X-ray astronomy

X-ray astronomy is a special kind of science that studies the universe using X-ray light. Because Earth’s air blocks X-rays, scientists need to use balloons, rockets, and satellites high above the planet to catch these hidden signals from space.

Very hot objects, like stars and gases that are millions of degrees hot, shine brightly in X-rays. Until scientists could send instruments up high, they could only guess about these hidden lights from the Sun and stars.

The first X-rays from beyond our solar system were found in 1962 by a rocket. This bright spot, called Scorpius X-1, shines much stronger in X-rays than what we see with our eyes. Since then, thousands of X-ray sources have been found, showing us the hidden heat and energy in space.

History of X-ray astronomy

Main article: History of X-ray astronomy

In 1927, scientists began thinking about using rockets to study the sky from above the Earth. They wanted to learn about light that we can't see from the ground, like special high-energy light called X-rays.

Later, in 1948, a rocket was launched to look for X-rays from the Sun. Since then, scientists have made better tools to see these X-rays, and they've discovered many amazing objects in space by using them.

Observational platforms

Sounding rocket flights

Main article: Sounding rocket

In 1949, scientists used a special rocket called a V-2 to study X-rays from the Sun. They sent a detector up high above the Earth's atmosphere, where it could capture X-rays that normally get blocked. This was the first time scientists used a rocket to study X-rays from space.

Later, in 1962, another rocket called an Aerobee detected X-rays from a star system outside our own solar system, called Scorpius X-1. These X-rays come from very dense stars, like neutron stars or black holes, where gravity heats up material to very high temperatures.

Balloons

Main article: balloon-borne telescope

Scientists also use balloons to carry X-ray detectors high into the sky. Balloons can stay up for much longer than rockets, allowing scientists to collect more data. In 1964, a balloon carrying special equipment detected X-rays from the Crab Nebula, a cloud of gas from an old star explosion.

High-energy focusing telescope

In 2005, a balloon carried a special telescope called HEFT, which could see X-rays with very high energy. It successfully observed two objects: Tau X-1 and the Crab Nebula.

High-resolution gamma-ray and hard X-ray spectrometer (HIREGS)

A balloon experiment named HIREGS flew over Antarctica in 1991 and 1992. It studied X-rays and gamma-rays from the Sun and other objects in space, with each flight lasting about two weeks.

Rockoons

Main article: Rockoon

A rockoon is a special kind of rocket that is first carried up into the sky by a balloon. Once high enough, the rocket separates from the balloon and ignites its engine. This way, it can reach even higher altitudes without having to push through thick air at the start.

Satellite observations

Satellites orbiting Earth can watch X-ray sources for long periods. Early attempts in the late 1950s didn’t work well, but in 1960, a satellite named SOLRAD 1 successfully measured X-rays from the Sun.

Instruments

X-ray telescopes and mirrors

Main article: X-ray telescope

We need special machines called satellites to study X-rays because Earth’s air blocks them. These machines can go up very high to catch the X-rays. X-ray telescopes are different from regular telescopes because they use special mirrors that bounce the X-rays just a little bit. This means they can only see a small part of the sky at one time.

The first time we used an X-ray telescope was to look at the Sun. In 1963, a telescope on a rocket took the first picture of the Sun using X-rays. Even earlier, in 1960, a simple camera on a rocket captured the very first X-ray image of the Sun.

To use X-ray mirrors to study objects beyond our solar system, we need two things: a way to know where the X-rays come from and tools that can detect the X-rays well.

X-ray astronomy detectors

Main article: X-ray telescope § Detection and imaging of X-rays

Scientists have made special tools to catch X-rays. These tools can count how many X-rays they see, measure how strong they are, or figure out their energy levels. This helps us learn about the objects in space that are sending out these X-rays.

Astrophysical sources of X-rays

Main article: Astrophysical X-ray source

Many space objects send out X-rays. These include groups of galaxies, black holes in active centers of galaxies, remnants left after stars explode, normal stars, pairs of stars where one is a dense, dead star, and even some objects in our solar system like the Moon. The Moon shines in X-rays mostly because it reflects X-rays from the Sun.

X-rays from these objects can come from many processes, such as high-energy particles moving close to each other, very hot gas, or light being bumped to higher energies by fast-moving particles.

Celestial X-ray sources

Main article: Astrophysical X-ray source

The sky is divided into 88 areas called constellations by astronomers. These areas contain amazing objects that give off X-rays. Some of these objects are huge collections of stars called galaxies, or even extremely dense points called black holes at the centers of galaxies. Others are rapidly spinning stars known as pulsars. Studying these X-ray objects helps us learn about our Sun, the entire universe, and how these distant lights affect our planet Earth.

One special area between the constellations Orion and Eridanus has a region that glows with soft X-rays. This area, called the Orion–Eridanus Superbubble, stretches across a wide part of the sky. The X-rays come from very hot gas inside this bubble. This glowing area shows the outline of a cooler filament of gas and dust, which blocks some of the X-rays. This filament might be part of a shell surrounding the hot bubble. Hot stars in this area send out powerful energy that helps create and shape this bubble.

Explorational X-ray astronomy

Usually, astronomers study objects from the ground or from space around Earth. But to study X-rays, we need to go far from Earth because its atmosphere blocks X-rays. So, special machines called X-ray telescopes are sent high above Earth on balloons, rockets, or satellites.

