Posted by: T. Boyd | March 22, 2009

Double Stars

“Who alone stretched out the heavens
and trampled the waves of the sea;
Who made the Bear and Orion,
the Pleiades and the chambers of the south?”
(Job 9:8-9)

Double Star in Cygnus: Albereo (Beta Cygni)

Double Star in Cygnus: Albereo (Beta Cygni)

Conversation with wife, written in the style of the classics:

Husband:  Have I told you lately … (wife waits eagerly for the rest of sentence) …  that about 1/3 of all the stars are double stars?
Wife:  (a little disappointed, sighs) I remember your telling me that years ago when you taught at Randolph-Macon.  Is that when 2 stars revolve around each other?
H:  That’s right.  But most of the stars are so far away that you can’t see two separate stars, even with the most powerful telescopes.  By the way, I looked at some (relatively) close-by ones last night – some are very beautiful, made up of two distinct colors.
W:  How do they know that the far away ones are made up of two stars?
H:  Good question.  There are several techniques to discover that.  One is when one star goes in front of the other and the light gets dimmer – those are called eclipsing binary stars.
W:  I see.  What’s another way?
H:  Well, when a star is moving away from us or toward us, the color (spectrum) of the star changes slightly.   When receding it gets more red, and when approaching, it gets more blue.  As they go around each other, one star in a double star will usually be moving away from us and its partner toward us.
W:  Is that called “red shift”?
H:  Yes, red and blue shift.  So in a double star, we see two distinct spectra.  I’m not explaining it very well, but that is the gist of it.   The complete story is much longer.
W:  Yes, that’s enough.  Well, what keeps the stars away from each other?  What happens if one falls into the other?  And if our Sun had a twin star, how far away would it be?
H:  Whoa… too many questions.  Actually, the Sun has a little sister, an “almost-star”: the planet Jupiter.  That was the point of the sequel to the movie 2001, whatever the second movie was called.  At the end of the story, Jupiter finally ignited and became a second star in the solar system.  Our Lord, who formed the solar system, fine tuned it so life is possible.
W:  But, what would happen to us if that did happen?
H:  You mean, if Jupiter became a star?  I think we would soon die because the life sustaining balance of the Earth would be too drastically changed with the extra radiation and with its coming from two different sources.
W:  Hmm … That sounds like a plot for another movie.
H:  Right.  Now back to your other questions.  The thing that keeps binary stars apart is the same thing that keeps Jupiter from the Sun – just regular orbital motion explained by Newton’s laws of motion.   The inward pull of gravity is offset by the tendency to go flying off in a straight line.  This tendency is called inertia, and wrongly labeled as a force as in centrifugal “force”.
W:  (Yawns). This is getting a little long, but one more question.  What happens if one star falls into another?
H:  A lot of different things could happen depending on the what was left after the collision and explosion.    But I expect that before that happened,  the stars would “die” (run out of hydrogen, then the heavier elements, in the fusion process) before the stars would get close enough to merge.
W: (Eyes getting glazed over). Sometime you can tell me about the life of a star.
H:  O.K.  Now… have I told you lately …. that I love you?
W:  Ah, that’s what I really wanted to hear….

Afterword by my wife: The odd thing is that we really did have the above conversation – which goes to show, I think, that if you can follow a man’s conversation down the roads of fishing lures, gadgets, and other assorted interests, he will eventually say something worth waiting for.

Note: This article copyrighted by Caroline Progress 2009


  1. I just posted an article on nuclear fusion and fission. The ang. momentum article will come after the life of a star posting. Keep on digging! Boyd

  2. Thank you so much sir . I was going through angular momentum for heavenly bodies and seems it will need a good study to understand all . Please let me know once your next article will be out :-).

  3. Hi, Chandra. Part of the life of the star is explained on another page (which I haven’t finished) entitled: Red Giants, White Dwarfs, … I’ll try to explain fusion soon.

    Gravity will not neutralize a head-on collision; conservation of angular momentum is what prevents the collision – another aspect of Newton’s laws.

    As two objects approach each other, unless their initial aim is exactly toward each other’s center of mass, they will wind up rotating around a common center outside of either one.

    Wow, that is hard to explain in a few words. Wikipedia has an article on Angular Momentum, but it is not easy. I will try to find a easier description, or write one of my own.


  4. Nice blog..very interesting :-). I would like to understand more about how stars die due to fusion process ?. Can you please bit explain as I think gravitational forces will try to neutralize a head-on-collision?.

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