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Distances ($a$) between binary stars vary wildly, from the order of the radius of the stars, to more than a light-year! The plot below (from here) shows a compilation of several surveys, with the color indicating the method by which they've been detected. Separations are roughly normally distributed in $\log a$, peaking at a typical separation of tens of AU....


32

Collapsing gas clouds fragment into multiple cores because the Jeans mass, that determines the minimum mass that becomes gravitationally unstable to collapse, becomes smaller if the cloud is able to contract without heating up too much. i.e. $$M_J \propto T^{3/2} \rho^{-1/2},$$ where $\rho$ is the cloud density. Thus if the cloud density can increase but ...


30

But why can't Jupiter be a Y Dwarf who is in the binary relationship with the Sun? There are two reasons: One is that Jupiter is too small to have ever undergone fusion of any sort. To qualify as a brown dwarf, an object needs to be large enough to have undergone deuterium fusion in its core. This requires a mass of at least 13 Jupiter masses. The other is ...


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𝛾 Vir (12h 42m, –01° 27′) Probably Porrima, $\gamma$ Vir, is the best candidate for most observers in the Northern Hemisphere to see changes in a binary orbit, particularly using a small telescope. It is a pair of stars with similar size and visual magnitude, of about 3.6. Their orbital period is about 169 years, but the orbit is eccentric, e = 0.88. They ...


20

There is no observational evidence that the sun is a member of a binary (trinary, or more) star system, where "star" means an object that is at least ~80 times the mass of jupiter and emits energy/light via standard hydrogen fusion. Some evidence that people point to is that the majority of stars in the Galaxy (perhaps 60% or so) are binary. However, that ...


17

Will Sirius B start accreting? Yes, it is doing so now. Sirius A will have a wind and some of that wind will be captured by the white dwarf. The effectiveness of wind capture is a strong function of relative wind speed. An analytic approximation to the accretion rate, known as Bondi-Hoyle accretion, goes as the inverse cube of the relative speed. In its ...


16

Black hole and main sequence star/giant star We can observe binary systems containing a black hole by looking for emissions from accretion disks which may form when matter is transferred from the companion star. X-ray binaries and microquasars are particularly notable types. The compact object doesn't have to be a black hole - neutron stars also sometimes ...


16

α Centauri (14h 40m, –60° 50′) The most obvious visual multiple system, where orbital changes can be observed is Alpha Cen A+B, (together with Proxima Centauri). The A/B system has an orbital period of 80 years, but because it is so close (1.34 parsec), the semi-major axis is a whopping 17.5 arcsecond. The two stars are currently separated by 5 arcseconds on ...


12

Well, if they have an angular momentum with respect to their center of mass, or in other words , if they are in orbit, gravitational collapse is not the issue. This is what happens when three bodies are in orbit around each other. Don't get disappointed so soon. There is indeed a way to avoid this. If the three bodies are of comparable mass, and one of the ...


12

Few important points about WISE: it was able to detect anything with a temperature above 70-100 K, whereas the coolest brown dwarfs are in the 500-600 K range (the coolest was discovered by WISE itself, see Mainzner et al., 2011); it was able to detect objects larger than 1km up to 3 AU from the Sun, or objects of 2-3 Jupiter masses in a distance up to 7-10 ...


12

There are two main theories for the formation of binary stars - one accepted, and one mainly deprecated. The fission hypothesis. The fission hypothesis states that the binary system forms after the collapse of the original gas cloud into a protostar. Angular momentum is conserved, so as the extremely large cloud slowly contracts, it spins faster. After ...


10

This is a difficult question, It's been changing through the years and it's difficult to calculate accurately. Now it's know that this percentage changes depending on the star type and ranges from 50% for Sun like stars up to a 80% for type O stars. Fonts: http://www.space.com/1995-astronomers-wrong-stars-single.html (2006) http://www.space.com/22509-...


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Yes. An example would be the Cataclysmic Variable stars (CVs), where the donor star (often in the main sequence), loses material via Roche Love overflow onto a compact white dwarf companion. The other obvious example (though less common) are the Low Mass X-ray Binaries (LMXBs), where the Roche Love overflow is onto a neutron star or black hole. Nelson & ...


9

Polaris consists of multiple stars, α UMi Aa is the main star. It is a supergiant, and a cepheid variable. α UMi Ab and α UMi B are smaller (but still larger than the sun) and both are in orbit with α UMi Aa, (the former is close in, the latter is further out). There are two further stars: α UMi C and D. These stars are thought to have formed before α UMi ...


