shrinking Betelgeuse
Phillip Helbig---remove CLOTHES to reply
(actually its online version) that Betelgeuse has shrunk by 15% in the
last 20 years or so (whether radius or volume or angular size isn't
clear, but in any case that's appreciable). I've known for a long time
that Betelgeuse is probably quite close to becoming a supernova, but of
course "quite close" in astronomical terms could be a rather, errm,
astronomical time for a human. I doubt one could even have said
hundreds of years or thousands or even tens of thousands (but probably
less than a million).
Does the measured shrinkage indicate that the time until supernova might
be relatively short even on a human timescale? If so, what are the
chances of it happening within, say, 30 years? Is such a shrinkage
expected? Apparently the luminosity hasn't changed; is this expected or
a surprise? (If the latter, then all other estimates are probably
rather uncertain.)
A web search for +betelgeuse +townes gives 1.860 hits, the first of
which is http://www.astronomy.com/asy/default.aspx?c=a&id=8342 which is
essentially what I read (in German) today.
Is there anyone here familiar with the late stages of stellar evolution
who can fill in the details? Popular-press accounts are sometimes
distorted, while an interested non-expert with little time like myself
can't read all the relevant refereed-journal papers.
Eric Flesch
actually shrinking. The article states:
"Since the 1921 measurement, its size has been re-measured by many
different interferometer systems over a range of wavelengths where the
diameter measured varies by about 30 percent," Wishnow said. "At a
given wavelength, however, the star has not varied in size much beyond
the measurement uncertainties."
Note the wavelength-dependent 30% variation in size. The final
sentence of the above paragraph directly clashes with the thrust of
the article, that there is now indeed a shrinkage of 15% in the one
wavelength. This is suggestive that if Betelgeuse undergoes natural
internal cycles (of heat or whatever), that the wavelength-dependent
visible diameter may migrate to longer or shorter wavelengths. The
article continues:
"The measurements cannot be compared anyway, because the star's size
depends on the wavelength of light used to measure it, Townes said.
This is because the tenuous gas in the outer regions of the star emits
light as well as absorbs it, which makes it difficult to determine the
edge of the star."
The article then describes that Townes finds a way around this
problem, but with Betelgeuse now said to be shrinking by 15%, it is
apparent that Townes has not got the answer after all. I trust the
star more than the astronomer.
Eric
Phillip Helbig---remove CLOTHES to reply
er...@flesch.org (Eric Flesch) writes:
> Interesting article, but there are clues that Betelgeuse is not
> actually shrinking. The article states:
>
> "Since the 1921 measurement, its size has been re-measured by many
> different interferometer systems over a range of wavelengths where the
> diameter measured varies by about 30 percent," Wishnow said. "At a
> given wavelength, however, the star has not varied in size much beyond
> the measurement uncertainties."
I think that refers to 1921--1991 or so, i.e. before Townes's
measurements. In other words, no shrinkage back then.
> "The measurements cannot be compared anyway, because the star's size
> depends on the wavelength of light used to measure it, Townes said.
> This is because the tenuous gas in the outer regions of the star emits
> light as well as absorbs it, which makes it difficult to determine the
> edge of the star."
Right---one can't straightforwardly compare older measurements made at
different wavelengths.
> The article then describes that Townes finds a way around this
> problem, but with Betelgeuse now said to be shrinking by 15%, it is
> apparent that Townes has not got the answer after all. I trust the
> star more than the astronomer.
Townes has been using the same setup and has seen the star shrink by 15%
at a given wavelength.
Gordon Stangler
to worry about. 15% at all wavelengths is still nothing to worry
about, since Betelgeuse is a red supergiant. Like all red
supergiants, the distended outer layers are very thin and tenuous. I
don't have the figures at my hands, but if I remember correctly, the
chromasphere has a density of only a few hundred atoms per cubic
meter.
Thus, a contraction of 15% is interesting, but it may involve only a
solar mass or so of material. Nothing to get excited over.
PS: I found these links:
http://www.sciencenews.org/view/generic/id/44573/title/Betelgeuse_shrinks
http://www.foxnews.com/story/0,2933,525695,00.html
Is either of these your links?
Eric Flesch
>Townes has been using the same setup and has seen the star shrink by 15%
>at a given wavelength.
Yes, but the star already shows variances of 30% apparent diameter at
different wavelengths. If the star is seen to be shrinking by 15% in
every wavelength, that woud be significant. The article states that
Townes operates at the wavelength of a calibrating laser, in the
mid-infrared. So, the article is based on one wavelength only.
So we have a choice of assumptions. If we assume that there are no
resonances or other internal process operating within Betelgeuse's
vapor-thin exosphere, which would cause the glow in that exosphere to
vary, then we deduce a huge physical contraction in the whole star.
On the other hand, if we assume no contraction, then we deduce that
there are resonances or other processes which cause variations in the
glow, like a neon lamp can glow faint or bright. And the gas in
Betelgeuse's "surface" is more attenuated than the gas in a neon lamp.
So I think a neon-lamp-like resonance in the surface of the star is
more likely than a massive contraction, in the absence of other data
beyond that of a single wavelength. That's all I've got.
Eric
Phillip Helbig---remove CLOTHES to reply
Stangler <gordon....@gmail.com> writes:
> PS: I found these links:
> http://www.sciencenews.org/view/generic/id/44573/title/Betelgeuse_shrinks
> http://www.foxnews.com/story/0,2933,525695,00.html
> Is either of these your links?
I originally read it at Spiegel Online (internet version of an
established German weekly news magazine) but search engines turned up
many links with essentially the same text. I'm not sure what the
original source is.