l o c a l   b u b b l e
the local bubble
Surveys of the region of space within 300 parsecs (1,000 light years) of the Solar System have shown that we occupy a kind of bubble that is largely empty of interstellar clouds of gas. However, our so-called “Local Bubble” is not a sphere but an irregular cavity resembling the internal structure of a sponge. (See a recent summary and follow-up article.) This void is bounded by “walls” consisting of huge gas clouds, and it extends into tunnels, tubes, and chimneys that lead to other empty regions farther away.
According to Lallement et al. (2003), the Local Bubble extends for about 60 parsecs (200 light years) in the direction of the Galactic Center, and for about 80 to 150 parsecs (260 to 490 light years) in the direction of the Galactic rim. Chimneys extend above and below the plane of the Galactic Disk for more than 200 parsecs (650 light years).
Supernovae explosions, which create powerful winds of superheated gas, have been invoked to explain this irregular network of matter and void. Such cosmic storms occurring over tens of millions of years have swept interstellar gases out of nearby space, blowing them away in all directions until they collide with similar fronts expanding out of neighboring cavities. Collisions between clouds create the tenuous walls that delimit each bubble.
The supernovae responsible for the Local Bubble were born in an expanding starburst formation known as the Gould Belt, an elliptical cloud of stars and molecular hydrogen whose long axis is about 750 parsecs (2500 light years). Although the origin and evolutionary history of the Solar System is completely unrelated to this structure, we are currently located near the center of the Gould Belt. Our night skies are dominated by its youngest generations of stars.
Across the Bubble & Beyond: The bright star near the left edge of the image is Alpha Centauri, a binary system consisting of 2 Sun-like stars only 1.34 parsecs away (4.36 light years). At image center is Crux, the Southern Cross, whose 4 brightest stars are located at distances of 25 to 115 pc (88-364 ly). Just left of Crux is the black cloud known as the Coalsack, a starless nebula at 150 pc (489 ly), near the boundary of the Local Bubble. The two rose-colored clouds to the right of Crux are IC 2944, a star-forming region at about 1800 pc (5900 ly), and the Carina Nebula or NGC 3372, an even larger star-forming at about 2300 pc (7500 ly).
Star populations in the Local Bubble
By definition, our local blown-out cavity lacks stellar nurseries – that is, the enormous clouds of molecular hydrogen in which new stars are formed. Thus the very hottest and most massive stars (blue dwarfs of spectral class O, with masses in excess of 20 MSOL) are absent from our neighborhood (Preibisch et al. 2002). The explanation is simple: these stars have such short lifetimes, generally on the order of a few million years, that they explode before they can travel far from their native nebulae.
The next hottest type, spectral class B, spans the range of masses from 3 MSOL to 20 MSOL (Preibisch et al. 2002). These stars are thinly scattered through the Local Bubble, especially at the low end of the mass range. About a dozen may be found within 40 parsecs (130 light years) of the Solar System (see The Sun’s Back Yard).
In order of decreasing mass and luminosity, the remaining stellar population consists of classes A (white), F (yellow-white), G (yellow), K (orange) and M (red). A stars span an approximate mass range from 1.6 MSOL to 3.0 MSOL (Johnson et al. 2007a). F stars occupy the next level down, from about 1.1 MSOL to 1.5 MSOL. At the lower end of the range they overlap with G stars, which have masses between 0.85 MSOL and 1.2 MSOL. Similarly, G stars overlap with K stars, whose masses fall in the range of 0.6 MSOL to 0.9 MSOL. Finally, M dwarfs range from 0.08 MSOL to 0.5 MSOL.
The population of each spectral class varies inversely according to mass. Red dwarf stars are the most common, comprising 75% of all stars in the Milky Way (Tarter et al. 2007), while hot white stars of class A comprise less than 1% (RECONS 2008). (See also Comparative Sizes of Nearby Stars.)
The sequence of spectral types from O through M has traditionally been remembered with the help of a mnemonic: “Oh, be a fine girl, kiss me!” Alternatives include “Out back a friend grows killer marijuana,” “Oh baby, a fat goat kicked me!” and “Only bored astronomers find gratification knowing mnemonics.”
The contents of the Local Bubble, like all objects in the Galactic Disk, are slowly orbiting the Galactic Center in Sagittarius. Many nearby stars are grouped together in open clusters or co-moving groups (also known as streams or associations) that reflect their common origin in some now-dispersed molecular cloud. The Local Bubble contains several, including the Beta Pictoris Moving Group, the AB Doradus Moving Group, the Ursa Major Moving Group, and the Tucana-Horologium Association (Lopez-Santiago et al. 2006, Zuckerman & Song 2004, King et al. 2003). These groups tend to break up within one billion years.
Stars formed in the inner regions of the Galactic Disk tend to migrate outwards over long time scales (Famaey et al. 2007, Ecuvillon et al. 2007). Thus the Local Bubble contains stars of many ages and origins whose orbits happen to coincide at this moment in the evolution of the Universe. Because stars formed closer to the Galactic Center generally exhibit a higher metallicity than do stars formed in the outskirts, and because more metallic stars are more likely to host giant planets than less metallic stars, a significant proportion of the planet-bearing stars in the Solar neighborhood have probably migrated here from spiral arms in the inner Milky Way (Ecuvillon et al. 2007).
Just as the shape of the Local Bubble is irregular, so is the distribution of matter inside it. Relatively small, low-density gas clouds exist even within this empty region, extending in clumps and filaments.
The Local Bubble: Overhead View
All text is copyright Raymond Harris 2006-2009. Image credits appear in the accompanying captions.