Do you ever wonder what the world, or the night sky, would look like if your eyes were sensitive to different kinds of light, like the VISOR vision of Star Trek fictional character Geordi La Forge, or perhaps radio waves, or X-rays?
In the past 60 or so years, astronomers have built telescopes and instruments which, together, can observe across much of the electromagnetic spectrum from low-frequency radio waves to high energy gamma rays. We now have multi-wavelength views of much of the sky, and many are very different from traditional visible light astronomy.
In this new light the sky is full of surprises.
Radio astronomy is one early example. When it was born in the 1950s, this “new vision” revealed extended, complex, jet-like structures tens to hundreds of times larger than our Milky Way galaxy, which filled the sky. Astronomers discovered that these structures emanate from active galaxies that are powered by a super-massive black-hole millions to billions of times our Sun’s mass. The black hole at the centre swallows stars, gas, and anything that comes too close. The closer the material gets the faster it spirals in, creating huge jets: columns of radiation that sometimes extend higher than the width of the galaxy itself. Because active galaxies shine much more brightly than the hundred million stars surrounding them, they often appear merely as points of light through the traditional, visible light telescopes through which I used to observe them as a PhD student at Cambridge. With radio astronomy we can see all this rich structure and the jets in action.
Similarly, the advent of X-ray astronomy in the 1970s revealed a different sky and many more surprises. X-rays originate in the hottest and most violent places in the Universe. In parallel with major events in our own lives, these include birth, death, collisions, capture and anything to do with black holes, albeit on much larger scales and longer timescales than here on earth. Throughout my professional career I have been investigating these violent events.
I recently retired as Director of NASA’s Chandra X-ray Observatory. Chandra, named for the Indian-born Nobel laureate Subrahmanyan Chandrasekhar, is the first and so far only X-ray telescope to provide high-resolution imaging which is comparable to ground-based telescopes on a clear night. It was launched on the NASA shuttle Columbia in July 1999 under the first female Commander, Eileen Collins.
Ever since, Chandra’s ultra-sharp X-ray images have revolutionized our understanding of celestial sources from young stars to clusters of galaxies, planets, exoplanets and the Universe as a whole.
As you know, when we break a leg we use X-rays to penetrate deep into thick, dense bone which visible light cannot penetrate. Chandra’s X-ray vision peeks at the youngest stars nascent in the clouds of gas and dust in our galaxy, reveals active galaxies at the farthest reaches of the Universe, probes through the debris of a massive star that blew up as a supernovae, images the otherwise invisible hot gas in galaxies and galaxy clusters, and has even imaged the aurorae shimmering with X-rays at the poles of Jupiter.
When we deployed Chandra in its orbit, more than a third of the distance from here to the Moon, our first target was a faraway active galaxy powered by a super-massive black hole.
This was guaranteed to be an X-ray source on which we could focus the telescope. While it indeed beautifully served this purpose, it also revealed something very curious.
It showed a long, narrow jet of X-ray emission on one side. The team of astronomers and engineers in the Chandra control room initially thought something had gone wrong with the telescope, causing X-rays to go astray. We were very relieved, and of course excited, when we realized that this X-ray emission coincided with a previously known jet of radio emission blasting out from the pole of the spinning black hole at close to the speed of light.
Our initial discovery marked the birth of a new field: multi-wavelength studies of radio jets, which continue to reveal the secrets of how they work.
Chandra also reveals complex and dynamic structures when such a jet sweeps the surrounding hot gas out of its path, creating bubbles in the X-ray gas which are filled with radio emission.
Chandra has merely looked at a few percent of the sky so far. Many discoveries await our sharp X-ray eyes as we continue our exploration of the X-ray sky.
Una, writer and educator
Radu Toma, Romania
Financier and CEO, USA
Letizia, Saudi Arabia
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