Shulou(Shulou.com)11/24 Report--

Last time, it was said that the polar craters on the moon, due to the absence of sunlight for many years, have become the coldest places in the solar system outside the laboratory. The coldest place in the solar system is in the lunar crater. Where will the coldest place in the universe be?

Note that there are two premises: 1, except for the laboratory. 2. It has been detected so far. That is to say, the place must be naturally formed and must have been observed.

Last time, many people mentioned stray planets, stray planets, this kind of interstellar object because it is far away from the star, if it is small enough and does not have an atmosphere to keep warm, it can indeed reach a very low temperature. In theory, as long as a celestial body is far away from all sources of radiation and cools itself thoroughly, the only thing that can bring heat to it is the "residual temperature" of the Big Bang.

As the afterglow of the Big Bang, the cosmic microwave background radiation has expanded with the universe, from more than 3000 degrees to less than 3K today. Since it is the "background temperature" of the universe, there should be no colder place than it under normal circumstances, right?

In addition, although the background radiation has remarkable isotropic characteristics, there are still differences between different regions to a certain extent. For example, you can see that the color in the picture is significantly bluer than the surrounding area, indicating that there is a lower temperature, so it is called "Cold Spot". Could this be the coldest place in the universe?

However, the background radiation is far more uniform than most people think. In fact, the precision of the picture with color difference that we see is very high. How high is it? After excluding the relative motion of the earth in the universe, the temperature difference here is only about 1/100000. According to Planck satellite data, the temperature of the "cold spot" is only about 100 μ K lower than the average temperature of the background radiation. So it's the coldest here, but it doesn't make much sense.

In addition, the temperature of the background radiation is only the background temperature of the universe, in fact, there are all kinds of heat sources in the universe.

If it is in a galaxy, stellar winds, supernovae and various cosmic rays all act as heat sources in space. Therefore, in the interstellar space of our Milky way galaxy, the temperature is very difficult to approach the background radiation temperature. Only out of the Milky way, in the broader intergalactic "intergalactic space", the microwave background may become the only source of heat.

But in fact, even the intergalactic space close to the background radiation is not the coldest place. There's a place where the temperature is even lower than the background radiation, and it's not anywhere else, just in our Milky way.

In the 1980s, astronomers discovered a somewhat strange nebula. Limited to the equipment at that time, people could not see its appearance very clearly at that time, and they could only vaguely see that there were two lobes inside it, and it was not quite symmetrical, which was a bit like a boomerang, so the nebula was named the "Spindle Dart Nebula", also known as the "Power Rod Nebula".

In 1998, through the Hubble Space Telescope, people finally saw the nebula, the whole shape is very symmetrical, like a "bow tie". So people gave it a more apt name-the Bowtie Nebula.

This is a protoplanetary nebula (PPN), or protoplanetary nebula, only 5000 light-years away.

The planetary nebula (PN) is a nebula formed after the death of sun-like stars; the proplanetary nebula is not yet a planetary nebula, but the star is at the end of its evolution and is about to become the intermediate state of the planetary nebula. Nebulae in this state generally last for a short time, usually only a few thousand years, so stars and nebulae at this stage are rare, and only a dozen cases have been found so far. But in these more than a dozen cases, the bow knot nebula shows its uniqueness.

About 1500 years ago, a sun-like star entered a late stage of evolution and gradually expanded into a red giant. When the red giant star was throwing its own matter out, something happened-a small mass companion star crashed into it. The impact made the otherwise ordinary nebula no longer ordinary.

Due to the sudden intrusion of the companion star, the outer layer of the red giant was badly torn, and nearby gas ejected from the center of the nebula at 164km/s speed, 10 times faster than the usual red giant throwing material. The outflow of gas spurted out for nearly two light-years, and the gas caused the nebula to expand rapidly along the way. The shape of boomerang and bow tie is made up of these high-speed outflows and extremely rapidly expanding gases.

This extraordinary speed means that the matter here has a very high momentum. In this way, these substances are supposed to lower their temperature by doing work, which is called adiabatic expansion or adiabatic cooling in thermodynamics.

Adiabatic expansion means that in an isolated system where there is no heat exchange with the outside world (or heat exchange can be ignored), the external work done by the expansion of the gas will cause the temperature to drop. The cosmic expansion leads to the decrease of background radiation temperature, which can be regarded as a kind of adiabatic expansion.

In fact, there are examples of this phenomenon in daily life:

I wonder if you ever dropped a lighter when you were a child. When the lighter is broken, the liquefied gas inside will evaporate quickly because of the loss of pressure, and you will find it very cold when you touch its shell. This is caused by adiabatic expansion.

And for example, why do you feel hot when you breathe and cool when you blow? Some people say the wind speed is different. Then try blowing slowly or breathing quickly, which is cold and which is hot. In fact, this can be explained from the perspective of adiabatic expansion: the original volume of the gas is relatively large, the diffusion rate will be relatively slow, while the volume of the gas blown out is very small, the diffusion rate is relatively fast, so it will feel cooler.

So if there is a similar phenomenon in this nebula, it will cool much faster than the radiation given to it by the star. Based on this, astronomers predict that the nebula can reach very low temperatures, even lower than any other naturally occurring temperature in the universe.

Because the spectral line intensity of CO molecules can accurately reflect the temperature of the gas cloud, astronomers first measured the temperature of the nebula-272.15 °C (1K)-- through the Atacama Radio Telescope (ALMA) in 1995, only 1 degree higher than absolute zero! The nebula has also become the only natural object found so far that the temperature is lower than the background radiation.

Because the background radiation is 2.7K, which means that the matter in this nebula is actually absorbing the energy of the background radiation! How outrageous this is. However, as the expansion rate of the gas cloud slows down, it will eventually reach thermal equilibrium with its surroundings. So far, it has been observed that there are signs of warming on the periphery of the nebula.

Maybe five billion years later, when our sun reaches this stage, the solar system may become another coldest place in the universe.

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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