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Variable Stars



 
 
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  #1  
Old November 17th 03, 02:57 PM
LazzaH
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Default Variable Stars

Hi all, first time post in this NG.

I was watching a documentary about the search for planets orbiting
other solar systems last night.The program explained how astronomers
searching for alien planets look for stars which show a varience in
the light they give out, which would indicate orbiting planets which
block the light reaching our telescopes. It suddenly dawned on me that
what they were talking about is a charactistic of so-called "variable
stars", which have been known about for years.

Am I thinking along the right lines?

Larry
  #2  
Old November 17th 03, 05:06 PM
Mike Dworetsky
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Default Variable Stars



"LazzaH" wrote in message
om...
Hi all, first time post in this NG.

I was watching a documentary about the search for planets orbiting
other solar systems last night.The program explained how astronomers
searching for alien planets look for stars which show a varience in
the light they give out, which would indicate orbiting planets which
block the light reaching our telescopes. It suddenly dawned on me that
what they were talking about is a charactistic of so-called "variable
stars", which have been known about for years.

Am I thinking along the right lines?

Larry


Same principle as eclipsing binaries, but the effect expected is far smaller
than what is usually seen in typical stellar binaries. The best known
example of a planet that transits a star results in drops in brightness of
the order of 2%. Detecting a terrestrial planet would require reliable
detection of drops of order 0.01%. This is difficult, but not impossible.
The main use would be to get statistical information on the likely number of
stars with planets.

Of course, it only gives you the stars with planets that happen to have
orbits with "nodes" in the plane of the line of sight. All the other
possible planets remain undetected. The main advantage of the method is
that precise photometry is easier to implement than ultraprecise positional
measurements.

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)


  #3  
Old November 17th 03, 07:54 PM
Mike Williams
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Default Variable Stars

Wasn't it LazzaH who wrote:
Hi all, first time post in this NG.

I was watching a documentary about the search for planets orbiting
other solar systems last night.The program explained how astronomers
searching for alien planets look for stars which show a varience in
the light they give out, which would indicate orbiting planets which
block the light reaching our telescopes. It suddenly dawned on me that
what they were talking about is a charactistic of so-called "variable
stars", which have been known about for years.

Am I thinking along the right lines?


Not really. What happens is that the light gets very slightly Doppler
shifted due to the wobble imparted by planets going round the star. Now
that we have ways of measuring the spectral lines we can see these
shifts. A planet doesn't cause the star's brightness to change.

The term "variable star" refers to a star which changes in brightness by
a noticeable amount.

The one thing that we did know about before 1995 was "spectroscopic
binary stars". These are caused by the same sort of Doppler effect, but
in this case there are two stars which orbit each other, causing
significantly greater Doppler shifts which could be detected with pre-
1995 technology. We could tell that there were two stars even though
they were too close together to be resolved by direct observation. The
light from one of the starts would be red shifted when it was in a part
of it's orbit moving away from us, and the other would be blue shifted.

--
Mike Williams
Gentleman of Leisure
  #4  
Old November 17th 03, 09:14 PM
Jonathan Silverlight
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Default Variable Stars

In message , Mike Williams
writes
Wasn't it LazzaH who wrote:
Hi all, first time post in this NG.

I was watching a documentary about the search for planets orbiting
other solar systems last night.The program explained how astronomers
searching for alien planets look for stars which show a varience in
the light they give out, which would indicate orbiting planets which
block the light reaching our telescopes. It suddenly dawned on me that
what they were talking about is a charactistic of so-called "variable
stars", which have been known about for years.

Am I thinking along the right lines?


Not really. What happens is that the light gets very slightly Doppler
shifted due to the wobble imparted by planets going round the star. Now
that we have ways of measuring the spectral lines we can see these
shifts. A planet doesn't cause the star's brightness to change.


All true, but as Mike Dworetsky has pointed out, the variation in light
as a planet transits the star is also being used to look for them.
It's even being used by amateurs, which is definitely not the case for
the Doppler technique.
--
Rabbit arithmetic - 1 plus 1 equals 10
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  #5  
Old November 17th 03, 09:24 PM
Stephen Tonkin
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Default Variable Stars

Jonathan Silverlight
wrote:
It's even being used by amateurs, which is definitely not the case for
the Doppler technique.


Not so. See the magnificent achievements of Tome Kaye and his
collaborators at:
http://www.spectrashift.com/

Best,
Stephen

Remove footfrommouth to reply

--
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  #6  
Old November 17th 03, 09:58 PM
Jonathan Silverlight
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Default Variable Stars

In message , Stephen Tonkin
writes
Jonathan Silverlight
wrote:
It's even being used by amateurs, which is definitely not the case for
the Doppler technique.


Not so. See the magnificent achievements of Tome Kaye and his
collaborators at:
http://www.spectrashift.com/


That was quick! As Samuel Johnson noted "you will find it a very good
practice always to verify your references!" and when I don't, I usually
regret it.
I'm very impressed. It seems there's very little well-equipped amateurs
can't do. It wasn't long before Jack Newton imaged the Hubble Deep
Field, for instance.
Which brings me to something I saw recently. How faint is Io in eclipse,
as imaged by Galileo and shown on the last "Sky at Night"? Could an
interference filter be used to isolate aurora light, for instance?
--
Rabbit arithmetic - 1 plus 1 equals 10
Remove spam and invalid from address to reply.
 




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