Cosmic Bubbles

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 (several thousand km/s) stellar wind from a single massive star of type O or B. Weaker stellar winds still blow bubble structures though, and these are also called astrospheres. The heliosphere blown by the solar wind, within which all the major planets of the Solar System are embedded, is in fact a small example of a stellar wind bubble.Stellar wind bubbles have a two-shock structure. The freely-expanding stellar wind hits an inner termination shock, where its kinetic energy is thermalized, producing 10⁶K, X-ray emitting plasma. The hot, high-pressure, shocked wind expands, driving a shock into the surrounding interstellar gas.

If the surrounding gas is dense enough (number densities n > 0.1 cm^-3or so), the swept up gas radiatively cools far faster than the hot interior, forming a thin, relatively dense shell around the hot, shocked wind.

Superbubble:

Superbubble is the astronomical term used to describe a cavity hundreds of light years across filled with 10⁶ K gas blown into the interstellar medium by multiple supernovae and stellar winds. The solar system lies near the center of an old superbubble, known as the Local Bubble, whose boundaries can be traced by a sudden rise in dust extinction of stars at distances greater than a few hundred light years.The most massive stars, with masses ranging from eight to roughly one hundred solar masses and spectral types 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 supernovae at the ends of their lives.



photo: The superbubble Henize 70, also known as N70 or DEM301, in the Large Magellanic Cloud.

The strongest stellar winds release kinetic energy of 10⁵¹ ergs (10⁴⁴ J), equivalent to a supernova explosion. These winds can blow stellar wind bubbles dozens of light years across.

Supernova explosions, similarly, drive blast waves that can reach even larger sizes, with expansion velocities of as much as several hundred km s^-1.

Stars in OB associations are not gravitationally bound, but only drift apart at speeds of around 20 km s^-1. As a result, most of their supernova explosions occur within the cavity carved by the strongest stellar wind bubbles. They never form a visible supernova remnant, but instead efficiently deposit their energy into the hot interior as sound waves. Large enough superbubbles can blow entirely through the galactic disk, releasing their energy into the surrounding galactic halo or even into the intergalactic 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, leading to the formulation of the theory of superbubble formation. They are also observed in X-ray 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 human optical 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.

Local Bubble:

The Local Bubble is a cavity in the interstellar medium (ISM) of the Orion Arm of the Milky Way. It is at least 300 light years across and has a neutral hydrogen density approximately one tenth of the 0.5 atoms per cubic centimetre average for the ISM in the Milky Way. The hot diffuse gas in the Local Bubble emits X-rays. The very sparse, hot gas of the Local Bubble is the result of supernovae that exploded within the past two to four million years. The most likely candidate for the remains of this supernova is "Geminga" ("Gemini gamma-ray source"), a pulsar in the constellation Gemini.



photo: Artist's conception of the Local Bubble (containing the Sun and Beta Canis Majoris) and the Loop I Bubble (containing Antares).

The Solar System has been travelling through the region currently occupied by the Local Bubble for the last five to ten million years. Its current location lies in the Local Interstellar Cloud a minor region of denser material within the Bubble. The cloud formed where the Local Bubble and the Loop I Bubble met. The gas within the LIC has a density of approximately 0.1 atoms per cubic centimeter.

The Local Bubble is not spherical, but seems to be narrower in the galactic plane, becoming somewhat egg-shaped or elliptical, and may widen above and below the galactic plane, becoming shaped like an hourglass.

The Local Bubble abuts other bubbles of less dense ISM, including, in particular, the Loop I Bubble. The Loop I Bubble was created by supernovae and stellar winds in the Scorpius-Centaurus Association, some 500 light years from the Sun. Other bubbles abutting the Local Bubble are the Loop II Bubble and the Loop III Bubble.

The Loop I Bubble contains star Antares (also known as Alpha Scorpii) as shown on the diagram above.

Do We Live in a Giant Cosmic Bubble?

Date: 30 September 2008

If the notion of dark energy sounds improbable, get ready for an even more outlandish suggestion.

Earth may be trapped in an abnormal bubble of space-time that is particularly void of matter. Scientists say this condition could account for the apparent acceleration of the universe's expansion, for which dark energy currently is the leading explanation.

Dark energy is the name given to the hypothetical force that could be drawing all the stuff in the universe outward at an ever-increasing rate. Current thinking is that 74 percent of the universe could be made up of this exotic dark energy, with another 21 percent being dark matter, and normal matter comprising the remaining 5 percent.

Until now, there has been no good way to choose between dark energy or the void explanation, but a new study outlines a potential test of the bubble scenario.

If we were in an unusually sparse area of the universe, then things could look farther away than they really are and there would be no need to rely on dark energy as an explanation for certain astronomical observations.

"If we lived in a very large under-density, then the space-time itself wouldn't be accelerating," said researcher Timothy Clifton of Oxford University in England. "It would just be that the observations, if interpreted in the usual way, would look like they were."

Scientists first detected the acceleration by noting that distant supernovae seemed to be moving away from us faster than they should be. One type of supernova (called Type Ia) is a useful distance indicator, because the explosions always have the same intrinsic brightness. Since light gets dimmer the farther it travels, that means that when the supernovae appear faint to us, they are far away, and when they appear bright, they are closer in.

But if we happened to be in a portion of the universe with less matter in it than normal, then the space-time around us would be different than it is outside, because matter warps space-time. Light travelling from supernovae outside our bubble would appear dimmer, because the light would diverge more than we would expect once it got inside our void.

One problem with the void idea, though, is that it negates a principle that has reined in astronomy for more than 450 years: namely, that our place in the universe isn't special. When Nicholas Copernicus argued that it made much more sense for the Earth to be revolving around the sun than vice versa, it revolutionized science. Since then, most theories have to pass the Copernican test. If they require our planet to be unique, or our position to be exalted, the ideas often seem unlikely.

"This idea that we live in a void would really be a statement that we live in a special place," Clifton told SPACE.com. "The regular cosmological model is based on the idea that where we live is a typical place in the universe. This would be a contradiction to the Copernican principle."

Clifton, along with Oxford researchers Pedro G. Ferreira and Kate Land, say that in coming years we may be able to distinguish between dark energy and the void. They point to the upcoming Joint Dark Energy Mission, planned by NASA and the U.S. Department of Energy to launch in 2014 or 2015. The satellite aims to measure the expansion of the universe precisely by observing about 2,300 supernovae.

The scientists suggest that by looking at a large number of supernovae in a certain region of the universe, they should be able to tell whether the objects are really accelerating away, or if their light is merely being distorted in a void.