APOD and General Astronomy Discussion Forum
https://asterisk.apod.com/
Are you asking about a binary star system? If so, the answer is that there simply isn't enough mass outside of the sun. The sun has 99.85% of the mass of the solar system. Jupiter has only 1/1000 as much mass as the sun, and Uranus has only 1/20000 as much mass as the sun. A brown dwarf (13 to 75 times the mass of Jupiter) can't burn hydrogen like a star. The least massive star is a red dwarf which is at least 75 times the mass of Jupiter, and the smallest one found yet is 93 times the mass of Jupiter. All of the contents of the solar system taken together (with the exception of the sun itself) are only 1.6 times the mass of Jupiter.mark swain wrote:Why did our binary never happen?
http://en.wikipedia.org/wiki/Binary_star wrote:Formation
While it is not impossible that some binaries might be created through gravitational capture between two single stars, given the very low likelihood of such an event (three objects are actually required, as conservation of energy rules out a single gravitating body capturing another) and the high number of binaries, this cannot be the primary formation process. Also, the observation of binaries consisting of pre main sequence stars, supports the theory that binaries are already formed during star formation. Fragmentation of the molecular cloud during the formation of protostars is an acceptable explanation for the formation of a binary or multiple star system.
The outcome of the three body problem, where the three stars are of comparable mass, is that eventually one of the three stars will be ejected from the system and, assuming no significant further perturbations, the remaining two will form a stable binary system.
Mass transfer and accretion
As a main sequence star increases in size during its evolution, it may at some point exceed its Roche lobe, meaning that some of its matter ventures into a region where the gravitational pull of its companion star is larger than its own. The result is that matter will transfer from one star to another through a process known as Roche Lobe overflow (RLOF), either being absorbed by direct impact or through an accretion disc. The mathematical point through which this transfer happens is called the first Lagrangian point. It is not uncommon that the accretion disc is the brightest (and thus sometimes the only visible) element of a binary star.
If a star grows outside of its Roche lobe too fast for all abundant matter to be transferred to the other component, it is also possible that matter will leave the system through other Lagrange points or as stellar wind, thus being effectively lost to both components. Since the evolution of a star is determined by its mass, the process influences the evolution of both companions, and creates stages that can not be attained by single stars.
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Research findings
It is estimated that approximately 1/3 of the star systems in the Milky Way are binary or multiple, with the remaining 2/3 consisting of single stars.
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Planets around binary stars
Science fiction has often featured planets of binary or ternary stars as a setting. In reality, some orbital ranges are impossible for dynamical reasons (the planet would be expelled from its orbit relatively quickly, being either ejected from the system altogether or transferred to a more inner or outer orbital range), whilst other orbits present serious challenges for eventual biospheres because of likely extreme variations in surface temperature during different parts of the orbit. Planets that orbit just one star in a binary pair are said to have "S-type" orbits, whereas those that orbit around both stars have "P-type" or "circumbinary" orbits. It is estimated that 50–60% of binary stars are capable of supporting habitable terrestrial planets within stable orbital ranges.
Simulations have shown that the presence of a binary companion can actually improve the rate of planet formation within stable orbital zones by "stirring up" the protoplanetary disk, increasing the accretion rate of the protoplanets within.
Detecting planets in multiple star systems introduces additional technical difficulties, which may be why they are only rarely found. Examples include PSR B1620-26 b and Gamma Cephei.
A study of fourteen previously known planetary systems found three of these systems to be binary systems. All planets were found to be in S-type orbits around the primary star. In these three cases the secondary star was much dimmer than the primary and so was not previously detected. This discovery resulted in a recalculation of parameters for both the planet and the primary star.
Uranus is not a rigid body and so it cannot wobble like a Billy Kilmer forward pass.mark swain wrote:I would love to know why Uranus is on its side though.
I can not see why, if it was hit by another planet, would put it on its side in a stable way.
