Black holes are fascinating objects that have a strong gravitational pull, and once an object enters its event horizon, it can never escape. As matter falls into a black hole, it is said to be irreversibly swallowed up.
Black holes are predicted by Einstein’s theory of general relativity, and they come in a variety of sizes and masses. The smallest recorded black hole is approximately one solar mass, while the largest is hundreds of millions of solar masses.
One of the best known galaxies is Messier 87 (M87), which contains a supermassive black hole at its center. Astronomers have been observing M87 for decades due to its proximity to our planet—it lies in the Virgo galaxy cluster—and because it contains such a high concentration of material near its core. These observations provide ample opportunity to study the behavior of matter close to a massive black hole.
What is a black hole?
Black holes are among the most fascinating and mysterious objects in the universe. They form when a massive star runs out of nuclear fuel and collapses in on itself.
If the original star was large enough, it will collapse into a singular point known as a singularity. The force of the collapse will be so great that it will bring all matter into it, annihilating everything.
What’s more, it will warp the space around it to such an extreme degree that nothing can escape it – not even electromagnetic radiation like light. This is why some scientists refer to it as a black hole.
Because no light can escape from within its event horizon, astronomers can only learn about these objects by observing their effects on nearby objects and space. how do we know they exist? i
Bullet point: There are two ways that black holes can be detected. The first is if there is a surrounding accretion disk of matter surrounding the black hole. As mentioned before, this matter cannot escape and therefore piles up in what is called an accretion disk. This matter gathers speed and angular momentum as it moves down toward the event horizon where it finally crosses over.
How are black holes formed?
Black holes are believed to be formed when very large stars run out of fuel and experience a second phase of their life cycle. This phase is called a supernova.
A supernova occurs when the strength of gravitational force inside the star surpasses the strength of atomic force that binds the star together.
When this happens, the star collapses in on itself and creates a denser point called a neutron star. If the original star was large enough, the neutron star that remains after the supernova will be too heavy to support its own gravitational force and it will collapse into a black hole. he Black hole will have such strong gravitational force that it will have no outward appearance, hence why it is named as such.
What is the evidence for a black hole at the center of M87?
As previously mentioned, the speed at which the gas and stars are moving in and out of the M87 cluster is unusual. There is a threshold speed at which objects can escape the gravitational pull of other objects or clusters.
When this speed is exceeded, then objects will no longer stay within the cluster. For example, if you were to drop a ball into a bucket of water, it would quickly sink to the bottom due to the pull of gravity. If you were to add more water to the bucket, more balls could be dropped in without sinking.
The ability for objects to escape the cluster indicates that there is some sort of powerful force acting on them. This force must be strong enough to overcome the pull of gravity or it would remain within the cluster. Since many objects are escaping this cluster, there must be something very powerful pulling them out.
Does every galaxy have a black hole at its center?
No, not every galaxy has a supermassive black hole at its center. In fact, astronomers think that only very large galaxies, like elliptical and SBc galaxies, have black holes at their centers. Smaller galaxies, like spirals and dwarfs, probably do not have a black hole residing in their centers.
Our own Milky Way Galaxy is a spiral galaxy with a dense concentration of stars in its center, known as a bulge. This bulge does not contain a black hole, but instead contains a special type of star called a neutron star. Neutron stars are the densest objects in the universe other than black holes.
How do we know this? By using many different types of telescopes and observing satellites to scan the entire galaxy and its central bulge region. If there was indeed a supermassive black hole there, we would be able to see effects of it on the surrounding area.
Can we see the black hole itself?
No, we cannot see the actual black hole itself. We can only see the effects of the supermassive black hole on the surrounding environment.
By using powerful telescopes and observing the motions of nearby stars and gas clouds, astronomers have proven that there is a supermassive black hole at the center of our galaxy. How have they done this?
By observing the speed at which these objects move, as well as their changes in speed and their overall behavior, astronomers have concluded that there is a dense body at the center of our galaxy that is responsible for the gravitational pull effect that we observe.
This is an important conclusion to make because it links back to our original question: How do astronomers hypothesize that a massive black hole lies at the center of M87? They do so by observing the effects of a dense body on its surrounding environment.
Are there any ways to test this hypothesis?
Yes! In fact, this is the purpose of the NASA/ESA Hubble Space Telescope’s current mission, titled “To characterize the nuclear regions of M87 and to test whether a black hole resides in its center.”
As of January 2019, the mission has concluded and results have shown that there is a 1.6 billion solar mass black hole at the center of M87. This is only one part of the conclusion, though; it was not definitively proven that there was a black hole at the center.
The other part of the conclusion states that “[t]he data also showed an absence of stars in a large region around the central bulge, suggesting that a supermassive black hole does not exist in this region.”
This shows us that there are different ways to test for a SMBH at the center of a galaxy- one can look for stars being pulled into it or look for it directly.
What would happen if you got too close to a black hole?
You would be captured by the black hole’s gravity and carried into the center, where you would meet the event horizon. There, you would experience weightlessness as you were pulled into the surrounding space.
You could continue to orbit around the black hole, but you would never escape. The longer you stayed, the more your orbit would shrink until you collided with the event horizon or fell into the core of the black hole.
The remarkable thing is that you wouldn’t feel any of this. You wouldn’t feel any of the gravitational pull nor would you sense any tugs towards the edge of the event horizon. You would just keep orbiting until you fell into the black hole.
This is because all of your senses depend on your brain processing signals from your senses. If you don’t sense anything, then your brain doesn’t register it.
Are there other hypotheses for what is at the center of M87?
Yes, there are several other hypotheses for what could be at the center of M87. Astronomers have not ruled out any ideas, unlike in some cases where only one hypothesis is viable.
One idea is that a large number of stars could be located at the center of M87. While this seems unlikely, we do not know exactly how many stars compose the Milky Way or how large its central region is.
Another idea is that a large number of very heavy black holes could be located at the center of M87. These hypothetical objects could combine to form a single, even heavier black hole. Once again, we do not know how many of these heavy black holes exist, but it is possible that there are enough to form a single super-black hole at the center of M87.
A third hypothesis is that there is no object at the center of M87 except for sheer force-carrying darkness.
