B What Makes Red Dwarf Stars Cooler Than the Sun?

[hyunxu]

Red dwarf stars are main sequence stars but they have such low mass that they’re much ' cooler' than stars like our Sun.

Then Simple question here : Sun is cool or hot??

 

[davenn]

Then Simple question here : Sun is cool or hot??

what do you think ?
have you googled something like ...
1) the surface temperature of the Sun or
2) the core temperature of the Sun ?

there's 2 things for you to do an report back with your findings Dave

 

[Ken G]

Although it is a strange term, the Sun is indeed regarded as a "cool star." The basic distinction is the different atmospheric physics you encounter when a star has a convection zone around its exterior, rather than a quiet zone of radiative diffusion there. "Hot stars" typically have convection in their cores, but not so much their exteriors. The discovery of magnetic fields in some "hot stars" has blurred the distinction somewhat, but there is a general tendency for higher mass stars to behave quite differently from lower mass stars, so this basic bimodal distinction between "hot stars" and "cool stars" is likely to persist. (For example, there is a yearly meeting called "
Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun", which more or less answers your question right there!)

 

[stefan r]

The terms cool and warm can be ambiguous. Artists, fashion designers and painters would say that "cool" colors have more blue in them. This is the opposite of a star's surface temperature.

 

[hyunxu]

The terms cool and warm can be ambiguous. Artists, fashion designers and painters would say that "cool" colors have more blue in them. This is the opposite of a star's surface temperature.

Quite different

 

[snorkack]

Red dwarf stars are main sequence stars but they have such low mass that they’re much ' cooler' than stars like our Sun.

Then Simple question here : Sun is cool or hot??

A simple question with no simple answer.
What is "cool" and what is "hot"?
Of the 93 brightest stars, 28 are cooler than Sun, 2 are about equal (Sun itself and Alpha Centauri A), 63 are hotter.
Of the 64 nearest stars, 6 are hotter than Sun, 2 are about equal (the aforesaid), 56 are cooler.

 

[Ken G]

Nevertheless, "cool" and "hot" are not really arbitrary distinctions for stars, they are an effort to notice that stars can be divided into two very different flavors. Of course any such division is artificial, and more divisions can certainly be made, but in a coarse sense you have stars that are more massive, less convective near the surface, and ultimately have core-collapse supernovae, and stars that are less massive, more convective near the surface, and become white dwarfs. So that's "hot" versus "cool" in a nutshell, though even the massive stars can puff out so much as they evolve that they temporarily become "cool" and develop surface convection. In that way of slicing things, the Sun is a "cool star." It's about helping researchers group together objects that may have similar physics that they are interested in, more so than some arbitrary line in the sand of surface temperature.

 

[stefan r]

... you have stars that are more massive, less convective near the surface, and ultimately have core-collapse supernovae, and stars that are less massive, more convective near the surface, and become white dwarfs...

Wikipedia on Wolf-Rayet stars:

Their surface temperatures range from 30,000 K to around 200,000 K, hotter than almost all other stars.

Massive main-sequence stars create a very hot core which fuses hydrogen very rapidly via the CNO process and results in strong convection throughout the whole star.

For example V429 Carinae has surface temperature 44,700K and is "

... one of the most massive and most luminous stars known...strongly convective all the way to the core and have dredged up fusion products to the surface

 

[Ken G]

The general trend is that if you start at the coolest surface temperatures and start increasing it, at first the core gets more and more convectively stable, while the outer layers remain convectively unstable. This continues until the outer convection zone gets smaller and smaller and eventually nearly disappears, but then the core starts to become unstable. At the highest surface temperatures (like the Wolf-Rayet phase), the core convection zone has expanded to include most of the star. But the pattern generally holds-- high surface temperature implies core convection, low surface temperature implies envelope convection. At the highest end, the core convection merely encompasses most of the star, and at the lowest end, the envelope convection encompasses most of the star. But the main point is, the people who are most interested in how convection affects the atmosphere are generally people who study cool stars, and it is far from established that all Wolf-Rayet stars have convection zones that reach the surface. Indeed, the surfaces of last scattering in Wolf-Rayet stars tend to be found within a supersonic wind, which is certainly not convective.

 

[snorkack]

Nevertheless, "cool" and "hot" are not really arbitrary distinctions for stars, they are an effort to notice that stars can be divided into two very different flavors. Of course any such division is artificial, and more divisions can certainly be made, but in a coarse sense you have stars that are more massive, less convective near the surface, and ultimately have core-collapse supernovae, and stars that are less massive, more convective near the surface, and become white dwarfs.

They are grossly arbitrary, because more distinctions can be made.
Counting nearby stars, I found that roughly half of them are hotter than M3.
It seems that it is roughly around M3...M4 that stars hotter than this possesses radiative stagnant core and accumulate some helium isotopes in core. Thus, all stars hotter than M3, including Sun, are hot stars in one important sense - but just one of many possible senses, therefore arbitrary.

