Black Holes
- 11th May 2023
- Author: Lucy Spencer
A simple guide to one of the most mysterious objects in the universe – let us take you through some of the commonly asked questions (and answers) about black holes.
What is a Black Hole?
A black hole is an astronomical object that is so dense, that nothing, not even light can escape its immense gravitational pull. Anything that has mass (i.e. is made up of material) exerts a gravitational pull on every other bit of mass. Standing with a friend, you’re pulling on them (and they’re pulling on you), but you have such low mass that the gravitational pull is really weak. Earth is more massive, and we feel it’s gravitational pull on us when we try to jump. But black holes are incredibly massive (they’re made of A LOT of material, albeit confined in a small amount of space) and as a result they exert a massive pull on objects around them.
There are two main types of black holes. Stellar black holes which have a mass of around three to 100 times the mass of our Sun, and supermassive black holes which have a mass of 100,000 to several billion times the mass of our Sun!
Most stellar black holes are very difficult to detect, but the number of stars large enough to produce a black hole suggest that there are as many as ten million to a billion black holes in the Milky Way galaxy alone. Supermassive black holes however are believed to be at the centre of virtually all large galaxies, including our own. The Milky Way’s supermassive black hole is called Sagittarius A* - but more on that later.
How Does a Black Hole Form?
Stellar black holes are formed at the end of a star’s lifetime. However, not all stars end up as black holes, only the most massive stars do. The Sun is a small to medium-sized star and would need to be about 20 times more massive to become a black hole. Instead, the Sun will become a white dwarf – a small, dense remnant of a star that glows from its leftover heat after the Sun runs out of fuel.
When massive stars run out of fuel, the star’s core collapses under its own gravity so forcefully, that the outer layers are expelled outwards in a violent supernova explosion. Left behind is a neutron star. A neutron star’s mass is around one to two times the mass of the Sun but is only around 20 km in diameter. Imagine all the material contained within our Sun being squashed into the distance between Leicester and Loughborough. A teaspoon of material from a neutron star would weigh as much as Mount Everest. That’s very dense - you’d need pretty strong arms to lift that!
The most massive stars explode in a supernova to leave behind a stellar black hole at the centre. Material has to be crammed into a very small space to become dense enough to form a black hole. For example the Earth along with everything in and on it would need to be squashed down into approximately the size of a marble – that’s how compact the material needs to be to form a black hole.
The formation of supermassive black holes (SMBH) however, is more complicated. Scientists are still researching how SMBHs form, but astrophysicists agree that black holes can grow by attracting more matter and merging with other black holes – find out more in the ‘Black Holes in the news section’ below.
What Would Happen if You Went Close to a Black Hole?
Things get very weird when you go near a black hole. The gravitational pull gets stronger and stronger as you get closer. But there is a point of no return called the event horizon. Once you cross this boundary, you cannot escape, even if you were travelling at the speed of light (the fastest possible speed). At the centre of the black hole there is a “singularity”. This is where the curvature of space-time becomes infinite and the laws of physics as we understand them break down, meaning it is impossible to describe what happens.
If you began falling into a black hole, the gravitational forces acting on your feet would be much stronger than on your head, so you would be stretched so much that this process is called “spaghettification”. Not only would you stretch into spaghetti, but time dilation would also occur. If you were to observe someone falling into a black hole from a distance , it would look like a slow-motion movie – they would appear to slow down as they approached the event horizon taking an infinite amount of time to reach it – essentially time appears to stop and observing from the outside you’d never see the person fall inside the black hole.
What happens when black holes merge?
If two black holes come close to each other they begin to orbit around each other faster and faster. This results in gravitational waves. Einstein believed that when two bodies orbit each other, it causes ripples in space. Imagine dropping a stone in a pond. This would cause ripples to spread out in the water. Gravitational waves are ripples in space-time, but these waves are invisible and travel at the speed of light. In September 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected the first direct evidence of gravitational waves, caused by the merging of two black holes around 30 times the mass of the Sun. LIGO consists of two identical detectors which have two perpendicular arms 4 km in length with mirrors at the end. Laser beams are then passed through them and reflected back to a detector – normally the two lasers from the identical beams arrives back in sync. But when a gravitational wave passes through the area, ripples in space-time cause the arms to stretch and compress – so the laser light returning from each arm comes back out of sync. This discovery was an incredible accomplishment, as the ripple changed the length of a 4 km LIGO arm by a thousandth of the width of a proton. To put this into perspective, this would be changing the distance to the nearest star outside of the solar system (40 trillion km away) by one hair’s width.
