Have you ever wondered what sits at the very center of our Milky Way, holding together hundreds of billions of stars in a cosmic dance? We're talking about Sagittarius A* (Sgr A*)—a supermassive black hole that's both incredibly massive and surprisingly compact.
If you're picturing something the size of our solar system, you're in for a surprise. While Sgr A* contains over 4 million times the mass of our Sun, its actual dimensions are far smaller than you'd expect. But here's where it gets interesting: depending on what you're measuring—the black hole itself, its shadow, or the glowing ring around it—the answer changes dramatically.
This article is crafted for you by FreeAstroScience.com, where we're committed to making science simple and accessible. We believe that keeping your mind active and alert fights the darkness—because as the saying goes, "the sleep of reason breeds monsters." So let's wake up our curiosity and explore one of the universe's most fascinating objects.
Table of Contents
- What Is Sagittarius A* and Why Should You Care?
- How Much Mass Does Sgr A* Actually Contain?
- What's the Size of the Event Horizon?
- How Big Is the Shadow We Actually See?
- Could Sgr A* Fit Inside Our Solar System?
- How Far Away Is It and How Do We Study It?
- What Have We Learned Recently About Our Black Hole?
What Is Sagittarius A* and Why Should You Care?
Sagittarius A* isn't just any black hole—it's our black hole. Located roughly 26,000 light-years from Earth in the constellation Sagittarius, this supermassive beast anchors our entire galaxy. Every star in the Milky Way, including our Sun, orbits around this gravitational giant.
For decades, astronomers tracked stars whipping around an invisible point at breakneck speeds. These stellar orbits provided "the best empirical evidence that supermassive black holes do really exist," according to research team leader Reinhard Genzel. But we didn't actually see Sgr A* until May 2022, when the Event Horizon Telescope (EHT) released the first-ever image of our galactic center's black hole. en.wikipedia
The image shows a glowing ring of superheated gas surrounding a dark center—exactly what Einstein's general relativity predicted we'd see. This wasn't just a pretty picture. It was proof that a supermassive black hole genuinely sits at our galaxy's heart.
How Much Mass Does Sgr A* Actually Contain?
Let's talk numbers. Sgr A* weighs in at approximately 4.1 to 4.3 million solar masses. Think about that for a second—that's the equivalent of more than 4 million Suns crushed into a single point.
kicker: despite this enormous mass, all of it is theoretically compressed into a point of infinite density called a singularity. We can't observe the singularity directly (nothing escapes from inside a black hole, not even light), but we can measure its effects on everything around it.
Research published in 2009 by Gillessen and colleagues, based on 16 years of monitoring stellar orbits, pinpointed the mass at 4.31 ± 0.38 million solar masses. These measurements came from watching stars zoom around Sgr A* at incredible speeds, their movements dictated entirely by the black hole's gravitational pull. en.wikipedia
Recent studies in 2024 and 2025 confirm that 99.9% of the mass at the galactic center belongs to Sagittarius A* itself, leaving almost no room for other matter. That's a staggering level of gravitational dominance.
What's the Size of the Event Horizon?
The Point of No Return
The event horizon is the boundary around a black hole where gravity becomes so strong that nothing—not even light—can escape. It's the ultimate "point of no return." For Sgr A*, this boundary is defined by something called the Schwarzschild radius.
The Schwarzschild radius of Sgr A* is approximately 12.7 million kilometers. Double that, and you get the event horizon's diameter: about 25.4 million kilometers across.
Putting It in Perspective
To give you a sense of scale, this diameter is less than half the distance between the Sun and Mercury, which sits about 58 million kilometers from our star. Mercury is the closest planet to the Sun, so we're talking about something that's surprisingly compact given how much mass it contains.
If you could somehow place a ruler across the event horizon, you'd measure roughly 25 million kilometers—small enough to fit comfortably inside Mercury's orbit. It's mind-boggling when you consider that this tiny region holds the gravitational reins of our entire galaxy.
How Big Is the Shadow We Actually See?
The Glowing Ring Phenomenon
When the Event Horizon Telescope captured Sgr A* in 2022, what we saw wasn't the black hole itself. Instead, we saw a bright ring of light surrounding a dark circular region—the black hole's "shadow."
This shadow appears larger than the actual event horizon because of gravitational lensing. The black hole's extreme gravity bends light rays that pass nearby, creating an optical illusion that makes the shadow roughly twice the size of the event horizon.
The shadow diameter measures approximately 52 million kilometers. The EHT observations recorded this as 51.8 ± 2.3 microarcseconds across in the sky —an astonishingly precise measurement for an object 26,000 light-years away.
The Accretion Disk's Reach
Beyond the shadow, there's the accretion disk—a swirling mass of superheated gas and dust spiraling into the black hole. This disk can extend for hundreds of millions of kilometers, comparable to Mercury's entire orbit or even farther.
The incandescent gas in this disk heats up to millions of degrees, glowing brilliantly in radio waves, X-rays, and infrared light. It's this glow that astronomers observe when studying Sgr A*.
Could Sgr A* Fit Inside Our Solar System?
Here's a fun thought experiment: what if we placed Sagittarius A* at the center of our solar system, replacing the Sun?
