Splitting Hairs (and Stars)


When we talk about the distance between stars there are 2 very different measures that might apply.  The actual distance between any two stars (typically measured in light-years, or astronomical units [the distance between Earth and our Sun] if they are very close) is generally not apparent by simple observation, whether aided by a telescope or not.  But we might also be concerned with the angular separation between stars.  Frankly, this information is most useful in measuring the quality of various optical elements, including human eyes.

Many stars are actually “binary stars” that are relatively close to each other in terms of angular separation.  But a pair of stars can appear to be close to each other simply due to our fixed perspective in viewing from Earth, while they are actually far apart (one much closer to us than the other).  Other binary stars are truly binary, meaning that they are physically very close to each other, which generally means that they interact with each other through gravity.  In extreme cases (and there are many of these), two or more stars spin rapidly around each other and their shapes are distorted by the gravity of the other(s).

Before going into this further, let’s review the units used in measuring angles.  In everyday human life we rarely need to use any units finer than a “degree” – 1/360th of a full circle.  In astronomy, we are very often concerned with angular measures much smaller than that, so we divide each degree into 60 “arcminutes” and each arcminute into 60 “arcseconds”.  So an arcsecond is 1/60 * 1/60 = 1/3600th of a degree, or 1/1,296,000 of a full circle.

I’ll use the familiar asterism, “The Big Dipper”, as an example.  You probably have a pretty good idea of how big this looks in the sky, but to be more precise, I’ll tell you that it is almost 26 degrees from Alkaid, the leftmost star, to Dubhe, the rightmost.  The second star from the left is Mizar (circled in red), and you might also be familiar with the star just above and a little to the left of Mizar, Alcor.  Alcor is just under 12 arcminutes (1/5th of a degree) from Mizar, a separation that seems small, but is actually very easy to see with the naked eye.  Except that Mizar is almost 6 times as bright as Alcor, so seeing it is both a test of our spatial resolution and our freedom from aberrations that would cause the brighter star to mask the dimmer one (cataracts will do that, for example).  In fact, many cultures around the world used this pair of stars as a test of visual acuity, and even general health.  Today, most of us live with so much light pollution that this can be a difficult test even for a person with perfect vision.

Big Dipper_WthLines_WthCirc

 

 

Big Dipper_WthLines_WthCirc_Zoom

Below is a close up (shot at higher magnification) of the Mizar/Alcor pair, with Alcor at the top.  As you can see, there is actually another star very close to Mizar – so close that we don’t see it in the wide-field shot.  In fact, these two stars are known as Mizar A and Mizar B.  They are separated by just 14.4 arcseconds, or 0.004 degrees.  Because they can be seen as separate stars through a telescope they are called a “visual binary pair”.  The last image is a close-up of this pair.

Mizar

Mizar

Finally, if we were to look at spectroscopic data from Alcor, Mizar A, and Mizar B we would see that ALL THREE OF THEM are binary stars, with a companion that is so close we cannot detect it visually, and therefore called “spectroscopic binary pairs”.  Mizar A’s two stars are about 1 arcsecond apart.  While that separation is theoretically within the resolution of the telescope and camera I used to take these pictures, other factors, such as atmospheric turbulence, make it impossible for me.  Professional observatories can resolve this separation, and even smaller ones, but they do it with equipment that costs many orders of magnitude beyond the typical amateur gear.

To put this in perspective, we have to remember that these points of light that we cannot separate are stars – massive things, and the distance between most spectroscopic binaries is massive in comparison to the size of the stars.  Which is to say that the distance from Earth to these stars, enough to render two stars as one, is awesomely massive.


About Greg Marshall

I am a retired electronics engineer and after a few months of enjoying my leisure I began to miss doing product development. My astronomy hobby always needed new solutions to unique problems, so I decided that whenever I came up with a good solution I would try to make it available to others.

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