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Star Distances



 
 
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Old July 1st 06, 12:03 AM posted to alt.astronomy,sci.astro,uk.sci.astronomy,sci.physics
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Default Star Distances

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Hurt wrote:
Such an expectation is still unrealistic. At a distance of, say, 4000 AU, a
star as bright as the Sun will still appear as bright as our Moon when at
half phase, or some 100 times brighter than Venus. And if 10,000 times
fainter, such a star would be easily visible to the naked eye anyway.



Maybe so. But who says somebody's eyes aren't seeing it.



Besides, a star at a distance of 4000 AU would show an annular parallax of
almost one full arc minute - an angle easily measured in telescope.



Good, than it should be easy to find. With astronomy being so dogmatic
who would believe it. You sure it hasn't been mislabeled as something
nebulous like a quasar. Which brings me back to the KISS principle.
To me, closer stars are more plausible than quasars and dark matter. I
mean I'm not opposed to such extravagances but they DO border on
unrealism.

http://en.wikipedia.org/wiki/Quasar


Sorry, but there are no other solar systems as close as several thousand AU
either. If there had been, they would have been quite obvious in the sky.



Yeah right. We're just finding planet size objects in the Kuiper Belt.
Granted, a star is bigger and brighter, but it's not that much more
obvious at the distances we're talking about considering our current
"understanding" of the universe.


Pardon me for jumping in here, but ....

Yes, we're finding "planet size objects" in the Kuiper Belt -- objects
which are around 1500 km in diameter. "Granted, a star is bigger and
brighter" -- but it IS that much more obvious. If it were otherwise, it
would have been trivial to find extrasolar planets: they'd be shining
right next to their primaries, just like a double star. But no, it's
extremely difficult to get an *image* of an extrasolar planet. We have
to rely on indirect detection methods instead.

Let's look at some numbers. A brown dwarf -- not massive enough to be a
real star -- would be about the size of Jupiter (radius 70,000 km or
so). The biggest things we're seeing so far in the KB are 1500 km.
That's a factor of 46 in radius. Square that and it's a factor of 2100
in angular area. That winds up producing a difference of 8.3 magnitudes
just due to size alone.

But KBOs shine by reflected sunlight. A brown dwarf emits its own light
(not from any significant fusion going on, but simply from shedding heat
generated from its formation: blackbody radiation). Its absolute
magnitude is going to be about +17 (see, for instance,
http://user.bahnhof.se/~davidgr/browndwf/bd_def.html for a discussion of
brown dwarfs). Put one of those at 4000 AU, or 0.2 percent of the
standard distance (10 pc) for computing absolute magnitude, and its
apparent magnitude would be 13.5 magnitudes brighter, or +3.5. It would
be an easy naked-eye object, very red in color, and there's no way on
earth that it could have been overlooked. We can *count* the number of
stars that bright. We have parallax data (from Hipparcos) for every one
of them. A parallax of 51 arcseconds -- which is what this object would
have -- is 70 times as much as the parallax of proxima Centauri.

And this is just for a brown dwarf. You said "star". Tack on another
5 magnitudes in that case, and we'd be looking at something as bright
as Sirius if it were just barely a star (an M dwarf), or something
much brighter than Venus if it had the same characteristics as the sun.

"Good, than it should be easy to find." Correct. And we haven't found
any. "KISS." Indeed so. What's the more logical conclusion then?
That generations of astronomers thoroughly scanning every part of the
sky have somehow missed something so obvious, or that such an object
does in fact not exist?

-- Bill Owen
 




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