Hi Folks
I am new here in this discussion forum and I would like to congratulate you all for having such a nice thing. I do have a question and I would like to ask you something about the triple star system HD 188753...
Remember... this is the star system with a gas giant orbiting the primary star . I hope that somebody can answer the question. In his publication (Nature 436, 230-233) the author mentioned that the primary star has about 1.06 sun masses. The orbiting binary pair has 1.63 sun masses in sum. Now the question opens to me why the binary star (wich has in sum the higher mass) is primary orbiting the smaller star and not vice versa?
Question regarding triple star system HD 188753
Question regarding triple star system HD 188753
cogito ergo sum
Re: Question regarding triple star system HD 188753
ChKy wrote:Hi Folks
I am new here in this discussion forum and I would like to congratulate you all for having such a nice thing. I do have a question and I would like to ask you something about the triple star system HD 188753...
Remember... this is the star system with a gas giant orbiting the primary star . I hope that somebody can answer the question. In his publication (Nature 436, 230-233) the author mentioned that the primary star has about 1.06 sun masses. The orbiting binary pair has 1.63 sun masses in sum. Now the question opens to me why the binary star (wich has in sum the higher mass) is primary orbiting the smaller star and not vice versa?
Depend of your referentiel frame...
Take an exemple:
The earth turn around the sun if you take the Heliocentric's (sun) referentiel frame.
And, the sun turn around the earth if take the Geocentric's (earth) referentiel frame.
I hope you understand that even if somes words are difficult for me to translate...
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Thanks for the reply 
@Fightspit
"Depend of your referentiel frame...
Take an exemple:
The earth turn around the sun if you take the Heliocentric's (sun) referentiel frame.
And, the sun turn around the earth if take the Geocentric's (earth) referentiel frame."
As far as I know is that two bodies are orbiting around the center of there mass/gravity (baricenter?). Depending on the ratio of mass the center is shifted to some extend towards the center of the body with the higher mass. This means that the orbits radius of the bigger body around the center is smaller than the orbits radius of the smaller body around the same center.
If two bodies have the same mass this paticular baricenter would be exactly the same distance apart between the centers of these bodies.
In your example the mass of sun is tremendeously higher compared to the earths mass. The baricenter between sun and earth is nearly at the center of the sun. So earth is orbiting the sun whereas our sun seems to make nearly no movement around the baricenter.
In case of the earth-moon system the baricenter lies 4000 km below the earths surface (if I remember correctly). While moon is moving around earth, our planet is tumbling like a boxer...
Now I wrote an essay nearly
Is it this what you ment?
[color=brown]@selden
"The orbits are specified relative to the brightest star. Although they have a total mass that's more, the two "satellites," HD 188753 B and HD 188753 C, are much dimmer than HD 188753 A."[/color]
Does this means that unless the baricenter is shifted towards HD 188753 B and C, the star HD 188753 A is the primary because it is brighter?
Thanks for your reply Astronomy is really and very interesting... Also for a biologist 
@Fightspit
"Depend of your referentiel frame...
Take an exemple:
The earth turn around the sun if you take the Heliocentric's (sun) referentiel frame.
And, the sun turn around the earth if take the Geocentric's (earth) referentiel frame."
As far as I know is that two bodies are orbiting around the center of there mass/gravity (baricenter?). Depending on the ratio of mass the center is shifted to some extend towards the center of the body with the higher mass. This means that the orbits radius of the bigger body around the center is smaller than the orbits radius of the smaller body around the same center.
If two bodies have the same mass this paticular baricenter would be exactly the same distance apart between the centers of these bodies.
In your example the mass of sun is tremendeously higher compared to the earths mass. The baricenter between sun and earth is nearly at the center of the sun. So earth is orbiting the sun whereas our sun seems to make nearly no movement around the baricenter.
In case of the earth-moon system the baricenter lies 4000 km below the earths surface (if I remember correctly). While moon is moving around earth, our planet is tumbling like a boxer...
Now I wrote an essay nearly
Is it this what you ment?
[color=brown]@selden
"The orbits are specified relative to the brightest star. Although they have a total mass that's more, the two "satellites," HD 188753 B and HD 188753 C, are much dimmer than HD 188753 A."[/color]
Does this means that unless the baricenter is shifted towards HD 188753 B and C, the star HD 188753 A is the primary because it is brighter?
Thanks for your reply
Astronomy is really and very interesting... Also for a biologist cogito ergo sum
Remember that we aren't the people who discovered that HD 188753 is a multiple star and that it has a planet, so we can't say for certain why the discoverers describe it the way they do.
However, bear in mind that it used to be described as a spectrographic binary. As best people could tell from looking at the spectrogram, it only consisted of two stars, A and B. B is very much dimmer than A. The standard way of recording a star's spectrogram doesn't include a standard stationary spectrogram for calibration. You can't determine which star is moving around the other, just that they're moving at different speeds as the lines separate and come together again over time as they're red- and blue-shifted relative to one another. The third star (C) is dim enough that, at the time, its lines couldn't be distinguished from B's lines. (The Simbad database still lists the star as being a double, not a triple.)
In general, dim stars are less massive than bright ones, so of course one would consider B to be orbiting around A.
However, bear in mind that it used to be described as a spectrographic binary. As best people could tell from looking at the spectrogram, it only consisted of two stars, A and B. B is very much dimmer than A. The standard way of recording a star's spectrogram doesn't include a standard stationary spectrogram for calibration. You can't determine which star is moving around the other, just that they're moving at different speeds as the lines separate and come together again over time as they're red- and blue-shifted relative to one another. The third star (C) is dim enough that, at the time, its lines couldn't be distinguished from B's lines. (The Simbad database still lists the star as being a double, not a triple.)
In general, dim stars are less massive than bright ones, so of course one would consider B to be orbiting around A.
Selden