chaos syndrome 19:45, 4 September 2006 (UTC)

Maybe simply use artist image from some NASA page?--83.144.95.66 13:44, 17 June 2007 (UTC)

I agree with your rationale. This article should be about the dynamics in general, but it currently says basically nothing about that (in fact, it says not much more than what's in the close binary star section. Moreover, there is no section on close binary stars in that article. --JorisvS (talk) 17:27, 19 February 2015 (UTC)

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## Is New Horizons’ next target a binary relic of the ancient solar system?

On July 14, 2015, the New Horizons spacecraft screamed past Pluto at a relative speed of nearly 14 kilometers per second, giving humanity its first close-up very of the distant, icy world.

At that speed, there was no hope of slowing New Horizons down it would’ve had to carry an immense amount of fuel to do that, and we don’t have the technical capability to launch that much mass to Pluto. Also, it would’ve meant a much longer time to get to Pluto, and it took nearly a decade as it was!

But this also means that the spacecraft is still zipping away from Earth, heading out into interstellar space. It’s not there, yet it’s currently passing through the Kuiper Belt, a roughly doughnut-shaped region far out from the Sun and dominated by icy bodies very much like Pluto, itself. Most are small, but some are many hundreds of kilometers across.

This was an opportunity too good to pass up. Astronomers looked for any Kuiper Belt Objects near the spacecraft’s trajectory, and, after using Hubble, they found one: Called 2014 MU69, it was a perfect target. Likely to be an ancient piece of solar system left over from its formation, it was reachable using some of what was left of the spacecraft’s fuel. The course correction was made, and New Horizons will blow past MU69 on January 1, 2019, passing just 10,000 km from it.

But such a close encounter means more information must be known. How big is it? Are there rings or moons around it that could be a hazard during the flyby?

Animation of the star blinking out as MU69 passed in front of it. This is actual data from the event the time between frames is 0.2 seconds. Credit: NASA / JHUAPL / SwRI / Emily Lakdawalla

To find this out, the New Horizons team sent two dozen telescopes to South America to observe a predicted occultation of a star by MU69. On July 17, 2017, five of the telescopes saw the star wink out for a fraction of a second.

Since we know how quickly MU69 is moving relative to the background stars, how long the star blinked out tells us how wide MU69 is as seen by that telescope. But there’s more: As I wrote in an earlier post, arranging the telescopes many kilometers apart in a north-south line allowed an estimate of the shape of the object to be obtained, as well.

That wasn’t released at the time, but now New Horizons Principal Investigator Alan Stern has revealed it on his blog. and. apparently, MU69 may be a binary object!

As MU69 passed in front of a star, it blocked the light, casting a shadow on Earth billions of kilometers away. Each telescope saw the star passing behind a different part of the object, allowing astronomers to roughly trace its shape. Credit: NASA/JHUAPL/SwRI/Alex Parker

The way the star blinked out from different locations makes sense if MU69 is not a single round object, but instead two roughly round objects, either orbiting one another as a close binary system or touching each other in what’s called a contact binary. We’ve seen both of these types of objects before and, in fact, it’s common to see this in asteroids and comets (67P/Chuyurmov-Gerasimenko, the comet visited by the Rosetta spacecraft, is a contact binary).

If this is the case, the two objects making up MU69 are about 20 and 18 km across. That’s a little bit like Pluto and its large moon, Charon, in fact the moon is about half the size of Pluto. We think Charon formed after a massive impact blew debris from Pluto into space, and it coalesced into the moon. Perhaps something similar happened with MU69. Another, perhaps more likely, scenario is a slow speed collision between two objects caused them to stick, or at least lose enough energy to become bound gravitationally to form a binary.

The icy object 2014 MU69 may be a binary, or even a contact binary, a double-lobed worldlet. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

To be honest, we don’t know that much about objects this far away they’re small and incredibly distant. MU69 is currently well over six billion km away from Earth, and the faintest star you can see with your naked eye is 15 million times brighter! It’s hard to learn a lot about such an object unless you go there.

