All objects emit electromagnetic radiation, and the amount of radiation emitted at each wavelength determines the temperature of the object. Hot objects emit more of their light at short wavelengths, and cold objects emit more of their light at long wavelengths. The radiation temperature of an object is related to the wavelength at which the object gives out the most light. We refer to the amount of light emitted at a particular wavelength as the intensity.

When you plot the intensity of light from an object at each wavelength, you trace out a smooth curve called a blackbody curve. For any temperature, the blackbody curve shows how much energy (intensity) is radiated at each wavelength. The wavelength where the intensity peaks determines the color of that the object. The intensity peak will be at shorter (bluer) wavelengths for hotter objects, and at longer (redder) wavelengths for cooler objects. Therefore, you can tell the temperature of a star or galaxy by its color because color is closely related to the wavelength at which its light intensity peaks.

Blackbody curves, for objects of all temperatures, have a similar shape, as shown in the graph below. However, the peak of the curve for a hotter object will be larger (more intense) than will the peak of the curve for a cooler object. For example, the intensity difference between the peak of the curve for an object at 30,000 K and the peak of the curve for an object at 300 K (body temperature) is a factor of 10 billion. This means that a star at 30,000 K puts out more energy by a factor of 10 billion than does a human at body temperature.

Because of the large intensity difference, it would be difficult to show both of these curves on the graph above without using logarithms. To plot blackbody curves with large intensity differences on the Heating Up page of Amazing Space's "Star Light, Star Bright," the scale of the intensity axis adjusts itself for each temperature change.

Electromagnetic radiation, or light, is a form of energy. Visible light is a narrow range of wavelengths of the electromagnetic spectrum. By measuring the wavelength or frequency of light coming from objects in the universe, we can learn something about their nature. Since we are not able to travel to a star or take samples from a galaxy, we must depend on electromagnetic radiation to carry information to us from distant objects in space.

The human eye is sensitive to a very small range of wavelengths called visible light. However, most objects in the universe radiate at wavelengths that our eyes cannot see. Astronomers use telescopes with detection devices that are sensitive to wavelengths other than visible light. This allows them to study objects that emit this radiation, which would otherwise be invisible to us. Computer techniques then code the light into arbitrary colors that we can see.

The Hubble Space Telescope is able to measure wavelengths from about 0.1150 to 2 micrometers, a range that covers more than just visible light. These measurements of electromagnetic radiation enable astronomers to determine certain physical characteristics of objects, such as their temperature, composition, and velocity.

The human eye is sensitive to a very small range of wavelengths called visible light. However, many celestial objects in the universe radiate at wavelengths that our eyes cannot see… and each type of radiation provides clues as to the nature of the object in question. Astronomers study celestial objects with detection devices that are sensitive to wavelengths other than visible light and then use computer techniques that code the light into colors that we can see.

Able to measure wavelengths from about 115 nanometers to 2500 nanometers, the Hubble Space Telescope looks at the energy that is not only visible, but also infrared and ultraviolet. These measurements better enable astronomers to determine physical characteristics of objects, such as their temperature, composition, and velocity.

Different wavelengths of light provide scientists with different information about the objects they are studying. For instance, infrared light can reveal details about objects shrouded in dust. Infrared light emitted by an object will pass through dust — unlike visible light, which is scattered. In contrast, ultraviolet light can reveal details about the stellar wind around stars. (When talking about our sun, this is called the solar wind.) Astronomers have ways of breaking light into a spectrum, which reveals a lot of information (including properties of the source of the light, the material through which the light passes, or the material off of which the light reflects).

However, sometimes scientists want to capture specific ranges of wavelengths of light, so they use a filter. A filter will allow only light within a small range of wavelengths to pass through. When the Hubble Space Telescope takes an image using a filter, that image shows only the varying intensity of light in that small range. In making color pictures, scientists usually use a red filter, a green filter, and a blue filter (the red filter allows light only in the red range to enter, etc.). By combining these images scientists can create full-color pictures.

Hubble uses professional-grade versions of the same detectors found in a digital camera. Each optical instrument on the telescope has a set of charge-coupled devices (CCDs) composed of a grid of pixels (picture elements) that measure the intensity of light that strikes them. For example, one instrument (the Wide Field and Planetary Camera 2) has four CCDs, each of which contains 640,000 pixels. Each pixel turns the light intensity it measures into a number.

These numbers are systematically downloaded to the Space Telescope Science Institute where they are translated into black-and-white images. Using computers, two or more of these images can be colorized and combined to produce a color image. CCDs are used only for optical light. Ultraviolet and infrared light have different detectors, which are not called CCDs but operate on similar principles.

The colors red, green, and blue are chosen because they are the primary colors of light. By combining these colors of light, white light is produced. Combinations of two of these colors produce other familiar colors: blue + green = cyan, red + blue = magenta, and red + green = yellow.

If red, green, and blue filters have been used, the red filter is assigned red light, the green filter is assigned green light, and the blue filter is assigned blue light. This changes the black-and-white scale into tones of red, green, and blue, respectively. Using computers, these images can be combined into one image, which represents (as close as possible) the true colors of the object being imaged. In general, when other filters are used, blue is assigned to the shortest wavelength light while red is assigned to the longest wavelength, with green being the wavelength in the middle.

The Hubble Space Telescope takes observations of almost every part of the sky, as long as it isn't toward the Sun. In our own solar system, the telescope can take incredibly detailed pictures of the outer planets and their moons. A crescent of Venus has been taken with the Hubble Space Telescope and the moon has been shot once, but Mercury is too close to the Sun to be imaged.

Farther out, Hubble takes images of stars and nebulae in our own galaxy as well as in galaxies in the larger universe. The majority of Hubble's time is used for scientific research, taking many observations of specific objects that the scientists are studying. However, a very small portion of Hubble's time is dedicated to taking beautiful pictures for public enjoyment.