In the constellations of Cygnus and Canis Majoris, astronomers see concentrations of newly-formed massive stars, created by the compression of the clouds of gas and dust from which stars are born. Details from ESA's star-fixing satellite Hipparcos, on the motions in the star associations, enable astronomers to judge the nature of the violent events in nearby regions of space which triggered the episodes of star formation.
Fernando Comerón of the European Southern Observatory (Garching, Germany) told the Hipparcos Symposium in Venice today (14 May) that the Cygnus starmaking was probably provoked by strong winds and explosions among previous generations of young hot stars, encouraging the formation of new generations. On the other hand, a smaller association in Canis Majoris seems to be due to the shock of a stellar explosion, a supernova, spreading through a single cloud of gas and dust and inducing starbirths. Comerón was reporting on a study by a team of astronomers in Barcelona, Paris and Garching.
The richest grouping of young stars known in the Milky Way Galaxy consists of the 400 members of the Cygnus OB2 Association, about 4000 light-years away. It is near the centre of the Cygnus Superbubble, a giant shell of very hot gas detected by X-ray astronomers. At least four other associations of young stars populate the region. The picture emerges of a shell of star formation spreading outwards from the central OB2 region for the past 5 to 10 million years. The stars too continue to travel outwards, after their formation, at speeds of up to 25 kilometres per second.
"We have to explain these high velocities," Comerón commented. "They represent a lot of kinetic energy. Thanks to Hipparcos we now can say with some confidence that the energy required, for the expansion of the shell in Cygnus, is compatible with the combined power to be expected from hot stellar winds and occasional supernova explosions. But some of the young stars of Cygnus may be products of the most violent chain-reaction in the contemporary Milky Way."
The Canis Major OB1 Association is smaller in scale, and the newborn stars flee from their birthplaces at a slower speed than in the Cygnus complex. The association is also lopsided, with a disturbance travelling outwards through a hemisphere, rather than a sphere as in Cygnus. Comerón and his colleagues therefore visualize a single supernova detonating at the edge of a dense cloud of gas and dust, about 3 light-years wide and with a mass about 1000 times the mass of the Sun.
Has the Sun wandered into the midst of a similar scene of star formation, but nearer and older? The ring of bright blue stars around the sky called Gould's Belt consists of hot young stars within 2000 light-years of the Sun and Earth. Members of Gould's Belt include blue stars in the constellation Orion and in the Scorpius-Centaurus region of the southern sky. According to the 19th-Century American astronomer Benjamin Gould, who gave his name to the belt, bright northern stars within (for example) Cygnus and Perseus are members of the same system. It slopes at about 20 degrees to the disk of the Milky Way Galaxy.
Per Olof Lindblad of Stockholm Observatory, Sweden, has made a lifelong study of Gould's Belt. Commenting on Hipparcos results on the Gould's Belt stars, as examined by a Swedish-Czech team, he confesses himself to be a little disconcerted.
"Gould's Belt was easier to describe, and to believe in, before we had the precise data from Hipparcos on the candidate stars," Lindblad says. "Now we are not even sure whether Benjamin Gould's northern stars are really part of the same system. As for the Orion and Scorpius-Centaurus associations, the primary members of Gould's Belt, they seem to be revolving around each other. This motion helps to explain why the belt is flat and how it has retained its inclination. We see a large system of hot, young stars, about 30 million years old, but we are still not sure how big it is, or how many of the bright stars are members. It is clearly associated with a huge expanding cloud of neutral hydrogen gas."
At the other end of the range of star ages, the first shots were exchanged in what promises to be a major battle among astronomers about the results from Hipparcos and their effects on estimates of the age of the oldest stars in the Universe. As mentioned in previous ESA Information Notes, several groups have already used Hipparcos data to arrive at relatively young age of 11-12 billion years. This is compatible with certain measurements of the motions of galaxies that indicate that the Universe too is relatively young. But in Venice today (14 May) Roger Cayrel of Paris insisted on an age of 14 billion years for the oldest stars close enough to the Sun and Earth for Hipparcos to measure their distances directly. Watch this space for further discussion of stellar ages at the Hipparcos Symposium tomorrow (15 May).
By fixing its distance at 850 light-years, Hipparcos firmly identifies the most massive star that exists in the relatively near vicinity of the Sun and Earth. Astronomers previously thought that Gamma-2 Velorum, in the southern constellation of Vela, was almost twice as far away, at 1500 light-years. At the Hipparcos Symposium in Venice, Wilhelm Seggewiss of the Bonn University Observatory reported the new evaluation of Gamma-2 Velorum on behalf of a world-wide group of astronomers in the Netherlands, Sweden, Germany, Belgium, Canada, Argentina and Indonesia.
Gamma-2 Velorum is easily visible to the naked eye. It is a massive star of the Wolf-Rayet type, discovered in 1867 by French astronomers. Wolf-Rayet stars are distinguished by strong emissions spread widely in wavelength by the motion of gas clouds. These tell of a massive star shedding mass at a very high rate. The mass of Gamma-2 Velorum is now estimated to be 32 times the mass of the Sun, but as it is diminishing at a rate of 3 solar masses per million years, it may be a remnant of a much more massive star. The new distance to Gamma-2 Velorum puts it close to the centre of huge spherical cloud of gas and dust, called the Gum Nebula.
This star has two ways of slimming. Some of its gas is dragged by gravity into its smaller companion, Gamma-1 Velorum, which orbits closely around it every 78 days. Gamma-2 Velorum is also blowing out a strong wind of gas at about 1500 kilometres per second. The results from Hipparcos enable astronomers to evaluate the massive Wolf-Rayet stars more surely than ever before, and to look for the shock of their activity in their surroundings in the Galaxy.
Built for ESA by Europe's aerospace industry, and sent into orbit by an Ariane 4 launcher, the Hipparcos satellite scanned the sky for nearly four years, 1989-93, measuring angles between stars. Protracted calculations by multinational teams then produced the Hipparcos Catalogue giving the positions and motions of 118,000 stars with 100 times the precision of previous surveys. The accompanying Tycho Catalogue contains a million stars charted with lesser but still unprecedented accuracy.
ESA will release the catalogues to the world's astronomers in June. Meanwhile astronomers who helped in the mission have had privileged access to the data. At the Hipparcos Symposium in Venice 13-17 May they give their early conclusions in 190 reports, in the form of talks or scientific posters. The results affect many branches of astronomy from asteroids to cosmology, but especially the theories of stellar physics and evolution, because Hipparcos provides, for the first time, accurate distances to many thousands of stars.