So, that’s what we’re doing. Right now, the New Horizons spacecraft is in hibernation to conserve energy, but engineers will wake it up this fall to tweak its path, then put it back to sleep until summer 2018. After that, it’ll wake up and start prepping for the encounter with MU69. And then, on New Year’s Day 2019, humanity will get its first close up look at primordial object from the earliest days of the our solar system.

Or, as seems likely now, two primordial objects! What will we see?

## Binary star

I will just skim the foam off this one---nice question.

Indeed close binary pairs do compromise each other's integrity.
Why shouldnt they.

Between the earth and moon there is a zero-gravity point
and likewise between two stars.
And one star can expand in a red giant stage to where its outer envelope of ionized gas touches this zero gravity point

and the expanded star can begin to flow material over to the more compact star.

Its great! Type Ia supernovas theoretically can occur this way---at the small compact partner which is receiving matter from its giant partner.

And ordinary novas too----Labguy has discussed this at least once so let him confirm or correct.

Stars even merge! A certain kind of merger is believed responsible for at least some of the observed gamma bursts.

When a red giant is feeding matter to a white dwarf partner the giant is distorted by the common gravitational field. It is no longer spherical.

At some point it may be better not to think of a binary system as two separate stars but as a collective distribution of mass that has two maxima. The center of mass is between the two and is found the same way with any other distribution of mass----roughly speaking a kind of average.

Maybe others will add detail. Always nice questions, wolram---combination of simple and evocative---always seem to lead somewhere. Good at it.

Marcus covered most of the bases, but on some sites, and books, you will get more detail about when binary stars can and can't exchange material. As with a Type Ia supernova, Marcus explained that material is "transferred" from a (usually) red giant to a white dwarf. When that dwarf passes its Chandrasekhar mass (we don't use the word "limit" much anymore.. ), it can become a Type Ia or Type Ib supernova, depending on chemical composition. Most white dwarfs would ignite the outer layers by fusion and become a "regular" nova, not a supernova.

BUT, that is not what you asked. You asked about contact binary stars, which takes in a few more factors. For any mass transfer, one or both stars in a binary system must expand its outer layers (atmosphere) beyond its Roche Lobe limit. There are millions of binary stars that cannot do this due to orbital distance and mass. The Roche Lobe of a star is dependant on the mass and orbital distance of both stars. It is the limit (distance) from either star, where the star's atmosphere has extended/swolen to where the gravitational pull of the companion has as much effect on the gasses as does the expanding star. Basically, it is just the place where the "gravity-point" effecting the gasses (either star) is equal. I am sure that you have seen some of the "artist's conceptions" showing a large, egg shaped star spilling matter in a stream to the companion. This will happen only in close binary systems. Also, if the orbit of either star is too elliptical (eccentric), this transfer effect will not happen at all, or only happen sporadically when the stars are at the closest point in orbit. This last description is what sometimes happens in a nova that repeats in regular intervals. Quite a few stars are known to go nova at repeated and equal time intervals.

A mass transfer as Marcus described for a Type Ia (or b) supernova only involves mass transfer in one direction from the giant to the white dwarf. The red giant's mass/size has reached the point where the gravity of the dwarf pulls matter through the Roche Lobe point onto the dwarf. All stars have a "Roche Limit", but transfer of matter through the Roche Lobe will only happen in binary star systems. Also, this "point of transfer" can only happen at the large star's L1, or inner Lagrangian point. This method of mass transfer occurs in many binary systems, but in common-usage terms, this still does not fit the description of contact binaries. In "contact binaries", both stars must have "atmospheres", which rules out any type of stellar remnant, or core, such as a white dwarf. Contact binaries are usually described as a situation where (a) the orbits are very close, therefore short period orbits and (b) where both stars have extended atmospheres beyond their Roche Lobes (limits), and material from both stars is intermixed. The center of gravity of such a system would be a common center of mass and would be no different than one calculated for any two orbiting bodies, This process will continue and increase until the stars eventually merge into one, larger star, very mixed up and unstable if the combined mass is high. In this case, sometimes the new, larger star will complete the evolution as a single star and may have enough mass to become a Type II supernova, not Type I.

