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Black Holes: What Are Black Holes?

Audience: 4 th Grade and older.

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This slide breaks down and explains what black holes are.

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Black Holes

The simplest definition of a black hole is an object that is so dense that not even light can escape its surface. But how does that happen?

The concept of a black hole can be understood by thinking about how fast something needs to move to escape the gravity of another object – this is called the escape velocity. Formally, escape velocity is the speed an object must attain to "break free" of the gravitational attraction of another body.

There are two things that affect the escape velocity – the mass of object and the distance to the center of that object. For example, a rocket must accelerate to 11.2 km/s in order to escape Earth's gravity. If, instead, that rocket was on a planet with the same mass as Earth but half the diameter, the escape velocity would be 15.8 km/s. Even though the mass is the same, the escape velocity is greater, because the object is smaller (and more dense).

What if we made the size of the object even smaller? If we squished the Earth's mass into a sphere with a radius of 9 mm, the escape velocity would be the speed of light. Just a wee-bit smaller, and the escape velocity is greater than the speed of light. But the speed of light is the cosmic speed limit, so it would be impossible to escape that tiny sphere, if you got close enough.

The radius at which a mass has an escape velocity equal to the speed of light is called the Schwarzschild radius. Any object that is smaller than its Schwarzschild radius is a black hole – in other words, anything with an escape velocity greater than the speed of light is a black hole. For something the mass of our sun would need to be squeezed into a volume with a radius of about 3 km.

Structure of a black hole

There are two basic parts to a black hole: the singularity and the event horizon.

The event horizon is the "point of no return" around the black hole. It is not a physical surface, but a sphere surrounding the black hole that marks where the escape velocity is equal to the speed of light. Its radius is the Schwarzschild radius mentioned earlier.

One thing about the event horizon: once matter is inside it, that matter will fall to the center. With such strong gravity, the matter squishes to just a point – a tiny, tiny volume with a crazy-big density. That point is called the singularity. It is vanishingly small, so it has essentially an infinite density. It's likely that the laws of physics break down at the singularity. Scientists are actively engaged in research to better understand what happens at these singularities, as well as how to develop a full theory that better describes what happens at the center of a black hole.

Seeing the unseen

If light can't escape a black hole, how can we see black holes?

Astronomers don't exactly see black holes directly. Instead, astronomers observe the presence of a black hole by its effect on its surroundings. A black hole, by itself out in the middle of our galaxy would be very difficult to detect.

Imagine you arrive home one night to find the kitchen a mess. You know that it was clean when you left, but now there are dirty dishes in the sink and crumbs strewn about the counter. From the evidence, you know someone used the kitchen while you were out – in fact, you can even say that they made a sandwich and chips because of the types of crumbs you see on the counter. You might even be able to identify who in your household was in the kitchen based on what kind of chips they had or what they put on their sandwich. You never saw that person in the kitchen, but their effect on the kitchen was evident.

Studying black holes relies heavily on indirect detection. Astronomers cannot observe black holes directly, but see behaviors in other objects that can only be explained by the presence of a very large and dense object nearby. The effects can include materials getting pulled into the black hole, accretion disks forming around the black hole, or stars orbiting a massive but unseen object.

Types of black holes

Traditionally, astronomers have talked about two basic classes of black hole – those with masses about 5-20 times that of the sun, which are called stellar-mass black holes, and those with masses millions to billions times that of the sun, which are called supermassive black holes. What about the gap between stellar mass and supermassive black holes? For a long time astronomers had proposed a third class, called intermediate mass black holes, but it was just in the past decade or so that they have started finding possible evidence of this class of black hole.

Stellar-mass black holes are formed when a massive star runs out of fuel and collapses. They are found scattered throughout the galaxy, in the same places where we find stars, since they began their lives as stars. Some stellar-mass black holes started their lives as part of a binary star system, and the way the black hole affects its companion and their environment can be a clue to astronomers about their presence.

