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X-ray Astronomy at JAXA Japan's Leading-Edge X-ray Astronomy: Unveiling the Mysteries of a Dynamic Universe
What is X-ray Astronomy? The Light That We Can't See - Exploring Space with X-Rays
Visible light - the light that humans can see with the naked eye - is very limited. There is also invisible light, which you can see only with a special telescope. The universe viewed with an X-ray telescope is full of dynamic energy created at extremely high temperatures, reaching hundreds of millions of degrees Celsius. The primary sources of this heat are neutron stars, black holes, supernova remnants, galaxies and galaxy clusters. X-ray observation is essential to uncover the secrets of the universe, but it is a relatively new method, only about 40 years old. That means X-ray astronomy is full of potential for new discoveries and the exploration of phenomena that we never before imagined.
What Are X-rays?

X-rays are a type of light. Light exists in the form of light waves, which are a type of electromagnetic wave, and travel in a vacuum at 300,000 km per second. There are visible light waves, and also invisible ones — such as ultraviolet or infrared rays — which we cannot see with the naked eye.

Infrared waves are produced at relatively low temperatures, such as that of the human body, around 36 degrees Celsius. We all emit infrared rays, but their energy level is quite low. The energy of X-rays, by comparison, is much more powerful. The temperature of objects that emit X-rays varies from a few million to 100 million degrees Celsius. Because their energy level is so high, the penetration of X-rays is also very deep. Fortunately for us, X-rays from space are absorbed by the Earth's atmosphere before they reach the ground. But that also makes them impossible to observe from the Earth's surface.



Why Are X-rays Essential?

Seeing with X-rays means viewing the universe in a very hot state.
The Sun's surface (photosphere) temperature is about 6000 degrees Celsius, which means it emits visible light. Meanwhile, the temperature of stars that emit X-rays can be more than 10 million degrees — up to 10,000 times as hot as the Sun. Before the era of X-ray observation, we had never imagined that the universe was so hot.

Before X-ray observation, we thought the universe was comprised only of stars and galaxies observed by visible light. X-rays revealed that the universe is instead a very hot and dynamic one, with supernova remnants, black holes and other extreme phenomena. In a galaxy cluster that contains hundreds or thousands of galaxies, scientists have discovered that galaxies are actually floating in hot gas that emits X-rays. Even in the Milky Way, our own galaxy, we are learning that X-ray emitting phenomena have been occurring for more that 10 billion light years and are still happening today. Without understanding these phenomena, we will not be able to truly understand space. Hence, X-rays are a very important key to the discovery of the origin, birth and fate of the universe.


1 | 2 | 3 The Universe Revealed by X-rays
1. What Is X-ray Astronomy? 2. Japan's Leading-Edge X-ray Astronomy 3. ASTRO-EII Launched into Space - Summer 2005
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