# Binary Stars (Part II)

Well, we rotate again within this topic.  Now in your view is on how we classify these binary stars, after those lovely properties described.  Also, remember, some past topics in the general/history sections still apply, like Kepler’s laws.  But now for types, we will even include binary stars that only appear to be binary stars.  Each type effectively uses a major astronomical technique to better understand the motion and properties of a binary system.  We will explain all the concepts that go behind it here, and then the math to apply it after this.

To start, let’s see what we have to work with.  What I mean by that is an optical double of course!  As stated, these are stars that just appear to move as binaries, but aren’t really.  They are produced when two stars fall into one’s line of sight, and they generally can pop up if you’re looking through a telescope at stars that aren’t even  within hundreds of parsecs of each other.  Some stars that appear close, like with a constellation, can do this at times.  Simply looking at a light curve (like we did with the variable star posts) reveals great distinction.

Seeing double again? From: UNL Astronomy.

Next we get physical with our binaries (no violence intended, disclaimer: this site is for all ages).  Physical binaries are bound by gravity; basically they are real binaries.  Therefore, we have various types based on the many ways we determine the properties of  systems.  This goes into finding the mass of the whole system or each object, rotational speed, recessional or radial velocity (speed towards or away from us), and various qualities of each star.

Yes, with this we can finally see the truth! From Discovery Space.

The first we shall talk about are the visual binaries and astrometric binaries.  We can spot the orbits of the binary stars in both these cases, and therefore the motions can tell us information about the period and separation of orbit for the two stars.  This is important since with that we can apply Kepler’s Laws (this is the perfect time to give a blast from the past with our Kepler post).  But the orbit can be at an angle, which is known as an inclination from our line of sight.  This has to be resolved when making calculations.  Afterwards, the total mass of a system and the center of the mass can be determined, which can lead to figuring out the mass of each star or object, and the types of stars or objects in the system.  Observing two stars can reveal a TON of data (which is a true theme of astronomy: making observations and calculations can lead to major discovery).  But you cannot do all this without knowing the system’s distance from us.  Without that, you cannot know the angular separation or inclination, linear distances, etc.

Also, you may be wondering whats the difference between visual and astrometric?  The visual part you can see, the astrometric part you measure.  Binary systems in a nutshell (yum, nuts…don’t call us astro people crazy now!).  The only real things to add on about that is much of this is because one part of a system can be brighter or too close to distinguish, and therefore basic laws of physics like Newton’s laws can be applied to find out much about the system.  This has extended to discovering exoplanets around stars, since these laws apply to any two orbiting bodies in the UNIVERSE.

Moving on, we have yet again those eclipsing binaries.  Okay, we’ve mentioned them a few times…they come up.  We already sort of discussed their set up, but let’s talk more explicitly about measuring them.  As stated, one object can block the other.  An interesting result is that we can find both the time of orbit or eclipsing and the brightness of an orbiting object.  Since we can find the brightness of each object, we can find distance, and all the other stuff mentioned with astrometric binaries.  So, a light curve plotting the brightness and eclipsing is extremely important here.  Looking at the shape and amount of dip after each eclipse can sometimes tell about relative size (whether one star much larger/more massive than another).  Very regular, very neat.  Eclipsing binaries can also be something called spectroscopic, another type of binary star.  But again, here eclipsing binaries and timed blocking can result in important exoplanet discovery.

Let’s talk quickly about spectrum binaries next.  This is the case where two stars cannot be resolved (sort of like the opposite of an optical double).  But the spectra produced by the stars immediately show the binary system.  Certain stars show more absorption lines of helium or calcium because stars have different masses, temperatures and different amounts of elements.  One star cannot be simultaneously hot and cold, so the presence of spectral lines associated with both hotter and colder stars would lead astronomers to consider a binary system.  The Doppler effect, where the lines of a star can be shifted, also takes place here so radial velocities of the stars can be measured by spectra.

