Here we are going to take a look at some cosmic couples, couples who have been married for millions or even billions of years, and most humans struggle to make it work for just a few years! What we are talking about here is binary pairs or double stars. Okay so what’s so special about them? I hear you ask. Ok I can’t hear you really but let’s just pretend. Well, binary pairs can tell us a lot about stars, their structure, size, power output, evolution. Also, if you have a binocular or a small telescope, then they are quite pretty to look at. Lets’s start by taking a look at the different kinds of binaries out there.
Optical Pairs. These are the Wannabees of the celestial sphere. They are not real binary pairs at all, but 2 stars that appear very close in the sky. These swindlers of the heavens can look just as pretty as their genuine cousins but are no more than a trick caused by the line of sight.
Astronomers test such pairs by measuring their distance from Earth and their velocity through space. If they prove to be about the same distance, and have more or less the same velocity through space, their ‘space motion’, then they might well prove to be a real double. If not then out they go!
Visual Binaries are just that, ones that we can see are actually two stars when we look at them through a telescope and which are proved to be a pair in the same ways that optical pairs are proved not to be real binaries.
Astrometric binaries. Ah, now this is where it gets interesting because these are pairs where one of the pair is too small and faint to be visible. So how can we know that there are two stars? This can be done by measuring the visible star’s motion through space. As we discussed in my article about orbits, two objects in space orbit around their common Centre of Mass or CoM. This means that the visible star will appear to wobble as it moves through the sky. This is also one of the methods used to find exoplanets, only on a larger scale as the invisible star will have a greater mass than a planet and will therefore make its brighter companion wobble more than a planet would. In fact this is how we discovered Sirius B, the very faint White Dwarf companion to the Dog Star.
Spectroscopic Binary. This is where a binary pair is too far away to separate them optically or to measure their wobbly motion through space. What we can do here is to look for blue or red shifts in their spectra. If the plane of their orbit is roughly edge on to us then as one of them is on the part of its orbit approaching us, its spectrum will be slightly blue shifted and then red shifted on its outward leg. So basically we’re back to our old friend Doppler Shift, this is where we usually get stories of racing cars and ambulances driving past so their sound changes etc. You know, the sound/light waves coming towards us are bunched up a bit and those going away get stretched a bit.
Spectrum Binary. Again we are talking about a binary pair that is too far away to be split by even a large telescope except that here the two stars are so different that their spectra are also very different so that when we measure them, we can see that we are looking at two stars and not one.
Eclipsing Binaries. These are orbiting roughly side on to us so that they eclipse each other. This means that the amount of light we measure from them will increase and decrease regularly as we see first the combined light of both stars as they are next to each other in the sky, and then a dimming as one passes in front of or behind the other.
Ok, so now we’ve got some different types of binary stars, so what? What use are they? Well as I said earlier, we can use measurements of the brightness and orbits of these pairs to find out important stuff about the stars themselves. Over to you Mr. Kepler- “Ok guys, as you know, my 3rd law says that the period of the orbit and the total mass of the system are related.” All very well and good, but then along came good old Sir Isaac, and gave this a little twist that makes it of real, practical use. He showed that if we know the time taken for one orbit, and we know the distance between the stars, then we can calculate the total mass of the system like this-
-The total mass of the 2 stars is equal to the distance between them cubed, divided by the period of the orbit squared. Also known as- M1 + M2 = A3 / P2. The orbital period is measured in years and the distance between the stars is measured in Astronomical Units, the average distance between the Sun and the Earth or 150,000,000 km, and this gives us a mass in Solar masses, 2 times the Suns mass, 3.3 times etc. Good on ya’ Sir Isaac!
Ok, so now we can find the total mass of the two stars together, what about the mass of each star? Well, if we know the absolute magnitude of one of the stars, then we can find its luminosity and then we can use something called the Mass- Luminosity Law, (L=M^3.5) to find its mass. The Mass- Luminosity equation I just showed you is in fact an approximation and the ^3.5 only applies to Main Sequence stars, but you get the gist.
Actually, binary star systems aren’t only of interest to nerdy astronomers. One of them, albeit a made up one, is world-famous.
But until recently it was thought that double, and treble, stars could not have any planets because the complicated gravitational situation around multiple stars made stable orbits impossible so if any planets did form along with these stars, their orbits would be so unstable that they would very quickly either be thrown out of the system or, more likely, crash into one of the stars. Fine. Great theory. But then as so often happens in science, along came Real Life and presented us with PSR B1620-26, a 2.5 Jupiter mass planet orbiting a Pulsar and a White Dwarf and later Kepler- 16 (AB)-b a Saturn sized planet with an orbit 3/4 that of Earth’s and where the two stars are closer to each other than Mercury is to our Sun.
So binaries are useful for Astronomers because they tell us a lot about themselves and about stars in general. But apart from all that practical, useful stuff, grab a telescope or a pair of binoculars and see how nice they look-