Astronomy C

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Astronomy C

Post by Knyte_Xjn » September 4th, 2018, 4:45 pm

Welcome to a new season! I'll start with something relatively simple.
What is the mass of a star that is orbited by a planet that has a period of 745 Earth days and a major axis of 4.6 AU?
Last edited by Knyte_Xjn on September 4th, 2018, 8:15 pm, edited 1 time in total.
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Re: Astronomy C

Post by Name » September 4th, 2018, 6:04 pm

Distance^3/Time^2 = Mass + Mass so 4.6^3/(745/365)^2 = 23.36 Assuming it orbits in a binary with the other star being equal in mass divide by 2 is 11.68 solar masses
Edit: It was major axis that was 4.6 au so semi major axis is 2.3, and correct answer is 1.46 Solar masses
Question: A star has a luminosity of 9540x that of the sun's. What is the apparent luminosity of that star at 1 kiloparsec?
Last edited by Name on September 4th, 2018, 8:41 pm, edited 1 time in total.
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Re: Astronomy C

Post by Knyte_Xjn » September 4th, 2018, 8:31 pm

Name wrote:
Distance^3/Time^2 = Mass + Mass so 4.6^3/(745/365)^2 = 23.36 Assuming it orbits in a binary with the other star being equal in mass divide by 2 is 11.68 solar masses
Question: A star has a luminosity of 9540x that of the sun's. What is the apparent luminosity of that star at 1 kiloparsec?
I apologize for my initial wording of the question as I didn't specify whether it was a binary system or a planet orbiting the star, but I intended for it to be the latter. In that case, the answer is 2.92 Msol, which you get as P^2 = a^3/M, M = a^3/P^2, and 2.3^3 / (745/365)^2 ≈ 2.92 Msol. Your answer would have been right as per your interpretation, but you should've divided 4.6 AU by 2 as a is the semi-major axis of the orbit. Using your interpretation of my question, the answer would've just been half of the total combined mass, or 1.46 Msol.
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Re: Astronomy C

Post by Name » September 4th, 2018, 8:38 pm

oops thought it said semi major axis, can't read lol

Anyways reposting the question:
Question: A star has a luminosity of 9540x that of the sun's. What is the apparent magnitude of that star at 1 kiloparsec?
Last edited by Name on September 5th, 2018, 3:38 am, edited 1 time in total.
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Re: Astronomy C

Post by Knyte_Xjn » September 4th, 2018, 9:54 pm

Name wrote:oops thought it said semi major axis, can't read lol

Anyways reposting the question:
Question: A star has a luminosity of 9540x that of the sun's. What is the apparent luminosity of that star at 1 kiloparsec?
m - M = 5log10(r/10)
m - (4.77 - 2.5log(l/lsol)) = 5log10(1000/10)
m = 5log10(1000/10) + 4.77 - 2.5log(9540)
m ≈ 4.82
By the way, apparent luminosity =/= apparent magnitude; apparent luminosity doesn't exist.
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Re: Astronomy C

Post by Name » September 5th, 2018, 3:38 am

Ah I meant apparent magnitude sorry
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Re: Astronomy C

Post by Knyte_Xjn » September 6th, 2018, 5:16 pm

Name wrote:Ah I meant apparent magnitude sorry
A star has a surface flux of 8.22 x 10^7 W m^-2. At what wavelength does it emit the most light, and what color is this star?
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Re: Astronomy C

Post by pb5754 » September 7th, 2018, 1:42 pm

Knyte_Xjn wrote:A star has a surface flux of 8.22 x 10^7 W m^-2. At what wavelength does it emit the most light, and what color is this star?
T^4 = (8.22 x 10^7) / (5.67 x 10^-8)
T = 6170 K
wavelength = 0.002898 / 6170
[u]wavelength = 4.70 x 10^-7 m = 470 nm[/u]
[u]color = yellow?[/u]
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Re: Astronomy C

Post by Knyte_Xjn » September 7th, 2018, 2:56 pm

pb5754[] wrote:
Knyte_Xjn wrote:A star has a surface flux of 8.22 x 10^7 W m^-2. At what wavelength does it emit the most light, and what color is this star?
T^4 = (8.22 x 10^7) / (5.67 x 10^-8)
T = 6170 K
wavelength = 0.002898 / 6170
[u]wavelength = 4.70 x 10^-7 m = 470 nm[/u]
[u]color = yellow?[/u]
The first part is correct. However, the second part should be blue, as 470nm is within the wavelength interval of blue(450-490nm).
Your turn.
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Re: Astronomy C

Post by pb5754 » September 7th, 2018, 4:09 pm

Knyte_Xjn wrote: However, the second part should be blue, as 470nm is within the wavelength interval of blue(450-490nm).
Your turn.
Oh ok makes sense.

I can't think of a good question right now, so the next person may post a question for me instead.
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