The sun warms up our planet but space is cold. If I lit a candle and stood 20 feet away, I could see the light but not feel the warmth. So, why do we feel warmth from the sun but space isn’t warmed by it?
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Heat is an interesting thing.
To understand heat, you have to understand what it is – and I find that a great example to use (and follow me here), is imagine you have an atom, but even more relatable; imagine an atom is a little bee that you put inside of a little ball. Now imagine that he’s in the center of a bunch of other bees that are also in their own little balls.
Now imagine you, as the sun, come on in and shake the little ball – the bee gets angry, and starts bzzing quite rapidly. Depending on how much he bzzes and shakes his little ball, the bees next to him will start bzzing, and depending on how much they bzz, the bees next to them will buzz and so on and so forth. What this represents is you putting energy into atoms – which the sun does with light.
When those atoms are next to each other, then their “excitement” and movement, can cause other atoms to also start moving around. Now looking back at the bees, your first bee is going to bzz the most, but the bees around him are probably going to bzz less, and the bees around those ones are going to bzz less. Just because you lose some of the “excitement” of the bees every layer you go down.
Some of the commotion you generated is going to be lost, because that energy has to used to move around and bees get tired. So eventually it tapers off. How well these bees – or atoms rather – can actually spread their buzzing around is a property of elements and materials that we measure.
So for instance, metals like Iron – are great at getting hot really quickly; this is why sticking metal spoons in a very hot dish is not a good idea. The bees that make up the spoon that are in the pot, with the energy will start bzzing fast, and the bees that make up iron are pro athletes at bzzing and don’t too tired, so they’re not losing much of that energy. So the spoon gets reaaalllly hot because most of the bees on the end of the spoon that you’re touching are also bzzing pretty hard.
Now that we’ve established that this concept of heat is actually just atoms buzzing around with energy – then space makes a bit more sense. Remember that space doesn’t have an atmosphere or any atoms around. It’s just empty. So there’s no bees to bzz and so no heat just around. UNLESS you’re getting directly hit by the sun.
The sun is a giant nuclear reactor – and it makes a looooot of energy. If you had an asteroid that was facing the sun in space, it would actually get incredibly hot, because all of those bees in that asteroid are being hit with the same level of shaking by the sun. That’s what happens with the moon, on the side that faces the sun – the dirt can get as hot as 260 F.
Now Earth’s atmosphere, might look empty, but remember that it’s actually full of stuff. We basically live in a soup of atoms; and those atoms are just like the bees. When the sun hits the bees with energy at the tippy top of our atmosphere, they start buzzing around and that buzzes around until you on the ground feel *warmth*. Now you feel warm and not burning, because the atmosphere atoms – or bees- around you are 1) not the ones being directly shaken by the sun; those are the ones sitting at the very edge of Earth and 2) just good enough at spreading the bzzing that you’re warm, but not so good that you’re on fire.
Going back to your question, this is why the candle doesn’t feel super hot either. We measure energy emitted over time – or how hard you shake the bee – in watts; we call it power!
A candle only generates about 80-100 W in power. So the bees in the atmosphere aren’t great at spreading that around. If you stuck a metal spoon on the candle though, you could easily feel that. Now they can if the candle becomes a fire of course, and that’s why we make a fire for warmth on cold nights. But remember that the bees are actually used to dealing with the sun – which makes around 44,000,000,000,000,000 W in power.
Hope my answer was helpful!
The sun’s rays (photons) need to interact (they carry kinetic energy) with the particles of gas in the atmosphere. It causes the particles to move (heat). The more kinetic energy they absorb, the more heat you feel. Think of how fast food would cook an oven being heated by the air versus in a sous vide being heated by water, which is denser. You can even cook chicken with a slap machine. It’s all just kinetic energy.
The only heat that space can hold is radiant energy from stars. There is no matter in space to hold conductive heat (kinetic energy) so it has no temperature besides the background radiation from stars, which is very little if you aren’t near one. Thus, the “temperature” of space is is very very cold. This doesn’t mean you’ll instantly freeze anything that goes into the vacuum, that is entirely dependent on the material put into space and its ability to radiate heat. This usually means hours before something will cool to freezing temperature.
Much of the universe is incredibly hot! The majority of the gas within the milky way is several thousand degrees. Even more substantially, much of the gas in between galaxies (which makes up almost half of all normal matter!) is tens of thousands to millions of degrees. (In some cases, it’s over a trillion!)
The issue is that, while it is hot, the density is so low that *it doesn’t transfer much heat*. Heat is what we feel as hot: my hand burns on a pan because the pan is hotter than my hand and transfers heat to my hand. The pan is very good at transferring heat because I’m grabbing a chunk of metal. The hot space gas, on the other hand, has only a handful of particles in a cubic centimeter. Iron would have almost 10^23 in that same volume, so even though it’s a trivial few hundred degrees, the sheer number and density of particles transfers more heat than the very rarefied, albeit extremely hot, space gas.
The Sun is different because we are actually directly receiving its radiation and it heats up an atmosphere that is much more dense than space.
Temperature is a representation of the average kinetic energy of the particles in a material.
Space has no particles. Space has no temperature.
Here on earth, we have a bunch of atoms and molecules and shit constantly flying around and bumping into each other. When it’s cold, they fly slower and bump into each other less. When it’s warm, they fly faster and bump into each other more. Temperature is a representation of the speed/collisions between those particles.
Space is empty.
Heat transfers in 3 manners: Conductive, Convective, and Radiative. Conduction and Convection both require a medium for the transfer of heat from point A to B. In space, there is no medium for Conduction/Convection. Radiation is the only manner. The Radiative Heat Transfer equation Q t = σ e A T 4 specifies the transfer rate.
Heat is a property of stuff, and there is very little stuff in space. In Earth orbit, like if you are on the moon or on a space station, you get a bit more sunlight then if you were outside at noon in the tropics.
If you are standing in Hawaii that energy you get from the sun moves away from you into the air and water and stuff all around you.
If you are in space, that heat just keeps warming you up, so you’d overheat.
Heat is the vibration of atoms. It requires matter to exist because it is an expression of the movement of matter. We experience heat on earth because the light energy from the sun starts knocking around the atoms in the earth’s atmosphere and ground, with different wavelengths of light smacking different atoms around. With little/no atoms to get smacked around, space is cold.
Space doesn’t really have a “temperature.” Temperature is the amount of energy in a substance’s atoms; faster-vibrating atoms mean higher temperature. Since space has no atoms, it can’t have a temperature.
Hot objects in space don’t make space warmer, because space can’t be hot (or cold)! However, hot objects still cool down in space, because they still give off heat in the form of infrared light. Thus we say it is “cold” because objects in space cool down to a very low resting temperature, which is cold for our purposes.
There’s a somewhat complicated equation for what temperature is in physics, too complicated for a five year old, but the best way to describe it is the *speed of molecules around you.* the hotter it is, the hotter these molecules are moving. So, these little molecules are moving around and banging into your skin (the hotter it is, the more times a minute these molecules hit your skin) and so your skin takes the energy from these molecules moving around and feels hot (or cold, when your body skin has more energy and is moving around more than the air around it).
In space, there’s no molecules moving around to make your skin feel warm, so it’s not hot because there’s nothing there to *be* hot.
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