Binary stars

[FLO]

Inspired by Ian (lunator), I recently split a binary star :mrgreen:

Now, I will confess that once the buzz of success wore off, I was left wondering...

What is the significance of binary stars?

Why are they important to astronomers?

Well, I looked them up in the encyclopedia Encarta and here it is:

INTRODUCTION

Binary Star, two stars that are bound to each other by gravity and orbit about a common center of mass. Binary star systems are quite common and the pairing of stars appears to be random in most cases. Astronomers estimate that approximately one-fourth of the visible stars belong to a binary system. The time it takes for one star to orbit the other can range from hours to centuries depending on the distance between the two stars and their masses. Some binary pairs, called interacting binary systems, are so close that they exchange material. Binary stars are very useful to astronomers because they are the only stars of which astronomers can directly determine mass.

IMPORTANCE OF BINARY STARS TO ASTRONOMY

The motions of visual binary systems are the most direct and easiest to determine, but astronomers have also developed techniques for deducing the motions of astrometric binaries, spectroscopic binaries, and eclipsing binaries.

Binary star systems are very important to astronomers because they offer unique opportunities to determine important information that cannot be obtained from single stars. In particular, with knowledge of the separation between the two stars of a binary system and the time they take to orbit each other, astronomers can determine the individual masses of the stars. Astronomers have found a strong correlation between the mass of a star and its luminosity, or light output. Therefore, once a star’s mass is known, astronomers can calculate its intrinsic luminosity and use this information to estimate its distance from the earth (see Magnitude).

Knowledge of the distance to a binary system allows astronomers to estimate the distance to other stars in its vicinity. This has been crucial to the development of stellar evolution theory, which has advanced primarily due to the study of star clusters. Star clusters are close groups of stars that appear to have evolved from the same material at about the same time. Knowing the distance to a star cluster allows astronomers to calculate the intrinsic luminosities of the stars in the cluster. From the mass-luminosity relationship, the masses of the stars in the cluster can then be inferred. Using information on the masses of the stars and information on their evolutionary states gained through spectroscopy, astronomers can determine the effect of mass on the evolution of the stars.

Contributed By:

Dennis L. Mammana

Microsoft ® Encarta ® Reference Library 2005.

[Greg]

I thought i'd explained that to you the other night Steve.....Pay attention in class please

Ok....Only joking...Very interesting...Could see myself hunting down some more..

[FLO]

OK,

Here's an as yet unanswered question:

If gravity pulls two stars together into an orbit; why don't they actually collide and become one?

[lunator]

Steve

It's all about angular momentum and stable orbits.

Just as the Earth doesn't fall into the Sun because we are in a stable orbit (and the closer the planet is to the Sun the faster it orbits) so 2 stars will orbit quicker the closer they are so they don't 'fall into' each other.

This is why close binaries can be seen to move over years observing.

Cheers

Ian

[Kaptain Klevtsov]

The orbit bit evades me as the "window" for two approaching objects to get into an orbit situation seems ludicrously small, unless I got the sums wrong years ago.

I wrote a BBC basic program to simulate a small object approaching a much larger object to see if I could get it to "slingshot" and gain speed. The small object started off at the left of the screen moving slowly to the right horizontally at first, and approached a big one just below the centre of the screen It was supposed to either acheive orbit, miss by miles, do a successful slingshot or just plain hit it. I ended up with a for next loop with a step size of 0.001 for the starting height up the screen before the adjustment was close enough to see the orbit happen.

For so many things to be in orbit araound each other implies that a lot more hit or miss, looking at the moon, I don't see anything in orbit around it, but there are plenty of dents in it.

Captain Chaos

[FLO]

Thanks chaps; that helps .... I think :?

[Astroman]

CC: The binary stars in question by and large formed together. As your experiment suggests, "captured" orbits are rare. Triton around Neptune is thought to be one, (since it orbits in retrograde), so they're not impossible, but very rare. That's one reason they study clusters, since they formed together, there's lots of gravitational interaction going that enables individual orbits to be determined, masses extrapolated, magnitudes extracted and the rest of what Steve posted.

Steve: The answer to your question is, they do. (But see above to CC, too.) But, since they formed together, and their orbits are very stable, as Lunator says, it takes a very long time indeed to bring them together. Pairs of stars that orbit close enough to exchange material, called "cataclysmic variable stars", or CV's, are one area I like to study. Their evolution from formation to eventual coalescence is fascinating and a hot topic among variable star observers. Unfortunately, there aren't that many "bright" pairs to observe. Most are in the ~14 to 18 mag range. (Light bucket required.)

[lunator]

Steve

The is one example where stars are believed to had merged.

These are generally in GC's. They are the merger of 2 stars.

It takes the crowded environs of a GC to create the conditions for mergers to take place.

They have a very high spin rate. The energy in the orbit has to go somewhere and it goes into spinning up the rotation of the star.

Cheers

Ian

[FLO]

All good stuff; thanks again chaps