This is a version of an article that originally appeared in Positively Naperville, where Fermilab scientist emeritus Mike Albrow contributes columns on scientific topics of general interest.
The left picture shows an infrared image of stars very close to the center of our galaxy through the giant Keck telescope in Hawaii. The right panel shows the same image after switching on the “adaptive optics” to correct 2,000 times per second for atmospheric turbulence. Stars in the small box are within 0.1 light-years of a supermassive black hole. Images: UCLA Galactic Center Group – W.M. Keck Observatory Laser Team
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 light-years away. The nearest star, apart from the sun, is four light-years away. We are in an outer suburb of the galaxy. In contrast, there are dozens of stars within one light-year of that black hole orbiting around it at speeds up to 8,000 kilometers per 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-meter-diameter Keck telescope in Hawaii. Make sure the mirrors are not deformed by gravity or temperature changes. Next, select infrared rather than visible light, since 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.
To focus a telescope on a faint star, focus on a fake star first: Instead of using a real star, astronomers shoot a pair of laser beams high up in the atmosphere, where they cause mercury atoms to glow, making a “fake star” above the turbulence. Photo: Ethan Tweedy Photography
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. Instead of using a real star, astronomers shoot a pair of laser beams high up in the atmosphere, where they cause mercury atoms to glow, making a “fake star” above the turbulence. This is better; it can be bright enough and is always in the field of view of the telescope no matter where it points. 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!