The Sky at Night – Black Holes: Searching for the Unknown

The Sky at Night - Black Holes: Searching for the Unknown

The Sky at Night – Black Holes: Searching for the Unknown – Delving into the vast mysteries of our cosmos, there’s never been a more electrifying period than now to immerse oneself in the enigma of one of space’s most captivating phenomena: black holes. This month, the illustrious team behind ‘The Sky at Night’ embarks on a profound journey, unraveling the sophisticated science behind black holes. Their mission is not just to uncover the guarded secrets of these space giants but also to potentially pave the way to answering some of the universe’s most profound questions.



Chris, with an insatiable curiosity, seeks out the expertise of Dr. Becky Smethurst at the prestigious University of Oxford. Together, they venture into the intricate process by which a star, nearing its end, transforms into a black hole. But Chris doesn’t stop there; he delves deeper into the representation of black holes in popular culture, questioning if they truly deserve the ominous and menacing image often depicted. With vivid detail, he paints a picture of the harrowing fate one might face when nearing a black hole’s event horizon. Yet, on a more luminous note, he also highlights the possibility of black holes playing a pivotal role in illuminating our vast universe.



On another intriguing front, Maggie takes us on a journey to decipher the evolving understanding of black holes. She meets Dr. Tessa Baker, a key player in the operations of LIGO – the Laser Interferometer Gravitational-Wave Observatory. Unlike any conventional observatory that sets its eyes upon the skies, LIGO stands apart, choosing to “listen” to the universe. As a new observational cycle unfolds, Maggie delves into the aspirations and goals of the LIGO team, uncovering what cosmic secrets they’re yearning to decode next.



Chris then rendezvous with Dr. James Nightingale, an astronomer who recently unraveled the presence of one of the largest black holes ever known. Together, they discuss his innovative use of computational technology combined with the timeless art of gravitational lensing. The duo then take a deeper dive into the symbiotic relationship between galaxies and black holes, particularly the theory suggesting that at the heart of every galaxy, a supermassive black hole resides, governing its dynamics.

We’re then escorted to the realm of our in-house celestial connoisseur, Pete Lawrence. He not only offers guidance on pinpointing a black hole in the night canvas but also showcases the resplendent beauty of Saturn in its most radiant glory.

Concluding this cosmic journey, George Dransfield steps into the groundbreaking environment of Dr. Silke Weinfurtner’s black hole laboratory. Here, on our very own Earth, they replicate and simulate certain aspects of black holes. Using intricate fluid systems, they conduct experiments that might just hold the key to confirming if the speculated phenomena surrounding black holes are indeed a reality.


The Sky at Night – Black Holes: Searching for the Unknown


There has never been a more exciting time to study one of the most mysterious phenomena in space. This month, The Sky at Night team investigate the science of black holes and discover the incredible techniques being used to uncover their secrets, and even help us answer bigger questions about our universe.


The Life and Death of Stars Leads to Black Holes

Chris meets with Dr Becky Smethurst at the University of Oxford to learn how a black hole forms from the death of a star. Stars come in a wide variety of sizes and masses. The smallest stars, with lower mass, burn their hydrogen fuel slowly and can exist for billions of years. On the other end of the spectrum, massive stars with higher mass burn very hot and fast, consuming their fuel quickly in just a few million years.

Our Sun is mid-sized, burning hydrogen into helium at a moderate pace, maintaining a balance against the inward crush of gravity. But eventually, all stars will use up their fuel. For stars around the Sun’s size, this means becoming dense stellar remnants called white dwarfs. However, when stars about ten times more massive than our Sun exhaust their fuel, the result is a gigantic explosion known as a supernova.

In a supernova, the extremely dense core of the massive star collapses, but lighter elements rebound outward with incredible energy. All that remains of the original star is a small, dense ball of material. What happens to this stellar core depends on how much mass it retains. For cores up to about 25 times the Sun’s mass, the inward crush of gravity is so strong that no known force can resist it. This results in the formation of one of the most mystifying objects in space – a black hole.


