A Conversation for An Amazing A-Z of Space

Wobbly Stars...

Post 1

Squange - Traveller of the USA

Aren't stars that we perceive as wobbling back and forth binary and trinary stars? I was always under the impression that planets outside of our own solar system are far too small to be seen with the naked eye - even giants like Jupiter.

Squange


Wobbly Stars...

Post 2

Gnomon - time to move on

Planets outside the solar system are two small to be seen with the strongest telescopes. Binary stars can be seen as two stars with a strong telescope. If a star is wobbling and it has no visible companion, we can estimate from the speed of the wobble how massive the orbiting object is. From this, we can determine whether it is an orbiting black hole, a neutron star or a planet. Most of the planets detected so far are much bigger than Jupiter but still far to small to be visible.


Wobbly Stars...

Post 3

Iuchiban

This does not particularly answer the first question posed in this conversation, but it should serve as a brief introduction to why wobbly stars indicate the presence of planets, or some other invisible companion.

The principle is this: the Doppler Effect when applied to this situation states that if the star has a velocity away from us relative to observers on Earth, its light will be slightly redshifted (that is, it will have lower frequencies). If it is moving towards us, it will be slightly blueshifted (higher frequencies).

This is fairly intuitive; the star is giving off light waves all the time (let us consider the wave mechanics of light in this instance rather than the particle characteristics, although they are related via the E=hf equation), and all those waves travel at the speed of light. If the star is stationary, an observer will see one wave front, then the next with a certain small time delay between the two wave fronts. If the star is traveling away from us, the wave front it emits at one time will be closer to us than the wave front it emits at a later time; the second wave will have to traverse a slightly greater distance, and the effect over a number of wave fronts is that the frequency seems to go down. For a star traveling toward us, the star will cover a little bit of the distance between the emission of one wave front and the next, which means we see the wave fronts more compressed than we would normally expect. Although this explanation isn't completely accurate, the effects are the same as if it was, and it is a relatively easy way to think about it.

So, what astronomers mean by a wobbling star is that it has a periodic redshift and blueshift in its spectrum, which means it must be orbiting something. In the case of a planet, the star is influenced by the planet's gravity, and the planet is influenced by the star's gravity. Thus, they both orbit the barycenter (center of gravity between the two), which is probably somewhere below the surface of the star for a Jupiter-sized planet, and the star seems to wobble slightly as it revolves about this point. While this effect happens for -any- mass orbiting the star, only sufficiently large and close planets will produce a detectable effect.

Thus, if we see a star wobbling, and no visible companion, we can determine by the level of redshifting or blueshifting that happens to the light the mass of the other object and the distance from it to the star (after making an assumption for the mass of the star) using Keplerian theory. As other techniques are more reliable for the detection of neutron stars and black holes, "wobbly stars" usually refers to those around which we have detected planets. As of yet, we do not have the means to detect Earth-like objects around any star.


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