However, if the neutron star exceeds the Tolman-Oppenheimer-Volkoff limit (which corresponds to eight solar masses for the parent star), even the neutrons can no longer counteract gravity sufficiently, and a black hole forms. The gravitational pull of the black hole is so strong that not even light can escape it - what disappears behind the so-called event horizon never comes to light again. This area would have a radius of several kilometers for a black hole with the mass of the sun. Because of the rotation of most black holes, however, it is rarely spherical, but has the shape of a rotational ellipsoid (a body spanned by a rotating ellipse).
Black holes were predicted as early as the nineteenth century. Until the 1960s, however, they were thought to be more of a mathematical curiosity, a special solution to Einstein's equations of general relativity for point-like masses. But when pulsars (neutron stars) were discovered in 1967, they moved into the realm of possibility. In 1971, Cygnus X-1/HDE 226868 was identified as the first candidate for a binary system of black holes.
Black holes cannot be observed directly - but they can be observed by their effects. For example, the orbits of other stars in multiple systems can be studied, suggesting the presence of an invisible object. Black holes can also act as gravitational lenses through their gravitational attraction, amplifying the light of stars that are actually far behind them - observation then reveals a difference between actual brightness and the brightness expected according to the nature of the celestial object.
What it looks like inside a black hole is as difficult to determine as the state of the universe shortly after the Big Bang. The main problem is that it is a singularity in which the currently known laws no longer apply. The density of a black hole goes towards infinity. The terms space and time lose their sense, therefore one speaks also of a hole in the space-time. Possibly black holes form transitions into other universes - however these are speculations, typical for science fiction authors.
But black holes do not only exist as remnants of a stellar explosion: Almost every galaxy has a superheavy black hole in its center. One assumes that these giants have eaten themselves big at matter from the galaxy on the one hand and through galaxy mergers on the other hand.
In the core of the Milky Way one suspects such a gigantic black hole called Sagittarius A*. It is about as heavy as four million suns. This mass is concentrated in a sphere with a Diameter of 44 million kilometers. In the solar system, this black hole would reach halfway to the innermost planet Mercury.
Even Stranger: Exotic Stars
Quark stars are made of Strange Matter, but they do not represent the strangest objects scientists can imagine. There are three other candidates for this award. First, there is the electroweak star. It generates energy by "burning" quarks into leptons (which include electrons), a process that can be attributed to the electroweak interaction. Such a star could be the last stage before the black hole, because the energy produced by it could possibly still stop the gravitational collapse. The nucleus in which the reaction takes place would be just about the size of an apple, but would have the mass of two Earths. The electroweak combustion could last up to ten million years as the last phase before the formation of a black hole.
Also pure theory are the preon stars. Preons are hypothetical, point-like particles, from which quarks and leptons are built up. Current experiments speak rather against their existence which was popular in the 1970-ies. According to the theory, preon stars should be tremendously dense, but they should have a radius of only 40 meters and weigh as much as 0.013 suns.
More likely are the boson stars. Bosons are elementary particles which have an integer spin (angular momentum). They include, for example, the photon (light particle) and the Higgs boson, which physicists have detected at the LHC particle accelerator. They can all assume the same quantum state - unlike their brothers, the fermions, which make up ordinary matter. That's why they can be packed particularly tightly. However, they cannot be considered as an alternative to the observed black holes in the core of the Milky Way, for example, because these objects would then have to be significantly larger.
How big can stars become? The hypergiants
Even more massive stars are rather rare in the observable part of the universe. This could have physical reasons: If the star generates too much energy, its own gravity can no longer withstand the pressure from its interior, and part of the envelope is blown into space - until a stable equilibrium is re-established.
This seems to have happened to R136a1, the heaviest star discovered so far, with 265 solar masses: At its formation, astronomers assume, it had 50 solar masses more. The hypergiant, which is 165,000 light-years away from Earth, shines 8.7 million times brighter than the sun.
At the end of its comparatively short lifetime, it is likely to go hypernova - a supernova characterized by a particularly high energy output, equivalent to about 100 supernovas. Some researchers suspect that the mass extinction on Earth 440 million years ago was triggered by the hypernova of a nearby star - but there is no conclusive evidence for this. Meanwhile, there is no sufficiently large star near Earth that could be a candidate for a hypernova.
Which blue hypergiants are there?
Deneb, the brightest star from Earth in the constellation of the Swan, which together with Altair and Vega forms the Summer Triangle.
P Cygni, also in the constellation of the Swan, whose extremely fluctuating absolute brightness currently exceeds even that of Deneb.
S Doradus in the nearby Large Magellanic Cloud.
Eta Carinae within the Carina Nebula (NGC 3372). Eta Carinae is extremely massive, possibly having a mass up to 120 or 150 times that of the Sun, and is 4 or 5 million times brighter than the Sun.
The Pistol star in the Quintuplet star cluster near the center of our galaxy in the constellation Sagittarius. It is possible that this star has 150 times the mass of the Sun and is 1.7 million times brighter than the Sun.
