“What’s a black hole?”, Lyra asked sheepishly while sipping her tea.

“Oh, it’s a term from astrophysics, designating an object so massive, it’s orbital velocity is the speed of light in vacuum”, Hermione explained. “Muggles did quite some advancements in philosophy of the natural world, as you would know it, having read ‘Principia’ in its original language”.

“And astrophysics is… let me guess, nature of the stars, if I translate directly from Greek?”, Lyra rubbed her chin.

“Close enough”, Hermione nodded. “Physics is the word they use for what used to be known as philosophia naturalis, or philosophy of the physical world, which was abbreviated to physics. Astrophysics is a term invented rather recently, but your library kind of doesn’t have anything since the establishment of the Statute of Secrecy, so I guess Newton’s ‘Philosophiae Naturalis Principia Mathematica’ counts as one of the most recent works of our joint civilization”.

“I mean, I know some of it – how the light breaks from white into a spectrum, and I can infer that the orbital velocity is the velocity at which speed of the object follows curvature of the Earth, or another celestial body, so that it keeps falling in perpetuity without hitting the ground. I don’t know that the light has a finite speed, or why it would be different in vacuum”, Lyra shrugged.

“So, you’re basically the greatest mind of seventeenth century science?”, Hermione giggled.

“Pretty much”, Lyra took a biscuit with apricot jam. “Would you please give me a crash course in the twentieth century physics, as they call it?”, she winked.

“Can I listen?”, Arthur’s eyes were wide.

“Sure, but keep up”, Hermione nodded.

“So, I’ll have to explain several concepts, but they rely heavily on what you already know. For instance, yes, the speed of light exists, and it’s different in vacuum because light consists of tiny luminous particles called photons, that travel straight and unimpeded in the vacuum of space, while they bounce into particles of matter when they travel in a liquid or gas, or they outright get absorbed in solid matter. The speed of light in vacuum is around 300000 km/s, and less in a medium”, Hermione looked at her to see if she’s keeping up.

“So, when you say that an object has an orbital velocity of the speed of light, this means that if you turn on a source of light on its surface, the particles of light would bend around it and keep falling in perpetuity rather than escape into space, which means no light can escape it, which explains why it’s called a black hole”, she nodded. “Things fall in, and nothing comes out, since I assume nothing is faster than light?”, she inquired, as Arthur looked at her with mouth agape.

“Correct”, Hermione smiled brilliantly.

“Does the speed of light depend on the colour of its components? For instance, is blue faster than red?”, Lyra frowned.

“No. The components differ in their specific energy, that can be measured when it is absorbed by matter, but their speed is the same. Also, and this will be weird, light particles propagate in waves, and so each photon has a specific wavelength, which is, I guess, a distance between two adjacent crests on a wave as it propagates. Red has a longer wavelength while blue has shorter, because, I guess, like a compressed spring that contains more energy”, Hermione struggled to explain.

“I figured out some of it from Newton’s ‘Opticks’”, Lyra nodded. “Light can interfere as if it were a wave, and yet it also behaves as if it were made of particles. I’m glad to hear that a definite conclusion was made later regarding that”.

It was now Hermione’s turn to stare with mouth open, but she collected herself. “Anyway, they learned other things, such as the fact that the wavelength of light stretches or compresses depending on whether the object radiating light is travelling toward you or away from you, and this is also the case with sound and other waves”.

“You mean, that’s why sounds become higher as I travel toward them fast on a broom, and lower as I travel away from them?”, Lyra perked up in realisation.

“Exactly so”, Hermione confirmed. “It’s called the Doppler effect”.

“How does this relate to astrophysics?”, Lyra asked.

“Well, most directly, it allows us to know whether some star is moving towards us or away from us”, Hermione answered.

“Oh, I get it. The light it produces would shift towards red or blue. If it is reddish, it would mean that it is moving away from us, and if it is bluish, it would mean it’s moving towards us. But how can you tell whether it’s a star that travels away from us, or it’s just a star of a naturally reddish colour?”, Lyra wanted to know.

