Over the past 13.8 billion years, gravity has driven matter to aggregate, compact, contract, and collapse, forming hundreds of billions of cosmic objects hurtling through space—solo or in gravitationally bound systems. In the Universe's vast expanse, trajectories often intersect, making collisions inevitable.
The standard cosmological model, Λ-CDM, outlines hierarchical structure formation, building from smaller to larger objects through mergers. This relies on cold, massive dark matter particles moving far slower than light, fostering this bottom-up growth.
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The collision of two stars will light up the night sky in 2022
Throughout cosmic evolution, our catalog has grown with planets, stars, galaxies, black holes, and more. These typically orbit within gravitationally bound systems that resist the Universe's expansion. Despite great distances, collisions occur frequently in dense environments.
About 4.5 billion years ago, as our Solar System formed, models indicate more than eight planets existed initially. Simulations suggest a fifth gas giant between Jupiter and Neptune, later ejected during orbital clearing.
The leading theory for the Moon's origin is a Mars-sized impactor striking proto-Earth, ejecting debris that coalesced into our satellite. Lunar samples from Apollo missions strongly support this, as do origins for Mars' moons Phobos and Deimos.
Current models show rocky planets frequently collide during solar system formation, merging into larger worlds while debris forms satellites—smaller with distance, as in the Pluto-Charon system.
The International Astronomical Union defines brown dwarfs as objects too light for sustained hydrogen fusion (13-75 Jupiter masses, or 1-7.5% solar mass) yet capable of deuterium fusion—neither full stars nor giant planets.
Composed of ~75% hydrogen, colliding brown dwarfs can exceed 0.075 solar masses, igniting hydrogen fusion and birthing an M-type red dwarf star.
Stars vary widely in mass: low-mass ones glow red, burn slowly; high-mass ones shine blue-hot, fuse rapidly, and die young. Cluster ages are gauged by stellar mass distributions on the Hertzsprung-Russell diagram.
In some clusters, anomalously blue, massive, hot stars defy expectations—these 'blue stragglers' arise from mergers.
A merger of, say, 0.7- and 0.8-solar-mass red dwarfs yields a 1.5-solar-mass blue giant, even in ancient clusters lacking such originals. Common in dense globular clusters, this renews stellar populations.
White dwarfs are ultra-dense remnants of evolved low- to medium-mass stars… (continued on next page)
