Chandra 101: An Overview

Chandra 101 provides basic information about topics pertaining to the Chandra X-Ray Observatory. Many sections contain links to more detailed information for those who wish to read further.

Contents

• A Look at X-Rays
  • What are x-rays?
  • Why do we need X-ray telescopes?
  • How do X-ray telescopes differ from optical telescopes?
  • Why can't we have earthbound X-ray observatories?

• A Look at Chandra
  • What makes Chandra unique?
  • What are the parts that make up Chandra?
  • What's a specific instance where Chandra is useful?

A Look at X-Rays

What are x-rays?

X-rays are a highly energetic form of light, not visible to human eyes. Light can take on many forms -- including radio waves, microwaves, infrared, visible, ultraviolet, X-ray and gamma radiation. Very low temperatures (hundreds of degrees below zero Celsius) produce mostly low energy radio and microwave photons, whereas cool bodies like ours (about 30 degrees Celsius) produce largely infrared radiation. Objects at very high temperatures (millions of degrees Celsius) emit most of their energy as x-rays.

• More information - Xrays & Light

Why do we need X-ray telescopes?

Much of the matter in the universe cannot be seen by any other telescope. X-ray telescopes are the only way we can observe extremely hot matter with temperatures of millions of degrees Celsius. It takes gigantic explosions, or intense magnetic or gravitational fields to energize particles to these high temperatures. Where do such conditions exist? In an astonishing variety of places, ranging from the vast spaces between galaxies to the bizarre, collapsed worlds of neutron stars and black holes.

• More information: Comparing X-ray telescopes to doctor's x-rays

How do X-ray telescopes differ from optical telescopes?
X-rays do not reflect off mirrors the same way that visible light does. Because of their high-energy, X-ray photons penetrate into the mirror in much the same way that bullets slam into a wall. Likewise, just as bullets ricochet when they hit a wall at a grazing angle, so too will x-rays ricochet off mirrors (see diagram below). These properties mean that X-ray telescopes must be very different from optical telescopes.

The mirrors have to be precisely shaped and aligned nearly parallel to incoming x-rays. Thus they look more like barrels than the familiar dish shape of optical telescopes.

• More Information - Reflection (java enhanced)

Why can't we have earthbound X-ray observatories?
X-ray observatories must be placed high above the Earth's surface because the Earth's atmosphere absorbs x-rays. This means that the ultra-precise mirrors and detectors -- together with the sophisticated electronics that convey the information back to Earth -- must be able to withstand the rigors of a rocket launch and operate in the hostile environment of space.

• More on X-ray Absorption

A Look at Chandra
What makes Chandra unique?

This 400 by 900 light-year mosaic of several Chandra images of the central region of our Milky Way galaxy
reveals hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog
of multimillion-degree gas.

Since its launch on July 23, 1999, the Chandra X-ray Observatory has been NASA's flagship mission for X-ray astronomy, taking its place in the fleet of "Great Observatories." Chandra detects and images X-ray sources that are billions of light years away. The mirrors on Chandra are the largest, most precisely shaped and aligned, and smoothest mirrors ever constructed. If the surface of Earth was as smooth as the Chandra mirrors, the highest mountain would be less than six feet tall! The images Chandra makes are twenty-five times sharper than the best previous X-ray telescope. This focusing power is equivalent to the ability to read a newspaper at a distance of half a mile. Chandra's improved sensitivity is making possible more detailed studies of black holes, supernovas, and dark matter. Chandra will increase our understanding of the origin, evolution, and destiny of the universe.

• View Chandra Images

What are the parts that make up Chandra?

1. The Mirrors
   The Chandra telescope system consists of four pairs of mirrors and their support structure. The mirrors have to be exquisitely shaped and aligned nearly parallel to incoming x-rays. Thus they look more like nested glass barrels than the familiar dish shape of optical telescopes. More Information
   
   
2. The Science Instruments
   The function of the science instruments is to record as accurately as possible the number, position and energy of the incoming x-rays. This information can be used to make an X-ray image and study other properties of the source, such as its temperature. More Information
   
   
3. The Spacecraft
   The spacecraft system provides the support structure and environment necessary for the telescope and the science instruments to work as an observatory. Parts of the system include solar panels to supply the instruments with power, a thermal system to control the temperature of the telescope, and a communications system to relay data to astronomers on Earth. More Information
   
   
   • Chandra Interactive Feature (requires flash)

What's a specific instance where Chandra is useful?

The Crab Nebula is an expanding cloud of gas leftover from a supernova explosion. Compare a Chandra X-ray image (Left) and an Optical image (Right) below.


The X-ray nebula shown in the Chandra image is about 40% as large as the infrared image of nebula. The X rays are more concentrated toward the center than the infrared emission. Also, it seems that the X-ray image is NOT centered on the pulsar, the star remnant from the supernova. There are all sorts of interesting loops and knots in the X-ray image, some of which appear to correlate with optical features and some that do not.

These X-ray features also change over time! View the timelapse here: https://chandra.si.edu/photo/2024/timelapse/

• More Information - Crab Nebula
• Crab Nebula Photo Album