Of course, now that I have typed this all out, you will find out several other (special) cases where stellar remnants, like neutron stars, can also qualify as contact binaries. It is the "merger" (collision?) of neutron stars, and maybe occassional black holes, that are thought to be the source of the huge energy release of gamma ray bursts. Hope some of this helped, but there are always exceptions to the rules or generalizations, especially if I type them.

This site has a good summary of binary types and a cool computer simulation you can run, near the bottom of the page:

## History

The observations of binary stars began with the invention of the telescope, with the first known recordings in the 17th century. Giovanni Battista Riccioli discovered in 1650 that Mizar was actually a binary. Christiaan Huygens found that that Theta Orionis was actually three stars in 1656, Robert Hooke made the same observation about Gamma Arietis in 1664, while in 1685 Father Fontenay observed that the star Acrux was really a binary pair. [9]

William Herschel was the first person to coin the term binary star. He defined the term in 1802 as:

The union of two stars, that are formed together in one system, by the laws of attraction. [10]

Herschel began his observation of binaries in 1779. The result was a cataloging of over 700 double stars systems as recorded in his book Catalogue of 500 new Nebulae . and Clusters of Stars with Remarks on the Construction of the Heavens in 1802. By the next year, he concluded that these double stars must be binary systems. [11] It was not until 1827 though that an actual orbit of a binary star system was calculated. This was completed by Félix Savary of the star Xi Ursae Majoris. Today over 100,000 binary star systems have been cataloged, although the actual orbits of only a few thousand of these are known, with some cataloged stars possibly being only optical binaries. [12]

## Do Stars Within A Galaxy Touch One Another?

Many body problems with contact forces, like grains of sand on a beach, are quite hard to solve satisfactorily with a computer. Is there a similar problem with galaxies where the stars are in gravitational contact, and could that be a factor in the problem with the galactic rotation curves not turning out as expected?

The largest NASA Hubble Space Telescope image ever assembled, this sweeping bird's-eye view of a . [+] portion of the Andromeda galaxy (M31) is the sharpest large composite image ever taken of our galactic next-door neighbor. Though the galaxy is over 2 million light-years away, the Hubble telescope is powerful enough to resolve individual stars in a 61,000-light-year-long stretch of the galaxy's pancake-shaped disk. It's like photographing a beach and resolving individual grains of sand. Image credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler

Grains of sand are still pretty tricky to model as convincing-looking sand. And as much as we use grains of sand as a metaphor for the number of stars in a galaxy, stars and sand can be a pretty poor comparison if you take the metaphor too far. Galaxies may appear to be solid objects in our skies, with tightly packed clusters of stars, but in fact, galaxies contain huge amounts of empty space. The problem is that for most galaxies, we don’t have the observing power required to distinguish the individual stars, and so the stellar multitudes blur their light together.

If we start to look around within our own galaxy - for instance around our own solar system - we can get a handle on just how much empty space we’re dealing with. Our Sun lives about two thirds of the way out from the center of our galaxy, so while we’re certainly not in the densest part of a galaxy, we’re by no means in a particularly vacant neighborhood either. The nearest stars to us, Alpha Centauri and Proxima Centauri, are 4 light years away. And looking at the diagram, you can see that that 4 light year distance is about as closely packed as the stars get in our neck of the galactic woods.

A diagram of Earth’s location in the Universe, in the Solar Interstellar Neighborhood. Image credit: . [+] Andrew Z. Colvin, CC A-SA 3.0

Now, Alpha Centauri raises an interesting point - Alpha Centauri is actually two stars. This is not uncommon in a galaxy. The two stars orbit each other as a binary system, and those two stars together travel around the galaxy’s core. However, the two stars are really not very close to each other at all. The two stars in Alpha Centauri are typically somewhere between 1 billion miles and three billion miles apart. That’s enough space to fit at least half our solar system between the two stars. At their narrowest, the two stars are separated by the distance between the Sun and Saturn.

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. Image credit: Wikimedia user Skatebiker, CC A-SA 3.0

Contact binaries between stars - where the surfaces of the two stars are actually touching - do exist. Stars are not solid objects, like grains of sand, so instead of wobbling unstably against each other’s surfaces, their atmospheres are pulled together into an irregular, double-lobed star blob. Contact binary stars are relatively rare. Not because it’s so hard to have two stars so close together, but because this is a very unstable arrangement, and is likely to rapidly morph into a single star, or to violently detonate itself in some kind of supernova.