If a binary pair orbits along our line of sight, a shift in spectra can be seen, known as a spectroscopic binary.  This is if the luminosities of the star can be compared, a double-line spectroscopic binary.  But there is also the case where one star can be brighter than the other, meaning only one set of spectra can be seen, a single-line spectroscopic binary.  In addition to looking at the spectra lines themselves, the velocity of a spectroscopic binary can be plotted to see their blueshift, redshift, or periods and radial velocities.  Therefore, an eclipsing binary’s analysis or photometry can show the brightness or period, and the spectroscopic aspect can show the Doppler effect, which can link to the speed of the system.  The double-line is better for analysis since you can actually see the whole system and analyze the speed and mass of the objects.

A double-line spectroscopic binary in all its beauty. First (double-line spectroscopy showing movement) and third (velocity curve) from Dept. of Phys and Astro at University of Tennessee. Second (showing Doppler effect in spectral lines and motion of a binary system) from Australia Telescope Outreach and Education

So you’re a normal piece of matter, say an electron, going along, and you suddenly get hit by a proton!  What happens when you get this high speed collision?  Well, you get X-ray radiation of course!  When you have two stars having a ton of this occur because of matter accretion (or accumulating matter from one star due to gravity pulling off a layer from another star) it’s called an X-ray binary.  This means that a compact object must have enough gravity to pull off material from its binary partner.  This can be created by a normal mass star turning into a white dwarf through stellar evolution while a star in the system is still a red giant, meaning the white dwarf will have enough gravity to pull off the loose Hydrogen layer from the red giant.  Alternatively, more massive stars can become neutron stars or black holes and still retain the system.  We mention them because they are quite important, and they too can be measured using a variety of physics between the rate of accretion and the angular motions and energy transfer that occurs with accretion.

This system sure is hot stuff! From Northern Arizona Meteorite Laboratory Glossary.

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TL;DR

The post summarized the different classifications of binary stars.  They are characterized by various methods of identification and analysis of data, all of which is very important.  In addition, many binary systems can have compact components and can exist in a variety of ways.  They can be a pair of any objects, be it brown dwarfs, white dwarfs, black holes, neutron stars, any type of star at any point of its stellar evolution except perhaps protostars, and planets and moons even.  Therefore, it is important to not only classify the systems by the type of objects, but by the way we see and understand our surroundings.

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Sources and links for further reading (links to images are below, some topics covered above that aren’t found are found in the General):

General

Eclipsing binary

Spectroscopic

Visual

Optical

Astrometric

Photometric

XRBs

Carroll & Ostlie, An Introduction to Modern Astrophysics, 2nd edition, p. 180-198

Images

Discovery Space: http://www.discoveryspace.net/index.asp?Cat_id=631

Dept. of Physics and Astronomy at University of Tennessee: http://csep10.phys.utk.edu/astr162/lect/binaries/spectroscopic.html

Australia Telescope Outreach and Education: http://outreach.atnf.csiro.au/education/senior/astrophysics/binary_types.html

Northern Arizona Meteorite Laboratory Glossary: http://www4.nau.edu/meteorite/Meteorite/Book-GlossaryX.html

# Binary Stars (Part I)

And now, we bring you a topic that hopefully will have you seeing double — in terms of stars, that is. Some astronomers say that a majority of stars are in binary or multiple star systems, but this is rather controversial, so suffice it to say that a significant number of stars are binaries or multiples. They are so important to astronomy that we can’t cover everything in one post, so we’ll split our discussion of binaries into a trilogy, with Part I (this post) covering general properties and evolution, Part II the types, and Part III the related math.

Before we really get into binaries, we should make it clear that a binary is not a “double star”, or optical double. An optical double is simply a pair of stars that, by chance, appear in nearly the same position in the sky but do not interact with each other in any way — perhaps the most famous example is Mizar and Alcor in Ursa Major. A true binary system has its stars gravitationally bound to each other, with both orbiting around the center of mass (also called the barycenter).

A schematic of a very basic binary star system (Credit: University of Oregon)

The formation of binary systems is still shrouded in mystery, with many competing theories all seeking to explain this stellar phenomenon. The old explanation for binary formation was that a rapidly rotating star could deform so much that it distorted into a “barbell” shape and eventually split into two stars that would then orbit around each other. However, this theory has been discredited in recent years due to simulations that show that stars tend to form accretion disks when spinning rapidly, rather than turning into barbells.

