2019-08-01_Sky_and_Telescope

X-RAY EXPLORERS by Monica Young

14 AUGUST 2019 • SKY & TELESCOPE

W

alk outside tonight and you’ll see stars twinkling in

their constellations, while planets wander predict-

ably through the star fi eld. Only the occasional

nova or eclipse mars the seemingly immutable heavens.

But what if we swapped this tame view with X-ray vision?

Hot and violent sources would fl are across the sky; Aristotle

would never have spoken of immutability under such a sight.

Fortunately for philosophers (and for life itself), Earth’s

atmosphere has long shielded against doctrine- and DNA-

destroying X-rays. Humanity had graduated to space before

we realized what we were missing. The fi rst hints of an X-ray-

emitting universe came from rocket and balloon observa-

tions of the Sun, starting in 1949. But solar emissions were so

weak — a million times fainter than visible light — that to see

even nearby stars, detectors would have had to be a thousand

times more sensitive. Scientists doubted other cosmic X-ray

sources could be observed. In fact, NASA rebuffed scientist

Riccardo Giacconi (then at American Science & Engineering)

As scientists and engineers worked over the decades

to access the X-ray sky, they revealed a hot and lively

cosmos — and revolutionized how we study astronomy.

when he submitted a proposal for a rocket to observe X-rays

from across the sky.

Undeterred, Giacconi reworked his proposal for the U.S.

Air Force. The solar wind, he argued, could fl uoresce off the

Moon with possible impacts on communications. The Air

Force accepted his proposal and in 1962, after two failed

attempts, Giacconi and his team fi nally launched an Aerobee

rocket from White Sands Missile Range in New Mexico.

The rocket fl ew above 80 kilometers (50 miles) — where

the atmosphere thins enough for X-rays to pass through — for

all of 5 minutes and 50 seconds. But during that brief time,

it caught something: a bright X-ray source nowhere near the

Moon, dubbed Scorpius X-1 for its location on the sky.

pPERSEUS CLUSTER After the Uhuru satellite fi rst detected hot gas

swirling within galaxy clusters, the Einstein Observatory imaged the

blob of gas within Perseus (inset). Ultimately, Chandra revealed intricate

details that are helping astronomers probe the cluster’s history.

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