What Are Black Holes and How Do They Form?

Black holes are regions of space with gravitational fields so intense that nothing, including light, can escape once it crosses the black hole’s boundary, known as the event horizon. At the very center of a black hole is the singularity, a point where matter is compressed to infinite density. Surrounding the singularity is the event horizon, encompassing the black hole. Any object that passes this threshold is forever trapped inside the black hole.

While the singularity remains a mystery, since known physics breaks down there, the event horizon is better understood. However, because no light can escape from within the event horizon, obtaining direct observations of black holes is extremely difficult. Nonetheless, by observing the gravitational effects on nearby stars and gas, astronomers can infer the presence of black holes.


Do Black Holes Deserve Their Sinister Reputation?

Perhaps because they cannot be directly seen, black holes are often depicted as terrifying monsters devouring everything around them. But are they really as sinister as their portrayal in popular culture? As Chris explains, black holes only pose a danger if you get too close to the event horizon. The difference in gravitational pull between your head and feet would rip you apart in a process called spaghettification.

While some theories propose black holes have intense firewalls around them, observations show black holes can peacefully coexist with surrounding stars. Stars orbiting the black hole at the center of our galaxy show no signs of being in peril. Like the Earth orbiting the Sun, they can orbit a black hole without falling in, as long as they maintain a safe distance beyond the event horizon.

Rather than menacing destroyers, Chris suggests black holes may even play a helpful role in powering the light of galaxies. As matter accelerates toward the black hole, it heats up and glows brightly. In this way, black holes act as cosmic light sources, energizing the universe around them.


LIGO Listens for Ripples in Space-Time

Maggie explores how scientists are trying to understand more about black holes by meeting Dr Tessa Baker, who works on LIGO. The Laser Interferometer Gravitational-Wave Observatory is one of the world’s largest physics experiments and is not your usual type of observatory; instead of looking – it listens.

LIGO uses laser interferometry to detect incredibly tiny ripples in space-time called gravitational waves. These waves are caused by energetic cosmic events like binary black holes orbiting each other. As the black holes spiral closer over millions of years, they drag space-time along with them faster and faster, creating gravitational waves that propagate across the universe. By the time the waves reach Earth, they cause fluctuations smaller than a proton, which LIGO can detect.

Maggie learns that each new observation run makes LIGO more sensitive, allowing more gravitational wave events to be detected. So far, over 90 black hole mergers have been observed. By analyzing the gravitational wave signals, scientists can determine properties like the black holes’ masses and spins. This gives insight into how and where black holes form in the universe. The latest upgrade extends LIGO’s range even further, helping reveal previously hidden black hole populations.


Hunting for Supermassive Black Holes with Gravitational Lensing

Chris meets with Dr James Nightingale, who has recently discovered one of the largest black holes in space using brand new computational technology and the age-old technique of gravitational lensing. Gravitational lensing occurs when the gravity of a massive object in the foreground distorts and magnifies the light from a background object behind it. This acts as a natural telescope, allowing astronomers to see very distant galaxies in greater detail.

Using Hubble Space Telescope images, Dr. Nightingale’s team identified a lensed galaxy with a counter-image that could only be explained by the presence of an extremely massive black hole – 33 billion times the mass of our Sun. By computationally modeling the lensing effects, they were able to infer properties like the black hole’s mass and size. At 33 billion solar masses, it is over 3 times more massive than the entire Milky Way galaxy.

This demonstrates gravitational lensing enables measuring black holes in galaxies not actively powering bright emissions, greatly expanding the number of black holes we can study. In the future, Dr. Nightingale plans to use data from the recently launched Euclid space telescope to discover many more lensed black holes, including potentially the most massive ones in the universe. This will shed light on the relationship between black holes and their host galaxies.


