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# Surface Temperature of the Sun

03/12/2010 12:33 AM

I was asked this question by some friends. Because it is cold in outer space and Mercury is so close. "Why doesn't the planet Mercury burn up? And why is cold in outer space? "

Yet we are warmed by the Sun. and we feel most heat in direct sunlight. And how do we measure the surface temp anyway? Not being sarcastic but it is kinda funny if you think about it from a child's perspective.

I have my own theories but would like some other thoughts or knowledge...

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#1

### Re: Surface Temperature of the Sun

03/12/2010 1:18 AM

It is all about surface area. If you look at the sun as a point in space with an infinite number of rays radiating out in all directions, the closer you are the more rays intersect your given surface area.

If the face of Mercury makes an arc 2º wide at its distance, the earth might be .25º at it's distance (these are just examples, I know the degrees are alot smaller) then the earth would absorb much less of the sun's rays than Mercury.

If you measured the amount of energy given by the sun for a set degree of arc you could compute how much energy a given planet could absorb. It is much like solar panels, the bigger solar panel, the more electricity it can provide.

As for why Mercury doesn't burn up, burning rocks is tough.

Someone with more knowledge might give better figures, but I think this explains the concept.

Drew

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#2

### Re: Surface Temperature of the Sun

03/12/2010 4:23 AM

First of all, the surface temperature of our sun is about 5500°C. (Interestingly enough, the temperature of the solar corona is about 5,000,000°C.) The surface temperature of the sun is directly related to it's color type. Our sun is a type G2, and all similar stars will be the same color and temperature. A star like Rigel on the other hand, a type B8 blue super-giant 17 times as massive as the sun, has a surface temperature of 11,000°C.

However, what is really important is the solar flux, which is to say, how much energy the sun is radiating. According to what I've been able to find, the sun radiates at a rate of about 9x107 watts per square meter of it's surface, for a total output of about 3.84x1026 watts. (Rigel's total output is 66,000 times that of the sun.)

Now remember, that energy radiated outward from any source obeys the Inverse Square Law, which simply states that the intensity of the energy reduces with the square of the distance. So an object such as a planet which is twice as far away as another planet will receive one fourth as much energy as the closer body, per unit area.

What this means is this. Here at Earth's orbit, solar flux is about 1.4Kw/m2. Mars is roughly 1.5 times as far out from the sun as Earth, so it follows that it's solar flux is 1/1.52 times ours, roughly .6Kw/m2.

Mercury on the other hand orbits at about 3/8ths of the distance that we do, which means that it receives the better part of 7 times the flux we do, roughly 9.4Kw/m2. Pretty fierce stuff, but nothing like the surface intensity of the sun itself. Also remember that this is a sphere. While the surface temperature at the equator can hit 430°C, this drops off to -170°C at the poles. So even at it's hottest, it's nowhere near hot enough to melt rock or metal.

As to why space is cold, this is because it has very little mass to heat. Heat may be thought of as measure of the energy contained in a mass. But in the absence of stuff to heat, there can be no temperature. Which can bring on problems all it's own.

For example. An automobile engine generates a tremendous amount of waste heat, which is carried by the coolant to the radiator, where it passes that heat to the surrounding air by conduction. But the only way a spacecraft can get rid of waste heat is to radiate it away as electromagnetic energy, since there is no air to conduct it to. And of course the spacecraft will be absorbing solar energy, which must be either used or gotten rid of as waste heat, though this is much less of an issue the further you get from the sun.

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#5

### Re: Surface Temperature of the Sun

03/12/2010 8:59 PM

VERY WELL SAID!! You are reminding me of my old teacher, who was a great guy.

Thanks for the Very Well Done class today. It is refreshing.

Best of luck!

jhunter1972

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#6

### Re: Surface Temperature of the Sun

03/12/2010 9:02 PM

High praise indeed. Thank you.

