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- Minneapolis, MN - Over the past year,
a star in the Caraina Constellation of the Southern Hemisphere has doubled
in brightness. How a star that is 7,500 light years away from earth could
brighten so dramatically baffles astronomers. In fact, of all the stars
that can be seen with the naked eye from earth - none are as mysterious
and confusing as Eta Carinae. If you live in southern Texas, southern Florida
or Hawaii, you can barely see the star peeking up a little above the horizon
straight south in June. It has a reddish-orange color and was completely
invisible to the naked eye only a few years ago.
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- How much more Eta Carainae will brighten
- and will it actually explode into a hypernova - are unknowns. But one
astronomer who devotes his research to massive stars like the unpredictable
Eta Carainae is Dr. Kris Davidson, Professor of Astronomy at the University
of Minnesota in Minneapolis.
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- I talked to him today about his interest
in studying Eta Carainae even before the recent and sudden brightening.
Professor Davidson:
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- Kris Davidson, Ph.D., Professor of Astronomy,
Univ. of Minnesota, Minneapolis, Minnesota: "Eta Carainae is a very
strange and extreme object. We can see about 6,000 stars with the naked
eye, more or less. And generally speaking, we understand all of them. We
understand enough about how stars work that any astronomers talking about
any mysteries of naked eye stars are talking about specialist details.
Eta Carainae is the exception. It's the naked eye star that we just don't
understand.
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- 1) First, it's extremely bright. To
see it from a distance of 7500 light years - that's quite a thing right
there. Most of the stars we see are much closer. But 150 years ago, it
blew up. It became one of the brightest stars in the sky for about 20 years.
It looked almost like a supernova explosion, but the star survived. It
seems to involve some type of instability we just haven't figured out yet
in an extremely massive star.
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- 2) Now the other thing is, that in addition
to being unstable, Eta Carainae from its brightness and its power appears
to be the most powerful - and therefore, the most massive star - in our
neck of the woods, in our part of the galaxy. It radiates something like
4 million times as much light as the sun. And that's extreme. There just
aren't any other stars that definitely do more than that. And it weighs
about 100 times as much as the sun does. And that also is extreme. There
are no other stars that we are certain are more massive than that.
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- So, it's a combination of instability,
extreme amount of power, extremely high mass - it puts it on the very limit
of everything we know about stars.
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- AND WHAT HAPPENED MOST RECENTLY THAT
SHOCKED YOU AND OTHER ASTRONOMERS?
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- Recently, it seems to be brightening.
Now, the problem with that is not just that we didn't expect it to brighten.
It's that we thought we had good reasons why it wouldn't. The story is
really that it was unstable - it was flickering a lot before that terrific
explosion 150 years ago. It's a lot like a geyser. When you see a geyser
beginning to go off, or a volcano. You can see hints that it's going to
happen - burbling, you know. Puffing away. It looks unstable.
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- Well, then it blew up 150 years ago.
It did a few other things before 1900. And since that time it's been relatively
quiet. And on the geyser and volcano analogy, that's just to be expected.
When a thing blows up, it gets steady for awhile after that when it stabilizes.
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- We thought Eta Carainae had stabilized.
It's becoming slowly brighter over many decades, many years, but that's
only because the dust is clearing away from its previous explosion. However,
in the past two years - space telescope data that we've obtained has shown
that at some wavelengths, the central star in it has doubled in brightness
within about a year. This is just completely unprecedented and shocks us
for a couple of reasons. One, it was unexpected, as I just said, and two,
the way it's brightening doesn't make sense. It's brightening at all wavelengths
and a non-technical interpretation of that is that it seems to be actually
increasing the amount of power it's producing. Usually when one of these
stars gets brighter, it's just because it's gotten to a different temperature
where we can see it better without changing the actual amount of power
coming out. And Eta seems to be actually increasing the amount of power
it'' radiating. Well, that just doesn't make sense with any theories we
have. We didn't expect this to happen. The other stars that are vaguely
like Eta Carainae, but not as extreme never do this. And yet it does seem
to be doing this. We are just completely puzzled by it.
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- DO YOU HAVE ANY HYPOTHESIS ABOUT WHAT
COULD BE GENERATING SUCH AN INTENSE INCREASE IN POWER?
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- I have sort of a technical hypothesis,
and that is - after the big explosion 150 years ago, it left the star in
a state where the energy trapped inside it was distributed all wrong, in
a funny way. There were bubbles of extreme amounts of energy trapped inside
this star. Stars are so big that it takes hundreds or even thousands or
even millions of years for the energy to leak out. And my suspicion - and
it's just a quarter baked idea - is that maybe a sort of bubble, a region
of extra energy that got trapped after the big explosion - has now reached
the surface and it's just puffing out.
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- BECAUSE IT'S SUCH A BIG STAR.
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- That's right. There is an alternative
idea and that is the dust around it is clearing very rapidly so we can
see it better suddenly. But that's not an easy idea, because the dust doesn't
just clear by itself that fast. Something would have to destroy the dust
and if something destroys it - well, the only thing we know that could
do that is the star and we're right back to the idea that the star has
increased in power so it can destroy the dust.
