Sizes of the Titans: M87* and Sagittarius A*

A Cosmic Showdown in Our Sky

The Event Horizon Telescope (EHT) Collaboration has once again delivered a breathtaking visualization, this time presenting a direct size comparison between two of the most enigmatic objects in the universe: M87* and Sagittarius A* (Sgr A*). These supermassive black holes, residing at the hearts of the galaxy Messier 87 and our very own Milky Way, are titans in their own right. While both appear roughly the same size in our sky due to their vastly different distances from Earth, their actual physical dimensions are worlds apart.

Understanding the Scale

The image beautifully illustrates the sheer scale of Sgr A* by placing it alongside M87* and familiar components of our Solar System. We see the orbits of Pluto and Mercury, the diameter of our Sun, and even the current location of the Voyager 1 spacecraft – humanity's most distant emissary.

The Giants Revealed:

• M87*: Located a staggering 55 million light-years away, M87* is an astronomical behemoth, ranking among the largest black holes known. Its mass is over 1000 times that of Sagittarius A*.
• Sagittarius A*: Situated 27,000 light-years away at the center of the Milky Way, Sgr A* possesses a mass approximately four million times that of our Sun.

Demystifying the 'Orange Ring'

Many of us see the "orange ring" surrounding the central black gap and wonder what exactly we're looking at. The black gap at the center represents the event horizon – the boundary beyond which nothing, not even light, can escape. The surrounding orange glow is not the black hole itself, but rather the accretion disk. This is a swirling mass of superheated particles – gas, dust, and plasma – orbiting the black hole at incredible speeds. As these particles collide and friction builds, they release energy in the form of radio waves, which are then detected by the EHT.

The Accretion Disk: More Than Just a Ring?

It's important to understand that the accretion disk isn't a simple flat ring. Due to intense gravitational forces and relativistic effects near the black hole, the material orbits and falls inward, often in a plane perpendicular to the black hole's angular momentum. While the event horizon is a spherical boundary, the accreting matter typically forms a disk-like structure, rather than a complete spherical shell. The EHT's images are captured at a favorable angle, allowing us to see the light bent and warped around the black hole's shadow.

A Closer Look at Sgr A*'s Dimensions

For Sgr A*, its mass of roughly 4.3 million solar masses translates to a Schwarzschild Radius (the size of its event horizon) of about 12.7 million km. However, what we observe as the 'hole' is actually the black hole's shadow, which is about 2.6 times larger than the event horizon itself. This shadow for Sgr A* would have a radius of approximately 33 million km – a scale that means it would cover most of Mercury's orbit! The innermost stable circular orbit (ISCO), which is essentially the hard inner edge of the accretion disk, is about three times the Schwarzschild radius, pushing inwards even further.

A Universe of Scale

This comparison offers a profound glimpse into the immense power and scale of these cosmic entities. M87* is truly colossal, dwarfing Sgr A* by an order of magnitude, yet both are mind-bogglingly vast when viewed against the backdrop of our familiar Solar System. It's a humbling reminder of the incredible diversity and grandeur of the universe we inhabit.

Credit: EHT collaboration (acknowledgment: Lia Medeiros, xkcd)