Several stars in the star cluster 1806-20. These are located on the opposite side of our galaxy from our solar system. One of these stars, LBV 1806-20, is the brightest star known so far, about two to 40 million times brighter than the Sun. At the same time it is one of the most massive stars ever.
Which yellow hypergiants are there?
Only a few are known in our galaxy:
Rho Cassiopeiae (7 Cas) in the northern constellation Cassiopeia is about 550,000 times brighter than the Sun and 10,000 light-years away from us. Based on its current pulsations, it is a candidate for the next supernova in our Galaxy.
V509 Cassiopeiae
V382 Carinae
IRC+10420, in the constellation Eagle
HR 5171 A, in the constellation Centaur
some stars in the star cluster Westerlund 1
What are the red supergiants?
RW Cephei
NML Cygni
VX Sagittarii
S Persei
WOH G64
By the way, among all astronomical objects a not quite unsightly star is the very best to observe - our sun, our life-giver. On average, 1367 watts per square meter reach us from it, about as much as a washing machine consumes. Our planet reflects just under a third of this, but the rest, together with the greenhouse effect of the atmosphere, is enough to heat the earth's surface to about 15 °C. So you'd best thank the sun the next time you look up at the sky.
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Excerpt: Into the Darkness
Darknight 28, 3890, Shuttle
“My shoulder joint’s still creaking.”
Marchenko raised his right load-arm and Adam shined his flashlight into the slot below the joint. The inner wall was covered with a thick slime.
“I’m going to need the compressed air again.”
With his touch-hand, Loknor passed the tube to Adam, who then held it in front of the opening.
“Turn it on, please,” he said.
<
br /> The tube twitched in Adam's hand as if it were alive. Adam aimed for the joint and some of the black material sprayed out and splattered on his forehead.
“Dammit!” he cursed. “Couldn't you have gotten the joint to be sealed? Who knows where else dirt has gotten into your body?”
“If I'm not mistaken, humans have one or two openings,” said Marchenko, roaring with laughter. “But in all seriousness, there’s got to be some direct heat dissipation. This body doesn’t have a circulatory system like yours does.”
Adam shone the light again into the slot below the joint.
“Nothing left to see,” he said. “I think that whatever didn’t land on my forehead has disappeared inside.”
“I’ll have to get opened up aboard the Draght at some point,” said Marchenko.
“Yeah, that I would like to see.”
“Seeing won't be enough. I'm going to need your help, Adam.”
“More S_$x air?” Loknor asked.
The Grosnop had learned his language with surprising speed. Loknor was going to be something else. Gradually it became embarrassing that they hardly spoke a word of the Grosnops’ language, even though they’d been in their company for so long. But they were simply unable to vocalize certain sounds in the ultra- and infrasound range. Marchenko was capable of this, but he was a robot, though he had a human being’s consciousness.
“Thank you, Loknor. I think we're done now,” said Adam.
Adam scratched at his stomach, then the crook of his arm, and finally between his legs.
“Did you get some vascular mites?” Gronolf asked.
“That can't be, General,” said Numbark. “It’s been so long since we were on Double Sun.”
“He could’ve caught them from one of us.” Gronolf pointed to the youngster Numbark, whom Eve had saved and secretly brought aboard the ship. “It’s unlikely he got a medical exam before departure.”
“I just need a shower,” Adam said.
“Marchenko, radio transmission!” hollered Loknor, who was working as the radio operator aboard this reconnaissance ship.
The heavy robot pushed past Adam.
“Adam, you should still get that checked out when we return to the Draght,” Gronolf warned. “I’m advising you as your friend. Vascular mites are no joke. They move around in your blood vessels for years, and once there’s too high a concentration of them, they find their way to the skin. Then that’s where it gets itchy.”
“Well, first of all, I’m going to take a shower. Then if it still itches, I’ll have Marchenko take a look at it. Your doctors don’t know that much about human physiology.”
“I wasn't done yet,” said Gronolf. “First it itches, and then they remove your skin layer by layer until the raw, green flesh is visible.”
Adam shuddered. “Our blood is red. And I’m still going to start with a shower.”
“I'm afraid the shower will have to wait a bit,” said Marchenko.
* * *
They stood around Loknor and stared at the screen, which had a sinusoidal signal moving across it. Marchenko's touch-hand rested on the console below, and the end of his arm was plugged into an interface to the right of the console. Marchenko had connected himself directly to the ship's computer. He kept his eyes closed and swung back and forth on his broad hips as if he were listening to an enchanting song.
“The signal is from the Messenger,” he said. “It's coming through loud and clear.”
“Then why haven't we received it yet?” Adam asked.
“We were probably in the radio shadow of the source.”
“Which is where, exactly?”
“At the moment, it’s not possible to locate it with much precision. But right now I'm determining what places have been in the radio shadow so far.”
“It must be a pretty large area.”
“We’ve got the advantage of being able to include the Draght and the scout ship, since previously neither had received anything from the source. And while the Draght got the signal yesterday, it didn’t reach us until today. This means the area is small.”
“And where is it?” Adam asked.
“Right over Luhman-16 B, the second brown dwarf. I was right! The Creator sent a Messenger ship into this system, too.”