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- SO, IN A WAY IT IS AS IF ETA CARAINAE
HAS SINCE 150 YEARS AGO APPEARED TO NOVA, IN A WAY, AND THEN WE'RE IN 1999
- AND IT APPEARS TO BE IN A SLOW NOVA BURST OF LIGHT AGAIN.
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- Could be. That's one of the interpretations.
We'll know in a few months. But in the meantime, it's behaving sort of
like that's what's happening. I don't think this means it's going to become
a supernova. In fact, there are some theories about very massive stars
like Eta Carainae in which they don't even become supernova - they become
something much more dangerous called a hypernova. But hose are largely
speculations. A hypernova releases a tremendous blast of gamma rays that
can be seen clear across the universe by modern equipment.
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- AND WE'VE HAD A COUPLE OF EXAMPLES OF
THAT, I THINK, IN JUST THE LAST SEVERAL MONTHS.
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- That's right. There have been some famous
gamma ray bursters, they are called, that happened far, far away - not
just in another galaxy far away as the phrase puts it, but really far away
- a large fraction of the distance across the visible universe.
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- AND STILL BEING ABLE TO AFFECT OUR SATELLITES.
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- That's right. There was another one that
did happen in our galaxy. It was just a little thing. It's called a magnetar.
It was just a little burst of gamma rays and that was enough to temporarily
disable the electronics aboard several spacecraft.
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- If Eta Carainae, for example, ever became
a hypernova - and some people imagine it might some time in the next 30,000
years - we don't have to worry about next week - if it ever did that, it
is conceivable according to some theories that the burst of gamma rays
would be sufficient to disable actually most spacecraft now in use that
happen to be on that side of the earth when it happened.
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- HOW SAFE ARE WE SITTING ON THE SURFACE
OF THE EARTH ROTATING AROUND THIS PARTICULAR SUN IN THIS PARTICULAR ARM
OF THE GALAXY IN RELATIONSHIP TO STARS OUT THERE THAT POTENTIALLY COULD
BURST MORE?
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- That's another fascinating question.
I've never looked into it in detail myself, but I've talked with other
people who've looked at the question a little bit. It looks as if we're
pretty safe because of the earth's atmosphere. The earth's atmosphere protects
us very well against gamma rays and x-rays. It's kind of like being under
30 feet of water - that's good shielding. And we can judge from the geological
record, the biological record, that life has been on earth for a long time
without being seriously disturbed. So, a very colorful and rather amusing
way to put it was given by one expert about a year ago when I was talking
about Eta Carainae and he was saying, "Eta Carainae - the star we
were talking about earlier - might become a hypernova. That's just somebody's
idea. And he said he was going to give some public talks about whether
it was going to be dangerous for us if Eta Carainae 7,500 light years away
from us were to become a hypernova and bombard us with gamma rays. And
his amusing way of putting it was, "We're probably safe."
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- Anyway, I wouldn't lose any sleep over
it. I am far more worried about being hit by a passing asteroid than having
a gamma ray burst that can penetrate earth's atmosphere. But the idea of
knocking out satellites is a completely different one - they are not shielded
the way we are.
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- AND THAT COULD BE A MAJOR PROBLEM.
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- Yeah, believe it or not, the earth's
atmosphere is much much thicker for gamma rays than the metal that is on
any spacecraft.
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- AND THIS PARTICULAR STAR HAS SURPRISED
YOU ALREADY IN 150 YEARS. SO IT SEEMS TO ME THAT WHAT YOU HAVE DESCRIBED
IS THAT IT'S THE MOST UNPREDICTABLE STAR THAT WE ARE WATCHING.
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- That's right. There are other stars that
are just as mysterious, but almost all of them are mysterious because they
are very faint and very far away or hidden behind thick clouds of dust
in the galaxy, so we can't study them very well. Eta Carainae is bright.
We can study it easily. And yet, its behavior just never matches what we
predict for it. It just never behaves itself. It's very extreme.
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- CAN PEOPLE IN AUSTRALIA AND THE SOUTHERN
HEMISPHERE NOW CLEARLY SEE THIS STAR AS IT BRIGHTENS BEING DIFFERENT IN
THE NIGHT SKY?
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- That's right. It's not a bright star
yet. The story is basically this: about 30 years ago you couldn't see Eta
Carainae at all with the naked eye. It was just too faint. The dust around
it from the explosion has cleared and two years ago someone with good eyes
out on the desert in Australia on a really dark sky could just see Eta
Carainae barely. Now, as far as I can tell, the recent brightening has
made it appreciably easier to see. And so I'd say now, you still have to
go out to a dark sky. But now a person with ordinary eyes could see it.
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- If it continues to brighten, it will
become relatively easy to see. We're not predicting it will become one
of the brightest stars in the sky again, although that could happen. But
if what's been happening lately continues, it will become a fairly easy
thing to see. And that's an amazing statement for an object that far away.
More Info
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- For more information about the mysterious
Eta Carainae and more photographs, visit the Hubble Space Science Telescope
web site at www.stsci.edu and the Home Page of astronomer Jon Morse at
the Center for Astrophysics and Space Astronomy, University of Colorado.
His web site shown below.
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- <http://casa.colorado.edu/~morseyhttp://casa.colorado.edu/~morsey
© 1999 Linda Moulton Howe
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