Why Is Space Cold If There Are So Many Stars?

Space is cold because it is inconceivably vast, so the heat radiated by stars dissipates almost completely in the empty space between them. The average temperature of deep space is about 2.7 K (roughly -270 °C or -455 °F), set by the leftover glow of the Big Bang. That is only a few degrees above absolute zero (0 K), the coldest temperature physically possible.

Have you taken the time to look up at the stars lately? Go out on any clear night and stare into the vastness of space, and at most points on the globe, you can see anywhere from 2,000-5,000 stars. That can make for an impressive sight for stargazers, and yet that is only a microscopic fraction of how many stars are out there.

Now, consider the star of our solar system (the Sun), and how hot it is (about 5,800 K on the surface, which is roughly 5,500 °C or nearly 10,000 °F). That star is roughly 150 million kilometers (about 93 million miles) away from Earth, which sounds like an incredible distance. As we know, without the warmth of the Sun, life wouldn’t exist on our planet, so it clearly pumps out a lot of heat.

With so many stars out there in the universe, pushing out unbelievable amounts of heat, you might think that space would be hot, and yet, space is extremely cold! How is that possible?

Our Tiny Corner Of The Galaxy

The simplest answer to this question is that space is inconceivably enormous, so the heat of stars dissipates to nearly absolute zero between them. However, let’s have a bit of fun and put this undeniable reality into perspective.

Sitting 150 million kilometers away, every second, our Sun radiates roughly 3.8 × 10²⁶ watts, energy equivalent to nearly 100 billion 1-megaton hydrogen bombs going off every single second. Even at the speed of light (about 300,000 km/s, or roughly 186,000 mi/s, the fastest speed anything in the universe can move), light from our Sun still takes about 8 minutes and 20 seconds to reach Earth.

If you wanted to wait for light from the next closest star to reach our planet (Proxima Centauri), you would have to wait about 4.25 years! There are only around 30 known stars (most of them faint red dwarfs) within 12.5 light years of Earth, and to put the idea of a light year in perspective, consider that 1 light year equals roughly 9.46 trillion kilometers (about 5.88 trillion miles), so our Sun is only about 0.000016 light years away from Earth.

If you haven’t gotten the impression thus far, even our minuscule corner of the Milky Way is huge, with only about 30 known stars within roughly 120 trillion kilometers (about 75 trillion miles) in any direction.

Stepping it up one more notch, the Milky Way Galaxy is 100,000 light years in diameter and contains more than 100 billion stars. You might not believe this, but the Milky Way galaxy is considered puny in comparison to other galaxies spread across the universe, some of which are more than 1.5 million light years in diameter! The distance between the Milky Way and our closest galactic neighbor (Andromeda galaxy) is 2.5 million light years.

For the final, mind-boggling level of universal scale, consider this: scientists estimate that the observable universe contains somewhere between a few hundred billion and up to 2 trillion galaxies, spread across a region that stretches roughly 46 billion light years in every direction from Earth.

Lots Of Stuff, But A Whole Lot More Space

With all those galaxies and stars filling up the void of space, it seems like space should be a sauna, particularly because our blazing Sun is just an average-sized yellow dwarf. The hottest known stars, such as the Wolf-Rayet star WR 102, can have surface temperatures above 200,000 K (more than 360,000 °F), tens of times hotter than the Sun.

However, the fact is, the most common temperature in the observable universe is 2.7 K (approximately -270 °C or -455 °F). The coldest theoretical temperature is 0 K (also known as absolute zero), which means that most of the universe is nearly as cold as physically possible.

It’s important to understand that space itself is not “cold” in the same way as we understand it, space is simply empty. If the entire universe was a cube with dimensions of 30 billion light years on each side, all of the matter in the universe could fit in a smaller cube in the corner with dimensions of only 1,000 light years on each side.

Radiation Vs. Distance

In a perfect vacuum, there is no temperature, because there are no molecules to possess heat. However, heat can be transferred through radiation (light energy, which includes the visible spectrum).

As radiation is shot out into space from the Sun, the radiant intensity at the Sun’s surface is roughly 63 million watts per square meter. By the time that energy reaches Earth, the radiant intensity has fallen to about 1,361 watts per square meter (a value known as the solar constant). I’ll do that math for you and tell you that the intensity of sunlight is roughly 46,000 times weaker by the time it reaches our planet.

Remember, that huge reduction in radiation energy occurs over a distance of .000016 light years! Pluto, for example, lies roughly 5.9 billion kilometers (about 3.7 billion miles) from the Sun, and the average surface temperature of that dwarf planet, which receives hardly any radiation from our Sun, is only about 40 K (roughly 40 degrees above absolute zero, or about -388 °F)! Almost all the radiation of the Sun is gone by the time it leaves our solar system!

In the space between stars in our galaxy, the radiation energy is even further dissipated, leaving the average temperature of interstellar gases and dust at approximately 10 degrees above absolute zero. The lowest temperature figure (2.7 K) in space is found in the vast empty void between galaxies and “blank spots” of the universe. 2.7 Kelvin is the temperature of “cosmic microwave background radiation”, which is the leftover radiation from the Big Bang that is still “hanging around” the universe and keeping the temperature just above absolute zero.

A Sunny Vacation In Space?

As this article showed you, space isn’t cold or hot, it’s simply too huge for radiation in most distant locations to warm up an object, meaning that any heat in an object would radiate outwards, thus making it feel “cold”. If you orbited low over a blazing star, you would certainly be able to get a sun tan and have a toasty vacation.

However, as you move further into the seemingly infinite void of space, less and less radiation would transfer, and the true icy nature of space would be felt!