Extrinsic Variable Stars

We don’t have quite the humongous post for you, sorry, we were both busy and unsure how to present these types of stars (but no worries, we are back in your view for this new post!).  Interestingly, it’s not always what’s on the inside that counts.  So, this post considers extrinsic variables.  Following weeks will probably cover either math, more about certain variable stars, or other general information.  But now, shall we move onto all our lovely extrinsic variables?  These things don’t have the same large number of types as their intrinsic cousins in terms of causes, but they are quite important in our search for life outside the solar system, making them quite close to home…

The power of the sun (varies over time). Yep, one of the closest variable stars. From: NASA, Sun picture

The principal topic here will involve binary stars, but some can be non-binary stars.  They may not seem common because they require all sorts of optical conditions or non-uniformity in a star, but they are definitely out there.  Events on the outside of the star are the essential cause.  For this, the two chief classes include eclipsing binaries and rotating types.

Let’s start by exploring the Rotating variables.  Circle around these  folks, things are about to get pivotal!  Rotating variables pretty much are the extrinsic variables that aren’t always in a multiple star system.  Interestingly, differences in luminosity can be created by magnetic fields, stellar spots (like the sunspots we talked about in the what are stars post, it makes a lot of sense considering what they are!), and non-spherical shapes for stars.  These stars really aren’t spotty at all in fact!  After all, sunspots are characteristic of this class since they are cooler regions on the outside of a star, related to magnetic fields created from rotation.  This makes the pulsations and discovery fairly consistent.

Yep, a real deal picture of starspots. Hopefully the magnetism here attracts you (hopefully not fatal attraction?)! From: University of St. Andrews

Here are the major types (sadly, we have to introduce…long names):

Alpha2 Canum Venaticorum (ACV): Main sequence stars of type B8p-A8p (the p means having peculiar spectral lines) with strong magnetic fields.

Aph10111814

Light curve. From: Planet Hunters, ACV light curve

Rapidly osccilating Alpha2 CVn (ACVO): These non-radially pulsate, and they are of type A spectral class (meaning they are massive) with relatively fast periods.

BY Draconis (BY): White dwarfs with occasional light changes and even flares.  This is from the rotation making the spots on the star, creating non-uniform surface brightness.  Our Sun could even be considered of this type, and sometimes they can be considered eruptive variables too.

Light Curve. From: BY Draconis light curve

Rotating ellipsoidal (ELL): Close binaries with ellipsoidal components (ones where the stars of a system can actually be stretched from gravity of each star in the system pulling eachother).  Spica is a famous example.

Light curves…again…it’s ellipsoidal? Sorry if this is getting repetitive. From: Ellipsoidal variable light curve

FK Comae Berenices (FKCOM): Rotating giants with non-uniform surface brightness (types G to K).  They can be spectroscopic binary systems, where the variability is caused by sunspots from rotation (in case you haven’t noticed, yes the pattern is as we said, you definitely want to read the stars post about sunspots for these rotating variables or perhaps we’ll make a post more on binary stars).

But yep, we’re back with another light curve. From: FK Comae Berenices light curve

Optically variable pulsars (CM Tau, PSR): These rotate very quickly and are known for strong magnetic fields (basically, pulsars are rotating, extrinsic variables, so yes even post-supernova objects can be classified as variable stars…classify all the things!).  Refer to the neutron star post for more on this (heh, not going to bother with another light curve).

SX Arietis-type (SXARI): Main sequence type B stars that are high temperature forms of ACV types.  Helium is prevalent in spectra, and they have those good ol’ sunspots and magnetic fields.

Now be ready, we don’t want to block your view from learning at all.  But we have eclipsing binary stars up next!  To start, the general definition is that the orbit has to be oriented so when we see it one object can pass in front of the other (this is termed lining up with the ecliptic, basically the stars line up with our line of sight).  They have VERY characteristic light curves and dips.  These stars are really nice in their periodic motions and observations, which makes O-C diagrams extremely useful here (in fact, their predicted variability can be theoretically easier than intrinsic variables).

I hope we are all inclined to see this. The system can be tilted to our view, sometimes one star can block another. From: University of Tennessee eclipsing binaries

Okay fine, that wasn’t the end of light curves. But hey…it’s what’s on the inside that counts, right? Oh wait…From: David Darling

Looking at the graph above we can derive two facts if values were given.  Pretty much, we could figure out the apparent magnitude of the stars depending on the dips (as seen, the larger dip is from the larger star, but you would have to assess that from data given) and the period of the system if not given (which as said can be useful for O-C diagrams).

We can name three major examples (this also could includes the not as stellar planetary transits used to detect exoplanets…those can be eclipsing, but we’ll leave those for another day):

Algol: Yes, a ghostly star, sometimes considered a demon.  A spectroscopic binary observed since Egpytian times.  They are ellipsoidal, with one B-type star and a K-type star (so it can be blue and red).  Aside from the usual, it presents what’s called the Algol paradox.  Normally a star with more mass evolves and reaches end stages more quickly.  Somehow, the lower mass star is a giant in this case, while the higher mass star is on the main sequence.  This is most likely due to accretion of matter.

The Algol system, notice the various dips. Hopefully this uneclipses all our views as to what this could look like. From: University of Tennessee the Algol system

Beta Lyrae: Yet another secretive star known since ancient times.  It follows traditional definition, but it has a particularly large rate of period change, making it most likely an active system (this could be similar to the Algol paradox).

Visual Light Curve

Beta Lyrae…not much more to say on this…light curve again? From AAVSO Beta Lyrae light curve.

W Ursae Majoris: Out of the examples, a relatively more recent discovery.  The peaks for this system seem almost equal.  But it most likely is also an active system like with Beta Lyrae.  This has probably created some period variation that has been detected over time, and the system also seems to have some star spots.

Hah, yes, another light curve! You really aren’t escaping these, sorry. From: AAVSO W Ursae Majoris light curve

Epsilon Aurigae:  Just to throw it in here because it is probably one of the most mysterious eclipsing binaries ever.  It has a REALLY long period, even the eclipse itself is long.  In fact, there was a year of astronomy around 2009 where people in part made lots of observations because of its rare period.

It may actually be caused by a gap in a relatively large accretion disk that occasionally doesn’t block the light. Yes, this is probably the definition of weird. It’s like having pizza dough in space spin, get really large, and normally it doesn’t happen, but it gets so thin you can sort of see through it. From: AAVSO Epsilon Aurigae

Another model of the system. Again, extremely unlikely. Hopefully you didn’t think we wouldn’t end with one last light curve! From: Hopkins Phoenix Observatory Epsilon Aurigae

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TL;DR

Oh, the beauty of stars.  They prove that it’s BOTH what’s on the inside AND the outside that matters!  Extrinsics, though, show what’s on the outside.  Whether it be pushing and pulling, magnetism, and rotation for rotating variables, or just straight on eclipsing lined up with our sight sadly blocking out stars on occasion, these stars are yet again far out.  While they may not seem exciting, some can be…quite weird.

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Sources and links for further reading (links to images are below):

General:

Rotating (ellipsoidal)

Eclipsing

Images:

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