X-ray astronomy

Space has a special kind of light.

This light comes from very hot gas. The air around Earth blocks it. We cannot see it from the ground.

To find this light, we must go high up. Scientists use big balloons. They also use rockets. Some use satellites in space.

One way is a rockoon. This is a rocket carried by a balloon. It flies very high.

These tools help us see far away. They see stars and gas. They can even see black holes. This light shows us a hidden world.

Space has a special kind of light called X-rays.

These rays come from very hot gas. This gas can be millions of degrees hot.

Most of this light comes from things like the Sun or stars. Some light comes from black holes. A black hole is a place where gravity is very strong. Gravity pulls gas and dust in. This makes the gas get very hot. This heat lets out X-rays.

We cannot see X-rays from the ground. Earth's atmosphere blocks them. To see them, we must go high up. Scientists use many ways to reach the sky. They use big balloons. They use sounding rockets.

They even use rockoons. A rockoon is a rocket carried by a balloon. The rocket starts after the balloon goes high. This saves fuel. Scientists also use satellites. These stay in space for a long time.

These tools use special mirrors to see the light. This helps us study the hidden parts of space.

X-ray astronomy is a special way to study the universe.

It looks for X-ray radiation, which is a type of light. This light is different from the light we see with our eyes. X-rays come from objects that have extremely hot gases. These gases can reach temperatures of one million kelvin or even hundreds of millions of kelvin.

By looking for these rays, scientists can find things that are hidden from regular telescopes. This helps us understand the most energetic parts of space.

There is a big problem with watching X-rays from the ground. The Earth's atmosphere is opaque to them. This means the air around our planet blocks the X-rays from reaching us. To solve this, scientists must take their tools high into the sky. They use large balloons to float instruments up to 40 km.

They also use sounding rockets for quick trips above the air. Some even use satellites that orbit the Earth to watch for a long time.

People have been working to solve this puzzle for a long time. In 1927, E.O. Hulburt and his team thought about using rockets to explore the upper air. Later, in 1948, a V-2 rocket was used to record the first solar X-rays.

Scientists also created something called a rockoon. This is a rocket that a balloon carries high into the sky first. Once it is high up, the rocket ignites to go even higher. This helps the rocket save fuel because it does not fight thick air.

Many amazing things have been found using these methods. In 1962, a rocket found the first source outside our solar system. It was called Scorpius X-1, located in the constellation Scorpius.

This source is much brighter in X-rays than the Sun is. In fact, its X-ray energy is 100,000 times greater than all the light the Sun sends out. We also know that space between galaxies is filled with very hot gas. This gas can be between 100 and 1,000 megakelvins.

You can think of X-ray astronomy like using a special heat camera. A regular camera sees colors, but a heat camera sees things that are very hot. X-ray telescopes work in a similar way for the universe. They use special mirrors to catch the rays. These mirrors often use a glancing angle to reflect the light.

This is different from how a normal mirror works. Because of this, X-ray telescopes often see a smaller part of the sky at once. This allows us to see the amazing details of hot stars and black holes.

X-ray astronomy is a specialized branch of observational astronomy. It focuses on detecting and studying X-ray radiation from celestial objects. X-rays are a type of electromagnetic radiation. They start at wavelengths of about 0.008 nanometers and extend to 8 nanometers.

These rays are invisible to the human eye. They also cannot be seen by standard optical telescopes, like those at the Mauna Kea Observatories. Instead, X-ray astronomy reveals the most energetic and violent parts of our universe.

To understand what X-ray astronomy finds, we must look at temperature. X-ray emission is expected from objects containing extremely hot gases. These gases reach temperatures from one million kelvin (K) to hundreds of millions of kelvin (MK).

In some cases, the gas is part of the interstellar medium. This is the matter that exists between star systems within a galaxy. The hot ionized medium (HIM) consists of coronal cloud ejections from star surfaces. These ejections reach temperatures of 10^6 to 10^7 K and emit X-rays.

A major challenge in this field is the Earth's atmosphere. The atmosphere is opaque to X-rays, meaning it absorbs them before they reach the ground. Because of this, scientists cannot use ground-based telescopes to study X-ray sources. They must send instruments to high altitudes to bypass the air.

This requires specialized platforms like balloons, sounding rockets, and satellites. Each platform offers different advantages for observing the cosmos.

Sounding rockets provide a quick way to reach the upper atmosphere. A detector is often placed in the nose cone for a suborbital flight. These flights are very short, lasting only a few minutes. Because of this short duration, they have a limited field of view. Rockets launched from the United States cannot see sources in the southern sky. Conversely, rockets from Australia cannot see the northern sky.

A unique method called a "rockoon" was developed to help. A rockoon uses a gas-filled balloon to carry a solid-fuel rocket to a high altitude. The rocket is ignited only after it separates from the balloon. This saves chemical fuel because the rocket does not fight thick, lower air layers.

Balloons offer a different approach to high-altitude observation. They can carry instruments up to 40 km above sea level. At this height, they are above 99.997% of the Earth's atmosphere. Balloons can stay aloft much longer than sounding rockets. However, they still cannot detect all X-rays. X-rays with energies less than 35 keV cannot reach these altitudes.

Some balloon experiments, like the High-energy focusing telescope (HEFT), use novel coatings. These coatings help extend the reflectivity of mirrors to observe "hard" X-rays.

Satellites provide the most effective way to study X-rays. They allow for continuous and long-term observations from orbit. The first successful satellite for measuring solar X-rays was SOLRAD 1, launched in 1960.

Unlike optical telescopes that use refraction, X-ray telescopes use glancing angle reflection. This means X-rays hit the mirrors at a very shallow angle to reflect. The mirrors can be made of metal or ceramic foil. This technique limits the field of view but allows for the detection of high-energy photons.

The history of this field is marked by incredible discoveries. In 1962, a sounding rocket discovered the first cosmic X-ray source. This source was Scorpius X-1 (Sco X-1), located in the constellation Scorpius.

The X-ray emission from Sco X-1 is 10,000 times greater than its visual emission. Its energy output in X-rays is 100,000 times greater than the Sun's total emission in all wavelengths. We now know Sco X-1 is a compact star, such as a neutron star or a black hole. In these objects, gravity heats infalling gas and dust to create X-rays. For these foundational discoveries, Riccardo Giacconi received the Nobel Prize in Physics in 2002.

Today, X-ray astronomy connects many different areas of science. It helps us study the intergalactic space in galaxy clusters. This space is filled with a hot, dilute gas between 100 and 1,000 megakelvins. The total mass of this hot gas is five to ten times the total mass of the visible galaxies.

By studying these high-energy signals, astronomers can map the structure of the universe. They can see how stars are born in molecular clouds and how supernovae inject energy into their surroundings.