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hard questions

ros515

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ros515

ros515

Posted
Posted

Hi Everybody! I'm lookin' for answers...

1. The absolute clarity of the star is:

a)brightness, which would be observed if the star is located at a distance of 10 pc from the observer

:D brightness, which would be observed if the star is to be located at a distance of 1 pc from the observer

c)light energy sent by the star in all directions in unit time

d) the total energy sent by the star in all directions in unit time

2. Hertzsprung-Russell diagram indicates the relationship between:

a)absolute brightness of a star and its size

b)absolute brightness of stars and a Stellar classification of the star

c)mass and size stars, and its temperature

d)radiant energy stars and the Stellar classification

3. The process of evolution of stars does not depend on it:

a)mass and density

b)chemical composition

c)temperature

d)angular velocity

:)

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Astronut

Astronut

Posted
Posted

Hi Everybody! I'm lookin' for answers...

1. The absolute clarity of the star is:

a)brightness, which would be observed if the star is located at a distance of 10 pc from the observer

:D brightness, which would be observed if the star is to be located at a distance of 1 pc from the observer

c)light energy sent by the star in all directions in unit time

d) the total energy sent by the star in all directions in unit time

2. Hertzsprung-Russell diagram indicates the relationship between:

a)absolute brightness of a star and its size

b)absolute brightness of stars and a Stellar classification of the star

c)mass and size stars, and its temperature

d)radiant energy stars and the Stellar classification

3. The process of evolution of stars does not depend on it:

a)mass and density

b)chemical composition

c)temperature

d)angular velocity

:)

1. If, by absolute clarity, you mean absolute magnitude then the answer is (a)

2. (:lol:

3. (d)

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narrowbandpaul

narrowbandpaul

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agreed but q2 is interesting...

since the magnitude is related to the flux which is in turn related to the stellar radius and temp through

L=4*pi*r^2*F

where F= sigma *T^4

in terms of axes it would be absolute magnitude versus temp

but variations do exist...luminosity vs temp; lum vs spectral class

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Ben Ritchie

Ben Ritchie

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You can argue question three two way, so I can see either © or (d) being the correct answer (or neither).

The spin of a star certainly does have an effect on evolution in some cases, in particular mass loss from high-mass stars will depend on the rotation rate which in turn critically affects their post main sequence evolution into the Wolf-Rayet phase and possibly - it's not really known - if they go 'bang' before ever becoming a WR. But for other stars it can be fairly insignificant.

You could answer ©, as the temperature can be regarded as a property derived from the evolutionary state of a star. For example, the effective temperature of a LBV undergoing an outburst, or a star moving up the AGB etc. varies rapidly, i'm studying a star that's changed (effective) temperature by more than 10,000K within 20 years at the moment. But that variation in temperature is a product of its continued evolution, not a property that affects it.

Of the two options i'd pick ©, but I don't think that's the answer the question expects. So I think it's a badly worded question, unless it was expected to create debate in a tutorial.

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Ben Ritchie

Ben Ritchie

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what star (stellar type) are you studying?

It's an LBV (Luminous Blue Variable), during outburst it has rapidly changed spectral type from a B2Ia blue hypergiant in its quiescent state to A2Ia during an eruption, which is roughly at the Humphreys-Davidson limit for its luminosity, a little under 10^6 L_sun. That's normal behaviour for an LBV, they're called S-Doradus type eruptions (as opposed to the Eta Carinae type of giant eruption). Something like AG Carinae is a well-studied example. But goes to show that temperature isn't trivially related to stellar evolution.

What I suspect the question meant was the simple zero age main-sequence relationship, i.e. hotter implies more massive, but that's a huge oversimplification :D

Sorry, rather off-topic :)

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Astronut

Astronut

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You can argue question three two way, so I can see either © or (d) being the correct answer (or neither).

...

I took 'angular velocity' to be it's motion in the Sky ie. proper motion and not rotational velocity.

Agree, it's badly worded - as WH points out it's a bad translation.

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Ben Ritchie

Ben Ritchie

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i have heard of the Eddington luminosity but the humphries? can u explain more

In general, when stars age and leave the main sequence they move redwards on the Hertzsprung-Russell and expand, increasing in luminosity in the process - i.e. they become familiar red giants or supergiants. However, if you look at the upper end of the main sequence the behaviour is different. The most luminous stars are all blue hypergiants; there are no red stars with equivalent luminosities.

Quite why this is is still a mystery, but it appears that the most luminous stars cannot become red hypergiants when they leave the main sequence, and once they start evolving redwards instead hit an instability where they literally blow themselves apart, losing mass very rapidly and evolving back bluewards on the HR diagram to a hotter state. The Humphreys-Davidson limit is the empirical "no-go" point for these stars. It's thought to be related to the Eddington limit, as the instability is believed to be driven by radiation pressure, but quite how is still unclear.

Attached is an example that I found online that plots the HD limit against a number of stars that lie close to, or across, it. Stars like IRC +10 420 appear to be able to make it just under the HD limit to become yellow hypergiants, AG Carinae is an example that crosses the HD limit during outbursts before evolving back bluewards, whereas Cyg OB2-12 is something of a mystery at the moment as it appears to be contrary to what the HD limit would suggest should be the case. As I said, it's empirical, and Cyg OB2-12 is a challenge for it.

