Posted by KeighleyAstro on Sep 26, 2022 in Main |

Thursday 22nd September 2022 was the first meeting of the Keighley Astronomical society’s 2022/23 season. The guest speaker was Dr. Mark Hughes from the School of Physics and Astronomy at the University of Manchester. The subject matter of his presentation was ‘Massive Black Holes’. His visit drew a large gathering of over 40 society members. Many new to the society.

Dr Hughes commenced his presentation by posing five questions

What are Black Holes?

Are there different types?

Where do we find them?

How do we detect them?

How do they fit into our picture of galaxies.

Dr Hughes started to answer these questions by explaining what we mean by Mass and massive

He described the differences between a fully suited Astronaut standing on the Earth and one standing on the Moon. So on the Earth the astronaut has a mass of say 85kg. His weight as he stands on the Earth is 187lb or 13.4 stones. When the astronaut is standing on the Moon he has a mass of 85kg; but his weight is now 31lb or 2,2 stones. Because weight is connected with gravity. As the Moon is a much smaller body than the Earth is attractive for called gravity is much less that of the Earth. If fact the Moon has only 1/6th of the Earths gravity.

So what we all know about Gravity & Mass is

‘What goes up must come down’, and the example Dr Hughes gave was the game of tennis. When the ball is struck with a bat. The ball travels forward and eventually falls to earth due to the force of gravity. The larger the mass of a body the greater is gravitational pull is.

He asked the question: What causes the force of gravity that we feel on Earth?
The answer being: The Earth’s mass.
The mass of the Earth is: 6,000,000,000,000,000,000,000,000 For the tennis ball to escape Earth’s gravity you need to make it travel at about… … 11 kilometres per second or about 24,600 mph.

“The Escape Velocity is how fast something has to go in order to escape the pull of gravity”

Dr Hughes posed another question. What would happen if you were on object where gravity is so strong that the escape velocity is greater than the speed of light?

Answer: You can never escape!!

So now we got to the part of the presentation where he looked at Black Holes. Dr Hughes explained that there are probably two main types of black hole.

Those Black holes that form from collapsing stars and

Massive Black Holes at the centres of galaxies.

His presentation then brought the members to look at the end points of Stars, when they die so to speak. Examples are White Dwarfs, Neutron Stars and Black Holes

Looking at Neutron stars they are about the size of a city. So that’s a whole star that has collapsed to that size.

The question was asked ‘What Conditions are necessary for a Black Hole to form’?
Dr Hughes explained, if you compress anything below a size known as the ‘gravitational radius’ you would make a Black Hole. To make a Black hole from the mass of the sun you would need to shrink it to a radius of about 3km. If you wanted to do that with the Earth it would be a radius of 9mm!

Are Black Holes inevitable?

J. Robert Oppernheimer the American theoretical physicist who is often credited as the “father of the atomic bomb” for his role in the Manhattan Project during world War Two. On 1st September 1939, the Nazi German army invaded Poland, triggering the beginning of the war that changed the world’s history forever. Remarkably, it was on this very same day that the first academic paper on black holes was published. The now acclaimed article, On Continued Gravitational Contraction, by J Robert Oppenheimer and Hartland Snyder, two American physicists, was a crucial point in the history of black holes.

The “most perfect objects” in the universe Black Holes are made of ‘space time’ (i.e. nothing!) From the outside we can only measure their Mass, Charge and Angular Momentum. All other information about the object that collapsed to form the Black Hole is lost.

Question: What happens if I fall into a Black Hole?

Answer; If you leapt heroically into a stellar-mass black hole, your body would be subjected to a process called ‘spaghettification’ (no, really, it is). The black hole’s gravity force would compress you from top to toe, while stretching you at the same time… thus, spaghetti.

Question: Will I even notice?

Answer: You would be able to see out from inside, but no one would be able to see you because any light would fall back on you.

Question: Will I get fried?
No matter what type of black hole you fall into, you’re ultimately going to get torn apart by the extreme gravity. No material, especially fleshy human bodies, could survive intact. So once you pass beyond the edge of the event horizon, you’re done.

