Our Supermassive Black Hole – Sagittarius A

Sagittarius A – as imaged by the team at the Event Horizon Telescope.

Imagine taking over 4 million copies of our Sun and cramming the combined mass into a region of space no bigger than the orbit of Mercury.

That’s Sagittarius A, the supermassive black hole at the centre of our own Milky Way galaxy. Evidence for Sagittarius A has been growing since the 1970s but now in 2022 the team at the Event Horizon Telescope have actually imaged it.

The term ‘supermassive’ when attributed to black holes is very misleading as black holes are incredibly low volume but dense regions of space. To give you a feel for this, if you took our Moon and somehow compressed it into a black hole, the resulting anomaly would have a diameter of 0.2 millimetres! That’s probably less than the size of a single pixel on the screen you’re reading this article on.

As black holes grow they can devour more mass and will slowly get bigger with the event horizon radius r defined by the famous Schwarzschild equation:

r = 2GM/c^2

In this equation G is the universal gravitational constant, c is the speed of light and M is the mass of the black hole. This is a simple linear relationship, so for example doubling the mass of a black hole will double its radius.

The bright central region of our Milky Way galaxy where Sagittarius A is located. Telescopes of the Atacama Large Millimeter/submillimeter Array in the foreground. Credit: EHT Collaboration

Given the relatively small volumes and areas of space involved, detecting even the most massive black holes in challenging to say the least. Sagittarius A, despite containing millions of solar masses, occupies a volume smaller than a single star in its giant phase of evolution. This gets compounded by the incredible distances involved. The centre of our Milky Way where Sagittarius A is located is a staggering 26,000 light years away. How then did the team capture the image?

The key was using multiple detectors spread across the planet, effectively constructing an Earth sized telescope. The data collected from these widely spaced arrays was then gathered together, producing many terabytes of data, and processed by banks of supercomputers called ‘correlators’. The final image was constructed using advanced algorithmic and statistical imaging techniques.

EVT’s detectors are spread around the planet effectively creating an Earth sized detection aperture.

Clearly by their nature black holes do not allow any light to escape so what we see in the final image is the infrared signature of super-heated gas rotating close to the black hole.  Black holes therefore reveal themselves by their indirect influence on nearby objects rather than direct observation.

Indeed, Sagittarius A’s existence was originally inferred by its influence on nearby stars, which are being thrown about at fantastically high speeds due to its intense gravitational influence. Fast enough for us to produce time lapse images over several years (see animation below).

Time-lapse from the ESO’s Very Large Telescope in Chile shows stars orbiting close to Sagittarius A over a 20 year detection period. 

Tidal Disruption Event

Amazing visualisation of a star captured and ripped apart by the immense gravitational well of a Black Hole.

As the outer atmosphere of the star is accelerated by the black hole’s gravity much of it reaches escape velocity and is strewn into space, while some becomes trapped in a highly eccentric orbit. Stellar material reaching the event horizon closer to the back hole is super heated by frictional heating and turbulent flow, generating a bright accretion disk.

Meanwhile jets of concentrated electromagnetic radiation and ionised particles are blasted deep into space along the axis of rotation – a so called astrophysical jet. This transfer of kinetic energy means the black hole system is slowly loosing angular momentum over time.

I often think the universe at this scale is like witnessing a vast machine running on the conservational of energy. A wonderful illustration of the exchange of gravitational potential, angular kinetic, linear kinetic, heat and mass energy.

All energy was created at the big bang singularity and all physical processes from star birth, star death, black holes and even organic human life is the transfer and redistribution of this original energy state.

Video credit: DESY, Science Communication Lab

Binary Black Holes – Gravitational Lensing

A visualisation of how extreme gravity can distort the light paths close to binary black holes. The blue and red halos are the accretion disks surrounding the black holes (material super heated close to the event horizon). The blue disk represents a black hole some 200 million times the mass of our Sun. The red one is a smaller black hole half this mass.

Gravitational lensing like this is a real and measurable consequence of general relativity and astrophysicists are now using sophisticated modelling techniques to make incredible predictions. One amazing application of gravitational lensing is predicting when duplicated but delayed images from the same supernovae will appear, allowing astronomers to study exploding stars in real time.

This happens when a single event – like a supernova – is projected into multiple copies of itself by a large intervening galactic mass, with each copy delayed due to different light paths through spacetime.⁣

Video Credit: @NASAGoddard