Superbubble is the astronomical

Astronomy

Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 term used to describe a cavity hundreds of light years across filled with 10⁶ K

Kelvin

The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

 gas blown into the interstellar medium

Interstellar medium

In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

 by multiple supernova

Supernova

A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

e and stellar wind

Stellar wind

A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...

s. The solar system

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 lies near the center of an old superbubble, known as the Local Bubble

Local Bubble

The Local Bubble is a cavity in the interstellar medium of the Orion Arm of the Milky Way. It is at least 300 light years across and has a neutral hydrogen density of about 0.05 atoms per cubic centimetre, or approximately one tenth of the average for the ISM in the Milky Way , and half that for...

, whose boundaries can be traced by a sudden rise in dust

Cosmic dust

Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...

 extinction

Extinction (astronomy)

Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation by matter between an emitting astronomical object and the observer. Interstellar extinction—also called Galactic extinction, when it occurs in the Milky Way—was first...

 of stars at distances greater than a few hundred light years.

Formation

The most massive stars, with masses ranging from eight to roughly one hundred solar masses and spectral type

Stellar classification

In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

s of O and early B are usually found in groups called OB associations. Massive O stars have strong stellar winds, and all of these stars explode as supernova

Supernova

A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

e at the ends of their lives.

The strongest stellar winds release kinetic energy of 10⁵¹ erg

Erg

An erg is the unit of energy and mechanical work in the centimetre-gram-second system of units, symbol "erg". Its name is derived from the Greek ergon, meaning "work"....

s (10⁴⁴ J

Joule

The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

) over the lifetime of a star, which is equivalent to a supernova explosion. These winds can form stellar wind bubble

Stellar wind bubble

Stellar-wind bubble is the astronomical term usually used to describe a cavity light years across filled with hot gas blown into the interstellar medium by the high-velocity stellar wind from a single massive star of type O or B. Weaker stellar winds still blow bubble structures though, and these...

s dozens of light years across.
Inside OB associations, the stars are close enough that their wind bubbles merge together, forming a giant bubble called a superbubble.
When stars die, supernova explosions, similarly, drive blast wave

Shock wave

A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

s that can reach even larger sizes, with expansion velocities up to several hundred km s^-1. Stars in OB associations are not gravitationally bound, but they drift apart at small speeds (of around 20 km s^-1), and they exhaust their fuel rapidly (after a few millions of years). As a result, most of their supernova explosions occur within the cavity formed by the stellar wind bubbles. These explosions never form a visible supernova remnant

Supernova remnant

A supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.There are two...

, but instead expend their energy in the hot interior as sound waves. Both stellar winds and stellar explosions thus power the expansion of the superbubble in the interstellar medium.

The interstellar gas swept up by superbubbles generally cools, forming a dense shell around the cavity. These shells were first observed in line emission at twenty-one centimeters from hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

, leading to the formulation of the theory of superbubble formation. They are also observed in X-ray

X-ray

X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

 emission from their hot interiors, in optical line emission from their ionized shells, and in infrared continuum emission from dust swept up in their shells. X-ray and visible emission are typically observed from younger superbubbles, while older, larger objects seen in twenty-one centimeters may even result from multiple superbubbles combining, and so are sometimes distinguished by calling them supershells.

Large enough superbubbles can blow through the entire galactic disk, releasing their energy into the surrounding galactic halo or even into the intergalactic medium.

Superbubble examples

• LHA 120-N 44 in the Large Magellanic Cloud .
• Anticenter shell
  Anticenter shell
  The Anti-center shell is the name of a region in space emitting 21 cm radiation near the anticenter of the Milky Way Galaxy in the constellation Auriga. It is located at , or l = 197°, b = +2° in galactic coordinates. It is a galactic supershell within our galaxy which is spherical in shape and...
  
  , a supershell once called "Snickers"
• Henize 70
• Local Bubble
  Local Bubble
  The Local Bubble is a cavity in the interstellar medium of the Orion Arm of the Milky Way. It is at least 300 light years across and has a neutral hydrogen density of about 0.05 atoms per cubic centimetre, or approximately one tenth of the average for the ISM in the Milky Way , and half that for...
  
  , which contains the Solar system
  Solar System
  The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
  
  
• Monogem Ring
• N44 Superbubble
• Ophiuchus Superbubble
• The Scutum Supershell
• Orion-Eridanus Superbubble

External links

• Tenorio-Tagle, G., & Bodenheimer, P. " Large-scale expanding superstructures in galaxies". 1988, Annual Review of Astronomy and Astrophysics 26, 145-197. General overview.