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#7

### Re: Surface Temperature of the Sun

03/12/2010 9:15 PM

Things well said deserve receiving praise. The article is very good and I will be sharing it with my youngest, he is going to be the next big astronomer I do believe, so he thinks at 6 years of age. lol

But hey, is that not how we get great people? We need to always set our view above the horizon.

jhunter1972

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#10

### Re: Surface Temperature of the Sun

03/13/2010 5:50 AM

I also did receive encouragement as a young boy... This is music to my ears that a young mind is fed!

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#12

### Re: Surface Temperature of the Sun

03/13/2010 9:40 AM

Cool...See we all benifet then.

Nice meeting you.

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#16

### Re: Surface Temperature of the Sun

03/15/2010 12:55 PM

If your son is interested in astronomy, two things I would suggest that you get for him: 1. A subscription to Astronomy magazine. 2. The book re the best photos from the Hubble telescope. I can send you further info re sources if you are interested.

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#18

### Re: Surface Temperature of the Sun

03/15/2010 2:03 PM

Thanks.

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#8

### Re: Surface Temperature of the Sun

03/12/2010 11:15 PM

GA. I would add that the universe has a background temperature of about 4 deg K. This is the Big Bang after cooling due to several billion years of expansion.

(That's why my windshield sometimes has frost even when the air temp is slightly above freezing. The glass radiates heat into space, cooling a few degrees.)

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#17

### Re: Surface Temperature of the Sun

03/15/2010 12:58 PM

Or... Is it the evaporation of some of the dew, or moisture on the windshield that removes heat from the glass, thereby freezing some of the remaining moisture?

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#14

### Re: Surface Temperature of the Sun

03/14/2010 9:17 AM

Hi DrMoose, GA from me too. So refreshing to go thro. Thanks. Rangasamy

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#19

### Re: Surface Temperature of the Sun

03/15/2010 2:19 PM

I would like to connect the dots here. There are several details being kicked around on this thread about heat, energy and light that are the dots and can be connected in many incorrect ways. For one thing, the temperature of stars(the sun being a star) is calculated from spectral data using blackbody law. This has been done for at least a century. It just does not make sense because using the same blackbody law for everyday things like an x-ray will indicate you are being toasted at 20,000 degrees every time you get an x-ray(One of the posts here states light has no temperature which is correct). Second, one of the posts here states heat cannot be radiated which is correct because heat is a property of matter and can be convected but not radiated whereas radiation is a property of energy and is also known as light or electromagnetic waves. Its important to define these details correctly if a clear understanding is desired.

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#20

### Re: Surface Temperature of the Sun

03/16/2010 1:14 PM

To connect the dots, remember a couple of simple principles.

First and foremost is the law of Conservation of Energy, which states very simply that energy cannot be lost or destroyed, but only converted from one form to another. So energy is energy, whether it's in the form of heat of light. Heat is not a property of matter, it is a way to measure how much energy is contained within a mass. And heat can be just easily radiated away by infra-red radiation, as conducted away by air or water.

Second is that radiation is quantized, in what we call photons, and that at any given frequency, a photon will have a specific amount of energy. However, different frequencies of light tend to do different things when they encounter a body. Infra-red light tends to be absorbed as heat. Visible light is reflected away to varying degrees, which is how we are able to see things. X-rays tend to go right through you without stopping, which is how we are able to take photos of broken bones.

This is why you are warmed by an infra-red heater, where visible light, even though it is of much higher energy, doesn't warm you much at all, and why getting an x-ray doesn't fry you to a crisp.

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#21

### Re: Surface Temperature of the Sun

03/16/2010 1:57 PM

Heat is an effect of energy interacting with matter as you indicate, but its not energy. An effect can be any of many manifestations occuring when energy interacts with matter. This a very important and overlooked detail whenever these effects and interactions are being kicked around. Everyone always uses heat and energy terms as if they interchangable-but the're not the same at all. Conservation laws don't effect how language is applied to subjects and details like these.

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#22

### Re: Surface Temperature of the Sun

03/16/2010 2:29 PM

Not true sir. Energy is energy, whether it manifests as heat, light or motion, and is easily inter-converted between these forms.

