Welcome to Three Alpha! Since last time: In the Solar System, samples returned from the asteroid Bennu show it is made of a mix of material from inside and outside the Solar System; in the Galaxy, a new quadruple star system has been discovered; and in the Universe, new studies suggest the masses of the earliest stars may have been more varied than thought.

Meanwhile, in this edition of the newsletter we’re focusing just how big the biggest black hole ever discovered really is. Read on for more…

Too massive to comprehend

The Universe has a new record for the largest black hole ever discovered. It lives in the centre of a pair of galaxies known collectively as the Cosmic Horseshoe. That’s a large, red elliptical galaxy and a more distant galaxy seen stretched around the elliptical thanks to gravitational lensing. The elliptical galaxy is huge: 100 times the mass of the Milky Way. So it’s not a surprise that it contains a large supermassive black hole at its centre. Even so, the mass of this black hole is startling: 36 billion solar masses.

Astronomers are often terrible at throwing large numbers around. 36 billion solar masses is equivalent to 12 quadrillion Earths. Incomprehensible. It’s about 10% of the mass of the Large Magellanic Cloud. Sure. I don’t think there’s a way to really get your head around how much mass that really is. Personally, I find it easier to think about the radius of the beast.

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A black hole doesn’t actually have a physical radius because it doesn’t have a surface, but by its radius we usually mean the Schwarzschild radius, also called the event horizon. That’s famously the point of no return. The edge of the gravitational abyss from which there is no escape. For a typical, stellar-mass black hole, the radius is a few miles across. For the black hole at the centre of our Galaxy, if you put it where the Sun is, it would stretch to somewhere between the orbits of Venus and Earth (just under 80 million miles, or less than 1 AU, which is the distance between the Sun and Earth).

Now, if you replaced the Sun with this new 36 billion solar mass black hole, how wide would it be? The black hole’s radius would stretch to about 70 AU. For reference, Jupiter is at about 5 AU and Pluto is between about 30 AU and 50 AU, so 70 AU is enough to engulf all the planets, and then some. Yikes!

Another fun way to think about it is the strength of gravity as you get near the black hole. The force of the gravitational attraction between two objects depends on their masses and how far apart they are. For the Earth and the Sun, that works out at a huge amount (35 sextillion Newtons, or 35 followed by 21 zeroes, if that is useful to you). So how far would the Earth need to be from this new black hole to feel the same force? That works out as just under 190,000 AU, or three light years. That’s about three quarters of the distance to the nearest star, Proxima Centauri, to experience the same force we feel from the Sun. Yikes again!

However you choose to think about it, this new black hole is so big there isn’t really an adjective suitable to describe it. Gigantic. Gargantuan. Bloody enormous. Perhaps we need to coin a new word. Let me know if you think of one!

Finally

If you haven’t already seen it, you need to take a look at this amazing new photo of a young planet clearing its orbit around a young star:

What is Three Alpha? Other than being the name of the newsletter you’re reading now, the name “three alpha” comes from the triple-alpha process, a nuclear chain reaction in stars which turns helium into carbon. Read more here.

Who writes this? My name is Dr. Adam McMaster. I’m an astronomer in the UK, where I mainly work on finding black holes. You can find me on BlueSky, @adammc.space.

Let me know what you think! You can send comments and feedback by hitting reply or by emailing [email protected].