Astronomy C

[raxu]

I have a question about HL Tauri: how old is it exactly? The webinar says it is farther along, about 1 million years old, while wikipedia says it is 100 thousand years old (this may seem like a big difference, but in astronomy we make big mistakes ).

[sciolymom]

The webinar slides have now been posted: https://www.soinc.org/astronomy_c.

Thanks for catching that, /u/sciolymom!

THANK YOU!

[Adi1008]

Well, on ZAMS they obviously don't expand because hydrostatic equilibrium. Past that you build up Helium -> higher fusion rates nearby the core (which I can explain if you want) -> higher L + expansion (more expansion happens than L inc) -> lower SURFACE T (as you suggest from the expansion). I guess they are saying that after ZAMS you still get the He build-up happening (which is true, just it's not a pure, inert He core until it hits the RGB).

I think the wording from that webpage is a bit funny, though maybe it is better (I guess it's perspective on how we should simplify things). Unless someone else has better input...either way, though. Your main question was along the main sequence what happens, and I think there are two points that. One is what happens when you look at the whole main sequence by mass, which follows from what was already mentioned. The other is what happens for an individual star, one would become cooler and have L changes etc etc. I do hope nobody tests in such a way about confusing terms like this, though...especially considering the topic is on exoplanets and pre-main sequence stuff anyway >.>.

Just to explain myself in case you were confused, I kept mentioning the sub-giant branch because if you look on your HR diagram and compare it to other stellar evo tracks, it seems a lot like the sub-giant branch...which makes me think of it like the core building up to that pure inert He core we know and love of red giants, but then this gets confusing (like wiki says sub-giants all have pure He at their core). I'd say be aware that the SGB is a thing, and just think of the main sequence like the ZAMS unless having to think of otherwise.

In astro you have to look at multiple things at once, but I hope I distinguished the concerns here (unless there's something I missed...).

I was under the impression that when on the main sequence, the core contracts while the outer layers expand slightly. As more and more hydrogen gets fused into helium, the amount of particles supporting the core decrease, which means there's less pressure holding up the core. The core contracts, which heats up the core and leads to a faster rate of core hydrogen fusion. This increases the pressure on the outer layers from the core, causing them to expand slightly. since the radius only expands slightly, the surface temperature must also rise (as shown by http://i.imgur.com/XVpnRby.png)

Also, doesn't shell hydrogen fusion ("higher fusion rates nearby the core") occur when the star is off the main sequence?

especially considering the topic is on exoplanets and pre-main sequence stuff anyway >.>

lol yeah I don't expect this to be a huge help in astro this year at least. I'm just learning it because I've never had a good understanding of stellar evolution (since Solar System has essentially no stellar evolution in it) and I've wanted to understand it for some time now.

Thanks for all of your help syo

The webinar slides have now been posted: https://www.soinc.org/astronomy_c.

Thanks for catching that, /u/sciolymom!

Thanks!!

on the answer key to the 2014 Pennsylvania state finals exam in astronomy, the correct answer to number 59-according to the key is 5.35 E36, this number seems incorrect, did anyone else notice this?

For what it's worth, every time I try and solve the question I get . I also don't know about #60 - isn't the total energy in an orbit just , which is half of the potential energy? On the answer key it seems to be about a fourth.

I have a feeling that the given periastron or apastron might be wrong. Taking a quick look at Wikipedia, the eccentricity of HD 1237b is 0.511 and the semi-major axis is 0.49 AU. Solving a system of equations to find the apastron and periastron, you get them to be 0.74 and 0.24 AU, respectively.

Additionally, even if they were 1.08 and 0.35 AU, isn't the average 0.715, not 0.716? Where does the 6 come from?

[Magikarpmaster629]

Do astronomy tests (in general) care about semantics between T Tauri stars and Herbig Ae/Be stars (or for that matter, FU Orionis stars)? I ask because wikipedia calls HL Tauri a T Tauri, wheras Simbad calls it a Herbig Ae/Be.

[syo_astro]

Adi, if you're still wondering we can talk it over to hash out the whole thing and make it less confusing (hey, it's good review for me).

Magikarp, as a proud research of HAEBE stars, I can tell you HL Tauri is not an HAEBE star. And yes, I am 95% sure about this at least to current knowledge because none of the identifiers are in my spreadsheet of all known HAEBE stars...SIMBAD makes mistakes, but so does Wikipedia. Also, I've just heard about HL Tauri here and there, and I believe it's a very famous T Tauri star like TW Hydrae.

For your question on distinctions I quote from 3a that you need to know about T Tau variables, FU Ori variables, and HAEBE stars. This includes "The processes and stages of stellar evolution, including spectral features and chemical composition, luminosity, blackbody radiation, color index, and the H-R diagram with an emphasis". For me, I would say that means you should be able to distinguish those features using the appropriate diagrams and tools mentioned in the rules, but how much you need to know changes by the level (invites, regs, states, nats) for sure. At regionals maybe you can get away with knowing it's a pre-main sequence star, but at states or nats or a hard invite you'd probably need to know what distinguishes the class and more specific distinctions.

