The Void

The Void

Bow Shocks

Wikipedia, Bow shocks in astrophysics

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.

Bow shock in the Orion nebula. The star's wind collides with the nebula flow.
Bow shock in the Orion nebula. The star's wind collides with the nebula flow.

The Heliopause

Wikipedia, Solar wind

The solar wind "blows a bubble" in the interstellar medium. The point where the solar wind's strength is no longer great enough to push back the interstellar medium is known as the heliopause and is often considered to be the outer border of the Solar System.

The Interstellar Medium

The interstellar medium is the vast space between stars. The perspective that you get when looking at things at this level is humbling, and tells you a lot about how difficult interstellar travel must be. You'd need some seriously amazing hardware to travel the distance.

The interstellar medium has a very low density, and consists mainly of clouds of hydrogen and helium gas. However, it is not 'empty'.

H.G. Wells, The War of the Worlds

Few men realise the immensity of the vacancy in which the dust of the material universe swims.

James Jeans, quoted in Skywatching

If you put three grains of sand inside a vast cathedral, that cathedral will be more densely packed with grains of sand than stars are found apart in space.

Wikipedia, Interstellar medium

The ISM is turbulent and therefore full of structure on all spatial scales. Stars are born deep inside large complexes of molecular clouds, typically a few parsecs in size. During their lives and deaths, stars interact physically with the ISM.

Stellar winds from young clusters of stars (often with giant or supergiant HII regions surrounding them) and shock waves created by supernovae inject enormous amounts of energy into their surroundings, which leads to hypersonic turbulence. The resultant structures – of varying sizes – can be observed, such as stellar wind bubbles and superbubbles of hot gas, seen by X-ray satellite telescopes or turbulent flows observed in radio telescope maps.

The Sun is currently traveling through the Local Interstellar Cloud, a denser region in the low-density Local Bubble.

Wikipedia, Interstellar medium

Voyager 1 reached the ISM on August 25, 2012, making it the first artificial object from Earth to do so. Interstellar plasma and dust will be studied until the mission's end in 2025.