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Variable stars

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#1 symbiosis

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Posted 20 March 2019 - 09:30 AM

Wondering if anyone knows the current thinking on variable stars. Are they all now thought the result of occultations of the star by extrasolar planets or do some have a genuinely variable output?



#2 gordtulloch

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Posted 20 March 2019 - 09:44 AM

Huge question - the best answer is here:

 

https://www.aavso.org/types-variables

 

You'll notice exoplanets aren't listed because the star isn't varying so it's not a variable. 


Edited by gordtulloch, 20 March 2019 - 09:45 AM.

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#3 Ian Robinson

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Posted 20 March 2019 - 10:39 AM

Lots of reasons why variable stars vary in brightness .
https://www.aavso.or... Manual 2.1.pdf

Edited by Ian Robinson, 20 March 2019 - 10:42 AM.


#4 symbiosis

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Posted 20 March 2019 - 12:02 PM

So, presumably there were many more stars once considered variables that are now stricken off the list since the discovery of accompanying planets?

Huge question - the best answer is here:

 

https://www.aavso.org/types-variables

 

You'll notice exoplanets aren't listed because the star isn't varying so it's not a variable. 



#5 Ian Robinson

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Posted 20 March 2019 - 12:34 PM

So, presumably there were many more stars once considered variables that are now stricken off the list since the discovery of accompanying planets?


No.

The stars with planets where transits happen of the star by the planet are not regarded as variable stars , because the changes in brightness are so small.

#6 Tim Hager

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Posted 20 March 2019 - 02:22 PM

When a planet transits a star, the light of the star is only dimmed by the tiniest amount - hundredths and more often thousandths of a magnitude.  That is undetectable with the unaided eye and very difficult to detect with photometric equipment unless very careful techniques are used.  Virtually every star that is designated as a variable star varies for reasons other than planetary transits.



#7 Tony Flanders

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Posted 20 March 2019 - 07:37 PM

Presumably, if you could study them sufficiently accurately, all stars would vary in brightness. Certainly, our own Sun does. And if you sample any piece of sky at random, you'll find that a large fraction of the bright stars in it are known variables, with amplitudes typically in the 0.1 magnitude range.

 

Stars with really big amplitudes, like Mira (Omicron Ceti), vary because of some kind of pulsation -- they actually grow and shrink in diameter.

 

Eclipsing binaries like Algol are fairly rare. They only occur for stars whose orbital plane happens to pass through our own location. Usually, when two stars orbit each other, one star passes "above" or "below" the other, just as the Moon usually passes above or below the Sun at new Moon, and rarely lines up to form a solar eclipse.

 

Exoplanet transits cause tiny drops in a star's brightness. They're only easy to pick out of the noise because they are periodic, whereas stuff like starspots (akin to sunspots) are not.



#8 gordtulloch

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Posted 22 March 2019 - 09:07 AM

The definitive guide for observing exoplanet transits is: http://astrodennis.com/Guide.pdf

 

AAVSO actually has a course you can take on observing exoplanet transits, which are not a lot different than observing variable stars - I've taken it, it's a great course!  

 

Regards,

  Gord




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