Can you picture something nine million miles across and yet is invisible? No, and neither can anyone. It is a “supermassive black hole” at the center of our Milky Way galaxy, some 27,000 lightyears away. The nearest star, apart from the Sun, is 4 lightyears away. We are in an outer suburb of the galaxy – social distancing applies. In contrast there are dozens of stars within one lightyear of that black hole orbiting around it at speeds up to 8,000 km/second! At that speed you could fly from Chicago to India in one second!
We cannot see the black hole – it’s really black – but this year’s Nobel prize for physics was awarded to astronomers Reinhard Genzel and Andrea Ghez who led teams that managed to see those stars. They watched one star make a complete orbit in 16 years.
You cannot simply look through a telescope to see such faint stars crowded together so far away. First, go up a mountain to get above most of our murky, turbulent atmosphere. Then use the huge 10 meters (30 feet) in diameter Keck telescope in Hawaii. Make sure the mirrors are not deformed by gravity or temperature changes. Next, select infrared rather than visible light because infrared is less affected by gas and dust in space.
Despite your efforts the image is still blurry because of atmospheric turbulence, the reason stars seem to twinkle. It’s not the stars twinkling but unsteady air, which is a good reason for putting telescopes in space, like the Hubble telescope.
But here’s the amazing trick. It’s called adaptive optics and it’s almost as good as being in space. Focus on a star as its image jiggles around and apply tiny deformations a thousand times a second to a small flexible mirror to keep that star’s image fixed. But they actually make a “fake star” by shooting laser beams high up into the atmosphere and exciting mercury atoms. All the stars in the field of view then stop jiggling. Et voila! It’s like when you have cataract surgery and suddenly everything’s clear!
Then keep watching. For two decades. Observe those stars orbiting that invisible black hole and calculate its mass. Can you believe it? Four million times the mass of the Sun (itself 300,000 times the mass of Earth)! We now think nearly all galaxies have giant black holes at their centers, with stars, planets, and even maybe civilizations falling in. Fortunately, the nearest black hole to us is 27,000 light years away!