 

[Ken G]

But all distinctions are arbitrary, it doesn't make them useless. Most of the stellar astrophysics community is divided into "hot star" and "cool star" studies. Better if everyone went to the same conferences, but then they'd just be too big and varied.

 

[stefan r]

...it is far from established that all Wolf-Rayet stars have convection zones that reach the surface. Indeed, the surfaces of last scattering in Wolf-Rayet stars tend to be found within a supersonic wind, which is certainly not convective.

The sun has coronal holes which are not convective. The corona further complicates the hot vs cool question.

The temperature of a star is a real number. There is no obvious gap in the hertzsprung-Russell diagram. The transition from not having a core to having a tiny core is very difficult to see. Somewhere in the M3 to M4 range. CNO burning of hydrogen is taking place in the Sun but at a rate too slow to cause core convection. You could find an overlap where a star has some core convection and some surface convection.

But all distinctions are arbitrary, it doesn't make them useless. Most of the stellar astrophysics community is divided into "hot star" and "cool star" studies. Better if everyone went to the same conferences, but then they'd just be too big and varied.

Are they divided by star types or by telescope/detector type? It would also be reasonable for Astronomers to group by the age of the stars they are studying. It is hard to see M-dwarfs that are far away.

Would a study of the Orion Nebula go to the "hot star" conference or the fit in with the "cool" astronomers?

 

[Ken G]

Are they divided by star types or by telescope/detector type?

Subfields are generally divided by star type, but astronomers often characterize their expertise by wavelength. It's odd, but you often find "radio astronomers" attending a "cool star meeting", or "X-ray astronomers" attending a "hot star meeting," or vice versa. The lines have historical purposes, but aren't always logical.

Would a study of the Orion Nebula go to the "hot star" conference or the fit in with the "cool" astronomers?

Possibly "hot stars," possibly "interstellar medium." If you are interested in the stars that illuminate the nebula, or the history of the supernovae that made it, you might hear about these at a "hot star" meeting. I doubt cool star astronomers care much about the Orion nebula!

 

[snorkack]

Possibly "hot stars," possibly "interstellar medium." If you are interested in the stars that illuminate the nebula, or the history of the supernovae that made it, you might hear about these at a "hot star" meeting. I doubt cool star astronomers care much about the Orion nebula!

They found a lot of young cool stars besides the young hot stars. Including (or excluding?) an eclipsing binary of two young brown dwarfs.

 

[Ken G]

Good point, I guess the real message is the unity of astronomy, and how the distinctions we make for convenience should never let us forget that it's all one cosmos.

 

[rootone]

I guess it is possible for two Jupiter sized objects to merge when a planetary disk collapses.
At that stage fusion starts, and it can be called a distinct star and not just a nebula..
The Sun is a bigger star than average, yes it is hotter than small stars.
It is not exceptionally bigger and hotter though, there are blue supergiants. although they are rare

 

[Ken G]

They would need to be quite some Jupiters-- our Jupiter has a mass that is 80 times too small to start fusion. But two of the largest exoplanets ever discovered (https://en.wikipedia.org/wiki/Exoplanet#/media/File:ExoplanetPopulations-20170616.png) could probably do it.

 

[stefan r]

They found a lot of young cool stars besides the young hot stars. Including (or excluding?) an eclipsing binary of two young brown dwarfs.

A paper link. Also a http://hubblesite.org/video/518/news_release/2006-11 from NASA.

They also found the temperatures reversed. The more massive brown dwarf is cooler. This could be caused by spots.

If you are interested in cold spots on hot stars do you go to the cool conference or the hot conference?

 

[Ken G]

If you are interested in cold spots on hot stars do to go to the cool conference or the hot conference?

Mind blown.

 

[Erribert]

I assume you are referring to the hottest part of the sun, which lies above the surface of the sun. As we get to the surface, temperatures drop significantly. Observations suggest that below the surface temperatures drop even more. Indeed, looking into holes indicates lower temperatures.

According to emmited heat, the consensus seems to be that our sun may be average or slightly below. It is about at the median. This is also complicated by the definition of a star.

There appears to be no consistent temperature throughout the sun and it is very difficult to get emissions deep in the sun. So, the answer is difficult. For most other stars I think we only measure emission temperatures.

At least that is my understanding.

 

[stefan r]

According to emmited heat, the consensus seems to be that our sun may be average or slightly below. It is about at the median. This is also complicated by the definition of a star.

76.45% of main sequence stars are classified as M-type. So median temperature is much lower than the sun. If you list the brightest visible stars then the sun is closer to median.

The milky way looks redder than the zodiacal light. That is not a precise number but you can collect the ballpark data yourself on a clear night using no equipment (if you can get away from city lights).