Supermassive black holes are found at the center of nearly every large galaxy. Exactly how supermassive black holes form is an active area of research for astronomers. Recent studies have shown that the size of the black hole is correlated with the size of the galaxy, so that the there must be some connection between the formation of the black hole and the galaxy.

With only a few candidate intermediate black holes, astronomers are just beginning to study them in any detail. These studies are complicated by the fact that many of the objects that initially looked like strong intermediate black hole candidates can be explained in other ways. For example, there is a class of object called ultraluminous X-ray sources (ULXs). These objects emit more X-ray light than known stellar processes. One model postulated that ULXs harbor an intermediate black hole; however, further study of these objects has favored alternate models for most of them. Stay tuned as astronomers work to unravel the mysteries of these elusive objects.

Updated: November 2016

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You are here, astr 160: frontiers and controversies in astrophysics,  - introduction to black holes.

The second half of the course begins, focusing on black holes and relativity. In introducing black holes, Professor Bailyn offers a definition, talks about how their existence is detected, and explains why (unlike in the case with exoplanets where Newtonian physics was applied) Einstein’s Theory of Relativity is now required when studying black holes. The concepts of escape and circular velocity are introduced. A number of problems are worked out and students learn how to calculate an object’s escape velocity. A historical overview is offered of our understanding and discovery of black holes in the context of stellar evolution.

Lecture Chapters

• Introduction
• Escape Velocity
• Defining Black Holes and the Schwarzschild Radius
• Gravity and Pressure in the Evolution of Stars
• From Electron Degeneracy Pressure to the Chandrasekhar Limit
• Neutron Stars

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What is a black hole (grades 5-8), nasa stem team, how big are black holes, how do black holes form, if black holes are “black,” how do scientists know they are there, could a black hole destroy earth, will the sun ever turn into a black hole, how is nasa studying black holes, more about black holes.

This article is for students grades 5-8

A black hole is a region in space where the pulling force of gravity is so strong that light is not able to escape. The strong gravity occurs because matter has been pressed into a tiny space. This compression can take place at the end of a star’s life. Some black holes are a result of dying stars.

Because no light can escape, black holes are invisible. However, space telescopes with special instruments can help find black holes. They can observe the behavior of material and stars that are very close to black holes.

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Words to Know

mass: the measurement for the amount of matter in an object

Black holes can come in a range of sizes, but there are three main types of black holes. The black hole’s mass and size determine what kind it is.

The smallest ones are known as primordial black holes. Scientists believe this type of black hole is as small as a single atom but with the mass of a large mountain.

The most common type of medium-sized black holes is called “stellar.” The mass of a stellar black hole can be up to 20 times greater than the mass of the sun and can fit inside a ball with a diameter of about 10 miles. Dozens of stellar mass black holes may exist within the Milky Way galaxy.

The largest black holes are called “supermassive.” These black holes have masses greater than 1 million suns combined and would fit inside a ball with a diameter about the size of the solar system. Scientific evidence suggests that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a ball with a diameter about the size of the sun.

Primordial black holes are thought to have formed in the early universe, soon after the big bang.

Stellar black holes form when the center of a very massive star collapses in upon itself. This collapse also causes a supernova, or an exploding star, that blasts part of the star into space.

Scientists think supermassive black holes formed at the same time as the galaxy they are in. The size of the supermassive black hole is related to the size and mass of the galaxy it is in.

A black hole can not be seen because of the strong gravity that is pulling all of the light into the black hole’s center. However, scientists can see the effects of its strong gravity on the stars and gases around it. If a star is orbiting a certain point in space, scientists can study the star’s motion to find out if it is orbiting a black hole.

When a black hole and a star are orbiting close together, high-energy light is produced. Scientific instruments can see this high-energy light.

