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.



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.


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


Eclipsing binary







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


UNL Astronomy:

Discovery Space:

Dept. of Physics and Astronomy at University of Tennessee:

Australia Telescope Outreach and Education:

Northern Arizona Meteorite Laboratory Glossary:


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