A question about star's luminosity, temperature and mass.

Vanadium 50

Yes. Technically, it's more luminous. The sun is brighter, but that's because it is closer.

qraal

And the opacity. In brown dwarfs the formation of dust and clouds obscures the purer black body output of very hot stars. In white dwarfs the thermal resistance of the crust means the star can have an average internal temperature much, much higher than its photosphere. Effective temperature is the equivalent purely blackbody temperature for a given luminosity and photospheric area and is usually pretty close to the spectral temperature. But deviations are what make astronomy interesting/challenging.

~jet

Sure; BG is more luminous but due to its bloated dimensions, that is spread out so much more dramatically than either the sun or proxima centauri... you average down (for lack of a better term) to a lower, redder, cooler temperature despite the greater output.

I am curious... I've ready that at outset, a star born here in this galaxy and at this time is going to have X percent hydrogen and Y percent helium and trace amounts of heavier castoffs. When all that aggregates to the point of fusion, (assuming a modest sized star) we speak of it as if it is only fusion hydrogen. Is that technically true? Is there no helium being fused at all until it reaches the end of the main sequence? My mind wants to default to believing that this is occurring in a gradient of a sort, rather than sudden flips of a switch where internal shells of alternately fusing substances materialize.

qraal

Each "step" has radically higher temperatures and densities at which its probability of occurring rises sufficiently to actually occur, plus each successive set of reactions involves heavier elements that can sink deeper - the heavier fusion "ash" of one reaction becomes the fuel of the next layer downwards because of that.

~jet

Of course; I'm familiar with the concept of shells of fusing different elements... what I wasn't as certain about is what is there that prevents a little helium from being fused along the way prior to the formation of the first helium core? I have some difficulty perceiving the mechanisms by which, SNAP, all of a sudden a helium core spontaneously exists.

So the heavier material sinks; that makes sense... it accumulates... does it reach, maybe, some critical mass where it begins to fuse? It just seems to me that at the center of a star, given higher temps and pressures, some of it would be fusing prior to the formation of a shell.

We should all go dissect a star. =) I don't think the natives will mind.

CoolYoungStar

Hello Jet, I've been reading along and I think I have a few answers. First, helium most certainly burns in the sun; however, the core is not hot enough to support large amounts of this cycle. I believe the p-p chain is responsible for 98% of the suns energy while the CNO is second with ~1.5%. Here is my source: http://neutrino.aquaphoenix.com/un-e...-chapter4.html

~jet

Hah! Much appreciated! I shall investigate.

turboguppy

Unfortunately the very first line on that source is not correct.

The sun does NOT shine because of fusion. The sun shines because it is hot. Fusion is a by-product and replenishes lost energy so that the sun may shine for a long time, but fusion doesn't cause the sun to shine.

I'm suspicious of the rest of the page as well... Most of the references are 10-20 years old. I think qraal's answer covered it. The temp and pressure must reach a critical value before helium fusion starts, and if I remember right this is very much a sudden process, hence the 'helium flash.'

Some of the more experienced members might be able to correct me though, if I have misunderstood this concept.

twofish-quant

Not true. Stars are not black bodies. You can approximate the output of a star by a black body to get a rough approximation, but they aren't. In particular, population II stars have heavy iron lines that cause their energy distribution to be wildly different than population I stars.

One problem here is that you end up with multiple definitions of "temperature", "luminosity" and "radius." You can *define* an effective temperature which is the temperature the star would have if it were a black body with the radius being the photosphere, but that effective temperature can be quite different from temperature defined some other way (for example, if you try to curve fit the spectrum against a BB spectrum).

Wikipedia does a good job explaining it

http://en.wikipedia.org/wiki/Effective_temperature
http://en.wikipedia.org/wiki/Color_temperature
http://en.wikipedia.org/wiki/Brightness_temperature

Also in astronomy the term luminosity is qualified with the wavelength of the luminosity. Bolometric luminosity is the totally energy output of the star at all wavelengths. Also talking about the "radius" of the star gets you into interesting questions. Stars are gaseous which means that there isn't a hard surface like there is on the earth. What people talk about when they talk about radius is the photosphere which is the point at which the star becomes opaque.

Chronos

There is no good way to calculate the size of a stars photosphere based on luminosity that I know of - too many variables.

twofish-quant

That's a semantic thing. If you turn off the fusion engine, the sun will go cold after a few thousand years.

Yup, but most of the processes were worked out in the 1940's, and solar mass nucleosynthesis hasn't changed in 10-20 years. The one place where the page is out of date is the last paragraph where it talks about the solar neutrino problem which has been solved.

twofish-quant

The problem is that CNO is a hydrogen-burning cycle not a helium burning one.

twofish-quant

Ignition temperature. A lot of these processes are very sensitive to temperature which means that if you are below a threshold, nothing happens, but once you cross it, the reaction rate increases by a huge amount. What's more the reactions release energy which cause everything to increase even more.

A good analogy is cooking oil. If you keep the oil below a critical temperature, nothing happens to it, but once you cross the limit, it smokes and ignites.

Critical temperature/pressure.

Depending on how hot the center is, it doesn't hit the critical temperatures.

Been done. Google for helioseismology and astroseismology

twofish-quant

Also there are two H-R diagrams. Theorists work with bolometric luminosity versus effective temperature. You run the computer model, that gives you energy output and a radius were pressure goes to zero, you then calculate an effective temperature.

Observers work with color versus absolute magnitude. You measure the brightness of the star with two colored filters and plot on one axis, you take the visual magnitude correct for distance, plot on the other axis.

These two scales are roughly the same, but if you want to do precision work, then you have to apply all sorts of correction terms. In the observers diagram you are measuring color temperature whereas in the theorists diagram, you are measuring effective temperature. People go spend a ton of effort getting all this right, see..

http://iopscience.iop.org/1538-3881/...0160.text.html

The pattern in which everything becomes more complicated when you look at it closely is common thing in astronomy. For example if I ask you want time is it, do you mean TCG, TCG, UT, or TAI?

JeffKoch

Luminosity in what energy band? A star isn't a blackbody of course, it has a distribution of temperatures and opacities at different depths and so does not have a single defined "temperature" and "surface".

~jet

What I am most interested in is having an acceptable approximation of luminosity, radius, surface temperature (and thus color) based on initial mass and approximate age. I'm not coming at the problem based on observed stars, but on initial mass.

Someone else 'round here mentioned metallicity influencing the visible output/temperature of a star, and that seems fair... but to what level? a percent? Are there terms I can research that would lead to understanding or measuring likely metallicity based on initial environment? or to what effect and degree their presence incurs?

robertrej

I have been finding formula's for just about everything about stars and
was wondering if core and surface temperature has a formula as well?

Its 2,771,043 for the Sun as an example.( core / surface)kelvin.


Thanks
Bob