Bow shocks form the boundary between a magnetosphere and an ambient magnetized medium. This occurs when the magnetic field of an astrophysical object interacts with the nearby flowing ambient plasma. For example, when the solar wind, flowing with a relative speed of order 400 km/s, encounters the magnetic field of Earth, a bow shape boundary forms. For Earth and other magnetized planets, it is the boundary at which the speed of the stellar wind abruptly drops as a result of its approach to the magnetopause. For stars, this boundary is typically the edge of the astrosphere, where the stellar wind meets the interstellar medium.
A protoplanetary disk is a rotating circumstellar disk of dense gas and dust surrounding a young newly formed star, a T Tauri star. The protoplanetary disk may also be considered an accretion disk for the star itself, because gasses or other material may be falling from the inner edge of the disk onto the surface of the star. But this process should not be confused with the accretion process thought to build up the planets themselves.
A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two, three, four, or even more stars are called multiple star systems. These systems, especially when more distant, often appear to the unaided eye as a single point of light, and are then revealed as double (or more) by other means. Research over the last two centuries suggests that half or more of visible stars are part of multiple star systems.
The search for extraterrestrial intelligence (SETI) is a collective term for the scientific search for intelligent extraterrestrial life. For example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other worlds. There are great challenges in searching the universe for signs of intelligent life, including their identification and interpretation. As various SETI projects have progressed, some have criticized early claims by researchers as being too “euphoric”.
Wolf-Rayet stars (often referred to as WR stars) are a heterogeneous set of stars with unusual spectra showing prominent broad emission lines of highly ionised helium and nitrogen or carbon. The spectra indicate very high surface temperatures of 30,000 – 200,000 Kelvin, surface enhancement of heavy elements, and strong stellar winds.