Quote: “Always two there are, no more, no less.” – Star Wars: Episode I – The Phantom Menace 
Well, stars do not always come in pairs (or triples, etc.). But binary stars – stellar pairs – are common. Estimates vary. Wiki uses an estimate “that approximately one third [33%] of the star systems in the Milky Way are binary or multiple , with the remaining two thirds being single stars.” Other estimates [Space.com 2018] for multiple star systems are even higher – at 80% or more, with binary the most common.
Detecting that a distant star system is binary may be challenging, for example, if a particularly wide pair. 
Close pairs are in the news lately. Mergers.
I am curious about these mergers of corpses of massive stars, namely, the state of quark matter before and after. As well as why stellar pairs are relatively common – as likely evolved systems vs. via rare problematical capture, for example.
EXTREME MASS MERGERS
• Caltech Weekly > “LIGO–Virgo–KAGRA finds elusive mergers of black holes with neutron stars” by Whitney Clavin (June 29, 2021) – detected by the National Science Foundation’s (NSF) Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States and by the Virgo detector in Italy after analyzing the LIGO signals from 2020 for January 5 (from ~900 million light-years away) and 15 (~1 billion light-years away).
(quote) For the first time, researchers have confirmed the detection of a collision between a black hole and a neutron star. In fact, the scientists detected not one but two such events occurring just 10 days apart in January 2020. The extreme events made splashes in space that sent gravitational waves rippling across at least 900 million light-years to reach Earth. In each case, the neutron star was likely swallowed whole by its black hole partner.
We can finally begin to understand how many of these systems exist, how often they merge, and why we have not yet seen examples in the Milky Way,” says Astrid Lamberts, CNRS researcher of the Virgo collaboration at Artemis and Lagrange laboratories, in Nice, France, and formerly a Caltech postdoctoral scholar.
LOWER MASS MERGERS
• Caltech Weekly > “A White Dwarf Living on the Edge” by Whitney Clavin (June 30, 2021) – discovery made by the Zwicky Transient Facility, or ZTF, which operates at Caltech’s Palomar Observatory.
(quote) White dwarfs are the collapsed remnants of stars that were once about eight times the mass of our sun or lighter. Our sun, for example, after it first puffs up into a red giant in about 5 billion years, will ultimately slough off its outer layers and shrink down into a compact white dwarf. About 97 percent of all stars become white dwarfs.
While our sun is alone in space without a stellar partner, many stars orbit around each other in pairs. The stars grow old together, and if they are both less than eight solar-masses, they will both evolve into white dwarfs.
The new discovery [ZTF J1901+1458] provides an example of what can happen after this phase. The pair of white dwarfs, which spiral around each other, lose energy in the form of gravitational waves and ultimately merge. If the dead stars are massive enough, they explode in what is called a type Ia supernova. But if they are below a certain mass threshold, they combine together into a new white dwarf that is heavier than either progenitor star. This process of merging boosts the magnetic field of that star and speeds up its rotation compared to that of the progenitors.
• Phys.org > “A white dwarf living on the edge” by W. M. Keck Observatory (June 30, 2021)
• Black hole systems – communicating the cosmos
• Photographing a black hole?
• Ergosphere – what?
 Except, according to fandom, no one actually followed the Rule of Two.
 The science fiction Three-Body Problem book series portrays a system consisting of “three solar-type stars orbiting each other in an unstable three-body system, with a single Earth-like planet unhappily being passed among them and suffering extremes of heat and cold, as well as the repeated destruction of its intelligent civilizations.”
 There’s even been speculation that our Sun is a binary star. Paired with a dim dwarf. Recent advanced sky surveys have made no such detection, however. But then there’s the question of a lost sibling, as noted by Space.com : “Recent computer-simulation research suggests that most if not all stars are born with companions.”
 Re stellar tango, Wiki:
(quote) It takes two to tango is a common idiomatic expression which suggests something in which more than one person or other entity are paired in an inextricably-related and active manner, occasionally with negative connotations.
2 thoughts on “Stellar pairs – when the tango stops”
Visualization of a future lower mass merger.
• Space.com > “Rare ‘teardrop’ star and its invisible partner are doomed to explode in a massive supernova” by Brandon Specktor (July 14, 2021) – The star system is the closest Type Ia supernova candidate [probably in millions of years] ever found near Earth.
Another visualization for some extreme mass mergers.
• NASA > APOD > “GW200115: Simulation of a Black Hole Merging with a Neutron Star” (7-14-2021)
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