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. Image credit: ESO/L. Calçada

The main force governing the behavior and orbits of stars around the galaxy’s core is plain old gravity. Your typical wide binary stars do not dominate the population of stars in the galaxy, and contact binaries, as the most extreme type of binary, are even less common, and not expected to stick around long enough to really change the galaxy as a whole. If the orbits of stars around the center of the galaxy look weird, and we can rule out other stars causing perturbations to their behaviors, the rotation curves of galaxies must look odd for another reason. Dark matter surrounding the galaxies, adding additional mass to the galaxy, and changing the shape of the gravitational distortion that each galaxy sits within, fits our requirements the best.

## 3 Discussions and conclusions

The orbital periods of 7 AF-type short-period NCBs are studied based on the analysis of their O–C observations. The secular period decreases of all systems are discovered and the decrease rate of each system is determined. This kind of change in the period is very typical for some NCBs, other example such as CN And ( Samec et al. 1998a), FT Lup ( Lipari & Sisteró 1986), RT Scl ( Duerbeek & Karimie 1979), AK CMi ( Samec, McDermith & Carrigan 1995), V1010 Oph ( Lipari & Sisteró 1987), AG Phe ( Cerruti 1994), and others also show such kind of period variation. The period of XZ CMi shows some complex variations, a possible periodic change is found to superpose on the long-time decrease. The periodic variation can be explained either by the light-time effect via the presence of an assumed third body or by the variation of the gravitational quadruple momentum via magnetic activity cycles of the cool components.

By the analysis of the photoelectric data with Russell model, a large amount of third light in XZ CMi was proposed by Wilson (1966) and was later confirmed by Rafert (1990) with the Wilson–Devinney method. Those results suggest that XZ CMi may be a truly triple system, which is in agreement with the period change of the system. In Section 2.3, orbital parameters of the third body are determined and are shown in Table 3. Rafter's photometric solution showed that the amount of third light is l3= 0.17 ± 0.03 for the V light curve and l3= 0.11 ± 0.04 for the B light curve in units of total flux of the system. If the third body is a main-sequence star, the parameters listed in Table 3 indicate that the orbital inclination of the third body should be very small (i′ < 15°), which is much smaller than that of the eclipsing pair . If this is in the case, we can conclude that the third body is captured by the eclipsing binary star. The triple system is not like planetary systems (and galaxies) as formed by contracting spinning gaseous clouds. The situation of XZ CMi is the same as that of the Algol-type eclipsing binary system S Equ ( Qian & Zhu 2002a). However, as that discussed by Qian & Zhu (2002a), third light may be the result of a numerical artefact in the solution of W-D code, since it is usually strongly correlated with many parameters. To check the presence of the third body, new photometric and spectroscopic observations and a careful analysis of those data are required.

The rates of the period decrease (dP/dt) of the present studied systems are listed in the fourth column of Table 5. Also shown in the same column are the period decrease rates for other NCBs collected from literatures, which are listed in the order of orbital period increasing. Of the 21 sample stars, the range of orbital period is from to with a mean value at . Those listed in the second column of this table are the mass ratio of the sample stars. No photometric and spectroscopic studies of V473 Cas were published. The mass ratio of VW Boo was given by Rainger, Bell & Hilditch (1990), and those of BE Cep, GR Tau, CN And, UU Lyn, FT Lup, BV Eri, RT Scl, AK CMi, XZ CMi, BO Peg, RS Ind, RU Eri, BF Vir, V1010 OPh, AV Hya, BL And, AG Phe, V388 Cyg and TT Her were from Samec et al. (1999), Lázaro et al. (1995), Van Hamme et al. (2001), Yamasaki et al. (1983), Lipari & Sisteró (1986), Gu (1999), Hilditch & King (1986), Samec et al. (1998b), Terrell et al. (1994), Yamasaki & Okazaki (1986), Marton et al. (1990), Nakamura et al. (1984), Russo & Sollazzo (1981), Leung & Wilson (1977), Qian et al. (2000), Kaluzny (1985), Cerruti (1996), Young et al. (2001) and Milano et al. (1989), respectively.

