What is the coldest place in the Universe?

Temperature is a scalar magnitude with which the average heat of a substance or any other physical system can be measured. The lowest temperature that can be reached is called Absolute zero and corresponds to -273.144 ºC or 0 K (zero degrees Kelvin).

Absolute zero, therefore, would be a state in which matter would not contain heat. This state derives from Third law of thermodynamics but it has only been achieved in theory and not in practice.

The coldest place in the Universe has been found, for now, in the boomerang nebulaalso known as the Bowtie Nebula, a planetary protonebula whose temperature is only 1 K1 ºC above absolute zero.

Artificially, scientists at MIT (Massachusetts Institute of Technology) have managed to cool isolated atoms of a sodium-potassium gas to the 500 nanokelvins (0.0000005K), and this is the lowest temperature ever recordedAt least for the moment.

Why is the Boomerang Nebula so cold?

The Boomerang Nebula is defined as a planetary protonebula. It is located in the constellation of Centaurus, about 5000 light years from Earth. The name was given by Keith Taylor and Mike Scarrot in 1980, as its shape reminded them of a boomerang.

Also known as the Bow Tie Nebula (bow-tie in English), since its shape in subsequent observations with higher resolution telescopes showed a more symmetrical shape and similar to a bow tie or bow tie. It has now been observed in greater detail and its actual structure appears to be much more complex.

Planetary protonbulae form in relatively small stars known as yellow dwarfs similar to our Sun, and appear just before they become white dwarfsthe last active stage of these stars before they die as black dwarfs.

Therefore, planetary protonebulae can be considered a fairly late stage in the evolution of a star. They last a very short time and appear between the end of the phase known as asymptotic giant branch (GAB)and the planetary nebula phase, near the end of the star's life.

Something similar will happen to the Sun when its death approaches. During the main stage of stellar life, in the core of the star there is consuming hydrogeneither in nuclear reactions that go turning into helium. Helium is heavier than hydrogen and accumulates in the nucleus, compressing it more and more.

The layers of hydrogen near the core are also being compressed, and all this increase in pressure makes the star more luminous. But the outermost layers fall behind from this pressure and begin to expand. The expansion does not stop until the star increases several hundred times its size and becomes a red giant.

In the case of the Sun, when it becomes a red giant it will engulf the Earth.

During the red giant phase, the buildup of helium in the core compresses it so much that it begins to fuse and form carbon and other heavier elements.

The reaction is very fast and energetic, helium can be consumed in a few hundred million years, compared to the total life of this type of star, which can be several tens of billions of years.

The formation of heavier elements causes a higher pressure in the nucleus and it is compressed even more. The outermost layers also expand faster and begin to expel large amounts of gas.

The core is exposed forming a white dwarf illuminating ejected gas. Thus begins to form the planetary protonebula with its characteristic appearance as large gas clouds.

The Boomerang Nebula is in this phase where the gas in the outer layers of the star is rapidly expanding. The rate of expansion in the Boomerang Nebula can be up to 600 thousand km/hwhich generates a very sudden drop in temperature.

The effect occurs in any compressed gas that expands. With the expansion, the pressure decreases and the gas cools. The greater the expansion, the faster the temperature drop. This effect is the basis for the operation of domestic refrigerators and freezers that use a gas compressor for cooling.

And for this effect The Boomerang Nebula is the coldest place in the Universe., at least that we know of. Its temperature is 1 K, just 1 ºC above absolute zero.

The Boomerang Nebula is even cooler than the microwave background radiationthe remnant radiation from the Big Bang, whose average temperature is 2.76 K.

Compared to similar nebulae, the Boomerang Nebula is considerably cooler than one would expect, and this may be due precisely to its rapid rate of expansion, some 100 times faster than other nebulae.

The Boomerang Nebula is not only the coldest place in the Universe, it is the second known astronomical object whose temperature is lower than the temperature of the microwave background, the other being the Eridanus supervoid or CMB cold spot, a region of the microwave background whose temperature is 2.7 K, about 70 µK lower than the average temperature of the rest of the microwave background. It may not seem like much, but on these scales it makes a big difference.

The lowest temperature achieved in the laboratory

The Boomerang Nebula is the coldest place in the Universe, it is the lowest temperature observed natural object, but it is not the lowest temperature achieved. On Earth, in 2015, MIT scientists managed to cool a gas of sodium and potassium to a temperature of only 500 nanoKelvins, that is, 0.0000005 K.

In the Cold Atom Laboratoryon board the International Space Station, it was possible to create a Bose-Einstein condensate at ultra-cold temperatures, although they were not as low as the MIT results, it is the coldest temperature achieved in space.

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