I assume you are referring to the hottest part of the sun, which lies above the surface of the sun. As we get to the surface, temperatures drop significantly. Observations suggest that below the surface temperatures drop even more. Indeed, looking into holes indicates lower temperatures...
...There appears to be no consistent temperature throughout the sun and it is very difficult to get emissions deep in the sun. So, the answer is difficult. For most other stars I think we only measure emission temperatures...

Most astronomers use the star as a point source. For far away stars it is impossible to separate the corona and photosphere except during eclipses. The temperature is determined by the spectrum. A star's spectrum is compared to radiation from a black body.

Core temperatures in red giants are much higher than core temperatures in similar mass main sequence stars. Red giants are considered cooler stars. The classification is entirely based on the light that is observed.

 

[snorkack]

76.45% of main sequence stars are classified as M-type. So median temperature is much lower than the sun. If you list the brightest visible stars then the sun is closer to median.

No.
On the other side of the median.
93 brightest stars:
https://en.wikipedia.org/wiki/List_of_brightest_stars
include:
6 M stars (all giants or supergiants)
22 K stars (ditto)
2 G stars (both dwarfs)
Grand total of 30 stars as cool or cooler than Sun, including Sun itself, and 63 hotter.
Obviously Sun is a cool star.
As just pointed out, by another criterion Sun is a hot star.

 

[stefan r]

No.
On the other side of the median.
93 brightest stars:
https://en.wikipedia.org/wiki/List_of_brightest_stars
include:
6 M stars (all giants or supergiants)
22 K stars (ditto)
2 G stars (both dwarfs)
Grand total of 30 stars as cool or cooler than Sun, including Sun itself, and 63 hotter.
Obviously Sun is a cool star.
As just pointed out, by another criterion Sun is a hot star.

There are 9,500 objects visible to (some) human eyes, magnitude 6.5. You could also use stars within 10 parsecs. This list has 4 As , 6 Fs ,22Gs and 35Ks

 

[snorkack]

There are 9,500 objects visible to (some) human eyes, magnitude 6.5. You could also use stars within 10 parsecs. This list

... which would give very different results!

Is Sun bright or dim?
See:
http://www.atlasoftheuniverse.com/stars.html
Of the brightest 50 (49 of the list, plus Sun), all of them apparent magnitude +2,00 or less, 10 have positive absolute magnitude. 11 have absolute magnitudes under -5.

 

[hyunxu]

This star regulus is hotter than sun.We conclude that blue stars are hotter than red and Orange.Why only "blue" stars make it

 

[russ_watters]

This star regulus is hotter than sun.We conclude that blue stars are hotter than red and Orange.Why only "blue" stars make it

Make what?

 

[hyunxu]

Make what?

Why only blue stars are hotter than red and Orange? What's the actual relationship between this "blue colour " and "hotness"

 

[russ_watters]

Why only blue stars are hotter than red and Orange? What's the actual relationship between this "blue colour " and "hotness"

https://en.wikipedia.org/wiki/Wien's_displacement_law

 

[rootone]

This star regulus is hotter than sun.We conclude that blue stars are hotter than red and Orange.Why only "blue" stars make itView attachment 226999

'Hotter' means more energetic photons are being emitted.
Usually this is a simple consequence of how massive the star is.
However other factors such as the composition of the star's elements play a part in stars luminosity profile.

 

[snorkack]

'Hotter' means more energetic photons are being emitted.
Usually this is a simple consequence of how massive the star is.
However other factors such as the composition of the star's elements play a part in stars luminosity profile.

And internal structure. Red giants may be a modest fraction of all stars, but they are a large part (though not a majority) of bright ones.

 

[hyunxu]

'Hotter' means more energetic photons are being emitted.
Usually this is a simple consequence of how massive the star is.
However other factors such as the composition of the star's elements play a part in stars luminosity profile.

So Does hotter star mean a massive star?

More hotter the star , more massive it is?
Or

More massive the star, more hotter it is?

 

[rootone]

In general yes, more massive stars emit much more energetic EM radiation (= hotter).
One consequence of this is that more massive stars have short lives compared to smaller ones.

 

[hyunxu]

One consequence of this is that more massive stars have short lives compared to smaller ones.

Yes. That's right. By the equation e= mc^2 , every second , the Sun slims down by 5 million tonnes. This may sound like a huge amount , but during the life of the Sun it only amounts to 0.01% of its mass. So Is Sun's mass average?

(As hotness depends on mass)

 

[snorkack]

So Is Sun's mass average?

Average over what?

Of the 50 brightest stars, 14 are dwarfs. All of them are more massive than Sun, save Sun itself.
Of the remaining 36 subgiants, giants, bright giants and supergiants, 12 are G or cooler. The rest are F or hotter.

