In June 1962, an Aerobee 150 sounding-rocket blasted above the Earth’s atmosphere from the White Sands Missile Range in the United States of America. During its five-minute flight, the small research craft aimed to detect X-rays fluorescing from the moon. What it found instead would take a decade to explain.
X-rays are an extremely high-energy form of electromagnetic radiation. While visible light, from violet to red, has a wavelength of between 400 and 700 nanometers, X-ray wavelengths stretch from only 0.1 to 10 nanometers.
Radiation from the sun extends over both spectrums, but the energy in X-rays is a millionth of that emitted in visible light. The X-rays that reach the Earth are unable to penetrate through our atmosphere, so exploration of cosmic sources needs to be done from space.
Despite being a hundred times more sensitive than previous attempts, no one expected the X-ray detector on board the Aerobee 150 to see many cosmic X-ray sources. Even if our nearest star, Sirius, emitted X-rays as luminous as its visible light (unlikely given the sun’s 1:1,000,000 ratio), it was still far too dim to be seen.
Instead, the rocket was hoping to see the moon’s fluorescence due to the incident X-rays from the sun. But the data rolled in to reveal another source in the sky.
A Mysterious Source
Named Scorpius X-1, this X-ray source was so strong that if its ratio to visible light had matched that of the sun, its brightness would have rivalled the moon from its position 9,000 light years away. This was a whole new type of cosmic engine and marked the birth of X-ray astronomy.
Scorpius X-1 would eventually reveal itself to be a binary of two stars in close orbit. One member of the pair had reached the end of its life and collapsed to form an immensely dense object known as a neutron star.
Its strong gravity was pulling gas off its stellar twin, which gained energy as it descended towards the neutron star, like a stone speeding up as it drops from a tall building. The energy was heating the gas to millions of degrees, causing it to radiate X-rays.
