How do we know it's the hottest thing?

LOL now explain a neutrino and gravitational wave to a 7years old

@BЈовић Tomorrow that same seven-year-old will be explaining a (somewhat imperfect) version of it to all their friends on the playground...

@BЈовић Gravitational wave: a wobble in the force due to gravity caused by the collison of two very massive objects - like the ripples on a pond when a stone is thrown in. Neutrino: a tiny, unseen particle, travelling at the speed of light, generated by extreme heat and that can pass through almost anything placed in its way.

"opaque to neutrinos" is certainly a phrase I didn't expect to see today.

@Hearth - The core of the SN has a density around $\rho \sim 10^{17}$ kg/m$^3$ and size of $\sim 30$ km, mainly in the form of neutrons and protons, so a baryon number density of $n \sim 10^{44}$ per m$^3$. The cross-section of a neutron/proton to neutrinos is about $\sigma \sim 10^{-47}$ m$^2$ at 1 MeV, so the mean free path of a neutrino is $1/n \sigma \sim 1$ km and the core is thus opaque to the neutrinos. But the cross-section falls rapidy with neutrino energy, so as the core cools, it becomes transparent to neutrinos.

@RobJeffries I'm not arguing the physics! Just saying that something being opaque to neutrinos is a rather foreign concept, as everything I've ever encountered and most things I can imagine would be transparent to them.

What is a billion for you? SI would be more appropriate.

@CacahueteFrito This is a science website. A billion is $10^9$. What do you mean by SI? Kelvins is an SI unit.

@RobJeffries For me a billion is 10^12. That's why. SI is the international system of units. Billion is clearly not SI. A European here :)

en.wikipedia.org/wiki/Billion @CacahueteFrito It is $10^9$ in English (which is what my answer is written in). And exclusively so in Astronomy, which is what this site is about. I have given the number in exponential format (not "SI") for the continentals.

@Hearth I just added this for interest.

@Hearth Check out Lethal neutrinos, and the ensuing forum discussion.

I'm in favor of "detectable by inference from $OTHERDATA" counting as observable.

@BruceBecker - science knows nothing, but you can " "observe" " things, and along with those observations will come sigmas which belies their validity. That's why, your sample sizes are too small and your standard deviations are too high, is a meme. That's also why science vs. religion is like a land war in Asia.

I thought it was going to be the inside of a black hole, but apparently it's really cold in there. Kinda hard to say one way or the other though... because it cannot be observed.

@BruceBecker: how can you accept this answer when you have already linked to a question, which states that 4*10^13 K have been reached in LHC?

@BЈовић My mother did some of her PhD work on neutrinos and I had learned about as much as a 7-year-old could learn about them by the time I turned 7...

@ThomasWeller it pays to read the question before commenting.

@Rob While I have no doubt you’re correct that astronomy, along with the rest of sciencedom, exclusively uses the short scale, it’s not quite correct to say that billion means 10^9 “in English” as such. The long scale meaning remains common enough colloquially in the UK to occasionally cause confusion, especially across generations.

@CacahueteFrito "Billion" hasn't meant 1012 in English (even British English) for about 50 years. My father would avoid "billion" because it meant 1012 to Britons of his parents' generation and 109 to Americans, but I have always used it to mean 109.

@JanusBahsJacquet I disagree. I'm British, and in my 60's. In my experience the long scale is completely dead.

@Martin Well, our experiences differ, then. I’m a good generation younger than you, but I have definitely experienced people talking past each other because one was using the long scale and the other the short scale.

@MartinBonner Well, I'm Spanish, and here we use exclusively the long scale, and we know that Americans only use the short scale (I didn't know the UK also adopted the short scale), so we have a big problem: Every time I read billion from someone that is not American, especially in Spanish news in Spanish, I don't know if they mean a billion (10^12 here), or if they just copied the information from some American source and didn't translate billion to milliard (10^9 here) (I've seen that happen enough times).

@CacahueteFrito But you are not reading in Spanish here. You are reading in English. It should not be a surprise that Spanish words are often meaningless in English, and if they exist in English quite often have different meanings. (Consider the Spanish and English meaning of negro.)

@MartinBonner Yeah, I just thought British English still used the long scale (that's what I was told at school), so I didn't know which English... :)

@JanusBahs Conversely I have NEVER read or heard any contemporary (last 20 years) usage of 1 billion that did not mean 1E9 and never, ever in a scientific context. The wikipedia link above clearly states that all forms of English regard 1 billion as 1E9. The word billion is used as such in all media, print, TV, and radio in the UK and never with an explanation that it means 1E9.