Why does the sun's heat & light intensity decrease over AU distances?

In summary: Earth.In summary, the sun's energy is spread out over a large area as it travels through space, resulting in a significant decrease in intensity. This is due to the inverse square law, where the energy is spread out as the distance increases. As a result, there is very little heat and warmth in outer space because there are very few particles to absorb or transform the energy. This is also why the universe is predicted to eventually reach a state of "big freeze," where there will not be enough material left to form new stars and the expansion of space will lead to a very cold and dark universe.
  • #1
Remon
85
0
I'm a bit confused because the sun's radiation & heat are passing through space (with some passing through planets' atmospheres), and there is almost nothing in space, therefore, how does that immense heat & light just diminish over AU distances when its going through almost nothingness? What absorbs or transforms both of these energies (especially heat, since we can still see the sun from much further away)? Do the planets, asteroids, gasses/dust, and other small objects in space absorb the sun's total energy? So basically, where does all that heat go?
I guess a more broad/general question would be why is outer space so cold when there's billions of stars constantly warming it? An even broader question would be why do some scientists believe that the universe is going to end in a "big freeze"? (you don't have to answer that last one lol)
 
Astronomy news on Phys.org
  • #2
The light doesn't decrease, it spreads out. Consider a spherical shell of a certain radius. All of the sun's energy hits it. If you double the radius, then the same energy hits a surface 4x the size and is thus 4x less intense.
 
  • #3
Remon said:
I guess a more broad/general question would be why is outer space so cold when there's billions of stars constantly warming it?
Space is huge. The light from some tiny stars is not sufficient to heat all of the universe significantly.

An even broader question would be why do some scientists believe that the universe is going to end in a "big freeze"? (you don't have to answer that last one lol)
At some point, there won't be enough material left to form new stars, and stars don't live forever. Together with the expansion of space, the universe will become a very dark, cold place.
 
  • #4
mfb said:
the universe will become a very dark, cold place.

well... that's inspiring lol. But seriously though, you guys are saying that the main reason that stars' heat diminish is because the energy waves simply just spread out everywhere? So instead of heating a small area significantly (such as a planet close to it), the stars heat large areas insignificantly (such as...well... space)?
 
  • #5
Remon said:
So instead of heating a small area significantly (such as a planet close to it), the stars heat large areas insignificantly (such as...well... space)?

Yes.

Surface that is heated grows as a square of the distance, but the power doesn't change. That means when you move twice further, amount of energy falling on a unit surface gets 4 times lower.
 
  • #6
Remon said:
well... that's inspiring lol. But seriously though, you guys are saying that the main reason that stars' heat diminish is because the energy waves simply just spread out everywhere? So instead of heating a small area significantly (such as a planet close to it), the stars heat large areas insignificantly (such as...well... space)?

A similar effect happens with water waves. Throw a rock into a still pond, lake, or other body of water. The amplitude of the water waves will be highest near the center and will gradually lessen as the wavefront propagates outwards.
 
  • #7
Ok, well thank you guys... such as simple answer for something I've been wondering about for a while
ahh...life never ceases to amaze lol
 
  • #8
When I was in high school, my physics teacher had what he called a "butter gun". It was a squirt gun with four sticks spreading out from the muzzle with places to put "toasts" inside them. The point was that there was a point where you could fit one piece of toast and "spray" butter on it. Twice that distance from the muzzle was were wires where you could put four pieces of toast- at twice the distance, by "similar triangles" (which we had learned the previous year in geometry!) the area between the sticks was four times as great. So the same amount of butter coming out of the muzzle had to be spread over fourth the distance. Twice the distance, 1/4 the density of butter, an inverse square relation!

In fact, spreading out a constant amount of anything in space results in an inverse square relation.
 
  • #9
There's nothing in space to keep warm. That's why it's called 'space'. All of the energy emitted by stars radiates unimpeded into the void.
 
  • #10
SteamKing said:
There's nothing in space to keep warm. That's why it's called 'space'. All of the energy emitted by stars radiates unimpeded into the void.

To expand on SteamKing's answer, what we call temperature is a measure of how fast the particles in a substance are moving or vibrating and is related to the amount of heat energy stored within that substance or gas. We feel things are hot when these particles are traveling at high speeds and collide with our hands or thermometers. These collisions transfer energy. That's what we feel.

