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Photo Album Tutorial -understanding the images

This page is meant to serve as a quick guide to help you achieve a basic understanding of the images and information you are browsing through in the Photo Album.

Definition Guide

Object Category

This is a general category under which a certain object falls. The types of categories are Solar System Objects, Normal Stars, White Dwarfs, Supernovas and Supernova Remnants, Neutron Stars, Black Holes, Normal Galaxies & Starburst Galaxies, Quasars and Active Galaxies, Galaxy Clusters, X-Ray Background and other miscellaneous objects. For example, object 0087, Hydra A, falls under the category of Galaxy Clusters. Visit our X-Ray Sources Field Guide for information on each category.


Right Ascension (RA) and Declination (Dec) are the coordinates that astronomers use to precisely locate stars in the sky. They are very similar to longitude and latitude coordinates used to describe geographical locations on the Earth. Unlike longitude, RA is measured from 0 to 24 hours instead of 0 to 360 degrees. RA measures the east - west position of an object, while Dec tells you how far north or south of the equator it is.


A constellation is a star pattern that can be found in the sky. Each constellation represents a picture of someone or something. The meanings of these constellations were made up over the years by all sorts of different people and cultures. Constellations were originally used by these people to tell what month or season it was. In 1929, the International Astronomical Union adopted official constellation boundaries (areas in the sky) which define the 88 official constellations today.

To find out what the constellations are named and their respective abbreviations visit IAU's Names and Standard Abbreviations of Constellations.

Multi-wavelength Images

All telescopes collect light from objects to make them visible to the eye. However, what we commonly perceive as light is only one type of electromagnetic radiation. Radiation comes in a range of energies, known as the electromagnetic spectrum. The spectrum consists of radiation such as Gamma rays, X-rays, Ultraviolet, Visible, Infrared and Radio. Radiation travels in waves, just like waves in an ocean. The energy of the radiation depends on the distance between the crests (the highest point) of the waves, or the wavelength. For example, Radio wavelengths can range from one centimeter (.40 inches) to 100 m (the size of a football field)! Visible light has a wavelength that is just right for the human eye to detect. Astronomers use telescopes that can detect different wavelengths in order to see what the universe truly looks like. Our Key to terms has descriptions of some of the telescopes used to produce images of objects at different wavelengths.

Observation Time

The exposure time is the time that the telescope is pointed toward the target and collecting data using the detector indicated. It is not focused elsewhere, although there may sometimes be more than one source - typically a foreground star or a background galaxy - in the field of view. The useful exposure time is usually somewhat less, typically a few percent, because of background flares which are generally due to energetic electrons.