A black hole’s gravity can sometimes be strong enough to pull off the outer gases of the star and grow a disk around itself called the accretion disk. As gas from the accretion disk spirals into the black hole, the gas heats to very high temperatures and releases X-ray light in all directions. NASA telescopes measure the X-ray light. Astronomers use this information to learn more about the properties of a black hole.

Black holes do not wander around the universe, randomly swallowing worlds. They follow the laws of gravity just like other objects in space. The orbit of a black hole would have to be very close to the solar system to affect Earth, which is not likely.

If a black hole with the same mass as the sun were to replace the sun, Earth would not fall in. The black hole with the same mass as the sun would keep the same gravity as the sun. The planets would still orbit the black hole as they orbit the sun now.

red giant star:  a star that is larger than the sun and red because it has a lower temperature

white dwarf star:  a small star, about the size of Earth; one of the last stages of a star’s life

The sun does not have enough mass to collapse into a black hole. In billions of years, when the sun is at the end of its life, it will become a red giant star . Then, when it has used the last of its fuel, it will throw off its outer layers and turn into a glowing ring of gas called a planetary nebula. Finally, all that will be left of the sun is a cooling white dwarf star .

NASA is learning about black holes using spacecraft like the Chandra X-ray Observatory, the Swift satellite and the Fermi Gamma-ray Space Telescope. Fermi launched in 2008 and is observing gamma rays – the most energetic form of light – in search of supermassive black holes and other astronomical phenomena. Spacecraft like these help scientists answer questions about the origin, evolution and destiny of the universe.

Space Place in a Snap: What Is a Black Hole? Black Hole Rescue Fall Into a Black Hole Black Holes: By the Numbers Slideshow Black Hole Travel Postcards

Read What Is a Black Hole? (Grades K-4)

What Is a Black Hole?

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8.5 x 11 inches, 8.5 x 13 inches, 11 x 17 inches, click here to read a transcript of this video..

Space is a pretty dark place. Even so, some areas are darker than others. Nothing is darker than a black hole.

A black hole is an area of such immense gravity that nothing—not even light—can escape from it.

Black holes form at the end of some stars’ lives. The energy that held the star together disappears and it collapses in on itself producing a magnificent explosion.

Here’s where things get crazy. All of that material left over from the explosion, many times the mass of our Sun, falls into an infinitely small point.

Black holes can form in many ways though, and large black holes can have tens to millions of times the mass of our sun trapped in a point smaller than the tip of a pin! Some black holes trap more and more material as their mass increases.

The point where all that mass is trapped is called a singularity. It may be infinitely small, but its influence is enormous.

Imagine a circle with a singularity in the middle. The gravity on the inside of the circle is so strong that nothing can escape—it sucks in everything, even light. That's why it's black!

This circle is known as the event horizon. An event horizon is probably what you are thinking of when you think of a black hole.

What would happen, you might wonder, if we took a spacecraft near a black hole’s event horizon? The answer—spaghettification! That’s the technical term, at least.

As our spacecraft approaches it, the gravity will be so much stronger on the side closer to the black hole than at the other side that it will get completely stretched out like a piece of spaghetti.

Try as you may, you would be hard-pressed to find anything weirder or cooler than a black hole…

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What do black holes look like?

As we investigate our Universe, black holes are some of the most violent and mysterious objects we find. Black holes are collapsed objects of incredible density that exert a gravitational pull so strong that not even light can escape. How can we take a picture if it doesn’t emit light?

Scientists at the Center for Astrophysics | Harvard & Smithsonian led the effort that created the first image of matter near the event horizon of M87’s supermassive black hole using the Event Horizon Telescope . In 2017, eight radio observatories, including the CfA’s Submillimeter Array , were linked to create an Earth-sized interferometer. These observations revealed the strong effects of gravity expected near a black hole and observed matter orbiting at near light speeds. The Event Horizon Telescope opens a new window to extreme physics at the edge of a black hole. Four more observatories, including CfA’s Greenland Telescope , are being added to the array for the next set of black hole images. 