No, you are confusing heat with temperature. When energy is absorbed by mass, it causes an increase in random motion of discrete particles within the mass, be it vibration as in a solid or linear motion as in a liquid or gas. The measurement of the energy content is temperature. However, masses can just as easily lose this energy via conduction through contact with other masses or via infra-red radiation, which goes on all of the time.

To say that heat is not energy is akin to the ancient Greek notion that movement in one direction was qualitatively different from motion in any other direction.

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#23

### Re: Surface Temperature of the Sun

03/16/2010 7:25 PM

DrM. So you would say then light is heat? You call heat energy in one case and then say heat is random motion of matter. So then light is random motion of matter? What is the temperature of light if heat and light are to be interchanged? Its you and everybody esle who fail to see the forest through the trees when heat and energy are not seen to be different. Heat and illumination are effects caused by energy acting on matter in very different ways.

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#24

### Re: Surface Temperature of the Sun

03/16/2010 7:42 PM

I suppose that this could be one way of looking at it. However, it is mathematically consistent to treat all forms of energy as being the same thing. In this way, energy is perhaps a bit of an abstract, but it does allow for us to do some fun things, such as building small computing devices.

But yes, in the case of heat, what you have is energy being absorbed by a mass. This energy is stored by the mass as random motion of the discrete particles within the mass. As has already been said, this energy is bled off by infra-red radiation, thus causing a reduction in the random motion and a lower temperature.

So yes, light, heat and motion do seem to be very different things, just as gravity and acceleration appear to be totally different things. Never the less, they are in fact the same thing, just doing somewhat different things. It is this realization, that the math actually means what it says, that leads brilliant men like Albert Einstein to come up with stuff like General Relativity.

Trust me on this. It's a very well proven principle.

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#27

### Re: Surface Temperature of the Sun

03/16/2010 8:23 PM

You are right. Thank you for keeping this topic straight. It is a very proven principle and has been followed this way for some time.

Again thanks.

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#25

### Re: Surface Temperature of the Sun

03/16/2010 7:51 PM

I thought Light was released as electrons descended into lower orbitals from an excited state. When you heat iron to a dull orange glow, the light is emitted as excess heat / energy is released as the electrons orbits fluctuate between higher and lower excited states.

Drew

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#26

### Re: Surface Temperature of the Sun

03/16/2010 8:20 PM

Heating anything to 2,000K or so is a very good example of the difference that energy, heat and light exibit. Its energy that interacts with matter causing the temp to rise that makes the molecules move. And as the temp rises the matter conducts and radiates something we call heat and electromagnetic waves. As more energy is applied the matter reaches critical points that are not well measured by science at this time(thats another detail). Anyway light begins to use more and more energy as the temp rises while little energy is used to make more heat. Thats why a very high temp is more effective for light than a low temp is. At low temps most of the energy is going to heat but at high temps more energy is going to light. At very low temps we get no light at all but lots of heat. So, as anyone clearly see if they look heat and light are not interchangable.

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#28

### Re: Surface Temperature of the Sun

03/16/2010 8:28 PM

Sir, have you ever taken a mathematics based physics course? Please tell me you're not a teacher! You aren't by any chance a flat-earther, are you? I give up.

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#29

### Re: Surface Temperature of the Sun

03/17/2010 1:33 PM
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#30

### Re: Surface Temperature of the Sun

03/17/2010 6:28 PM

You posted an excellent link, Anon.

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#31

### Re: Surface Temperature of the Sun

03/22/2010 1:37 PM

Could we then, knowing these figures, determine the size of space??? It appears to me that the temperature of space is directly proportional to its size.

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#3

### Re: Surface Temperature of the Sun

03/12/2010 5:43 AM

The people in about 1930 were faced with the same question.

They knew that heat cannot pass through a vacuum.

Their solution was that space was filled with a calcium cloud - ether (visible as the milky way) whereby heat can be conveyed to earth.

Teachers in the 50's were adamant about it (And that a atom is a solid indivisible particle.)

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#4

### Re: Surface Temperature of the Sun

03/12/2010 9:13 AM

They knew that heat cannot pass through a vacuum. I would have thought that touching any light bulb or vacuum tube would have dissuaded them from this idea.