[Magikarpmaster629]

I'm looking for information on the formation of planets around white dwarfs and neutron stars, can someone direct me to a source?

[Adi1008]

tfw MIT is in only a few weeks T_T_T_T_T_T_T_T_T_T_T_T_T_T

I'm looking for information on the formation of planets around white dwarfs and neutron stars, can someone direct me to a source?

I haven't found anything, sorry :/

Do astronomy tests (in general) care about semantics between T Tauri stars and Herbig Ae/Be stars (or for that matter, FU Orionis stars)? I ask because wikipedia calls HL Tauri a T Tauri, wheras Simbad calls it a Herbig Ae/Be.

I actually wasn't sure about this too, but I'd say it would be a T Tauri star like what syo said

Adi, if you're still wondering we can talk it over to hash out the whole thing and make it less confusing (hey, it's good review for me).

Magikarp, as a proud research of HAEBE stars, I can tell you HL Tauri is not an HAEBE star. And yes, I am 95% sure about this at least to current knowledge because none of the identifiers are in my spreadsheet of all known HAEBE stars...SIMBAD makes mistakes, but so does Wikipedia. Also, I've just heard about HL Tauri here and there, and I believe it's a very famous T Tauri star like TW Hydrae.

For your question on distinctions I quote from 3a that you need to know about T Tau variables, FU Ori variables, and HAEBE stars. This includes "The processes and stages of stellar evolution, including spectral features and chemical composition, luminosity, blackbody radiation, color index, and the H-R diagram with an emphasis". For me, I would say that means you should be able to distinguish those features using the appropriate diagrams and tools mentioned in the rules, but how much you need to know changes by the level (invites, regs, states, nats) for sure. At regionals maybe you can get away with knowing it's a pre-main sequence star, but at states or nats or a hard invite you'd probably need to know what distinguishes the class and more specific distinctions.

Thanks syo

[sciolymom]

Regarding 2MASSJ22282889-431026, there is info on the wiki page giving right ascension/declination etc. Does anyone know where this was found? I'm having a hard time finding any real data on that object, just news stories. I searched in Simbad even and got nothing.

Thanks!

[andrewwski]

on the answer key to the 2014 Pennsylvania state finals exam in astronomy, the correct answer to number 59-according to the key is 5.35 E36, this number seems incorrect, did anyone else notice this?

For what it's worth, every time I try and solve the question I get . I also don't know about #60 - isn't the total energy in an orbit just , which is half of the potential energy? On the answer key it seems to be about a fourth.

I have a feeling that the given periastron or apastron might be wrong. Taking a quick look at Wikipedia, the eccentricity of HD 1237b is 0.511 and the semi-major axis is 0.49 AU. Solving a system of equations to find the apastron and periastron, you get them to be 0.74 and 0.24 AU, respectively.

Additionally, even if they were 1.08 and 0.35 AU, isn't the average 0.715, not 0.716? Where does the 6 come from?

Looks like this is from the 2015 PA test, but yeah, I just checked and your answer for 59 (-3.7e36 J) looks right. Total energy is only half the potential energy for a circular orbit though. The eccentricity of that orbit is 0.51, so at periastron you need to divide by (1-e), which gives -1.51e37 J.

If you assumed total mechanical energy was -5.35e36 J as in the key, then you get the answer in the key for 60 (-2.19e37 J) - but this is based off an incorrect answer for 59.

[Adi1008]

Regarding 2MASSJ22282889-431026, there is info on the wiki page giving right ascension/declination etc. Does anyone know where this was found? I'm having a hard time finding any real data on that object, just news stories. I searched in Simbad even and got nothing.

Thanks!

I haven't really found anything concrete about it except for press releases too :/

on the answer key to the 2014 Pennsylvania state finals exam in astronomy, the correct answer to number 59-according to the key is 5.35 E36, this number seems incorrect, did anyone else notice this?

For what it's worth, every time I try and solve the question I get . I also don't know about #60 - isn't the total energy in an orbit just , which is half of the potential energy? On the answer key it seems to be about a fourth.

I have a feeling that the given periastron or apastron might be wrong. Taking a quick look at Wikipedia, the eccentricity of HD 1237b is 0.511 and the semi-major axis is 0.49 AU. Solving a system of equations to find the apastron and periastron, you get them to be 0.74 and 0.24 AU, respectively.

Additionally, even if they were 1.08 and 0.35 AU, isn't the average 0.715, not 0.716? Where does the 6 come from?

Looks like this is from the 2015 PA test, but yeah, I just checked and your answer for 59 (-3.7e36 J) looks right. Total energy is only half the potential energy for a circular orbit though. The eccentricity of that orbit is 0.51, so at periastron you need to divide by (1-e), which gives -1.51e37 J.

If you assumed total mechanical energy was -5.35e36 J as in the key, then you get the answer in the key for 60 (-2.19e37 J) - but this is based off an incorrect answer for 59.

If you use the semimajor axis of an elliptical orbit, isn't it the same as using the radius of a circular orbit? I don't know why, but I've heard that it has something to do with the energy of an orbit staying constant. Not sure though, but thanks a ton!