Astronomy C

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SciolyHarsh
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Re: Astronomy C

Post by SciolyHarsh » March 6th, 2019, 9:05 pm

Yup. I didn't specify whether it was a galaxy with a single cepheid or not but that's what I meant so props to you.

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Re: Astronomy C

Post by syo_astro » March 7th, 2019, 9:48 am

SciolyHarsh wrote:Yup. I didn't specify whether it was a galaxy with a single cepheid or not but that's what I meant so props to you.

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Normally I don't check these, but it's simpler to just say the galaxy contains a cepheid, and you measure that cepheid's apparent magnitude to be whatever it is (to everyone...I figure people "get this", but it helps to be explicit instead of "the galaxy is a single cepheid" when reading the answers above...really not a big deal). BUT that leads to the interesting question of how you separate the apparent brightness of cepheids in a galaxy from other stars, or where that might be relevant...but you should probably ask a different question you're actually wondering since that's kinda vague. There's all kinds of issues you get into when you're actually trying to go from a galaxy's overall apparent brightness to that of stars, but I'm honestly not a pro at it.

When I'm writing questions one neat thing you can do is assume your answer (make it up, like 20.8 kpc) and work backwards to get appropriate values. That way you KNOW your answer will fit the context or qualitative aspects you want.
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Re: Astronomy C

Post by ET2020 » March 9th, 2019, 12:35 pm

Two stars orbit in a binary system. Star A's orbit traces a circle with an apparent radius of 600 milliarcseconds, while star B's orbital traces a circle with an apparent radius of 200 milliarcsecond. Star A is a main sequence star with a luminosity of 4.2 solar luminosities and an apparent magnitude of 7.65. Find the mass of Star B and its distance from the center of mass of the binary system.
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Re: Astronomy C

Post by Giantpants » April 13th, 2019, 3:18 pm

ET2020 wrote:Two stars orbit in a binary system. Star A's orbit traces a circle with an apparent radius of 600 milliarcseconds, while star B's orbital traces a circle with an apparent radius of 200 milliarcsecond. Star A is a main sequence star with a luminosity of 4.2 solar luminosities and an apparent magnitude of 7.65. Find the mass of Star B and its distance from the center of mass of the binary system.
Not sure about my answer but may as well try to revive the thread.
So first I made the absolute magnitude of star A to be 3.21, which makes the distance to star A 77.2 pc.
Using something I learned from a test, if the star is 77.2 pc away, then one arcsecond of distance should be 77.2 AU.
So the radius of Star A is 600 mas, .6 arcsec, or 46.3 AU, and by that logic, Star B is .2 arcsec away from the center of mass, making it [b]15.4 AU[/b], which [b]should[/b] be the first answer (?)
As for the mass of Star B, I dont have an exact answer. The mass of Star B must be 3 times greater than Star A because it is 3 times closer to the center of mass, but I dont know how i could figure that out without the period, which i could use to find the combined mass of the system using MsubA + MsubB = a^3/p^2, but im unsure about my value for a (30.4 AU?) and I have no idea how to get p. So, my answer is gonna be [b]3 times greater than the mass of Star A.
Anywhere close? We'll see ig haha[/b]
Last edited by Giantpants on April 13th, 2019, 3:26 pm, edited 1 time in total.
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Re: Astronomy C

Post by Giantpants » April 13th, 2019, 3:20 pm

(this post was made by an accidental misclick oops)
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Re: Astronomy C

Post by Name » April 13th, 2019, 7:26 pm

Giantpants wrote:
ET2020 wrote:Two stars orbit in a binary system. Star A's orbit traces a circle with an apparent radius of 600 milliarcseconds, while star B's orbital traces a circle with an apparent radius of 200 milliarcsecond. Star A is a main sequence star with a luminosity of 4.2 solar luminosities and an apparent magnitude of 7.65. Find the mass of Star B and its distance from the center of mass of the binary system.
Not sure about my answer but may as well try to revive the thread.
So first I made the absolute magnitude of star A to be 3.21, which makes the distance to star A 77.2 pc.
Using something I learned from a test, if the star is 77.2 pc away, then one arcsecond of distance should be 77.2 AU.
So the radius of Star A is 600 mas, .6 arcsec, or 46.3 AU, and by that logic, Star B is .2 arcsec away from the center of mass, making it [b]15.4 AU[/b], which [b]should[/b] be the first answer (?)
As for the mass of Star B, I dont have an exact answer. The mass of Star B must be 3 times greater than Star A because it is 3 times closer to the center of mass, but I dont know how i could figure that out without the period, which i could use to find the combined mass of the system using MsubA + MsubB = a^3/p^2, but im unsure about my value for a (30.4 AU?) and I have no idea how to get p. So, my answer is gonna be [b]3 times greater than the mass of Star A.
Anywhere close? We'll see ig haha[/b]
Use the mass luminosity relationship. For a main sequence star from a mass of .43 to 2 sm (Relationships are different with different masses), the equation is luminosity is approximently equal to mass^4. Luminosity of star A is 4.2, the fourth root of that is 1.43 which would be mass of star A in solar masses. Then multiplied by 3 to get [b]4.29[/b] solar masses for mass of star B
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