The Center for Astrophysics | Harvard & Smithsonian works on all aspects of black hole research, across all wavelengths and scales, from the observational to the theoretical.

NASA’s Chandra X-ray Observatory observes the X-rays from the superheated material falling onto the black hole. This high-energy radiation can penetrate obscuring gas and dust, giving us an X-ray view of the action. CfA scientists monitor Sagittarius A* and are currently observing a star, or pair of stars, being shredded by the supermassive black hole.

The Black Hole Initiative joins CfA astronomers with other colleagues from Harvard University to form the first center in the world to focus on the study of black holes. The research draws on astronomy, physics, mathematics, history of science, and philosophy to better understand these fascinating objects.

The Institute for Theory and Computation also models, among many black hole research topics, the high-energy conditions that occur when gas clouds or stars fall onto a black hole. Scientists have been studying black hole accretion flows for many years using state-of-art numerical simulations, which allow us to properly follow the black hole physics and evolution of magnetic fields.

Getting the Picture

There is a lot we don’t know about black holes . For example, what happens at the center of a black hole? Or, how do the biggest black holes form? And how do these giant black holes and their host galaxies coexist?

But this much is clear—you wouldn’t want to see one up close. NASA’s Chandra X-ray Observatory observes X-rays from material falling into a black hole as it heats up to millions of degrees and the gravity is sufficient to stretch apart an unfortunate passerby in a process known as “spaghettification.”

All of the black holes we know about are either a few times more massive than the Sun, or supermassive, millions to billions of times more massive than the Sun. Strangely, we have not found any confirmed medium-sized black holes. The nearest supermassive black hole, known as Sagittarius A* (pronounced Sagittarius A-star) is about four million times the mass of the sun. It is a monster that lurks at the center of the Milky Way and has been observed tearing apart and devouring stars that venture too close. The black hole at the center of the galaxy M87 is even larger, billions of times more massive than the Sun.

Black holes themselves are fundamentally unseeable. There’s no way to bring back light from beyond the event horizon—the point at which light itself is irrecoverably lost to the object’s gravity. The only way we know of their existence is to observe their effects on light and other objects. But we are working on a solution to see right up to the event horizon.

The Event Horizon Telescope is an Earth-sized virtual telescope called an “interferometer”, created by linking radio telescopes from all over the world. This long baseline allows us to make ultra-high resolution images of the event horizon, comparable to counting individual dimples on a golf ball in Los Angeles from New York. Using the power of the Event Horizon Telescope, we captured the first-ever image of matter swirling around the supermassive black hole at the center of the nearby galaxy M87, and are working to do the same thing for the black hole at the center of the Milky Way.

The first image of a black hole in human history, captured by the Event Horizon Telescope, showing light emitted by matter as it swirls under the influence of intense gravity. This black hole is 6.5 billion times the mass of the Sun and resides at the center of the galaxy M87.

What We Know

Black holes are:

• Small.  Despite how massive black holes can be, they are quite compact. If our Sun were to turn into a black hole, it would measure less than two miles across. Sagittarius A* could fit within Mercury’s orbit, and M87’s supermassive black hole is about three times the size of Pluto’s orbit.
• Powerful. The largest black holes can be billions of times as massive as our Sun. The force of their gravity on nearby matter can heat infalling gas to millions of degrees, warp light, and slow the passage of time compared to someone far away from the black hole.
• Prevalent. The Milky Way galaxy is known to contain about a few hundred million stellar mass black holes, roughly one per every thousand visible stars. Supermassive black holes are found at the center of most large galaxies and evidence suggests that they may be crucial to galaxy formation.
• Mysterious. Einstein’s Theory of General Relativity starts to break down when it ventures inside a black hole. This is the realm of quantum gravity, merging general relativity, the theory of massive objects, with quantum mechanics, the theory of how things act on very small scales. These theories have proven frustratingly hard to merge, demonstrating a holy grail of science.
• Black Holes
• Very Long Baseline Interferometry
• Computational Astrophysics
• The Milky Way Galaxy
• Stellar Astronomy
• Theoretical Astrophysics