Teachers in the 50's were adamant about it (And that a atom is a solid indivisible particle.) I would have thought that all that nuclear bomb testing would have put this idea to rest by then.

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#9

### Re: Surface Temperature of the Sun

03/13/2010 1:04 AM

Hi, all.

Good answers and observations overall, except for that thing about why space is cold. There's one thing to understand: it isn't.

Space will FEEL cold to someone not warmed by starlight or some other heat source because he'll be losing heat by radiation.

There are three ways to transfer heat between non-reacting bodies (so no chemical reactions or anything else): conduction, convection, and radiation.

Conduction's easy: two bodies touch each other and heat flows from the hotter to the cooler through the surfaces in contact

Convection's a bit harder to visualize: a fluid in contact with two bodies that aren't in contact with each other pick up heat by conduction from the hotter body, flows in the space between, and eventually transfers heat into the cooler body. For example, in a room heated by baseboards, air near the a wall and the floor gets heated by the baseboard, rises up along the wall (its density drops become its temperature rises ... hot air is 'lighter' than cool air), cools as it rises, cools further when it hits the ceiling, moves along the ceiling (again getting cooler), and drops down away from the heated wall. The air flow distributes heat from the the baseboard to whatever's in the room in this manner. That's convection.

The last way to transfer heat is by radiation. Any body whose temperature is above absolute zero radiates heat. The hotter a body is, the higher up in the spectrum (and the higher in energy content) the light radiated is. At room temperature, bodies pretty much radiate in infrared, which we can't see unless we use special 'night vision' devices. Those devices are sensitive (that is, they can 'see') in infrared and generate images at frequencies that we can see. However, heat a body enough (say a lump of iron) and it'll start to glow in red, then ornage and yellow as the temperature rises, etc. all the way into blue and white at some point. Hence why 'white-hot' is hotter than 'red-hot'.

So, why does space seem 'cold'? It because a body (say an astronaut floating on space) radiates heat. If he's lit by a close-enough star, then his side facing the star is being warmed by the star's radiation (and possibly by it stellar wind, which would be convection). However, in all directions including the side facing the star, the astronaut would be radiating heat away. If he's too far from a star for it to compensate his radiative heat loss, the astronaut will freeze to death because he'll continually be losing energy.

So, other space isn't 'cold', in the sense that vacuum has no temperature. Particles surrounded by vacuum DO have temperatures, which are functions of the particles speed (the faster, the hotter). Light doesn't have a temperature, but it does contain energy that it can transfer to bodies that absorb it; the higher the frequency of a photon, the more energy it carries and the more it'll transfer to a body that absorbs it (so a blue photon absorbed by a body will warmit more than will a red photon).

All that to say: you can't conduct between two bodies that don't touch eaach other, and you can't convect heat through vacuum. But you DO lose in space by radiating light, which is inevitable. And that light that you're radiating bleeds you of your heat and kills you unless you heat yourself somehow.

Cheers! DZ

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#11

### Re: Surface Temperature of the Sun

03/13/2010 5:56 AM

Thanks to all who gave a response. My thoughts are confirmed and advanced.

Now for an even greater challenge. Translate all this to German...

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#13

### Re: Surface Temperature of the Sun

03/13/2010 1:19 PM

Johannes ... hahahaha .. Oh my.

Make my answer simple and just say that all bodies radiate light whose frequency and energy content depends on their temperature, and that this light carries away energy from a body in space.

Cheers! DZ

P.S. By the way, this radiation explains why there's no need for space to be filled by 'ether' ... heat isn't lost by conduction and convection to a fluid that fills space, it's lost to vacuum through radiation.

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#15

### Re: Surface Temperature of the Sun

03/15/2010 11:01 AM

Well, as it happens....

DZ's reply in German = "alle Körper strahlen Licht aus, dessen Frequenz und Energieinhalt von ihrer Temperatur abhängt und dem dieses Licht Energie von einem Körper im Raum wegschafft."

Ok! I admit I used Babelfish to get the above. It doesn't always translate technical language very well but I suppose it gives you a good starting place.

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