interstellar-medium

The solar wind is a continuous outflow of charged particles, mostly protons and electrons, from the Sun's hot corona. It moves at roughly 400 to 800 km/s, bends the interplanetary magnetic field into a spiral, inflates the heliosphere, pushes comet ion tails away from the Sun, drives auroras, and helps strip planetary atmospheres. Voyager 1 crossed the heliopause, where the wind gives way to interstellar space, in 2012.

The interstellar medium (ISM), the gas and dust filling the space between stars, is not empty. It is a multiphase medium ranging from 10 K molecular clouds where stars form to million-kelvin plasma heated by supernovae. Dust hides the centre of our galaxy from visible light, and the ISM is both the raw material for new stars and the place where dying stars deposit their products.

Nebulae are vast clouds of gas and dust between stars. Some are stellar nurseries where new stars form, some are shells shed by dying stars, and some are the debris of supernova explosions. The Orion Nebula is a nursery visible to the naked eye; the Crab Nebula is the expanding wreckage of a star that exploded in 1054.

Stars do not sit quietly inside galaxies. Their radiation, winds, and supernovae heat, push, and enrich the gas around them. This stellar feedback is why star formation is slow, why dwarf galaxies are faint, and where much of a galaxy's heavy elements end up.

Interstellar dust grains, typically far thinner than a human hair, dim starlight, build molecular hydrogen, shield star-forming clouds, carry the isotopic signatures of dead stars, and supply the solids from which planets grow.