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We got it! Astronomers reveal first image of the black hole at the heart of our galaxy

Washington, D.C.  –  During a press conference hosted by the U.S. National Science Foundation with the Event Horizon Telescope Collaboration in Washington, D.C. today, astronomers unveiled the first image of the supermassive black hole at the center of our own Milky Way galaxy. This result provides overwhelming evidence that the object is indeed a black hole and yields valuable clues about the workings of such giants, which are thought to reside at the center of most galaxies. The image was produced by a global research team called the Event Horizon Telescope, or EHT, Collaboration, using observations from a worldwide network of radio telescopes. 

Download a high resolution version of the Sgr A* image.

The image is a long-anticipated look at the massive object that sits at the very center of our galaxy. Scientists had previously seen stars orbiting around something invisible, compact, and very massive at the center of the Milky Way. This strongly suggested that this object — known as Sagittarius A* (Sgr A*, pronounced "sadge-ay-star") — is a black hole, and today’s image provides the first direct visual evidence of it.

Although we cannot see the black hole itself, because it is completely dark, glowing gas around it reveals a telltale signature: a dark central region (called a “shadow”) surrounded by a bright ring-like structure. The new view captures light bent by the powerful gravity of the black hole, which is four million times more massive than our Sun.

“We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity," said EHT Project Scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. "These unprecedented observations have greatly improved our understanding of what happens at the very center of our galaxy and offer new insights on how these giant black holes interact with their surroundings.” The EHT team's results are being published today in a special issue of The Astrophysical Journal Letters. https://iopscience.iop.org/journal/2041-8205/page/Focus_on_First_Sgr_A_Results

Because the black hole is about 27,000 light-years away from Earth, it appears to us to have about the same size in the sky as a donut on the Moon. To image it, the team created the powerful EHT, which linked together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope [1]. The EHT observed Sgr A* on multiple nights, collecting data for many hours in a row, similar to using a long exposure time on a camera.

The breakthrough follows the EHT collaboration’s 2019 release of the first image of a black hole, called M87*, at the center of the more distant Messier 87 galaxy.

The two black holes look remarkably similar, even though our galaxy’s black hole is more than a thousand times smaller and less massive than M87* [2]. "We have two completely different types of galaxies and two very different black hole masses, but close to the edge of these black holes they look amazingly similar,” says Sera Markoff, Co-Chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam, the Netherlands. "This tells us that General Relativity governs these objects up close, and any differences we see further away must be due to differences in the material that surrounds the black holes.”

This achievement was considerably more difficult than for M87*, even though Sgr A* is much closer to us. EHT scientist Chi-kwan (‘CK’) Chan, from Steward Observatory and Department of Astronomy and the Data Science Institute of the University of Arizona, U.S., explains: “The gas in the vicinity of the black holes moves at the same speed — nearly as fast as light — around both Sgr A* and M87*. But where gas takes days to weeks to orbit the larger M87*, in the much smaller Sgr A* it completes an orbit in mere minutes. This means the brightness and pattern of the gas around Sgr A* was changing rapidly as the EHT Collaboration was observing it — a bit like trying to take a clear picture of a puppy quickly chasing its tail.”

The researchers had to develop sophisticated new tools that accounted for the gas movement around Sgr A*. While M87* was an easier, steadier target, with nearly all images looking the same, that was not the case for Sgr A*. The image of the Sgr A* black hole is an average of the different images the team extracted, finally revealing the giant lurking at the center of our galaxy for the first time.

The effort was made possible through the ingenuity of more than 300 researchers from 80 institutes around the world that together make up the EHT Collaboration. In addition to developing complex tools to overcome the challenges of imaging Sgr A*, the team worked rigorously for five years, using supercomputers to combine and analyze their data, all while compiling an unprecedented library of simulated black holes to compare with the observations.

