Black holes are mysterious, with their ability to warp space and terminate time. They sound like science fiction, rather than facts, but there is no doubt that they exist.

In 1964, this kind of stellar mass was discovered for the first time through X-rays , which was also the first confirmation of black holes. 

But many questions remain, ranging from their mass to how they are formed. And surprises just keep coming.

Scientists from the national astronomical observatories said that they have found a stellar mass black hole with 70 times the mass of the Sun, twice as big as the current theory on black hole formation, proving once again how little we know about our universe.

Looking up at the night sky, all those stars, just like the Sun, are giant nuclear fusion reactors that pump energy outward, carefully keeping the balance by counteracting the star's gravity that crushes inward. But when a star runs out of fuel, balance is broken and gravity wins. The star collapses upon itself, creating a supernova, the biggest explosion that humans have ever seen. If there is enough mass left behind, it will be crushed by the gravity all together until there is nowhere to go but a point of infinite density, forming a black hole.

In the singularity, time and space are much distorted. Any matter that falls into the black hole, including light, cannot escape, so this is why black holes cannot be directly observed.

Even so, black holes have been found through their interactions with their surroundings. Many black holes have a companion star or are immersed in a cloud of gas. In both cases, the black hole can pull gas or even entire stars down into it. As the material falls into the hole, it heats up and emits X-rays.

Using X-ray, the astronomers found a total of about 20 stellar mass black holes in the Milky Way. But theory predicts that there are hundreds of millions of stellar mass black holes.

However, if a black hole isn't feeding on its surroundings, finding it is much harder. The vast majority of black holes may exist in isolation, which are undetectable since they do not emit X-rays. So black hole hunters need to change another way. 

They can observe a visible star for signs of orbital movement and further confirm if a black hole hides there. They call it the radial velocity method. It sounds simple, but it isn't easy.  

"If you want to find a black hole by using the radial velocity method, it means that you need to identify this kind of binary systems among many stars. If you use only one telescope, it will be very time-consuming," says Bai ZhongRui from the National Astronomical Observatories.

It was like looking for a needle in a haystack, until the astronomers built the LAMOST, or the Large Sky Area Multi-Object Fibre Spectroscopic Telescope.

Stars are composed of chemical elements, mainly with hydrogen and helium. Astronomers call other heavier elements "metal," and use metallicity to describe their abundance. Different elements emit different colors of light when they burn. With the spectrum of starlight, observed by LAMOST, scientists can access much information, such as velocity, about any shining star.

Weak light from stars will first reach the mirror in the dome. The light is reflected, through a tunnel, then on to a larger primary mirror. Reflected again, the light is eventually imaged on a focal surface, which is connected with 4,000 optical fibers. The special design allows LAMOST to observe 4,000 stars at any one time. The next best telescope can only observe 640 stars.

"But LAMOST has 4,000 eyes. It greatly improves the observation efficiency and increases our sample size, so the chances of discovering black holes have been greatly increased," Bai said. 

With the advanced features of an instrument like LAMOST, the painstaking job of looking up at the same area of night sky with its patch of the same stars, at the same time, every single day is made a bit easier. And one little abnormal move is enough to get the astronomer's attention. That move comes from a star which astronomers call a B-star.

"It was in the winter of 2016, we selected an area near Gemini and wanted to find some binary systems. And we found that there is one strange star, with the spectrum of B-star. Its mass is very large. We observed it 27 times, and we found that its radial velocity shows regular sinusoidal variation. That means the star is orbiting something, which, however, is invisible to us," Bai said. 

With the help of LAMOST, calculations show that this "thing" is as massive as 70 Suns. Astronomers ruled out a companion star, as a star of that mass would have been readily visible. 

Just one possibility remained.

When the scientists got the result of 70 solar masses, they could believe it. So they got in touch with Keck telescope in the U.S. and also contacted the Gran Telescopio Canarias in Spain, which also made more than a dozen of observations. The trend of radial velocity calculated from their observations was almost the same as that from LAMOST.

"It proved that we were right. And you must believe it," the scientist said.

They concluded that their discovery, which they named LB-1, is a black hole with nearly 70 solar masses. 

There are many black holes that could be heavier than that. For example, scientists have found evidence that at the center of most galaxies there are so-called supermassive black holes. But for LB-1 which is a Stellar Mass Black Hole, 70 solar masses mean it is a monster. Because scientists believe that a star with the metallicity similar to our sun, like what formed LB-1, should not create a black hole more than 25 solar masses.

The mass of a black hole is related to a supernova explosion. During the explosion, the star throws out an enormous amount of mass. With higher metallicity, the star's explosion can generate stronger star winds, more mass will be blown away and fewer remnants will be left. The black hole that it will eventually form will be smaller.

Heavier stars will form heavier black holes, but gradually, the curve will become flat, which means no matter how large a mass the star has, the mass of the black holes cannot get larger. 

But we still have no idea how such a massive black hole could be formed. 

The most "exciting possibility" is that LB-1 might actually consist of two black holes orbiting each other, instead of just one. If that is the case, this would be the first report of a binary black hole.