Orbital period decrease ratesdP/dtfor some NCBs.

Orbital period decrease ratesdP/dtfor some NCBs.

The sample stars can be divided into four groups: (1) semidetached systems (e.g. RT Scl, V388 Cyg, and TT Her) that have their primary components filling the critical Roche lobe (2) semidetached ones with lobe-filling secondary components (e.g. CN And, RS Ind, BF Vir and AV Hya) (3) systems with both components nearly close to the critical Roche lobe (e.g. RU Eri and UU Lun) and (4) the remaining group with both components in marginal contact (e.g. VW Boo, FT Lup and BL And). However, it should be noted here that for some NCBs their configurations are difficult to determined exactly. A given system can be divided into several groups and different investigators usually obtain different configurations even by using the same data. This may be caused by the photometric disturbances and asymmetries on the light curve via mass and energy transfer between the components.

The dP/dt of the NCBs are displayed graphically against P in Fig. 11 where diamonds refer to systems [usually members of groups (1) and (4)] that displayed evidence for primary to secondary mass transfer (PSMT), which is indicated by a primary-filling configuration or by a hotspot on the secondary, and solid dots refer to other systems. For systems belonging to groups (2) and (3), the period decrease may not be reasonably explained by mass transfer between the components. The primary components of the NCBs are usually A- and F-type, while the secondary ones are G- or K-type cool stars. The period decreasing may result from angular momentum loss (AML) via magnetic braking. It is shown from Fig. 11 that the dP/dt of NCBs with PSMT are usually larger than those of the other systems. This indicates that apart from AML, PSMT in these systems also contributes to the period decrease. For overcontact binary systems, the studies of Qian (2001a, b) have shown that mass ratio is a key variable for orbital period change. The parameters listed in Table 5 indicate that there is no significant correlation between the mass ratio and the period change for NCBs.

A possible correlation between dP/dtandP for NCBs with decreasing period. Diamonds refer to systems with PSMT and solid dots to the others. See text for detail.

A possible correlation between dP/dtandP for NCBs with decreasing period. Diamonds refer to systems with PSMT and solid dots to the others. See text for detail.

Two evolutionary paths to overcontact binaries were discussed by Hilditch et al. (1988). One was from detached system directly evolving into initially shallow overcontact and the other was via a case A mass transfer to semidetached, then to overcontact systems. The components of the NCBs listed in Table 5 are main-sequence stars. They may be the results of a Case A evolution in which a magnetic stellar wind has resulted in drastic AML. The secular period decreases of the NCBs indicates that the AML is continuous, and a overcontact configuration would seem inevitable. Thus the present sample stars may evolve into overcontact binaries via the second path of Hilditch (1988). As the orbital period is decreasing, the shrinking of the critical Roche lobe can cause the formation of a common convective envelope (CCE). Once the CCE is formed, these NCBs become to overcontact binary stars where both components share the CCE with its nearly uniform surface brightness resulting from energy transfer between the two stars and thus the systems display EW-type light curve. Therefore, these NCBs are at the beginning of the overcontact phase. They are very important source for understanding the formation of CCE and for studying the dynamical evolution of close binary star.

To explain the period variations of W UMa-type overcontact binary systems, an evolutionary scheme was proposed by Qian (2001a). The scheme assumed that the change of the depth of overcontact can cause the variation of magnetic activity that changes the AML rate (see also Vilhu 1981 and Smith 1984). This model predicted that cooler overcontact binaries (M1 < 1.35 M) will oscillate around a critical mass ratio because a period increase can cause a decrease of depth of overcontact and result in a rather higher AML rate, and finally the period will decrease again. For the hotter overcontact binary systems (M1 > 1.35 M), since the AML rate is lower, a period-increasing system is expected. Once overcontact is broken, an NCB will be formed and the system will oscillate around a marginal-contact state as predicted by TRO ( Lucy 1976 Flannery 1976 Robertson & Eggleton 1977).