Question: Why did we think there would be Massive Black holes?

Answer: Active galaxies

Question: Will a Black Hole swallow up everything in a galaxy?

Answer: The pull of gravity doesn’t always win! Remember when you where a child and you would play on the roundabout in the local park. It was always difficult to get closer to the centre when it was spinning around due to centrifugal force pushing you away from the centre. That is the apparent outward force on a mass when it is rotated.
Galaxy is spinning and so objects far from the black hole (like us!) don’t fall in.

Question: How important are galaxy mergers? and are Galaxy Mergers Important for Making Galaxies ‘Active’?

Answer: Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the merger rate is a fundamental measurement of galaxy evolution. The merger rate also provides astronomers with clues about how galaxies bulked up over time. A supermassive black hole (millions or even billions of times the mass of the sun) sits at the core of most if not all large galaxies. But only a fraction of these are the bright radiation sources known as active galactic nuclei, which light up when the central black hole is actively gobbling up nearby gas clouds. The galactic centre glows brightly at all wavelengths because the gas heats up as it falls into the black hole and emits intense radiation.

Question: How do we see a black hole?

Answer: We don’t! We look for its effect

In 2019, the Event Horizon Telescope team released the first image of a black hole. The black hole at the centre of this galaxy, named M87*, is a behemoth 2,000 times larger than Sagittarius A* and 7 billion times the mass of the Sun. But because Sagittarius A* is 2,000 times closer to Earth than M87*, the Event Horizon Telescope was able to observe both black holes at a similar resolution – giving astronomers a chance to learn about the universe by comparing the two.

The similarity of the two images is striking because small stars and small galaxies look and behave very differently than large stars or galaxies. Black holes are the only objects in existence that only answer to one law of nature – gravity. And gravity does not care about scale.
For the last few decades, astronomers have thought that there are massive black holes at the centre of almost every galaxy. While M87* is an unusually huge black hole, Sagittarius A* is likely pretty similar to many of the hundreds of billions of black holes at the centre of other galaxies in the universe.

Detecting Massive Black Holes using Gas.

The simple model is of black hole accretion consists of a black hole surrounded by a sphere of hot gas, and that gas accretes smoothly onto the black hole, and everything’s simple, mathematically. But the most compelling evidence that this process is not smooth, simple, and clean, but actually quite chaotic and clumpy. Black holes probably have two ways of feeding: For most of the time, they may slowly graze on a steady diet of diffuse hot gas. Once in a while, they may quickly gobble up clumps of cold gas as it comes nearby.

Question; Is there a relationship between the Black Hole Mass and the Bulge of the Galaxy?

The results now show a close relationship between the black hole mass and the stars that comprise an elliptical galaxy or the central bulge stars of a spiral galaxy. But surprisingly, an even tighter correlation is found. ‘Other observations of the entire stellar mass of the bulge show a very tight relationship between a black hole’s mass and the depth of the gravitational potential well as measured by the magnitude of random velocities of stars in the galaxy’s hub. This bolsters the conclusion that the mass correlation is real.

Question: Are merging black holes – a source of gravitational waves

Answer: Only cataclysmic events involving the heaviest objects, such as black holes and neutron stars, can create gravitational waves big enough to be detected on Earth. They radiate out across the universe at the speed of light, passing through almost everything in their path.

Prospects for the future – spaced based gravitational wave detection

The Laser Interferometer Space Antenna (LISA) is a proposed space probe to detect and accurately measure gravitational waves—tiny ripples in the fabric of spacetime—from astronomical sources. LISA would be the first dedicated space-based gravitational wave detector. In 2017 the suggested mission received its clearance goal for the 2030s, and was approved as one of the main research missions of ESA.

Summary

1) Black Holes have gone from mathematical curiosities to essential components of astrophysical theories.

2) Massive Black holes appear to be common at the centres of galaxies – but which came first, the black hole or the galaxy?

3) Gravitational wave detectors and the Event Horizon Telescope have the potential to revolutionise our picture of Black Holes and even gravity itself.