Harvard-Smithsonian Center for Astrophysics
Release No. 03-02:
FOR RELEASE: 9:20 AM PST, Tuesday, January 7, 2003
SEATTLE, WA--For more than half a century astronomers have been mesmerized by the mysterious outburst in 1946 of one of the most luminous supergiant stars of our Galaxy. In the fall of 2000 professional and amateur astronomers worldwide observed a new and long-expected strong dimming of Rho Cassiopeiae. For over half a year the star changed its usual yellowish-white color to deep orange, resulting from a massive explosion of its entire mammoth atmosphere.
An international team of professional astronomers, led by Alex Lobel, an astrophysicist of the Harvard-Smithsonian Center for Astrophysics (CfA), announced here today at the 201st meeting of the American Astronomical Society, that the new outburst of Rho Cas blasted off nearly ten thousand times the mass of the Earth in atmospheric gases into space, or about three percent of the mass of our Sun. The team directly observed the largest rate of mass-loss in any stellar object so far. Their measurements reveal that the temperature at the surface of the star plummeted by more than three thousand degrees for about a hundred days during the phenomenal event.
New discoveries in astronomy often require observations with telescopes in space that are not influenced by Earth's disturbing atmosphere, or with very large ground-based telescopes. Lobel's team however, succeeded to make the discovery using five modest-sized telescopes in the US and Europe. For nearly a decade they regularly observed the spectral fingerprint of the queen of all irregular variable stars.
When asked about the observations of Rho Cas, Lobel replied: ``Monitoring a few hypergiant stars occupies a major part of an astronomer's active career, and we are humbled to have finally witnessed one of these stunning explosions after years of silent persistence.''
Radiating more than half a million times more light than the Sun, at an estimated distance of an amazing ten thousand lightyears away, Rho Cas still shines at about 4.5 magnitudes in the visible part of the spectrum. It is the most distant luminous cool star one can easily observe with the unaided eye. Hypergiants as Rho Cas do not live long, and their short existence may end tomorrow with one final big bang; a supernova. These super-luminous monster stars, with half the size of our solar system, are the only celestial objects known to astronomers for which a supernova explosion can be expected any time soon. They are very scarce. Less than one in a million stars of our Galaxy is a hypergiant. Bright Rho Cas is among them, and the team caught it suffering in the final stages of its tumultuous life. Not yet the final catastrophic blow, but still an eruption so violent that the supergiant atmosphere temporarily more than doubled its usual size of four- to five-hundred times the radius of the Sun.
In the summer and fall of 2000 the team observed how the optical spectrum suddenly revealed dark bands attributed to certain molecules in its atmosphere, but which are normally not observed in Rho Cas. ``These peculiar spectral bands were also temporarily observed during the deep brightness minimum of 1946. We know now that the star produced them because the atmosphere chilled by more than 3000 degrees during the outburst, pumping a tremendous amount of gas away. That gas is lifted into space by a blast wave, or a loud shock wave, as one can sometimes hear when a jet plane flies over, but much stronger and wrapping the entire star'', says Lobel.
By monitoring Rho Cas the astronomers are trying to solve one of the most enduring riddles of stellar physics: Why are there no cool stars that are more luminous than about one million times the luminosity of our Sun? The new outburst has helped them to lift a tip of the cosmic veil. Based on the observations, their calculations show that the shock wave can only propagate so fast and far into space because the atmospheric gas produces huge amounts of neutral hydrogen atoms. Hydrogen is the most abundant chemical element in space, but also in this type of heavy hypergiant star. When a proton and an electron (the building blocks of matter) combine into a hydrogen atom it gives up a small amount of energy. If many hydrogen atoms suddenly form, while the gas cools down, large amounts of freed energy help to push the shock wave outward which chills the inflating atmosphere even more in its wake, yielding an avalanche of gas expelled by the star.
``It makes the atmosphere of stars with a surface temperature like our Sun -- but pouring out about a million times more radiation -- very unstable. Stars more luminous don't make it for long. We don't get the chance of finding them because they perished in their galactic cradles long ago, or because these recurrent mass ejections prevent them from subsisting such large luminosities'', says Lobel.
More recent observations by the team show that the colossal atmosphere of Rho Cas continues to behave tamelessly in the aftermath of the outburst two years ago. Lobel's team proceeds with monitoring the enigmatic hypergiant in the transatlantic scientific joint venture, awaiting its next eruption. Maybe next year. Maybe over half a century. It will happen again. This time perhaps with one final fatal blow.
This work is supported by NASA through the Space Telescope Science Institute and the Smithsonian Astrophysical Observatory. The science team mentionned in this press release consists of Drs. Alex Lobel, Andrea Dupree, Robert Stefanik, Guillermo Torres (Harvard-Smithsonian Center for Astrophysics, MA), Garik Israelian (Instituto de Astrofisica de Canarias, Spain), Nancy Morrison (University of Toledo, OH), Cornelis de Jager, Hans Nieuwenhuijzen (Space Research Organization of the Netherlands, The Netherlands), Ilya Ilyin (University of Oulu, Finland), and Faig Musaev (Special Astrophysical Observatory, Russia).
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