X-ray astronomy
Observing the universe in X-rays from above Earth's atmosphere.
X-ray astronomy is a branch of observational science focused on detecting and studying X-rays from celestial objects. Because Earth's atmosphere absorbs X-radiation, telescopes designed to observe these wavelengths must be carried to high altitudes using balloons, sounding rockets, or satellites. Unlike standard optical telescopes—such as those at the Maunakea Observatories—X-ray space telescopes are built to see this otherwise invisible radiation.
X-rays are expected from astronomical objects containing extremely hot gases, with temperatures ranging from about one million kelvin to hundreds of millions of kelvin. The existence of the E-layer of ionized gas high in Earth's thermosphere also hinted at a strong extraterrestrial source of X-rays. Theory predicted that the Sun and other stars would be prominent X-ray sources, but Earth's atmosphere blocked most extraterrestrial X-rays, making verification impossible until instruments could be sent to high altitudes.
Solar X-rays were confirmed in the mid-twentieth century using converted V-2 sounding rockets. Since 1958, detecting extraterrestrial X-rays has been a primary or secondary goal of many satellites. The first cosmic X-ray source beyond the Solar System was discovered by a sounding rocket in 1962. Named Scorpius X-1 (the first X-ray source found in the constellation Scorpius), its X-ray emission is 10,000 times greater than its visible light output, while the Sun's X-ray emission is about a million times less than its visible light. The energy output of Scorpius X-1 in X-rays alone is 100,000 times greater than the Sun's total emission across all wavelengths.
Thousands of X-ray sources have since been identified. Additionally, the intergalactic space within galaxy clusters is filled with a hot, extremely dilute gas at temperatures between 100 and 1000 megakelvins. This hot gas totals five to ten times the mass of all visible galaxies in those clusters.
**History of X-ray astronomy**
In 1927, E.O. Hulburt of the U.S. Naval Research Laboratory, along with Gregory Breit and Merle A. Tuve of the Carnegie Institution of Washington, considered equipping Robert H. Goddard's rockets to study the upper atmosphere. Two years later, Hulburt proposed an experimental program where a rocket could carry instruments to detect ultraviolet radiation and X-rays at high altitudes.
- First solar x rays recorded
- August 5, 1948, by T. Burnight on a V-2 rocket
- First cosmic x ray source discovered
- Scorpius X-1, June 19, 1962, by an Aerobee 150 rocket
- X ray emission of scorpius x 1 vs visual
- 10,000 times greater than its visual emission
- Sun x ray emission vs visual
- about a million times less
- Energy output in x rays of scorpius x 1
- 100,000 times greater than total emission of Sun in all wavelengths
- First balloon based detection of discret
- Crab Nebula, July 21, 1964
- Heft maiden flight
- May 2005 from Fort Sumner, New Mexico
Lore & Background
The history of X-ray astronomy began with early proposals in 1927 by E.O. Hulburt and associates to equip rockets for upper atmosphere exploration, including detection of X-rays. In the late 1930s, a hot tenuous gas surrounding the Sun was inferred from optical coronal lines. The first solar X-rays were recorded on August 5, 1948, by a V-2 rocket as part of Project Hermes. The first cosmic X-ray source, Scorpius X-1, was discovered by a sounding rocket on June 19, 1962. Its X-ray emission is 10,000 times greater than its visual emission, while the Sun's is about a million times less. The energy output in X-rays from Scorpius X-1 is 100,000 times greater than the total emission of the Sun in all wavelengths. Through the 1960s, 70s, 80s, and 90s, detector sensitivity increased greatly, and the ability to focus X-rays developed enormously.
Reader's Guide
X-ray astronomy is significant because it reveals extremely hot gases at temperatures from about a million kelvin to hundreds of millions of kelvin, which standard optical telescopes cannot see. The discovery of Scorpius X-1 in 1962 opened a new field, leading to Riccardo Giacconi receiving the Nobel Prize in Physics in 2002. Many thousands of X-ray sources have since been discovered, including compact stars such as neutron stars or black holes, where infalling gas and dust is heated by strong gravitational fields. The intergalactic space in galaxy clusters is filled with hot, dilute gas at temperatures between 100 and 1000 megakelvins, with total mass five to ten times that of visible galaxies. Observational platforms include sounding rockets, balloons, and satellites. Balloon flights can carry instruments to 40 km altitude, above 99.997% of the atmosphere, but X-rays with energies less than 35 keV cannot reach balloons. The High-Energy Focusing Telescope (HEFT) imaged astrophysical sources in the hard X-ray band (20–100 keV) from a balloon in May 2005. The High-resolution gamma-ray and hard X-ray spectrometer (HIREGS) observed X-rays from the Sun and other objects from Antarctica in 1991 and 1992.
Did You Know?
- The first cosmic X-ray source, Scorpius X-1, emits 10,000 times more X-rays than visible light.
- The first balloon-based detection of a discrete cosmic X-ray source was the Crab Nebula on July 21, 1964.
- The intergalactic gas in galaxy clusters is five to ten times the total mass of visible galaxies.
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