All the dwarfs brighter than Sun are also more massive. And hotter. On main sequence, it is fairly consistent tat a more massive star has bigger total luminosity, higher temperature as well as larger area.
About the giants: as stars exhaust part of their fuel (protium in centre first), they tend to fuse the remaining fuel at increasing rate. At equal mass, an older star is brighter.
The temperature varies widely. Many old stars cool down - they become bright, but acquire a large area and low temperature. Yet most giants are still hotter than Sun.
Most of the giants that are bright in sky are still more massive than Sun. Not all. Some, notably Arcturus, are suspected of being only as massive as Sun, but older, brighter, bigger and cooler.

 

[hyunxu]

Some, notably Arcturus, are suspected of being only as massive as Sun, but older, brighter, bigger and cooler.

Here mass is same , but why does the area varies?

 

[snorkack]

Here mass is same , but why does the area varies?

Because the luminosity varies.

 

[hyunxu]

Because the luminosity varies.

High luminosity or low luminosity ?
How surface area depends on the luminosity?

 

[snorkack]

High luminosity or low luminosity ?
How surface area depends on the luminosity?

As the core of the star produces increasing amounts of heat, the surface layers of the star must emit the light somehow.
They might emit it by remaining the same size and getting hotter. But gases expand on heating, so surface area necessarily increases at least somewhat.
For red giants, the expansion is actually so large that the star finally comes to equilibrium where its surface area has increased even more than luminosity and the surface temperature has therefore fallen.

 

[stefan r]

Here mass is same , but why does the area varies?View attachment 227071

The core of Arcturus is not the same as the core of the sun. When astronomers say "temperature of a star" they mean the observable surface of the star. The Sun's corona is much hotter(5 million°K) than the Sun's photosphere(5772°K). The Sun's core is closer to 16 million°K. The hydrogen at the core in Arcturus burned up. Hydrogen fusion occurs in a shell around a much smaller core made of mostly helium. The core in red giant branch stars is less than around 100 to 200 million °K. When the temperature gets higher the helium will fuse and Arcturus will move to the next stage.

Because Arcturus has a hotter core the fusion rate in the shell is higher than the fusion rate in the Sun's core. The surfaces of both stars are held up by radiation pressure. Increased reaction rates inflate the star and makes the surface puff outward. If you take any gas and compress it you change the temperature. Gas law. Arcturus has around 25 times the Sun's radius so around 16,000 times the volume. If you created a sample of Arcturus gas in a laboratory and then you compacted it to the density of the Sun's gas you would expect the temperature/pressure to rise by a factor of 16,000 if it was an "ideal gas". The internal structure is different and the gases are not ideal but the general idea applies. Air conditioners and refrigerators work using temperature change in a gas that expands and compresses. If you took a sample of solar gas at 5772°K in a laboratory container and expanded to 16,000 times the volume the temperature will be much lower than Arcturus (4286°K).

 

[Ken G]

Because Arcturus has a hotter core the fusion rate in the shell is higher than the fusion rate in the Sun's core. The surfaces of both stars are held up by radiation pressure. Increased reaction rates inflate the star and makes the surface puff outward.

You're right that there is a high shell fusion rate in Arcturus, but neither the Sun nor Arcturus experiences any significant radiation pressure anywhere in the star. It's well over 99% gas pressure.

 

[davenn]

The Sun's corona is much hotter(5 million°K) than the Sun's photosphere(5772°K).

ohhh and by the way, just for your physics learning ...
It isn't deg Kelvin, it is just plain Kelvin ... 5 million K ... 5772 K etc

Dave

 

[hyunxu]

Still we say that radiation from Sun is dangerous. But what about the radiation from the star which is hotter than Sun? Is there any relationship between hotness and radiation?

(Still it's about hotness of the sun)

 

[glappkaeft]

It depends on which radiation you are talking about. Electromagnetic or particle? If EM what frequency are you discussing.? If particle, which particle? Are you concerned about average levels or peaks associated with things like solar flares?

 

[Drakkith]

Still we say that radiation from Sun is dangerous. But what about the radiation from the star which is hotter than Sun? Is there any relationship between hotness and radiation?

(Still it's about hotness of the sun)

The hotter the star, the more UV radiation is emitted. For example, at 5,000 K the Sun emits about 3.8 W/sr/m² at a wavelength of 100 nm, while a star at 20,000 K would emit almost 9x10⁹W/sr/m² at the same wavelength. That's about a 3-billion-fold increase. It's even 1,000 times more than the Sun emits at 500 nm, which is approximately green light. Basically, if you go outside at noon on a sunny day and feel the heat from the Sun, a star at a temperature of 20,000 K would put out more than 1,000 times that much energy just in the UV portion of the spectrum.

 

[rootone]

what about the radiation from the star which is hotter than Sun?

Sun bathing somewhere near a blue giant isn't anything to worry about really.
All of your water molecules would be disassociated in less than a microsecond.