Shulou(Shulou.com)11/24 Report--

This year, scientists showed us pictures of supermassive black holes at the center of the Milky way galaxy. It is about 27,000 light-years away and has a mass 4 million times the mass of the sun. However, its mass is actually at the low end of supermassive black holes, some of which are of the order of magnitude of billions of suns. But the problem is that supermassive black holes should not exist in existing theory.

First of all, we need to accurately understand the meaning of "super mass". Black holes are divided into three categories according to their mass: stellar mass black holes, which are about dozens of times the mass of the sun; medium-mass black holes, which are 100 to 100000 times the mass of the sun; and finally, supermassive black holes, 100000 to billions times the mass of the sun. Generally speaking, we will find these supermassive black holes in the center of the galaxy.

Our own Milky way has a black hole with a mass of about 4 million solar masses, and we have known about it for some time, mainly because it affects the motion of nearby stars, and these stars also help us calculate its mass. Although this supermassive black hole has the mass of 4 million suns, its diameter is at most the same as the orbit of Mercury. Its diameter can only hold 17 suns, which is smaller than most active giants. But the universe is not old enough to turn any stellar-mass black hole into a supermassive black hole, so how do they exist?

First of all, we need to understand how constant star black holes are formed. Inside an active star, there is a very delicate balance at work, called hydrostatic equilibrium. The outward radiation pressure of the core prevents gravity from collapsing inward. At the end of the star's life, the nuclear fuel is exhausted and the radiation pressure stops. If the star is large enough, the core collapses into a black hole, releasing huge amounts of energy during the collapse and blowing the rest into a supernova. So only the core becomes a black hole, which accounts for about 10% to 15% of the mass of the entire star, which limits the mass of the black hole when it forms.

The outward radiation pressure from the core of the star not only overcomes gravity, but also pushes particles from the surface of the star into space, which we call "stellar wind". The greater the mass of a star, the hotter its core, which means that the greater the radiation pressure, the more particles are emitted. If the mass of the star is high enough, the stellar wind will be so strong that the mass of the star will drop significantly, which is called the Eddington limit. If a star tries to form a higher mass, the mass of the star will be reduced to that limit, and we estimate it to be about 130 solar masses. If the core is only 15% of that mass at most, it will eventually form a black hole with only about 20 solar masses.

This 20 solar mass limit applies only to the formation of stellar mass black holes, but stars are usually formed in binary systems, sometimes even three-star systems. If both stars in the binary system are within the upper mass limit, they will both become black holes. If we give them enough time, they will emit gravitational waves that cause the orbits to get closer and closer and eventually merge with each other. If there were more time, the larger merging black holes could find each other and merge again. According to our estimates, the black hole exists long enough for the process to reach a maximum of about 200 solar masses.

However, even if there is a merger, the universe does not have enough time to create a black hole with more than 200 solar masses, let alone any supermassive black hole. To create a supermassive black hole, we need to start with a star with a mass of tens of thousands of suns. We need this star to exist during a stellar period that does not exist in the universe.

The early universe was a very different place. Before stars and galaxies, the universe was very uniform. But it is not completely uniform, and these slight inhomogeneity eventually become superclusters full of galaxies. Suppose matter collapses so fast in a particular place that it skips nuclear fusion and immediately becomes a tiny black hole. The black hole is surrounded by matter, which falls in and causes the black hole to grow. This continuous collapse releases a lot of energy, but the gravity of the matter around it is so strong that it can achieve hydrostatic equilibrium like a star.

It will be a quasar, but it is more appropriately called a black hole star. It is not the outward pressure caused by nuclear fusion, but by the matter that falls into the black hole, and the core of this quasar will be a black hole. The Eddington limit does not apply at this point because it assumes that most of the space is empty and allows particles to escape. But in the early universe, the quasi-star was surrounded by matter, but with a slightly lower density than the star itself. Any particles emitted in the stellar wind will be made up for by the surrounding matter. As time goes on, the surrounding matter will fall into the black hole. By then, the black hole will grow to at least a thousand solar masses, possibly tens of thousands of solar masses.

Over time, these medium-mass black holes accumulate more matter, occasionally discover and merge with each other, and eventually become the supermassive black holes we see at the center of galaxies today. However, there is no evidence to support all this. According to current physics, "black hole stars" are just a guess about the origin of supermassive black holes. We haven't even made confirmed observations of the first generation of real stars, let alone these hypothetical quasars.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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