Of those supercomputers, the analysis in paper five includes nearly 80 million CPU hours on the NSF Frontera supercomputer and 20 million CPU hours on the NSF Open Science Grid. NSF’s South Pole Telescope (SPT) and the international Atacama Large Millimeter/submillimeter Array (ALMA), a telescope managed under NSF’s National Radio Astronomy Observatory (NRAO), were two of the seven telescopes used to collect the image data in 2017.

Scientists are particularly excited to finally have images of two black holes of very different sizes, which offers the opportunity to understand how they compare and contrast. They have also begun to use the new data to test theories and models of how gas behaves around supermassive black holes. This process is not yet fully understood but is thought to play a key role in shaping the formation and evolution of galaxies.

“Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works,” said EHT scientist Keiichi Asada from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. “We have images for two black holes — one at the large end and one at the small end of supermassive black holes in the Universe — so we can go a lot further in testing how gravity behaves in these extreme environments than ever before.”

Progress on the EHT continues: a major observation campaign in March 2022 included more telescopes than ever before. The ongoing expansion of the EHT network and significant technological upgrades will allow scientists to share even more impressive images as well as movies of black holes in the near future.

“This image is a testament to what we can accomplish, when as a global research community, we bring our brightest minds together to make the seemingly impossible, possible. Language, continents and even the galaxy can’t stand in the way of what humanity can accomplish when we come together for the greater good of all. This is a historic moment where we see the black hole at the heart of our Milky Way as a capstone achievement following decades of intense curiosity-driven discovery research. NSF is proud to be an international partner that invests in this innovative research and the infrastructure that makes such fantastic discoveries possible,” said NSF Director Sethuraman Panchanathan.

[1] The individual telescopes involved in the EHT in April 2017, when the observations were conducted, were: the Atacama Large Millimeter/submillimeter Array (ALMA), the Atacama Pathfinder Experiment (APEX), the IRAM 30-meter Telescope, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope Alfonso Serrano (LMT), the Submillimeter Array (SMA), the UArizona Submillimeter Telescope (SMT), the South Pole Telescope (SPT). Since then, the EHT has added the Greenland Telescope (GLT), the NOrthern Extended Millimeter Array (NOEMA) and the UArizona 12-meter Telescope on Kitt Peak to its network. 

ALMA is a partnership of the European Southern Observatory (ESO; Europe, representing its member states), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan, together with the National Research Council (Canada), the Ministry of Science and Technology (MOST in Taipei), Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), and Korea Astronomy and Space Science Institute (KASI; Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, the Associated Universities, Inc./National Radio Astronomy Observatory (AUI/NRAO) and the National Astronomical Observatory of Japan (NAOJ). APEX , a collaboration between the Max Planck Institute for Radio Astronomy (Germany), the Onsala Space Observatory (Sweden) and ESO, is operated by ESO. The 30-meter Telescope is operated by IRAM (the IRAM Partner Organizations are MPG (Germany), CNRS (France) and IGN (Spain). The JCMT is operated by the East Asian Observatory on behalf of the Center for Astronomical Mega-Science of the Chinese Academy of Sciences, NAOJ, ASIAA, KASI, the National Astronomical Research Institute of Thailand, and organizations in the United Kingdom and Canada. The LMT is operated by INAOE and UMass, the SMA is operated by Center for Astrophysics | Harvard & Smithsonian and ASIAA, and the UArizona SMT  is operated by the University of Arizona. The SPT is operated by the University of Chicago with specialized EHT instrumentation provided by the University of Arizona. 

The Greenland Telescope ( GLT ) is operated by ASIAA and the Smithsonian Astrophysical Observatory (SAO). The GLT is part of the ALMA-Taiwan project and is supported in part by Taipei’s Academia Sinica (AS) and MOST. NOEMA is operated by IRAM and the UArizona 12-meter telescope at Kitt Peak is operated by the University of Arizona.

