Date: April 30, 2009

Podcaster: The Ordinary Guy

Organization: Brains matter: http://www.brainsmatter.com.

Description: Up until recently, it was thought that binary stars were special or unusual. As it turns out, they’re very common. Listen in and find out what binary stars are – how they are formed, and how we have been able to discover them.

Bio: The Brains Matter podcast has been producing and communicating science stories and interviews since September 2006. The show is based out of Melbourne, Australia, and takes an everyday person’s perspective of science in easy-to-understand language.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Joseph Brimacombe, a hyper-enthusiastic amateur astronomer based at the Coral Towers and Macedon Ranges observatories in Australia, and New Mexico skies in the United States and is dedicated to Lawrence Brimbacombe, my father and Sir Patrick Moore, the father of amateur astronomy in the United Kingdom, who inspired me as a boy to look upward and wonder. For more information go to: Northerngalactic.com or Southerngalacatic.com

Transcript:

G’day everyone, and welcome to the 365 Days of Astronomy podcast. I’m the Ordinary Guy, the host of the Brains Matter podcast. You can find information about Brains Matter at www.brainsmatter.com.

Today, I’m going to talk to you about binary stars. The concept may sound simple – it’s just a pair of stars together, right? Well, that’s right. But, there’s a lot more interesting information about binary stars than one might think.

You might think of a binary star system as having one star orbiting around another – such as the way we think about planets revolving around our sun in the solar system. In actuality, binary stars orbit around a common centre of gravity, which in most cases in situated somewhere in the space between the two stars – but not always. This is actually the situation for the solar system as well, but given the mass of the sun in comparison to the planets, it’s hardly noticeable. It IS noticeable with Pluto and it’s companion satellite Charon. But back to binary stars…

Given that we live in a solar system with one star, up until not that long ago we used to think that binary stars were special, and that most stars in the galaxy were singular stars. But in actuality, it turns out that binary stars are the norm!

Stars between 5 to 30 times the mass of the sun tend to all be in binary systems. Lower mass stars – the ones between 1/10th and 1/2 of the sun, have a much lower fraction of those stars in binary systems. For stars around the mass of our sun, around 2/3 of them are in binaries.

The existence of a binary star is fraught with danger – a pair is stable as long as they don’t get close enough to interact. On the other hand, if they are too far apart, they run the risk of the binary system being affected by other objects within the vicinity. You can also actually have higher order systems – so, 3 or more stars – as long as there’s a kind of hierarchy – in other words – two stars could orbit closer together, and a third orbit at a larger distance from the first two – otherwise the gravitational effects interfere with each other, and the higher order system breaks apart.

So why are most stars in a binary configuration? It’s not known for sure at the moment, but astrophysicists suggest is is related to the star formation process. Stars tend to form from molecular clouds, and if an area of the molecular cloud forms basically a lump – or what’s referred to as ‘overdensity’ in a region, then obviously it attracts more material towards itself via gravity, and material starts to accrete around the infant proto-star – kind of like a plate or a disc – and somehow, some kind of process interferes with this accretion disc to allow it to form another object, which also starts accreting – the end result is a pair of stars. What is that interfering process? That, unfortunately, it not clear just yet.

You might think – what about capturing of another star to form a binary system? After all, it happens with satellites within our solar system (or so it’s thought) – can’t it happen with stars? There’s nothing really that rules this possibility out, but almost all binary systems that have been observed actually formed together. In most instances where two stars approach for an interaction, the gravitational and kinetic energy is such that they’ll swing right by each other, rather than become captured in a cosmic waltz. For this to actually happen, there needs to be another influence which dissipates some of that energy.

So how do we figure out if a star is a binary? They’re so far away – isn’t it too hard to see two distinct objects, which in comparison to their distance from us, are very close together? Well sure, – normally both stars are in view, but because their separation is so little in comparison to their distance to the earth, it looks like a single point of light – but there are some smart ways to find out whether a star is actually a binary star.

Think about eclipses – when the moon passes in front of the sun, it blocks out some of the light, or a lot of the light, depending on what type of eclipse it is. So if you’re measuring the amount of light received on earth, you would know that there was an object in between the sun and the earth. This same principle was used to measure stars – and hence, discover that they were actually a binary star system.

If one star moves in front of the other during it’s orbit, there’s a slight change in brightness – and also a slight change in colour, if the colours of the two stars are different – and this is what can be measured, and hence a determination that the star is actually a binary system can be made.

Another method is called the Radial Velocity Method – where you look at the star and measure it’s speed using the Doppler Shift. Measuring the star’s spectrum, if it’s moving towards you it looks slightly more blue, and if it’s moving away, the spectrum looks slightly more red. If you have a binary system whereby one of the stars is a lot brighter than the other, then the emission lines move back and forth between slightly redder to slightly bluer. And from that, astronomers can work out things right down to details like whether the orbits are circular or elliptical, and sometimes even the mass of the star!

Usually, what we find is that the two stars in a binary aren’t the same size – one tends to be larger than the other. And the lifetimes of larger stars are actually shorter – despite the fact that they may have initially more hydrogen and helium. The larger they are, the hotter they burn, the more internal pressure they have, so nuclear burning occurs faster. The more massive will run out of hydrogen, and then it forms a giant. As the outer layers swell, they become closer to the other star, and then the companion star can actually accrete material from the outer layers of the giant, and become bigger itself.

Can binary systems have planets? The answer to that is – binary stars CAN have planets, but it’s a much more difficult situation, given there’s a large external gravitational effect in place. The binary system effectively needs to be a wide binary – so the second star in the binary does not interfere with possible planetary formation.

Well, I hope you enjoyed this little foray into binary stars – keep enjoying the 365 Days of Astronomy podcast – and be sure to check out the Brains Matter podcast as well. Keep your eyes on the sky, and bye for now.

End of podcast:

365 Days of Astronomy
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