How Cold Is Space? What Is Its Temperature?

How Cold Is Space
The temperature of space is around 2.7 K.

One of the most fascinating aspects of space is its temperature. When people think of space, they likely imagine a freezing, desolate void. But just how cold is space? Let’s explore how cold space is, the science behind the answer, and the factors that contribute to its extreme conditions.

  • Space is very cold. Its temperature is 2.7 Kelvin, which is not much higher than absolute zero.
  • The reason space is cold is because it is a vacuum. Temperature comes from the vibrations of atoms and molecules.
  • The coldest parts of space are expanding nebulae, while the hottest parts are stars and regions surrounding black holes.

How Cold Is Space?

The temperature of space is approximately 2.7 Kelvin (-270.45 degrees Celsius or -454.81 degrees Fahrenheit). This temperature is just slightly above absolute zero, the theoretical lowest temperature possible, where all molecular motion is at its minimum.

Why Is Space Cold?

Space is cold primarily because it is a vacuum, meaning it lacks enough matter to hold and transfer heat. In space, there is no atmosphere that traps heat. The vast distances between atoms mean that heat does not travel easily.

Heat transfer occurs in three ways: conduction, convection, and radiation. In space, conduction and convection are virtually nonexistent due to the lack of matter. Intergalactic space is about as close to a perfect vacuum as nature gets, with only around 10-6 particles per cubic meter of space. This leaves radiation as the primary mode of heat transfer. Objects in space, such as planets and stars, emit heat in the form of infrared radiation, but the vacuum of space does not retain this heat, leading to the extremely low temperatures observed.

Cosmic Microwave Background (CMB)

Yet, the temperature of space is still higher than absolute zero. Why? The Cosmic Microwave Background (CMB) is a critical factor in understanding the temperature of space. The CMB is the residual thermal radiation from the Big Bang, which occurred approximately 13.8 billion years ago. This radiation fills the entire universe and is a snapshot of the infant universe, providing a clue to its conditions at that time.

The CMB is incredibly uniform in all directions, with a temperature of about 2.7 Kelvin. There are fluctuations at the level of microkelvin (millionths of a Kelvin). These fluctuations are tiny, on the order of one part in 100,000. For instance, if the average temperature is 2.725 K, the anisotropies might range from 2.72499 K to 2.72501 K. It is this uniformity that gives space its consistent cold temperature. The CMB represents the afterglow of the Big Bang and is a crucial piece of evidence for the Big Bang theory. It permeates the entire universe and provides a baseline temperature for the cosmos.

The Coldest Known Part of Space

The coldest known place in space is the Boomerang Nebula, located about 5,000 light-years from Earth in the constellation Centaurus. The temperature in the Boomerang Nebula is approximately 1 Kelvin (-272.15 degrees Celsius or -457.87 degrees Fahrenheit), which is colder than the average temperature of space.

The reason for this extreme cold is due to the rapid expansion of gas from a dying star at the center of the nebula. As the gas expands, it cools, similar to how air expands and cools when it escapes from a punctured tire. This rapid cooling creates an environment that is colder than the surrounding space.

The Hottest Part of Space

In contrast, some of the hottest temperatures in space occur in the cores of stars and near black holes. The core of a typical star like our Sun reaches temperatures of around 15 million Kelvin (about 15 million degrees Celsius or 27 million degrees Fahrenheit). In even more extreme environments, such as the accretion disks around black holes, temperatures soar to billions of Kelvin.

These high temperatures are a result of immense gravitational forces and the fusion reactions occurring in stellar cores. In the case of black holes, the intense gravitational pull generates high-energy collisions and friction in the accretion disk, resulting in extremely high temperatures.

Measuring the Temperature of Space

Scientists determine the temperature of space using various methods. The temperature of the CMB comes from satellite-based instruments such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP). These instruments detect and measure the faint microwave radiation that permeates the universe, providing precise data on the CMB’s temperature.

For localized temperature measurements, such as those of stars or nebulae, astronomers use spectroscopy. By analyzing the light emitted or absorbed by objects in space, scientists determine their temperatures based on the spectral lines. These lines indicate the presence of specific elements and their ionization states, which correspond to particular temperatures.


Space is an incredibly cold environment, with an average temperature of about 2.7 Kelvin, primarily due to its vacuum nature and the presence of the Cosmic Microwave Background. The coldest known region, the Boomerang Nebula, demonstrates the extremes of cosmic temperatures. Meanwhile, the hottest areas, such as stellar cores and black hole accretion disks, showcase the incredible range of thermal conditions in the universe. By using sophisticated instruments and techniques, scientists expand our understanding of these extreme environments, unveiling the mysteries of the cosmos.


  • Fixsen, D. J. (2009). “The Temperature of the Cosmic Microwave Background”. The Astrophysical Journal. 707 (2): 916–920. doi:10.1088/0004-637X/707/2/916
  • Gupta, Anjali; et al. (May 2010). “Detection and Characterization of the Warm-Hot Intergalactic Medium”. Bulletin of the American Astronomical Society. 41: 908, Bibcode:2010AAS…21631808G
  • Mathiesen, B. F.; Evrard, A. E. (2001). “Four Measures of the Intracluster Medium Temperature and Their Relation to a Cluster’s Dynamical State”. The Astrophysical Journal. 546 (1): 100. doi:10.1086/318249
  • Partridge, R. Bruce (2019). “The cosmic microwave background: from discovery to precision cosmology”. In Kragh, Helge; Longair, Malcolm S. (eds.). The Oxford Handbook of the History of Modern Cosmology (1st ed.). Oxford University Press. pp. 292–345. ISBN 978-0-19-881766-6. doi:10.1093/oxfordhb/9780198817666.013.8
  • Tadokoro, M. (1968). “A Study of the Local Group by Use of the Virial Theorem”. Publications of the Astronomical Society of Japan. 20: 230. Bibcode:1968PASJ…20..230T