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Pulsating variable star YZ Boo.


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Pulsating variable star YZ Boo. All of my previous variable star measurements have been of eclipsing binary stars where the change in brightness is due to one star coming in front of the other. YZ Boo is a single star that pulsates radially and so causing a brightness fluctuation.

The process behind this is essentially the same as in Cepheid variables. Main sequence stars are in equilibrium. There is an outward force due to the internal pressure which depends on the temperature and an inward force due to gravity.  These forces are normally balanced but if for some other reason the star were smaller than the equilibrium position the temperature inside would increase and hence the pressure would overcome the gravity and the star would expand. There could possibly be an oscillation but it would be short lived due to energy loss due to friction.  If however there was a means of energy being supplied to the oscillation it could then be maintained. In pulsating stars this can come from helium atoms within the star losing their remaining electron but when the star expands it regains an electron and emits its ionisation energy which helps to maintain the oscillation.

I don’t fully understand this but is the best explanation found so far.

These results were obtained with my Megrez 90 and Atik 460EX CCD camera. Multiple exposures of 20s were taken with the aid of SGPro and the data analysed with the free software Muniwin.

While the results were coming in I spent the time doing some Citizen science  using Zooniverse to identify animals in Kenya (where I once lived for 4 years)

YZ Boo2018-02-08.jpg

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10 minutes ago, lukebl said:

Interesting stuff. I didn't realise that naturally-variable stars (as opposed to eclipsing binaries) had such short periods. The science of it is above my pay grade.

New to me also. Apparently there are plenty with shorter periods but smaller amplitude, which makes sense.

Dave

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Just found a better explanation from the AAVSO website,

The driving mechanism behind "self-excited" oscillations is a special region of the stellar interior where atoms of either hydrogen or helium transition from partly to completely ionized. If the star is compressed, the ionization fraction of these regions increases, raising the opacity of the material and blocking the luminous energy trying to escape from the interior. The increased heat and pressure built up in this layer push the outer layers of the star outward. As these outer layers fall back inward again under the force of gravity, the ionization region gets compressed again, restarting the cycle. The variation in brightness is caused by changes in temperature and radius caused by these motions.

Dave

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