Real geodes are handball-sized, hollow rocks that start out as bubbles in volcanic or sedimentary rock. Only when these inconspicuous round rocks are split in half by a geologist, do we get a chance to appreciate the inside of the rock cavity that is lined with crystals. In the case of Hubbles 35 light-year diameter celestial geode the transparency of its bubble-like cavity of interstellar gas and dust reveals the treasures of its interior.
In this unusual image, the NASA/ESA Hubble Space Telescope captures a rare view of the celestial equivalent of a geode - a gas cavity carved by the stellar wind and intense ultraviolet radiation from a young hot star.
The object, called N44F, is being inflated by a torrent of fast-moving particles (what astronomers call a "stellar wind") from an exceptionally hot star (the bright star just below the centre of the bubble) once buried inside a cold dense cloud. Compared with our Sun (which is losing mass through the so-called "solar wind"), the central star in N44F is ejecting more than a 100 million times more mass per second and the hurricane of particles moves much faster at 7 million km per hour (as opposed to less than 1.5 million km per hour for our Sun). Because the bright central star does not exist in empty space but is surrounded by an envelope of gas, the stellar wind collides with this gas, pushing it out, like a snow plough. This forms a bubble, whose striking structure is clearly visible in the crisp Hubble image.
The nebula N44F is one of a handful of known interstellar bubbles. Bubbles like these have been seen around evolved massive stars (so-called Wolf-Rayet stars), and also around clusters of stars (where they are called "super-bubbles"). But they have rarely been viewed around isolated stars, as is the case here.
Lars Lindberg Christensen | EurekAlert!
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