We know that the thermal radiation emitted by the Sun plays an extremely important role in sustaining life on our planet. The fiery sphere – the center of the Solar System has a surface temperature reaching up to 5527 degrees Celsius, transmitting heat to Earth across an astonishing distance of 150 million kilometers. Clearly, the amount of heat produced by the Sun is immense.
Considering only the Solar System, if we take the entire universe into account, where numerous stars and planets generate a heat output that humanity cannot fully calculate, we can believe that the environment in space is extremely hot. However, contrary to this assumption, the temperature in outer space is actually extremely low. How can this be?
The temperature in outer space is extremely low.
A Small Corner of the Milky Way
The first and simplest reason that can explain the question above is: The universe is too vast. Imagine it as a gigantic, limitless box, vast and extending infinitely. The universe that we can currently observe has a diameter of about 91 billion light-years, while the distance from the Sun to Earth is approximately 0.000016 light-years. Due to the infinite nature of space, the heat radiated from stars will almost dissipate before reaching other stars or planets.
Let’s approach this issue from a more specific perspective: At a distance of 150 million km from Earth, the Sun emits thermal energy equivalent to a trillion megaton bomb every second (while the most powerful hydrogen bomb ever created, the Tsar Bomba, has an explosive yield of about 50 megatons). Calculating at the speed of light (almost 300,000 km/s), it takes about 8 minutes for sunlight to reach our planet. If we want to avoid sunlight from the Sun, we would have to wait at least 4 to 5 years for light from the nearest star, Proxima Centauri, to reach our planet.
Currently, in a very small corner of the Milky Way (where our Solar System is located), there are only 25 stars within a radius of 110 trillion km from Earth. If we expand this to the entire Milky Way, which has a diameter of 100,000 light-years, there are currently about 100 billion stars. However, this area is almost insignificant compared to other galaxies that stretch across the universe – places with diameters of over 1.5 million light-years. The distance between our Milky Way and our nearest neighbor, Andromeda, is about 2.5 million light-years.
The distance between our Milky Way and our “neighbor” is 2.5 million light-years.
Finally, let’s use our imagination to grasp the following issue: Scientists estimate that within the observable universe, there are up to 100 billion galaxies existing, spanning a distance of 46 billion light-years in every direction. These figures illustrate the vastness of the universe, where our Earth is merely a tiny grain of sand in that vast “desert” of space.
Countless Planets, but Infinite Space
As mentioned above, with an infinite universe filled with galaxies and stars, it should resemble a gigantic steam boiler. Not to mention, while our Sun is brilliant, it is just an average-sized yellow dwarf star, whereas out there exist numerous stars with masses over 60 times that of the Sun and surface temperatures reaching nearly 25,000oC.
However, reality has proven that the most common temperature in outer space measured by scientists is only about -270.15oC, while the lowest temperature conventionally is around -273oC. Therefore, it can be said that the universe is nearly at the coldest limit that humans can measure.
The universe is nearly at the coldest limit that humans can measure.
So where does this contradiction come from? It seems we were mistaken when we perceived that the entire galaxy and stars are “crowded” in the universe; in reality, space is considered “empty”. If we imagine the entire universe as a cube with a side length of 30 billion light-years, then all the matter within it would only fill a cube with a side length of 1000 light-years.
Radiation – Distance
Outer space is an almost perfect vacuum, so the ability to transfer heat through molecules is impossible. However, we know that heat can be transferred in three different ways: conduction, convection, and radiation. Therefore, in space, the most optimal method for temperature to spread is through thermal radiation.
The radiation from the Sun that shoots out into space has a strength of about 64 million W/m2. When this radiation reaches Earth, the energy has almost completely dissipated, leaving only 1370 W/m2. If we perform a division calculation in this case, the intensity of sunlight has weakened over 47,000 times by the time it reaches Earth.
The radiation from the Sun that shoots out into space has a strength of about 64 million W/m2.
For the space between stars in the galaxy, the radiative energy is even more dissipated. The temperature of gas and dust measured in the interstellar space is only about -263.15oC. The figure of -270.15oC is taken from the vast empty regions between galaxies and the “voids” in the universe, believed to be the temperature of the “cosmic microwave background radiation” leftover from the Big Bang 13.798 billion years ago.
The Dream of a “Sweltering” Summer in Space?
With the information above, we can understand that the universe is neither cold nor hot; it is simply because space is so vast that thermal radiation from most planets or stars cannot reach us, while at the same time, the thermal radiation from within our bodies escapes, causing a feeling of coldness. However, if we were standing on a planet orbiting a bright star, it would surely be a truly sweltering summer, creating a tan on our skin just as we desire.