Hermione looked at her with complete amazement, thinking that’s probably how Newton would look at Archimedes or Aristotle if they got the chance to talk. “Now, there’s a thing. The stars have types, and they have a life cycle, and there’s a specific characteristic they tend to have depending on their mass and their place in their lifecycle. If you know that a star is naturally yellow, and you perceive it as reddish, it means its spectrum is shifted”.

Lyra nodded. “I get it, but I guess you would have to know first in order to extrapolate the transformation”.

“Exactly, which is why there’s lots of assumptions in astrophysics, and occasionally it gets corrected, when they realise that something isn’t a very big red-shifted object very far away, but a much smaller, naturally red object much closer”, Hermione nodded.

“I get it”, Lyra nodded in return. “What are stars, actually?”

“They are in essence the same category of objects as our Sun. They are big, very massive, and they produce light by fusing light atoms of matter into heavier ones due to the extreme pressure in their cores”.

“There are many things I don’t understand there, for instance how that actually works, why atoms need to be pressed together and why they produce light when they are joined together”, Lyra frowned.

“Well, what you need to understand is that atoms don’t actually exist, in the way Democritus imagined them. Sure, matter consists of smaller particles, but those particles consist of even smaller particles. Every atom consists of a nucleus and a swarm of electrons, that have a negative electric charge. The nucleus consists of protons and neutrons that are all bound together. Protons and neutrons have about the same mass, and all the mass of an atom is contained in its nucleus. Protons differ from the neutrons in that they have a positive electric charge. There are forces in the atom that keep the electrons attracted to the core at one distance, and repelled when they come closer”, Hermione struggled to explain.

Lyra gave it a thought. “What is an electric charge?”

Hermione started fidgeting and pulling at her hair as she tried to figure out an explanation. “The ancient Greeks discovered electric charge when they were rubbing amber on wool and it would produce glowing sparks”.

“Ah, so that’s why those negatively charged particles are called elektron, which means amber”, Lyra perked up.

“Exactly. Such sparks happen when there’s a difference of electrically charged particles in different objects, and when they equalize with a spark. Lightning is merely a very big spark. The muggles made all sorts of discoveries regarding electricity, like the way electricity can travel through metal and produce magnetism. You know what that is, right?”, Hermione was curious.

“Yes, certain stones tend to point in a north-south direction and attach themselves to iron”, Lyra nodded.

“When electric charge travels through a metal wire, it produces the same effect, and it’s called electromagnetism. Similarly, as Earth spins, making a full circle every day, some of its molten interior layers spin at different speed due to drag, and it creates a magnetic field around the entire Earth, which manifests as magnetic poles, which attract the magnets to themselves and create the phenomenon you are familiar with”, Hermione confirmed.

Lyra thought about it. “I guess different kinds of matter have different structure, which is why it works for metal and not for wood or dirt. I guess those electrons around a nucleus in an atom, that are attracted at one distance and repelled at another, can also be lost, or exchanged between atoms, or expelled to create an electric charge”.

Hermione was even more stunned, but tried to continue. “Very much so. All the matter we normally see is made when two atoms share electrons between them. Those are called chemical bonds, and they combine atoms of different elemental substances, also called elements, into chemical compounds, such as water or salt”, she nodded.

“And I guess the atoms can have more or less energy – if something is cold, it would have less energy and the atoms would come closer together and form a solid, but if you heat it up, it would form a liquid and then a gas, as the atoms get far enough apart from the excess of energy that they fly off”, Lyra offered.

Hermione’s eyes went wide. “Exactly so. And if you compact the atoms together by force, introducing outside pressure, they will bleed the excess energy until they become cold enough to naturally be able to function at such proximity without repelling each other”.