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narrowbandpaul

narrowbandpaul

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very interesting...

the masive stars really are fascinating...

are u doing a phd?

since F goes as T^4 one would expect the red stars to be less luminous than a hot blue star of the same radius, so to make a red star the same lumonisty it has be be physically larger in size...

so what stops the red stars expanding to this size?

during expansion the stars cool hence why we have the red giants, so the Flux falls as T^4, but the radius grows. Since the luminosity is surface area times flux, to achieve a constant luminosity the star has to expand such that R is proportional to T^2.

Dont know what I'm getting at, but I would have thought that a red hypergiant is harder to achieve, simply due to the temperature being lower.

Very interestiong though..I enjoyed the stellar structure and evolution course did last year

The answer to this question cannot be simple, as it would have been known already...

Cheers

Paul

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narrowbandpaul

narrowbandpaul

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all good mate, sarcasm is hard to put in type, plus I'm actually an idiot!

in some places (unions) a pint is still less than £2...i have some fairly strong viewpoints on unions and bar/clubs offering cheap booze. i wont air these here as I will get chucked off for sure. Its at the very least irresponsible as it encourages binge drinking, which is a culture the young ones buy into. An as anyone who knows me will agree, I distance myself from the drinking crowd..those that go out and get smashed 4 times a week.

But think in two years time, when I am flying airliners, and they are still doing this. Oh how I will laugh.

rant over...

but isnt stellar structure/evolution fascinating

Sorry for misinterpretising your post, like I say..im not all there at times (ask ally)

Best Wishes

Paul

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EA2007

EA2007

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Yeh, I think Ally will vouch for that :),

nah I wasn't being sarcastic btw. Oh you sure didn't go to my union, they did everything possible to stop you drinking there, they had nooo deals and the ony time you could use your NUS for reductions on your drinks were at stupid times like between 5 and 7 on a tuesday evening.

I get what your saying, there are the groups of fools that drink from 2pm to 2am but on a student budget and wanting to have a good night out, sometimes its best if the prices are reduced a little. Our uni was always complaining about not having enough dosh, if they had some sweeteners they would have mad a killing.

I guess I had better get bk to astrophysics here..... staying on an alcohol theme, I used to explain orbital paths and stellar ages etc to chums using beer mats and bottles hee hee...always worked (I think)!

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Ben Ritchie

Ben Ritchie

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during expansion the stars cool hence why we have the red giants, so the Flux falls as T^4, but the radius grows. Since the luminosity is surface area times flux, to achieve a constant luminosity the star has to expand such that R is proportional to T^2.

It depends a bit on where you start on the main sequence, but in general, R grows faster than this at first. So you get a significant increase in luminosity from an increase in radius coupled with lesser cooling. After that evolution across to a red giant is at near-constant bolometric luminosity. So a 20 solar mass star will double its brightness (to around 100,000 times solar luminosity) while evolving from something like O4 to B0, and then evolve from B0 to M0 at near-constant luminosity.

But that doesn't work for a 100 solar mass star, which are an order of magnitude more luminous. They hit the H-D limit at spectral type B0ish, and don't evolve any further redwards ... or, at least, nobody has ever seen any evidence that they do, there aren't any known M-type hypergiants with luminosity of 10^6 solar. As I said, it's an empirical limit deduced from the apparent absence of M-type hypergiants coupled with stars like AG Car evolving just beyond the limit but not continuing redwards.

Quite what happens when they hit the (apparent) instability limit is unknown, but it appears to be a 'messy' process, not a clean symmetric mass-loss. Eta Carinae, for example, shows a number of clumps known as 'Weigelt blobs' that have been ejected from the star, so it's clearly not a homogenious process. But exactly what's happening isn't understood.

are u doing a phd?

Did one some time ago, now using the VLT to study this stuff. Totally different to the backgarden variety, but I enjoy both forms and mark one eyeball's a useful reminder of why i'm interested in the first place, the other stuff can degenate into endless analysis of Fe II multiplet tables and it's easy to forget you're dealing with a star.

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narrowbandpaul

narrowbandpaul

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that sounds really cool ben...

in fact it reminds me how unknown a lot of stellar physics is...

the one I am reffering to in particular is stellar formation...

the jeans mass for collapse, around the star forming epoch is much greater than mass of stars...infact I think it is of order globular clusters...

so it is not clear why stars should form.

I think i am reffering to the first star forming epoch (the one that spwaned the oldest stars), the universe was too hot and dense to allow matter of star size to collapse. the collapsing structures allowed by the jeans mass are much higher.

I think this is right, but a coherent explanation escapes me!

Fascinating work though

Paul

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narrowbandpaul

narrowbandpaul

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hi EA...

im not against drinking...far from it!, just against that peculiar hardcore who seem to equate a good time with getting smashed, and illiteracy with charm!

I do like a good beer, nothing better than chilling out watching The Bill with a nice cold ale!

Just dont see the point in going out getting paralytic, falling over drunk, being sick next morning and having a hangover till 5 the next day.

Nothing wrong with a few down the pub...but 30 units is pushing it a bit.

paul

Hope ally hasnt being slagging me off...haha. Thing is it probably would have been the truth. i lack a bit of common sense..:-)

PS spot the tom lehrer line, that I st..., ehem adapted...and name the song for 5 bonus points

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