[2] Black holes are the only objects we know of where mass scales with size. A black hole a thousand times smaller than another is also a thousand times less massive. 

More Information

The EHT consortium consists of 13 stakeholder institutes; the Academia Sinica Institute of Astronomy and Astrophysics, the University of Arizona, the University of Chicago, the East Asian Observatory, Goethe-Universitaet Frankfurt, Institut de Radioastronomie Millimétrique, Large Millimeter Telescope, Max Planck Institute for Radio Astronomy, MIT Haystack Observatory, National Astronomical Observatory of Japan, Perimeter Institute for Theoretical Physics, Radboud University and the Smithsonian Astrophysical Observatory. 

Additional Links

• High Resolution Sgr A* Image
• EHT website
• NSF Exploring Black Holes

Black Hole in 3-D

This three-dimensional illustration shows how the rotating space around a black hole twists up the magnetic field in the plasma falling toward the black hole. The black sphere at the center of the figure is the black hole itself, and the yellow region around it represents the area where space is being twisted. The red tubes depict magnetic field lines threading this twisting space, while the green ones show magnetic field lines which have not yet entered that space. This simulation was conducted using supercomputers at Japan's National Institute for Fusion Science.

Keep Exploring

Black Hole Background

Background for black hole interactive hotspot module. Credit: NASA’s Goddard Space Flight Center/Jeremy Schnittman

BH_AccretionDisk_Sim_Stationary_1080

(m4v) (2.14 MB)

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Why 100 million black holes could be hidden across our galaxy

Let’s hope for all our sakes we don’t stumble across any…

Image credit: Getty

Dr Alastair Gunn

‘Stellar’ black holes form when massive stars collapse under their own gravity at the end of their lives. 

We know that a star must have an initial mass of between 15–20 solar masses in order to collapse and form a stellar black hole . 

We also have a pretty good idea of the distribution of stellar masses in the Milky Way. 

These two facts tell us that about one in 1,000 stars has the potential to become a black hole. 

An estimate of the number of stars in the Milky Way, 100 billion stars, then implies there could be up to 100 million stellar black holes in the Milky Way.

This article is an answer to the question (asked by [Aled Weaver, Wigan]) 'How many black holes are there in the Milky Way?'

To submit your questions, email us at [email protected] , or message our Facebook , X , or Instagram pages (don't forget to include your name and location).

Check out our ultimate fun facts page for more mind-blowing science.

• Stunning new image of black hole at centre of our galaxy revealed
• Who really discovered black holes?
• Why we don’t have to worry about being sucked into the supermassive black hole at the centre of the Milky Way

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Black Hole Mysteries: Unanswered Questions About These Cosmic Phenomena

B lack holes, those enigmatic regions in space where gravity becomes so strong that not even light can escape, have fascinated scientists, astronomers, and the public alike for decades. While significant strides have been made in understanding these cosmic behemoths, many questions remain unanswered, shrouding black holes in mystery and intrigue. In this article, we will delve into some of the most perplexing and tantalizing questions surrounding black holes, exploring the current state of knowledge and the ongoing quest to unravel their secrets.

What Are Black Holes?

Before diving into the mysteries, let’s briefly review what we know about black holes. At their core, black holes are regions in space where matter has been compressed to such an extent that the gravitational pull becomes infinitely strong. This extreme gravitational force creates a point of no return, known as the event horizon, beyond which nothing can escape, not even light.

• Formation: Black holes can form through various processes, including the collapse of massive stars, the merger of binary black holes, and the accretion of matter onto supermassive black holes at the centers of galaxies.
• Stellar Black Holes: Formed from the collapse of massive stars, these black holes have masses ranging from a few to several tens of times that of the Sun.
• Intermediate Black Holes: With masses between stellar and supermassive black holes, these are relatively rare and their formation mechanisms are still a subject of debate.
• Supermassive Black Holes: Found at the centers of galaxies, these black holes have masses millions to billions of times that of the Sun and play a crucial role in galaxy formation and evolution.

