Neutron stars are typical astrophysical objects that many have heard about. Like black holes by the way.

Who has not heard anything about those guys, especially in this gastrophysics era where the detection of gravitational waves is all over the place.

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![](https://s18.postimg.org/yzz6ytqnd/neutronst.jpg)
<sub> [image credits: [Wikipedia](https://en.wikipedia.org/wiki/Neutron_star)]</sub> 
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One of the interesting outcome of the recent observations of gravitational waves is that lots of activities have been triggered around **compact stellar objects**.

This morning, browsing the new research papers of the day, I have read [**a very interesting article**](https://arxiv.org/abs/1802.06685) about **pion stars**.

This triggered my attention as I have never thought about pion stars before. As it can be guessed from the wording, pion stars are potential stars made of  [**pions**](https://en.wikipedia.org/wiki/Pion). Surprize, surprize…

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Stars made of bosons (pions are  [**bosonic particles**](https://en.wikipedia.org/wiki/Boson)) have been studied for more than 50 years (see [**here**](https://arxiv.org/abs/1202.5809) for a review, not too accessible to anyone without any background in physics). Although this works nicely theoretically-speaking, not a single observation has allowed today to conclude to their existence.

So why bothering about pion stars? I will let you know, but before, let me re-explain what is a pion.

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## WHAT IS A PION? ##

Before going further, let us discuss pions. In the world of the elementary particles, the tiniest bits of matter are some guys called **quarks** (see [**this post**](https://steemit.com/steemstem/@lemouth/a-crash-course-on-particle-physics-to-get-prepared-for-the-steemstem-meetup-at-cern-1-the-structure-of-matter) for more information). 

**Quarks are strongly interacting particles**. This means, that they are interacting with each other by virtue of strong interactions, the strongest of all fundamental forces of nature that is also known as **quantum chromodynamics** (see [**here**](https://steemit.com/steemstem/@lemouth/the-mysteries-of-quantum-chromodynamics-from-quarks-to-the-sexy-sexaquark)).

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<center>  ![](https://s1.postimg.org/1t1uqb6j7z/quarks.png)
<sub> [image credits: [Nobel Prize](http://www.nobelprize.org/nobel_prizes/physics/laureates/2004/illpres/)]</sub> </center> 
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And thanks to the properties of the strong interactions, quarks can form composite systems, *i.e.* particles made of several quarks. 

This is sketched on the figure on the right where three quarks are glued together thanks to strong interactions. The most well-known of these systems are incontrovertibly the protons and the neutrons, but many other options are possible.

In particular, it exist a class of particles named [**mesons**](https://en.wikipedia.org/wiki/Meson) that are **made of one quark and one antiquark**.

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<center>  ![](https://s1.postimg.org/7ylfrm1gnj/pion.png)
<sub> [image credits: [Wikipedia](https://en.wikipedia.org/wiki/Pion)]</sub> 
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We have discovered a whole bunch of mesons during the last hundred years, and those we know the best are the **pions**, *i.e.* the guys in which we are interested in this post (see the picture on the left for an example).

Pions are particles **made of up and down quarks and antiquarks** and can thus be either neutral (up-antiup or down-antidown) or charged (up-antidown or down-antiup). 

I admit that life is in reality much more complicated than this, but let’s stick to this for the sake of simplicity.

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## FROM PIONS TO PION STARS ##

A few **0.0001 seconds after the Big Bang**, in Standard Cosmology (see [**here**](https://steemit.com/science/@lemouth/standard-cosmology-in-a-nutshell) for more information), **the Universe is comprised mainly of a gas of pions mixed with electrons, muons and neutrinos**. The universe is indeed old enough so that all quarks have started to form composite objects.

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<center>![](https://s29.postimg.org/4gh3yvps7/univ.jpg)
<sub> [image credits: [pixabay](https://pixabay.com/en/milky-way-rocks-night-landscape-1096815/)]</sub> </center>
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We can therefore imagine that some of those pions could condensate and **form a pion star**. The conditions for this mechanism to occur could indeed be realized.

The only drawback is that pions are unstable. How could then the star survive? This is where [**the research paper**](https://arxiv.org/abs/1802.06685) mentioned earlier comes into the game. 

The authors have shown that pion stars were objects for which standard (and computer-intensive) lattice simulations of quantum chromodynamics were achievable. In other words, **the properties of the pion stars can be evaluated from first principles thanks to simulations on super-computers**.

They did the calculations and demonstrated that there were options in which the weak and electromagnetic decays of the pion condensates were not allowed, although usually, pions are known to be unstable and to decay either via weak or via electromagnetic interactions. 

In other words, the authors have shown that it was possible that the pion condensate was stable, or that **pion stars could exist!** 

Just to give some details, pion stars are actually stabilized by the presence of the surrounding electrons and muons. The stars will then still decay, but potentially after billions of years so that stars of masses reaching 250 times the solar mass could still be present in our universe today.

What is left to do? Well, pion stars have not been observed so far so that they are still classified among the ensemble of hypothetical astrophysical objects. This nonetheless does not contradicts the fact that they could exist.
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## SUMMARY ##

In this post, I introduced a new class of astrophysical objects that I have never about before: pion stars. I explained how they were formed and why this was thought impossible until know. What has changed? Supercomputers that allowed to run simulations and show that there were theory configurations in which those stars could exist!

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