What X-Rays Have Done for Astronomy

A Google search for anything today should immediately alert you to the fact that the software giant is celebrating the 115th anniversary of the discovery of x-rays.

The penetrating radiation—a very high-energy form of light—was first documented by German scientist Wilhelm Conrad Roentgen in 1895. His famed x-ray picture of his wife’s hand made the medical applications of x-rays immediately clear.

The beauty of x-rays is that they blast right through materials that visible light bounces off of, such as skin, but they can be stopped if a material is dense enough, such as metal or bone.

In addition to the wonders of inner space, x-rays can teach us quite a bit about mysterious objects in outer space, such as black holes, neutron stars, supernova remnants, and the effects of the solar wind.

This, for example, is the first x-ray picture of Earth, taken by an orbiting NASA satellite:


—Image courtesy Polar, PIXIE, NASA

Most of the planet is dark, because Earth’s relatively thick atmosphere does a stellar job of absorbing x-rays. That bright spot near the North Pole is where auroras are glowing, not just as curtains of visible light, but also as swirls of x-rays.

Auroras appear when charged particles from the sun flow along Earth’s magnetic field lines and slam into the atmosphere, exciting (aka, adding energy to) atoms in the air.

These excited atoms spit the extra energy back out as light, giving us the colorful displays visible to the human eye. But auroras also generally emit x-rays, ultraviolet light, and radio waves, any of which we would be able to see with the right filters.

Turning our x-ray vision out into the cosmos, the high-energy radiation gives us a glimpse of the details in otherwise dark objects, such as black holes.

This picture, for instance, combines data from NASA’s Chandra X-ray Observatory and ESO’s Very Large Telescope to reveal a newfound microquasar in the galaxy NGC 7793:


—X-ray image courtesy NASA/CXC/Univ of Strasbourg/M. Pakull et al; optical image courtesy ESO/VLT/Univ of Strasbourg/M. Pakull et al; H-alpha image courtesy NOAO/AURA/NSF/CTIO 1.5m

In this zoomed-in part of the picture, the faint greenish-blue blob near the middle marks the position of a stellar-mass black hole.


—Image courtesy NASA/CXC/Univ of Strasbourg/M. Pakull et al

But how can we see something that by definition is so dense not even light can escape?

The trick is that this black hole is chowing down on matter from a nearby companion star. The material gets so hot as it falls into the black hole that it emits scads of x-rays, effectively providing a sign-post for the black hole.

What’s more, the act of eating causes the black hole to belch jets of hot gas, creating a bubble a thousand light-years wide. The jets are heating the surrounding material to the point that it’s also emitting x-rays, making the end points of the jets visible as orange and yellow blobs in this picture.

In the broader scheme of things, x-rays can tell astronomers a ton about objects already visible in optical wavelengths. Combining data from several observatories shows not just the state of things today, but can help trace the evolution of a particular object.

Take the Antennae galaxies, a pair of colliding galaxies discovered by Friedrich Wilhelm Herschel in 1785. Through a wide-field telescope, what you see is this:


—Image courtesy Digitized Sky Survey

Taken alone, optical data from the Hubble Space Telescope, infrared data from the Spitzer Space Telescope, and x-ray data from Chandra give these frames:


But add them up, and you get this:


—X-ray image courtesy NASA/CXC/SAO/J.DePasquale; IR image courtesy NASA/JPL-Caltech; Optical image courtesy NASA/STScI

Not too shabby, eh? And in addition to being an eye-popper, the combined picture tells astronomers what’s been going on during a collision that started a hundred million years ago.

X-rays tell us that the merger is causing huge clouds of hot, interstellar gas to be injected with elements from supernova explosions, such as oxygen, iron, magnesium, and silicon. The enriched gas will likely be incorporated into new generations of stars and planets.

Meanwhile, the Spitzer data show infrared light from dust clouds being heated by newborn stars, while the Hubble data reveals where older stars and clusters now reside.

As for the future of x-ray astronomy, the next big proposal is the International X-ray Observatory, a joint effort between NASA, the European Space Agency, and the Japan Aerospace Exploration Agency.

The probe is one of three missions competing for funding, and only one can move forward to launch. If IXO makes the cut, it would launch in 2021 and would study what happens close to the event horizons of black holes, how supermassive black holes grow, and how the violent processes that produce x-rays give rise to large-scale structures in the universe.

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