The two black holes eventually merge making a more massive black hole, releasing an enormous amount of energy and an intense burst of gravitational waves.
Could supermassive black holes be the source of dark energy?
First of all we need to answer the question: what is dark energy? We don’t know much. But what we do know is that it makes up around 70% of the universe. Scientists also believe that dark energy is accelerating the expansion of the universe. This means that dark energy is essentially an unknown force working against gravity.
Earlier I mentioned that supermassive black holes grow in two ways: by attracting matter or merging with another black hole. Scientists studied black holes in young galaxies with lots of stars forming which the black holes could feed on. They also look at black holes in giant but dormant galaxies that have very little to devour. They found that the black holes in the large dormant galaxies were more massive than they should be, so the growth of those black holes must be through a different process. One explanation proposed in February 2023 is that perhaps black holes grow with the expansion of the Universe, which could be the case if dark energy is contained in their cores. But this is still a controversial theory, and more research needs to be done.
The First Picture Taken of a Black Hole
M87 is a massive elliptical galaxy. It contains several trillion stars, making it one of the most massive galaxies in the local universe, housing one of the largest supermassive black holes known, M87* which has a mass of 6.5 billion times the mass of the Sun. But that’s not all. A giant jet of high energy particles extends for thousands of light-years from M87*. The jet is thought to be produced by a process known as accretion, in which matter is drawn into the black hole and heated to extremely high temperatures. As the matter spirals into the black hole, it gives off intense radiation which accelerate charged particles to nearly the speed of light. These particles are ejected from the black hole forming the jet!
What was previously thought to be impossible was achieved when scientists released the very first image of a black hole in April 2019. But how did they do it? A telescope the size of Earth was needed for this task. M87* the supermassive black hole in the centre of the galaxy M87, located around 55 million light-years away was captured by the Event Horizon Telescope - a global network of eight ground-based radio telescopes that worked together to make a single Earth-size dish.
Sagittarius A* - the supermassive black hole at the centre of our galaxy
On 12 May 2022, scientists captured the second image of a black hole – Sagittarius A* (Sgr A*), taken by the Event Horizon Telescope. Scientists found that M87* and Sgr A* look very similar, even though Sgr A* is more than a thousand times smaller than M87*. These observations have greatly improved our understanding of how black holes interact with their surroundings.
The supermassive black hole located at the centre of the Milky Way galaxy is around 26,000 light-years away from Earth – so we won’t be falling into it any time soon. Its mass is 4.3 million times the mass of the Sun, but its diameter is only 17 times that of the Sun.
Black holes are fascinating objects that challenge our understanding of the universe. Many questions remain unanswered, but scientists continue to study black holes to unlock these mysteries.
Full image credits / references:
(Banner image) An artist's impression of a black hole. Credit: NASA’s Goddard Space Flight Center; background, ESA/Gaia/DPAC
(1a) A black hole consuming a star. Credit: NASA/CXC/M. Weiss
(1b) An artist's rendering of the Milky Way. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)
(2) Our Sun captured by NASA's Solar Dynamics Observatory. Credit NASA/Solar Dynamics Observatory
(3) The Crab Nebula. Credit NASA/ESA/J. Hester and A. Loll (Arizona State University)
(4) A diagram of a black hole. Credit: ESO/ESA/Hubble/M. Kornmesser/N. Bartmann
(video) Simulation of a supermassive black hole merger. Credit: NASA https://www.youtube.com/watch?v=uDhDZi9Qxhk
(5) The energy distribution of the universe. Credit: NASA/CXC/K. Divona
(6) The first image of a black hole - M87*. Credit: Event Horizon Telescope Collaboration
(7) An image of Sagittarius A* at the centre of the Milky Way. Credit: Event Horizon Telescope Collaboration
