LEFT: The first infrared image ever obtained of a pre-stellar core shows the egg-like object from which a new star will form. It illustrates ISO's role in tracing the earliest stages of star formation. Pre-stellar cores are very cold and hidden by dust from all telescopes except those working with very short radio waves and long infrared waves. Within a core, gravity squeezes gas and dust towards the centre to make an embryonic star, but by the time other instruments can see anything, the star-making processes are far advanced. The object L1689B is judged to be on the very brink of collapsing under gravity, to begin to form a star.

Derek Ward-Thompson of the Royal Observatory Edinburgh (UK) with British and French colleagues detected L1689B and other objects by sub-millimetre radio observations, as small, dense features within a much larger cloud of gas and dust located in the constellation Ophiuchus. Scans by ISO's photometer ISOPHOT, at various wavelengths up to its limit of 200 microns, now give different impressions of L1689B and enable the astronomers to measure the temperature of the pre-stellar core at about minus 260 degrees C (13 K). The image is colour-coded for intensity, with the bright core in yellow-white.

RIGHT: Also from ISOPHOT on ISO comes the last measurement of the size of an infrared-emitting disk of dust that surrounds the bright star Vega. When detected in 1983 by the Dutch-US-UK Infrared Astronomy Satellite IRAS, this dust disk excited astronomers because it matched their ideas about the surroundings of the Sun at the time when the planets of the Solar System were being born. The ISO scan of Vega enables Ingolf Heinrichsen of the ISOPHOT Instrument Dedicated Team (Spain, Germany) and Helen Walker of the Rutherford Appleton Laboratory (UK) offer the latest measurement of the radius of the Vega disk. It is between 80 and 140 times the distance of the Earth from the Sun, and the edge the disk may stretch as far as 400 times the Earth-Sun distance. For comparison Neptune, the most distant large planet in the Solar System, is at 30 times the Earth-Sun distance.

A corresponding scan across the star Gamma Draconis, used as a control specimen, shows no dust disk. The infrared emissions are concentrated in the visible star and they are very bright on the ISOPHOT detector, but the peak brightness is restricted to a thin strip in the middle of the scan, unlike Vega. In both of the colour-coded images, red denotes the strongest emissions from the source.

Credit: (L1689B) : ESA/ISO, ISOPHOT and D. Ward-Thompson et al.

(Vega and Gamma Draconis): ESA/ISO, ISOPHOT IDT and H. Walker

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