Binaries may also form from the fragmentation of molecular gas clouds as they collapse into protostars. However, the original cloud may not be able to immediately fragment into multiple clumps, so it may have to first collapse, then stop collapsing before it can divide into smaller chunks that then give birth to a gravitationally bound multiple star system. Alternatively, an accretion disk around a protostar may continue to…accrete…more mass from the molecular cloud around it. If this disk grows more massive than the star it orbits, it becomes unstable, and may clump together under its own gravity to form a second star and therefore produce a binary.

Stars that have formed separately may interact with each other to form a binary system, but this requires very high densities of stars, such as in globular clusters. Gravitational capture of an object requires a loss of energy from the system (referring to the two stars that will eventually become the binary), because of the principle of conservation of energy. In tidal capture, the excess energy goes into distorting the interior of the two stars as they pass each other at close quarters. However, this method of binary formation requires the two stars to interact at a very precise distance — too great a separation and the interaction won’t drain enough energy from the system to form a binary, but too small a separation and the two stars will just smash into each other to form a single, larger star. In three-body gravitational capture, excess kinetic energy is transferred to a hapless third star, which is then flung away at high speed while the other two stars become a binary system.

Castor sextuple star system, made up of three pairs of binary stars… because if you’re going to do it, you might as well overdo it. (Credit: Jodrell Bank Center for Astrophysics, University of Manchester)

The evolution of binary systems depends heavily on the degree to which the two stars in the system transfer mass. Each star has a Roche lobe, which is basically the space where a star has gravitational influence. If a star expands outside its Roche lobe, then material can flow to the companion star and lead to odd stellar evolution such as the Algol paradox, named for a binary system composed of a K0 subgiant and a B8 main sequence star. The theories of stellar evolution predict that the more massive B8 star should have evolved off the main sequence to the giant phase before the K0 star, but this is not the case — thus a paradox. However, astronomers have resolved the paradox by positing that the Algol system started out as a pair of main sequence stars, with one much more massive than the other. As the more massive star entered its red giant phase, it overfilled its Roche lobe and transferred away so much mass that it ended up as a subgiant while its companion became a massive blue main sequence star.

Gas flow simulations in the Algol system (Credit: M. Ratliff and M. Richards, PSU)

In a detached binary (wide binary), the two stars are both within their Roche lobes, so stellar evolution proceeds just as it would if the two stars evolved separately.

A semi-detached binary occurs when one star fills its Roche lobe and transfers mass to the other. Semi-detached binaries can produce interesting objects such as novae or x-ray binaries. Novae form from binary systems of a white dwarf and a main sequence or giant star, where mass streams onto the white dwarf and eventually ignites a nova outburst. An x-ray binary, on the other hand, forms from a system of two massive stars, where one has gone supernova — without disrupting the binary system — and left behind a neutron star or black hole. When the second star becomes a red giant, it streams mass onto an accretion disk surrounding the NS/BH, which emits strongly in x-rays. The x-ray radiation may even be powerful enough to vaporize the companion star that powered it in the first place.

Contact binaries are the strangest of the lot. The two stars share much of their mass (both are overfilling their Roche lobes) and orbit within a common envelope. The components may spiral in towards each other, due to loss of orbital energy to friction of orbiting within an atmosphere, and eventually merge into a single rapidly-rotating star. For more examples of what may happen to interacting (semi-detached and contact) binaries, check out the links below, especially this paper by P. Podsiadlowski of Oxford University.

Types of binary systems (Credit: David Darling)

Once both stars in a binary system have reached their end stages of evolution, end results vary wildly. One binary system made of two low-mass stars may end up as a pair of orbiting white  dwarfs (remember RX J0806.3, 2012 Astronomy folks?). Meanwhile, another binary system composed of a neutron star and a supergiant might turn into two runaway stellar remnants heading in opposite directions at high speeds, if the system is blown apart when the supergiant eventually goes supernova.

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TL; DR — Binary systems are pairs of stars that are gravitationally bound together. They may form due to fragmentation of molecular clouds or protostellar disks, or more rarely, from gravitational capture. Stars within binary system may transfer mass to each other if they expand outside their Roche lobes, and mass transfer leads to fascinating examples of stellar evolution in semi-detached and contact binary systems.  Even more than that the amount of mass in the system or for each component can also change the properties, leading to many variations of the system.

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# 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.

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).

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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