9

Two massive bodies orbiting each other form stable orbits. This is called the "two-body problem." Add a 3rd body to the system and the results are unstable orbits. It's akin to the motion of a singe pendulum. A single pendulum swinging back and forth is a linear system with motion that is repeating and predictable. Similarly two bodies orbiting each ...


9

"Satellite type" and "Planet type". The terms seem to have been coined by Rudolf Dvorak in 1982 paper "Planetenbahnen in Doppelsternsystemen" Due to the fact that quasiperiodic orbits exist around stable orbits 3 different types of possible planetary orbits are found: S-types (satellite type orbit around one primary), P-types (...


8

I don't know about names for the planets, specifically, but the orbits are called S-type and P-type: S-type: The planet orbits around one star, and the host star has a binary companion (i.e., "the other kind" in the XKCD comic) P-type: The planet orbits around both stars of the binary


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I'm not aware of any official (i.e., IAU) definition of a multiple star system. However, as someone who used to do research in that field, I would interpret multiple star system as encompassing systems with two or more stars. So a binary system, a triple system, and a quadruple system are all multiple star systems.


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It's worth noting that in many cases, if not most, we simply don't know the exact answer to such excellent questions as the one you ask. Note that the book you mention (ISBN-13: 978-0471409762) was written around 2000 and appeared in 2002: that's a really long time ago in terms of all the amazing new instruments which have become available. For example, ...


8

Go to the exoplanets.org database. Top left hand dropdown menu - select "Stars" You get a table listing all the known stars with exoplanets. Click on the big plus sign in the top right hand of the table and select "Binary Flag" This will now show a boolean flag that indicates if the star is known to be part of a binary (or multiple) system. In the ...


8

Bond et al. (2017) measure the orbital period of the Sirius system to be $50.1284 \pm 0.0043$ years. I believe this is the most precise and accurate value (I cannot find any more recent papers, with new determinations, that cite this paper). An earlier, comprehensive study by Gatewood & Gatewood (1978) gave $50.090\pm 0.056$ years; consistent with the ...


7

At least 7. We currently know of two star systems with 7 stars: Nu Scorpii and AR Cassiopeiae. The two have different structures, both of which are complicated but appear to be stable on stellar timescales. Nu Scorpii This system has two components, Nu Scorpii AB and Nu Scorpii CD. Nu Scorpii A is a triple star system itself, with a binary system in the ...


7

Okay! Here are some of the popular targets, search for the season of visibility yourself please. The above list describes the angular seperation between the stars and also the magnification required to resolve it, both explicitly. So, this list is not only for amateurs, but there are some intermediate ones which you can try if you have a good enough ...


7

I just found the answer to my question at this place: http://astronomynow.com/2015/02/18/suns-close-encounter-with-scholzsstar/ Here is the answer: Currently, Scholz’s Star is a small, dim red dwarf in the constellation of Monoceros, about 20 light-years away. However, at the closest point in its flyby of the solar system, Scholz’s Star would have ...


7

Your best bet for finding relevant information on this is to look up actual published papers. I'll walk you through my research process to help in the future, as well as provide the results I found. Step 1: Google Scholar is your friend I start out using Google Scholar. This is much like Google, but rather than returning any old website, it specifically ...


7

In general those binaries will be hard to detect, but there is no reason why they wouldn't form. A whole class of objects, X-Ray binaries are in fact thought to be hosting a pair of BH and non-BH objects. The idea here is that the X-ray luminosity originates from mass accreted from the companion through Roche-Lobe-overflow. The mass from the companion or '...


7

Prior to the discovery of gravitational wave sources, the only confirmed stellar-sized black holes, were those detected in binary systems with more "normal" stars. These were most detected via X-rays emitted by hot material that is accreted from the "normal" companion. The two main categories are high-mass and low-mass X-ray binaries: in the former, the ...


7

You will always get a "stable" orbit if the stars have less than escape velocity relative to each other. (unless you are modelling the stars as having non-zero radii so they can collide) The stars will enter into elliptical orbits around a barycentre. But I guess you want a circular orbit. For a circular orbit the speed $v$ is given by $$v^2=GM/r$$...


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No. Such an arrangement is at best "metastable". That is, although there are periodic solutions to the three body problem (stable orbits) an infintesimal perturbation (eg the proverbial butterfly flapping its wings) will push the system off the stable orbit and into chaos. Getting a planet to remain at the barycentre is like trying to balance a ...


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