Observing Black Holes and Saturn in August’s Night Sky

We visit our in-house stargazing expert, Pete Lawrence, who shows us how to find a black hole in the sky, and Saturn at its brightest and best. The constellation Cygnus is easy to spot by its distinctive Northern Cross shape. Within Cygnus lurks Cygnus X-1, the first identified black hole, discovered because of its powerful X-ray emissions.

Though the black hole itself is invisible, we can locate its position in the sky by finding the supergiant star it orbits. Pete explains how to star-hop from Deneb and Albireo, the tail and beak of the Swan, to identify the stars marking Cygnus X-1, about 7,000 light years from Earth.

Saturn will also put on a good show this month, reaching opposition on August 27th. This is when Saturn lines up on the opposite side of Earth from the Sun, so it rises as the Sun sets and appears biggest and brightest. Binoculars will reveal Saturn’s oval disk, while a small telescope is needed to spot its iconic rings. Saturn remains well-placed for evening viewing throughout September.


Simulating Black Holes in an Earth Laboratory

Finally, George Dransfield visits Dr Silke Weinfurtner at her black hole laboratory, where they are simulating features of black holes here on Earth. They use fluid systems to perform experiments to try to determine if phenomena we think occur around black holes could actually happen.

One such effect is superradiance – a spinning black hole can amplify the energy of waves interacting with it under the right conditions. While not directly observable in space, Dr. Weinfurtner’s lab creates vortex flows in water that behave identically to light waves around a spinning black hole. By studying these fluid systems, her team was able to experimentally confirm superradiance from a black hole-like vortex for the first time.

This demonstrates how analogue experiments can provide insights into exotic astrophysical systems we cannot easily observe or measure directly. As we improve techniques for detecting effects like superradiance in the lab, it may guide future space-based observations to find them in the real universe.



This month, the Sky at Night team employed some revolutionary methods to uncover new findings about one of space’s biggest mysteries – black holes. From listening to ripples in space-time with LIGO to magnifying distant galaxies with gravitational lensing, scientists are discovering more black holes than ever before. This is helping reveal their role in stellar life cycles, galaxy formation, and the cosmos.

While black holes remain invisible, the stars surrounding them provide clues to their mass and location. Improved observational data is enabling more accurate simulations of black hole environments, right here in university labs. As we confirm theories experimentally, it inspires new ways to search for phenomena like superradiance occurring in the vicinity of real black holes. There are sure to be many more discoveries ahead as we open this new gravitational window on our dynamic universe.


Frequently Asked Questions – The Sky at Night – Black Holes


What are black holes?

Black holes are extremely dense regions of space with gravitational fields so strong that nothing, including light, can escape past their boundary or event horizon. At the center is an infinitely dense point called the singularity. Black holes form from the collapsed cores of massive stars after supernova explosions.


How does LIGO work?

The Laser Interferometer Gravitational-Wave Observatory uses laser interferometry to detect minute ripples in space-time called gravitational waves, caused by energetic events like two black holes orbiting and merging. LIGO has two perpendicular arms several kilometers long. Lasers bounce off mirrors at the ends, allowing tiny changes in arm length to be measured.


What are supermassive black holes?

Supermassive black holes have masses millions to billions times that of the Sun and lie at the center of most large galaxies, including our Milky Way. Their origins are uncertain, but may result from mergers of smaller black holes over billions of years or rapid accretion of matter in the dense early universe.


What is gravitational lensing?

Gravitational lensing occurs when light from a distant object is bent and distorted by the gravity of a massive foreground object, like a galaxy cluster. This acts as a natural telescope, magnifying the background light source into arcs and multiple images. It enables studying faint distant galaxies as well as measuring black hole masses.


What is Saturn?

Saturn is the sixth planet from the Sun and the second largest in our Solar System. It is a gas giant mainly composed hydrogen and helium. Saturn is best known for its magnificent ring system and many moons. Saturn takes about 29 Earth years to orbit the Sun.


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