The Mysteries of Black Holes

Despite decades of research and observation, black holes continue to pose numerous questions that challenge our understanding of physics, gravity, and the nature of the universe.

• Singularity: At the heart of a black hole lies a point of infinite density and curvature known as the singularity. General relativity predicts that all matter and energy consumed by the black hole converge at the singularity, where the laws of physics as we know them break down.
• Spacetime Curvature: As objects approach the event horizon, spacetime becomes increasingly curved, leading to extreme gravitational time dilation and tidal forces. What happens to an object or information that crosses the event horizon remains one of the most profound mysteries of black hole physics.
• Hawking Radiation: In 1974, physicist Stephen Hawking proposed that black holes are not entirely black but emit radiation due to quantum mechanical effects near the event horizon, known as Hawking radiation. This discovery raises fundamental questions about the fate of information and the conservation of quantum information in black hole evaporation.
• Information Loss: Theoretical predictions suggest that black hole evaporation via Hawking radiation leads to the loss of quantum information, violating the principle of quantum mechanics known as unitarity. Resolving this information paradox remains a central challenge in reconciling general relativity with quantum mechanics.
• Seeds of Supermassive Black Holes: The origin of supermassive black holes found in the centers of galaxies remains a topic of debate. Proposed mechanisms include direct collapse of massive gas clouds, rapid accretion of matter onto stellar black holes, or the merger of intermediate black holes in dense galactic environments.
• Black Hole Mergers: Observations by LIGO and Virgo observatories have detected gravitational waves from black hole mergers, providing insights into their formation, growth, and dynamics. However, the frequency, mass distribution, and evolutionary pathways of black hole mergers remain uncertain.
• Role in Galaxy Evolution: Supermassive black holes play a crucial role in regulating star formation, galaxy growth, and the evolution of galactic structures. Understanding the feedback mechanisms, accretion processes, and interaction between black holes and their host galaxies is essential for deciphering the cosmic connection between black holes and the universe’s evolution.

The Quest for Answers

Addressing the mysteries of black holes requires a multifaceted approach, combining theoretical physics, observational astronomy, computational simulations, and experimental tests of fundamental theories.

• Theoretical Advances: Researchers continue to develop and refine theories of black hole physics, quantum gravity, and the nature of spacetime within and beyond the event horizon. Exploring alternative theories, such as string theory, loop quantum gravity, and holographic principles, offers promising avenues for resolving existing paradoxes and uncovering new insights.
• Observational Challenges: Advancements in observational techniques, telescope technology, and data analysis tools enable astronomers to probe deeper into black holes’ secrets. Future missions, such as the Event Horizon Telescope’s continued observations and the launch of next-generation space telescopes, promise to capture high-resolution images, detect gravitational waves, and explore black hole environments with unprecedented detail.
• Collaborative Efforts: Collaborative initiatives, including international partnerships, interdisciplinary research teams, and public engagement programs, foster innovation, creativity, and shared knowledge in the pursuit of understanding black holes and the cosmos.

Black holes, with their extreme gravitational fields and mysterious interiors, stand as some of the universe’s most enigmatic and fascinating phenomena. While significant progress has been made in unveiling their secrets, many questions remain unanswered, challenging scientists and inspiring curiosity across generations.

As we continue to explore, study, and unlock the mysteries of black holes, we embark on a journey of discovery, innovation, and exploration that transcends boundaries and expands our understanding of the universe’s fundamental nature. Through collaboration, perseverance, and a shared passion for knowledge, we draw closer to unraveling the cosmic enigmas that shape our universe and inspire wonder, awe, and a renewed appreciation for the grandeur and complexity of the cosmos that surrounds us in the vast expanse of space and time.

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