(SNES) Fwd: star about to blow its top

[Timetrav2@webtv.net (Andy and Barry)]

Brad sent this to me a couple of days ago.

Stars in your eyes,
Barry

http://community.webtv.net/Timetrav2/TIMETRAVELERS
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From: "WA1BSB(Brad Brown)" <wa1bsb@cox.net>
To: "Andy and Barry" <Timetrav2@webtv.net>
Subject: star about to blow its top
Date: Sat, 11 Jan 2003 11:57:22 -0500
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                        Just off the familiar W shape of the constellation
Cassiopeia glimmers 4th-magnitude Rho Cassiopeiae - a yellow-white hypergiant
star probably about to undergo a new episode of eruption, fading, and mass
ejection. Photo by Akira Fujii.




January 10, 2003 | Keep an eye on Cassiopeia - it contains a naked-eye star
that may brighten and dim dramatically in the coming months.
That was the message at a Tuesday press conference at the American
Astronomical Society meeting in Seattle. Alex J. R. Lobel and Andrea Dupree
(Harvard-Smithsonian Center for Astrophysics) both reported observations of
the active hypergiant star Rho Cassiopeiae, which is visible to the naked eye
at magnitude 4.5. That it shines so brightly from 10,000 light-years away
means that it must be huge. Rho Cas is about as hot as the Sun but roughly a
million times more luminous, which makes it is so big that, if it replaced our
Sun, its surface would lie beyond the orbit of Mars.

According to Lobel and Dupree, the star has had a chaotic recent past. In 1946
astronomers watched it fade to 6th magnitude and cool from 7,0000 to 3,0000
Kelvin, changing from spectral type F to M. Astronomers at the time speculated
that it had undergone an enormous internal eruption that caused it to swell
and cool, but they could not tell much else. The star eventually returned to
normal. Then in 2000 astronomers caught Rho Cas acting up again. It brightened
by 20 percent (0.2 magnitude), then dimmed by about two magnitudes while again
cooling by more than 3,0000 K.

This time astronomers were better prepared to study what was going on. The
star turned out to be having the largest stellar mass ejection ever recorded.
"It ejected 50 Earths per day for 200 days," says Dupree. When the entire
event was over, about 5 percent of a solar mass was gone - roughly a
thousandth of Rho Cas's mass.

It wouldn't take many such ejections to have life-altering effects on the
star. Its behavior may hold the answer to one of astronomy's lingering
questions - why are there no stars more luminous than a million Suns? "Maybe
these mass losses constrain the luminosity," suggests Dupree. If all
hypergiants have episodic eruptions like Rho Cas, Lobel suggests, it could be
why they can't sustain superbright luminosities.

Clearly the star's days are numbered. "Rho Cas is in the very last stages of
its evolution," says Dupree. It could go supernova in as little as 50,000
years.

It also seems to have an encore planned. The telltale spectral changes that it
showed before its 2000 events have showed up again, only this time they are
happening much faster. While Rho Cas isn't expected to change much in coming
days, "We are looking at months rather than years," says Dupree.


                        Naked-eye and binocular starwatchers can estimate the
brightness of Rho (r) Cassiopeiae using the magnitudes given here for several
of its neighbors. As of January 9th it was visual magnitude 4.6, according to
Janet Mattei of the American Association of Variable Star Observers (AAVSO).
Sky & Telescope chart.






"We have advised amateur groups to continue to monitor it," adds Lobel. "Many
national groups are looking, and an observing alert was also issued in Japan
to watch." When the star does erupt, Lobel and Dupree plan to alert the
amateur community.

"We know this star did an amazing thing," says Dupree. It may be poised to do
so again. Keep watch.
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