One famous satellite was Ulysses, launched in 1990. It flew past Jupiter and studied the Sun’s X-rays and powerful explosions of energy from space. It was the first to carry special tools to detect these explosions far from Earth. These tools helped scientists learn more about the Sun and space weather.

Dynamos

Main article: Dynamo theory

See also: Solar dynamo

Dynamo theory explains how spinning, moving, and electrically conducting liquids, like the Sun’s plasma, can create and keep magnetic fields. This helps us understand why stars and planets have magnetic fields.

Astronomical models

By looking at the X-rays from space objects and comparing them to light and radio waves, scientists can build models to guess where the X-rays come from. For example, the object Scorpius X-1 shows a pattern of X-rays that suggests it is very hot gas, maybe from a star system close together.

The Crab Nebula looks very different. Its X-rays are stronger, and it is much larger, about light-years across. Its X-rays might come from a spread-out ball of very thin gas, with energy much greater than what we see in light or radio waves.

Analytical X-ray astronomy

High-mass X-ray binaries are systems made up of very large stars and dense objects like neutron stars or black holes. In supergiant X-ray binaries, these dense objects orbit their large companions every few days. These systems give off strong X-ray light and can be very bright, reaching up to 10³⁶ erg·s⁻¹ (10²⁹ watts) in X-ray brightness.

Scientists are still discussing what causes the different behaviors seen in these systems compared to a newer group called supergiant fast X-ray transients.

Stellar X-ray astronomy

The first time we detected X-rays from a star was on April 5, 1974, from a star called Capella. A special rocket flight aimed at Capella and detected X-rays. This showed that Capella shines very brightly in X-rays, much more than our Sun.

Scientists have studied X-rays from many stars using special tools on spacecraft like Skylab and Copernicus. They found that many stars, including Sirius, give off X-rays. These X-rays come from the very hot areas around stars, called coronae. Young stars, before they settle down, also shine brightly in X-rays. This helps us find young stars in space because X-rays can pass through clouds that block ordinary light.

Some very hot stars, like Eta Carinae, also give off strong X-rays. Recent observations show three different structures of hot gas around Eta Carinae, created by material being blown away from the star at very high speeds. These structures help scientists understand how such hot and bright X-rays are produced.

Amateur X-ray astronomy

Amateur astronomers watch many objects in space, sometimes using tools they build themselves. The United States Air Force Academy helps students create experiments that can fly on rockets for free.

But there are big challenges for amateurs who want to study X-rays. Building a rocket or balloon to carry a detector high up is very expensive, and making a good X-ray detector also costs a lot.

Major questions in X-ray astronomy

X-ray astronomy helps us solve many space mysteries by looking at special kinds of light called X-rays.

One big question is about the magnetic fields around stars. We know stars have these invisible forces, but we don’t fully understand why or how they work. Some stars keep old magnetic fields from when they formed, while others make new ones.

Another question is about finding X-ray sources far from Earth. When scientists find an X-ray source, they ask what it is. They look in other types of light, like visible or radio waves, to find matching objects. This can be tricky because of problems with measuring positions and changes in the sources themselves.

Solar X-ray astronomy

Main article: Solar X-ray astronomy

All X-ray sources near the Sun seem connected to its outer atmosphere, called the corona. One big puzzle is why the corona is much hotter than the Sun’s surface. The Sun’s surface is about 5,570 K, but the corona can be 1–2 million K, with some spots even hotter. Scientists think energy from movements below the surface heats the corona, possibly through waves or magnetic activity.

Another topic is coronal mass ejection (CME), where huge amounts of particles burst out from the Sun. These can affect Earth’s magnetic field. The first CME was detected in 1971, but we now know older observations were the same thing.

Exotic X-ray sources

Main article: Astrophysical X-ray source

See also: Be X-ray binaries

Some special objects in space give off X-rays. One example is a microquasar, which is smaller than a quasar and sends out radio waves. It also has two jets that we can sometimes see. Another interesting object is a magnetar, a kind of neutron star with a very strong magnetic field. This strong field helps it give off lots of energy, including X-rays.

X-ray dark stars

Main article: Astrophysical X-ray source

See also: Supergiant

Sometimes, the Sun hardly gives off any X-rays, while other times it shines more in X-rays. Big, red stars like Betelgeuse almost never give off X-rays. X-rays start to appear more clearly in stars that are a little hotter, around types A7 to F0. Some of these stars shine much brighter in X-rays than others. Scientists study these stars to learn more about their magnetic fields and activity.

X-ray dark planets and comets

X-ray observations can help us find planets when they pass in front of their star, blocking part of the star's bright outer atmosphere. This method works especially well for smaller stars, where a planet like Jupiter could cover a big part of the star's glow.

As tools for spotting X-rays have gotten better, we've learned that some planets and other objects in space that usually don’t give off X-rays can sometimes shine, glow, or reflect these high-energy lights under certain conditions.

Comet Lulin

Main article: Comet Lulin

NASA’s Swift Gamma-Ray Burst Mission watched Comet Lulin as it came close to Earth—about 63 million kilometers away. For the first time, scientists saw both ultraviolet and X-ray pictures of a comet at the same time. The stream of particles from the Sun, called the solar wind, mixes with the comet’s wide cloud of atoms. This mixing makes the solar wind glow with X-rays, which the Swift satellite’s X-ray telescope can see. This happens because of a process called charge exchange, and it causes most comets to give off X-rays when they come within about three times the distance from the Earth to the Sun. Comet Lulin was very active, so its cloud of atoms was very thick. Because of this, the area glowing with X-rays reached far toward the Sun from the comet.