Marchenko's arm rotated several times in the elbow joint and then he withdrew himself from the interface. The robot put his hand back on the stump and screwed it on tightly.
“Don't keep calling him the Creator,” Adam said. “He was a criminal.”
Marchenko took the arm he’d just reattached and placed it on Adam’s shoulder. It was warm. Even though the palm was made of hard metal and thus not at all soft, Adam was comforted by the gesture. Marchenko was the closest thing Adam had to a father. His genes had come from a criminal on Earth who probably died long ago, but this didn’t have to play any role in his life.
“I'm sorry, Adam. Without him, you wouldn’t exist, and I think that would be a real shame. But you’re right. Ethically, this man's behavior was unacceptable. He made all the decisions about your lives for you.”
Marchenko pressed on his shoulder. The hand was heavy, but Adam didn’t resist.
* * *
“Gronolf? $-X_grok_ *x_sham Luhman-16 B,” said Marchenko.
This made Adam sit up and take notice. The fact that Marchenko was speaking in the language of the Grosnops meant he must have been worried
“I understand, my friend,” Gronolf answered. “Loknor, *x_sham Luhman-16 B.”
A force started pushing him against the armrest. Loknor must have started up a thruster. They were probably headed for the planet known as Luhman-16 Bb, which was in orbit around the star known as Luhman-16 B. Eve would be irritated, since she was still waiting aboard the Draght and was missing out an yet another expedition. At the moment, Adam wouldn't have minded if she’d been there to come along. He didn’t need another adventure like the one he’d had on Luhman-16 A.
“Jolk x-*hok, Gronolf,” said the robot.
Adam interpreted these words as, “Thank you.”
“I trust you, Marchenko. But I’d like to ask another question.”
“Of course. I’m sure you want to know what kind of message we intercepted.”
Gronolf struck his abdomen in confirmation. “Is it an emergency?”
“It's a farewell message,” said Marchenko.
“Who’s out there saying goodbye?” Adam asked.
“It’s me. That is, the replica of me that’s aboard the Messenger spacecraft that landed on Luhman-16 Bb.”
“But why? Has the ship moved on to the nearest star?”
“No, Adam, that's not possible. Once the Messenger’s slowed down, nobody can get it up to interstellar speeds again. It would take thousands of years.”
“So they flew to Luhman-16A?”
This is where they’d just came from. Surely they would have noticed a spacecraft from Earth.
“There was no launch,” said Marchenko. “The ship must still be on Luhman-16 Bb. The other Marchenko is saying goodbye because he wants to power down.”
All Messenger spaceships had an AI named Marchenko. Each one had the mission of raising a woman and a man – Eve and Adam – and then go exploring their destination planet with the two of them. This was how he and Eve had ended up on Proxima Centauri.
“Will power down or wants to?”
“From the message, it isn’t clear. But in his situation, I wouldn’t repeatedly announce something if I was still just in the planning phase. And since the AI down there must be similar to me structurally…”
“You can't know that,” Adam said. “Many years have gone by since we launched. You’ve evolved, and so has that Marchenko down there. It would be a great coincidence if you’d changed in the same way.”
“That’s what I’m hoping,” said Marchenko. “I would only consider turning myself off, if at all, so long as you were provided for.”
“Then what’s happened to his Adam and Eve?”
<
br /> “We'll have to find that out. When the AI powers down, they'll be on their own. If they're still alive. I hope we don’t get there too late.”
Darknight 29, 3890, Shuttle
“Do you even have enough fuel?”
Adam turned down the volume a bit. When Eve was upset, she got instinctively louder, and he didn’t want to be responsible for his fellow travelers waking up. They had a grueling expedition behind them, and the days ahead were bound to be equally as stressful.
“Yes, the planet is considerably smaller and lighter than the one we’re coming from. We'll need two-thirds less energy for takeoff and landing than on Double Sun.”
“It would have been safer if you’d refueled at the Majestic Draght.”
“And then we could have taken you with us. I know. But that would have taken at least five days, and Marchenko's in quite a hurry.”
Eve sighed. “It’s pretty boring without you on board. I don't know anyone here. Even Ragnor is with you.”
“Next time you’ll fly along. I'll do everything I can to make sure it happens.”
“Hey, you idiot, if you hadn't tricked me, I'd be there with you now!”
“I'm really sorry.”
“Easy for you to say.”
“You could talk to the Omniscience.”
“I've thought of that, too. Without the Omniscience, I probably would have died of boredom long ago.”
“I’ve got to sign off now, or I'll wake up someone here.”
“Enjoy the rest of your trip, you traitor.”
* * *
Adam lay down on his side, pulled the blanket up to his chin, and then fastened the belt over it so that something pressed him onto the lounger. There were new problems that came with sleeping in zero gravity. Or was it the strange sounds that the Grosnops made as they slept, grunting and groaning as if they were constantly fighting in their dreams? There was also a regular gurgling noise that came from their stomach opening and drowned out the sound of the life support system.
Proxima Logfiles 1: Marchenko's Children: Hard Science Fiction Page 17