acidbaseextract

Just out of curiosity, why'd you leave? MSR sounds pretty great.

Watch that youtube link in my sibling comment first, but I actually wanted to take a crack at an answer. What's the change in mass, ∆m, as the star cools? We'll assume the mass of the neutron star is 1.5 solar masses.

    E = mc^2
    ∆m = ∆E/c^2

So really, what's ∆E? A hyper hand-wavy estimate:

    ∆E = Q = mc∆T (c being specific heat)

Specific heat by mass is really hard to predict, but by moles it is fairly constant, well within an order of magnitude. So we'll discuss mass in moles.

    m = neutron star moles
    m = (mass of neutron star / mass of neutron) / Avogadro's #
    m = 2.9580163e33 mol

    c = 24 J / (mol * K)

    ∆T = 1e12 K - 1e6 K

→

    ∆E = 24 * 2.9580163e33 * (1e12 - 1e6) J = 7.0991681e45 J

Substituting that back into the original, as a neutron star cools:

    ∆m = 7.0991681e45 J / c^2 = 7.89888982e28 kg

Which is like 2.6% of the mass of the original star, so a pretty solid chunk. But that number is pulled out of my ass—I am not a physicist.

But there are even more weird effects going on, due to the warping of gravity the mass of neutron stars can be up to 20% less than you'd expect based on its baryonic (neutron) constituents (questions 4 and 7):

https://www.astro.umd.edu/~miller/teaching/questions/neutron...

"The True Nature of Matter and Mass" - https://www.youtube.com/watch?v=gSKzgpt4HBU

This whole channel will be up your alley, but this video and "The Real Meaning of E=mc^2" one directly answer your question.

Nima Arkani-Hamed is an amazing speaker, and if you're interested in modern physics I highly recommend watching his talks. A great place to start is his 5 part Messenger lecture series. It starts with the discovery of quantum mechanics, and the last 3 talks all cover different frontiers of fundamental physics today.

http://www.cornell.edu/video/playlist/nima-arkani-hamed-on-f...

Introduction ends and Nima starts about 4 minutes into the first lecture. :)