Because there are very few particles in space, there is very little capacity to store heat energy and if we put a temperature there, we would register very few of these collisions and would therefore measure almost no temperature.

That is why the Earth is hot (dense atmosphere with lots of particles) while empty space at the same distance from the Sun is very cold.
 
  • #11
BOYLANATOR said:
Because there are very few particles in space, there is very little capacity to store heat energy and if we put a temperature there, we would register very few of these collisions and would therefore measure almost no temperature.
We would still get the energy by radiation,increasing the temperature of the liquid in the thermometer.No,almost no temperature means it's just above absolute zero,It can't be unless sun's or other star's radiation doesn't reach us
 
  • #12
For the right triangle where "A" is the radius of the Earth and "B" is 1AU, then the similar triangle is where "a" is the radius of the radiating spot on the Sun's surface, and "b" is the radius of the Sun (approximately).

This makes the diameter of this central radiating spot on the Sun 592m.

So a spot on the Sun about 27,500 square meters is projecting to half the surface of the Earth, about 510 million square kilometers.

That is a proportion of reduction of 0.00000005 at 1AU
 
  • #13
SteamKing said:
There's nothing in space to keep warm. That's why it's called 'space'. All of the energy emitted by stars radiates unimpeded into the void.
The gas in that "nothing" has 20 times more mass than all stars combined.

BOYLANATOR said:
Because there are very few particles in space, there is very little capacity to store heat energy and if we put a temperature there, we would register very few of these collisions and would therefore measure almost no temperature.

That is why the Earth is hot (dense atmosphere with lots of particles) while empty space at the same distance from the Sun is very cold.
That is just wrong.
Few collisions are also few opportunities to lose energy, so the equilibrium does not depend on the particle density. In addition, there is always thermal radiation, incoming and outgoing. Far away from stars, that equilibrium temperature is currently 3K, and mainly coming from the cosmic microwave background.

Source: The cosmic energy inventory@bahamagreen: I don't know how you got that number, the reduction is (radius of sun / distance to sun)^2 = 2*10-5. Did you calculate the reverse quantity (or the fraction of the solar radiation that reaches earth)?
 

1. Why does the sun's heat decrease over AU distances?

The sun's heat decreases over astronomical unit (AU) distances because of the inverse square law. This law states that the intensity of heat and light decreases as the distance from the source increases. Since the sun is the primary source of heat and light in our solar system, its intensity decreases as we move further away from it.

2. What is the inverse square law?

The inverse square law is a principle in physics that states that the intensity of a physical quantity, such as heat or light, is inversely proportional to the square of the distance from the source. This means that as the distance from the source increases, the intensity decreases exponentially.

3. How does the distance from the sun affect the temperature on Earth?

The distance from the sun plays a crucial role in determining the temperature on Earth. The Earth's orbit around the sun is elliptical, meaning it is not a perfect circle. This means that at certain points in its orbit, the Earth is closer to the sun and receives more heat and light, resulting in warmer temperatures. At other points, the Earth is further away and receives less heat and light, resulting in cooler temperatures.

4. Can the sun's heat and light intensity vary over time?

Yes, the sun's heat and light intensity can vary over time. This can be due to natural phenomena such as sunspots, solar flares, and solar winds. These variations can affect the amount of heat and light reaching the Earth's surface, resulting in changes in temperature and weather patterns.

5. How is the sun's heat and light intensity measured?

The sun's heat and light intensity are measured using a unit called the solar constant, which is the amount of solar energy that reaches the Earth's upper atmosphere per unit area. It is measured in watts per square meter (W/m²) and is currently estimated to be around 1361 W/m². This measurement is important in understanding the Earth's climate and how it is affected by the sun's energy.

Similar threads

  • Astronomy and Astrophysics
2
Replies
49
Views
2K
  • Astronomy and Astrophysics
Replies
3
Views
1K
Replies
34
Views
6K
  • Astronomy and Astrophysics
Replies
5
Views
2K
Replies
8
Views
1K
  • Astronomy and Astrophysics
Replies
1
Views
1K
  • Introductory Physics Homework Help
Replies
10
Views
2K
Replies
7
Views
906
  • Astronomy and Astrophysics
Replies
6
Views
1K
Replies
7
Views
4K
Back
Top