Astronomy C

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bhavjain
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Re: Astronomy C

Post by bhavjain » November 14th, 2016, 10:06 pm

One of the questions from Captain's Tryouts: What is the last element formed in a star's core through nuclear fusion?

The answer key says Iron, but I always thought stars can form Nickel-56 via nuclear fusion. Any help?

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Re: Astronomy C

Post by Unome » November 15th, 2016, 4:58 am

bhavjain wrote:One of the questions from Captain's Tryouts: What is the last element formed in a star's core through nuclear fusion?

The answer key says Iron, but I always thought stars can form Nickel-56 via nuclear fusion. Any help?
This is the sort of thing that sometimes it's better not to take too literally, and just give the ES what they're looking for. Usually it'll be iron, but if you judge from the rest of the test that you think the ES might want nickel-56, then I'd suggest you put that down with a specific note about it.

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Re: Astronomy C

Post by syo_astro » November 15th, 2016, 9:23 am

The question to me just seems badly phrased. I agree the answer should be iron just because it's one of those "I want the duh answer" type of things, but the question itself doesn't necessarily make sense. In a low-mass star, what is the last element fused to in its core? What about a red dwarf, etc? Unfortunately, this often happens at quite a few levels (hopefully less so at states/nats).

A more satisfying reasoning if you're curious:
I believe iron is the "last" element to form through fusion. Though, Ni is sort of too. You can get both out of Si fusion, but I believe Ni can decay into Fe, which is why we mostly talk about Fe cores (though, there can be some Ni there too?). This bit gets confusing and very technical, so I think that's about all that needs to be said.

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Re: Astronomy C

Post by Adi1008 » November 15th, 2016, 2:11 pm

bhavjain wrote:One of the questions from Captain's Tryouts: What is the last element formed in a star's core through nuclear fusion?

The answer key says Iron, but I always thought stars can form Nickel-56 via nuclear fusion. Any help?
Just to add to what Unome and syo_astro already said: Yes, Nickel-56 is produced through silicon burning; it decays into Cobalt-56 and then finally into Iron-56. Now, you might be asking, "if Nickel-56 is produced, why don't we observe it as much and say that as the answer instead of Iron-56?". Well, Nickel-56's half life is a mere 6 days, and Cobalt-56's is 77 days (you can find this on Wikipedia), which is almost nothing in astronomical terms. By the time astronomers get around to observing anything, almost all of the Nickel and Cobalt are gone, and only Iron-56 is remaining.

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Re: Astronomy C

Post by bhavjain » November 15th, 2016, 3:52 pm

Adi1008 wrote:
bhavjain wrote:One of the questions from Captain's Tryouts: What is the last element formed in a star's core through nuclear fusion?

The answer key says Iron, but I always thought stars can form Nickel-56 via nuclear fusion. Any help?
Just to add to what Unome and syo_astro already said: Yes, Nickel-56 is produced through silicon burning; it decays into Cobalt-56 and then finally into Iron-56. Now, you might be asking, "if Nickel-56 is produced, why don't we observe it as much and say that as the answer instead of Iron-56?". Well, Nickel-56's half life is a mere 6 days, and Cobalt-56's is 77 days (you can find this on Wikipedia), which is almost nothing in astronomical terms. By the time astronomers get around to observing anything, almost all of the Nickel and Cobalt are gone, and only Iron-56 is remaining.
But, TECHNICALLY, the last element formed is NIckel-56. Anyways, you all recommend putting Iron, so that's what I'll do on the test if it shows up :P

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Re: Astronomy C

Post by bhavjain » November 17th, 2016, 2:34 pm

Anyone know what the difference between an AM CVn and x-ray binary system are? Both involve white dwarfs :P

Example of AM CVn: J075141 and J174140
Example of x-ray binary: HM Cancri

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Re: Astronomy C

Post by Unome » November 17th, 2016, 2:44 pm

bhavjain wrote:Anyone know what the difference between an AM CVn and x-ray binary system are? Both involve white dwarfs :P

Example of AM CVn: J075141 and J174140
Example of x-ray binary: HM Cancri
As I understand it, an X-ray binary system is any binary star system that emits X-rays (not sure about this one), whereas an AM CVn has two white dwarfs, a more massive, smaller (radius) one made of carbon & oxygen, and a lighter one made of helium (?), where the more massive star accretes helium from the less massive one.

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Re: Astronomy C

Post by Magikarpmaster629 » November 17th, 2016, 3:54 pm

Unome wrote:
bhavjain wrote:Anyone know what the difference between an AM CVn and x-ray binary system are? Both involve white dwarfs :P

Example of AM CVn: J075141 and J174140
Example of x-ray binary: HM Cancri
As I understand it, an X-ray binary system is any binary star system that emits X-rays (not sure about this one), whereas an AM CVn has two white dwarfs, a more massive, smaller (radius) one made of carbon & oxygen, and a lighter one made of helium (?), where the more massive star accretes helium from the less massive one.
X-ray binaries are binary systems where there is some combination of white dwarfs, neutron stars and black holes.

EDIT: Or could be made up of two individual objects of the three listed above.
Last edited by Magikarpmaster629 on November 17th, 2016, 7:55 pm, edited 1 time in total.

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Re: Astronomy C

Post by bhavjain » November 17th, 2016, 6:09 pm

Magikarpmaster629 wrote:
Unome wrote:
bhavjain wrote:Anyone know what the difference between an AM CVn and x-ray binary system are? Both involve white dwarfs :P

Example of AM CVn: J075141 and J174140
Example of x-ray binary: HM Cancri
As I understand it, an X-ray binary system is any binary star system that emits X-rays (not sure about this one), whereas an AM CVn has two white dwarfs, a more massive, smaller (radius) one made of carbon & oxygen, and a lighter one made of helium (?), where the more massive star accretes helium from the less massive one.
X-ray binaries are binary systems where there is some combination of white dwarfs, neutron stars and black holes.
Pretty sure HM Cancri only has white dwarfs...? Yet is is an xray binary.

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Re: Astronomy C

Post by y_utsumi » December 12th, 2016, 2:01 pm

How can this be solved? I know it has to do with Kepler's laws, but I keep getting a very large answer..

Suppose that the mass of Sirius B is 0.98 solar masses, and the semi-major axis between Sirius A is 7.5 arcsec observed from Earth, and that we are viewing the system face-on. Calculate the period for the Sirius system to complete one orbit in Earth years.
(The previous questions gave the following information: mass of Sirius A = 2.18 solar masses, Sirius parallax (observed from Earth) = 0.38 arcsec, apparent magnitude of Sirius A = -1.47.)

The answer is 48.4 +/- 1.0 years.

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