“And if you apply enough pressure, the cores of different atoms, that are normally separated by a layer of electrons, will bump into each other and form a different kind of atom, with all their combined protons and neutrons forming a new nucleus?”, Lyra inquired, to Hermione’s amazement. Arthur, on the other hand, lost them very long ago and just looked confused.

“That’s exactly right”, Hermione confirmed. “And in some cases, when the atoms are light because they contain only one or two protons, this will produce an excess of energy as some of the bonds within the nucleus release in form of light, which is what happens in the stars. On the other hand, when very heavy atoms are broken apart, which can also happen, the excess of energy is released and two lighter atoms are produced”.

“So, I guess depending on how heavy a star is, it will produce slightly different light. Also, as it keeps spending the light atoms and producing the heavy ones, it will eventually run out of light atoms and this is probably what you meant as stars aging through their life cycle”, Lyra offered.

“This is exactly right”, Hermione confirmed, with profound amazement. “And when a star grows very old, and runs out of all atoms that are light enough to produce an excess of energy in a process of nuclear fusion, the balance of pressure and gravity is disturbed because production of light no longer creates an outward pressure and all the matter collapses inwards, creating an incredibly strong explosion”.

“Wait, are those the stellae novae the ancients wrote about? When a new star appears, or becomes much brighter for a while, only to fade away later?”, Lyra remembered.

“It is exactly so, and this is why in astrophysics, those are indeed called stellae novae, or just novae. There’s also a category of “super nova”, which is a particularly large explosion that happens with stars much massive than our Sun, because the energies involved are much greater. Also, when those extreme energies and masses are involved, there gets to be a remnant of the exploding star, that is so dense, that all the mass collapses together with such force, that the protons lose their electric charge and transform into neutrons, and the entire object consists of neutrons tightly pressing together, with the entire thing rotating as fast as thousands of times per second”.

“Oh, so that’s what you meant by a ‘neutron star’. It’s a remnant of a dying star that spins very quickly and is made of only neutrons”, Lyra nodded.

“Exactly. And this object is very small and incredibly dense, and this creates incredibly sudden, violent rises in gravity, which means that if you were to fall into it, it would pull your feet so much stronger than it would pull your head, that it would exceed the attractive forces between the atoms of your body and draw you out into a string of liquid. This is actually called ‘spaghettification’ in astrophysics. When we add the effects like frame-dragging caused by the extremely fast spin of the neutron star and a black hole, you get the effect where the spaghetti-ribbon consisting of extremely overheated atoms of the unfortunate thing falling in start wrapping around the massive object as a glowing ring”, Hermione explained.

“Which is what you did to those unfortunate idiots”, Lyra grinned.

“Exactly”, Hermione smiled.

“And I guess the sole difference between a neutron star and a black hole is in its gravity, because a black hole is so massive that even the light can’t escape it, while a neutron star is close enough but not there yet”, Lyra raised an eyebrow.

“Exactly so”, Hermione nodded. “The distance from the black hole at which the escape velocity is the speed of light is called the ‘event horizon’, because it’s the point after which nothing can escape the black hole to tell the tale of what’s inside”.

Lyra nodded. “I think I got it. Can you please get me some muggle books about that, so that I can learn it in more detail? It sounds very interesting, and I’m completely ignorant of it all, obviously”.

“You may be ignorant, but this was the most impressive feat of raw intelligence that I ever witnessed in my entire life, and if I didn’t already worship you as my personal deity, I’d start now”, Hermione looked at her sister adoringly.

“Are the two of you even human?”, Arthur managed to say. “Because I don’t think so”.

“Oh, I’m just a very smart human”, Hermione winked. “I still don’t know what she is”, she added.

“Hermione, you held my hand and checked my cervix for dilation when I was giving birth. You know exactly how human I am”, Lyra smiled.

“True, but you just may be some deity taking human form”, Hermione pointed out. “It’s actually the most parsimonious explanation”, she grinned as her sister hid her head under the sweater and whined.