The event horizon—that 25.4-million-kilometer-wide boundary—wouldn't even reach Mercury. Mercury orbits at about 58 million kilometers from the Sun, so the black hole's point of no return would sit comfortably inside that orbit.
However, the shadow (at 52 million kilometers across) would come close to Mercury's orbit, and the accretion disk could extend well beyond it. The glowing disk of superheated matter might stretch past Mercury, possibly reaching toward Venus or even Earth's orbital distance.
But here's what's truly remarkable: despite occupying such a relatively small volume, Sgr A*'s gravitational influence extends across the entire Milky Way. It's what holds our galaxy together, orchestrating the cosmic ballet of hundreds of billions of stars.
How Far Away Is It and How Do We Study It?
A 26,000-Year Journey
Sagittarius A* sits approximately 26,000 light-years from Earth. That means the light we see today left the black hole 26,000 years ago, when our ancestors were living in the Stone Age.
Despite this vast distance, Sgr A* is one of the closest supermassive black holes to us, making it an ideal target for study. Only a handful of black holes in the universe allow us to observe the flow of matter near their event horizons with such clarity.
The Event Horizon Telescope
The EHT isn't a single telescope—it's a global network of eight radio observatories working together. By combining data from telescopes across six geographical locations, astronomers created an Earth-sized virtual telescope capable of resolving incredibly fine details.
The 2022 image of Sgr A* took five years to process. Why so long? Because Sgr A* changes rapidly. Its radio emission varies on timescales of minutes, making it far more challenging to image than M87*, the first black hole ever photographed. en.wikipedia
In 2024, the EHT captured Sgr A* in polarized light for the first time, revealing organized magnetic fields around the black hole—similar to those around M87*. These magnetic fields likely play a role in launching powerful jets and controlling how matter falls into the black hole.
What Have We Learned Recently About Our Black Hole?
It's Less Destructive Than We Thought
Recent observations using the ERIS instrument at the Very Large Telescope in Chile revealed something surprising: several "dusty objects" follow stable orbits around Sgr A*. This suggests our supermassive black hole is less destructive than previously
"Our results show that Sagittarius A* is less destructive than was previously thought," explains Dr. Florian Peissker from the University of Cologne. "This makes the center of our galaxy an ideal laboratory for studying the interactions between black holes and stars."
In December 2024, scientists discovered a pair of binary stars (designated D9) orbiting each other close to Sgr A*. These findings challenge our assumptions about how violent and chaotic the galactic center truly is.
Surprisingly Faint Emission
Despite its enormous mass, Sgr A* is remarkably faint. Studies using NASA's Chandra X-ray Observatory found that less than 1% of material within the black hole's gravitational influence actually reaches the event horizon. Most of it gets ejected before crossing the point of no return.
This explains why Sgr A* doesn't shine as brightly as we'd expect. The black hole is essentially on a starvation diet, consuming only a tiny fraction of the material swirling around it.
Flares and Orbital Motions
High-resolution infrared observations with the GRAVITY instrument have caught Sgr A* producing bright flares. During these events, hot spots of gas appear to orbit the black hole at super-Keplerian speeds—faster than standard orbital mechanics would predict. arxiv
These flares might result from magnetic reconnection events, where tangled magnetic field lines snap and reconnect, releasing enormous amounts of energy. Understanding these flares helps us learn how black holes interact with their immediate environment.
Historical Variations
New research suggests that Sagittarius A* hasn't always been as quiet as it is today. The X-ray light coming from the galactic center may have changed dramatically over time, with the black hole potentially shining 10,000 times brighter in the past.
Why This Matters to You
Understanding Sagittarius A* isn't just an abstract exercise. It tells us about our place in the cosmos. This black hole, sitting quietly at the center of our galaxy, has shaped the Milky Way's structure and evolution over billions of years.
Every time you look up at the night sky, you're part of a galaxy held together by a supermassive black hole that's only 25 million kilometers across—smaller than Mercury's orbit, yet powerful enough to command hundreds of billions of stars.
The recent images from the Event Horizon Telescope prove that Einstein was right about general relativity. They show us that black holes aren't just theoretical constructs—they're real objects we can observe and study. And Sgr A* serves as a cosmic laboratory, helping us understand how gravity works at its most extreme.
At FreeAstroScience.com, we're passionate about sharing these discoveries with you. Science doesn't have to be complicated or intimidating. By exploring objects like Sagittarius A*, we keep our minds sharp and our curiosity alive. Remember, staying intellectually engaged is how we fight the darkness—because the sleep of reason breeds monsters.
We hope this article has given you a clearer picture of the supermassive black hole at our galaxy's heart. Come back to FreeAstroScience.com anytime you're curious about the universe. We'll be here, ready to make science simple and exciting.
Sources
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- NSF. "Astronomers confront massive black hole at the heart of the Milky Way, Sagittarius A*." June 13, 2022. nsf
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- arXiv. "A 'coronal-mass-ejection' model for flares in Sagittarius A*." January 27, 2023. arxiv
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- Scientific American. "Supermassive Black Hole Sagittarius A* May Have Once Shone 10,000 Times." January 13, 2026. scientificamerican
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Written by Gerd Dani for FreeAstroScience.com
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