Date: April 18, 2012
Title: Binary Stars
Podcasters: Mel Blake & Deb Bailey
Organization: UNA Planetarium
Link: http://www.una.edu/planetarium
UNA Planetarium Facebook Page
@unaplanetarium
Bios: Dr. Mel Blake. I am from Newfoundland, Canada. I received a PhD from York University in Toronto, Canada, and I am currently director of the University of North Alabama Planetarium and Observatory. I am an Associate professor in the department of Physics and Earth Science. My research interests include binary stars and star clusters.
Deb Bailey is an undergraduate honors student from Alabama who majors in professional writing and English. She plans to pursue graduate studies in the fall.
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Transcript:
Dr. B. – Hello. I am Dr. Mel Blake.
Deb – And I’m Deb Bailey.
Dr. B. – And we’re at the University of North Alabama Planetarium and Observatory, and today we’ll be discussing binary stars.
Dr. B. – The Sun travels through the galaxy on an orbit that lasts hundreds of millions of years. With the exception of the planets, it travels this path alone, with no other star nearby.
Deb – It turns out that this is the exception rather than the rule. Most stars are members of binary star systems, where two stars orbit around one another. Half the stars in the sky are binaries. Because there are so many, we can learn a great deal of information about them.
Dr. B. – The constellation Ursa Major, the Great Bear, contains the seven stars that make up the “Big Dipper”. In the bend of the handle of the Big Dipper is the star Mizar. Those with good eyesight will also notice a fainter companion next to it. This star is called Alcor. The Arabs called the two the “horse and rider,” and it was used as a test of keen eyesight. It turns out that the two stars are not a “double star” or a chance alignment of stars in the sky. Alcor and Mizar are in orbit around one another and move through the galaxy together.
Deb – They are what we call a visual binary. We can actually see the two stars in the pair separately on the sky as two distinct objects. Studies of their motion on the sky confirm that they are companions. This is true for the nearest binary stars. However, if two stars are farther away, instead of studying their motion on the sky, we use our knowledge of starlight.
Dr. B. – The light that we see with our eyes is in fact composed of all the colors of the rainbow, from blue to red. We can break up the light into its colors using an instrument called a spectrograph. The pattern of light is called a spectrum.
Deb- We can use this spectrum to detect the elements that make up a star because they leave gaps in the light called lines. The lines from the element Hydrogen are different from that of Helium, Oxygen and all the other elements which are each unique.
Dr. B. – If we have two stars orbiting one another, we can make use of the Doppler effect to detect and study the two stars. The Doppler effect is caused by the motion of an object. If an object is moving towards you, the signal from the star will be sent a little closer to you each second. This causes the waves of light to look a little shorter than they would if the object were not moving. Likewise, if an object is moving away from you, the light will be a little longer.
Deb – Red light is longer in wavelength than blue light. So objects moving towards you have their light blue-shifted and objects moving away have their light red-shifted.
Dr. B. – We can use this to detect binary systems. If the two stars are in orbit around one another, one object will be moving towards you while the other is moving away. As a result, the lines in their spectrum will be separated.
Deb – When we examine their spectra we will see the pattern of lines of the two stars separate from one another. As the two stars move around one another, we will see the lines move around back and forth as they orbit.
Dr. B. – Remembering Alcor and Mizar, our visual binary, we discover that there are more stars there. The spectra of the stars reveal that Alcor is in fact a binary system, and Mizar is a quadruple system, with a pair of binaries in orbit around one another. We call the two stars in a binary “A” and “B”, so we have Alcor A and Alcor B and so on.
Deb – The back and forth motion of the stars’ lines can also produce data on how much mass is in each star. Isaac Newton studied gravity and motion. One of his basic laws of motion simply states that the same force will accelerate a more massive object less than a less massive object. In addition, his law of gravity says that the force of gravity depends on their masses and how far apart they are.
Dr. B. – The force of gravity between two objects in a binary star is the same, but the more massive star will undergo less acceleration, and will therefore have a smaller orbit than the less massive star. This means that the less massive star will need to cover a larger orbit in the same time the more massive star covers a small one. The less massive object will have to move faster. As a result, the lines in this spectrum will shift more.
Deb – By watching the shift of lines in the spectra due to the Doppler effect, we can figure out how much more massive one star is than the other. Much of what we know about the masses of stars is obtained exactly this way. If the stars are a visual double, then we can use this to get the size of the orbit in kilometers and therefore, the distance to the stars and their masses.
Dr. B. – There is another way to detect a binary star. If the orbits are exactly right, and the stars are close, as the two orbit one another, one star can pass between us and the other star. Such an eclipse results because instead of the combined light from two stars, we now see only one or perhaps only part of the other.
Deb – During the eclipse, there is a drop in brightness. The length of the drop in brightness will depend on the size of the star that is blocking the other. As a result, we can get the size of the star. We can measure the length of the eclipse and take spectra to see how fast the stars are moving.
Dr. B. – This then gives us the diameter of the star, assuming it is a total eclipse. Partial eclipses must be carefully modeled. Much of what we know about the radii of stars comes from studying these spectroscopic eclipsing binaries.
Deb – We can see that binary stars are useful to study for many reasons. The stars’ masses and radii can be obtained by studying binary stars. We need this information to compare to our models of stars of different types and improve our ideas about how stars work and evolve. However, given that a single star like our Sun, traveling through the galaxy on its own is the minority, it just makes sense to study the most common stars, those is pairs.
Dr. B. – So, the next time you look at a star in the sky, its pinpoint sending its message across the galaxy, remember that the star may be two stars, held together by mutual attraction.
End of podcast:
365 Days of Astronomy
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