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[myscientificblog]

Black Holes

Black holes are the most extreme objects in the universe. They are the densest objects in the universe, each with a mass that exceeds that of our sun by a factor of millions. As if this weren't enough, they also have a gravitational field so strong that nothing—not even light—can escape from them.

The discovery of black holes was one of the most important achievements in astronomy in the 20th century. When we look up at the night sky, we see stars and nebulae that give us clues to how the universe evolved over time. But there is one object that cannot be seen because it devours everything around it: a black hole.

A black hole forms when an object with a very large mass (like an exploding star), collapses under its own gravity. Even though these objects are incredibly dense, they also have very sharp edges and very little volume compared to their mass; therefore they can never become completely spherical as planets or stars do.

Black holes are the ultimate source of gravitational energy, which is why they are so important in astrophysics. Not only do they weigh as much as a million suns, but they also have the power to bend light around them. But we don't need to wait for the black hole's next discovery to learn about how it works: we can actually see black holes and study their effects on stellar orbits and jets.

We know that black holes exist because we can observe their effects on nearby stars, but we don't know exactly what causes those effects. To understand how these objects work, scientists must first understand what makes them so powerful.

Black holes were discovered in the early 20th century by Karl Schwarzschild in 1916 and by Allan Sandage in 1960. Since then, scientists have found thousands of black holes throughout our galaxy and beyond. These discoveries have led to an ongoing revolution in astronomy and cosmology — one that has brought us closer than ever before to understanding these mysterious objects.

Black holes are the remnants of dead stars. They're incredibly dense and can have a mass of over 10 billion times the mass of our Sun.

When a star dies, it becomes a black hole because it has no more fuel to burn. The outer layers of the star collapse inward, while the center collapses into a singularity—a point where all matter is crushed down to nothingness.

Black holes are so dense that light can't escape them. If you were to look directly at a black hole, you'd see an event horizon—the point beyond which not even light can escape. But there are other ways these objects affect our universe: Black holes emit X-rays and radio waves as they suck in material from nearby stars and galaxies.

So far, we've found about 100 black holes in our galaxy, but NASA's Spitzer Space Telescope has discovered more than 1,000 in other galaxies throughout the universe!

We've known for a long time that black holes are weird. They're so unstable that they quickly evaporate and emit X-rays — so much so, in fact, that it's been hard to study them in detail.

This is changing. In the past few years, two teams of astronomers have discovered two new black holes that were hidden behind clouds of dust and gas. And these weren't just any black holes; they were two of the most massive ones ever seen outside our galaxy.

The first was discovered by a team led by Matthew Evans at Caltech in Pasadena, California. The second was founded by a team led by Guido Risaliti at the University of Leiden in the Netherlands. Both teams published their results this month in The Astrophysical Journal Letters.

When astronomers first began to study black holes with X-ray telescopes like NASA's Chandra X-ray Observatory, they assumed that all black holes would be similar to Sagittarius A* (Sgr A*), which lies about 26 000 light-years away in our local neighborhood. At about 8 billion times the mass of our Sun, Sgr A* is one of only three active supermassive black holes known outside our galaxy — and it's been doing its thing for about 14