Images by Date
Images by Category
Solar System
Stars
Exoplanets
White Dwarfs
Supernovas
Neutron Stars
Black Holes
Milky Way Galaxy
Normal Galaxies
Quasars
Galaxy Clusters
Cosmology/Deep Field
Miscellaneous
Images by Interest
Space Scoop for Kids
4K JPG
Multiwavelength
Sky Map
Constellations
Photo Blog
Top Rated Images
Image Handouts
Desktops
Fits Files
Visual descriptions
Image Tutorials
Photo Album Tutorial
False Color
Cosmic Distance
Look-Back Time
Scale & Distance
Angular Measurement
Images & Processing
AVM/Metadata
Image Use Policy
Web Shortcuts
Chandra Blog
RSS Feed
Chronicle
Email Newsletter
News & Noteworthy
Image Use Policy
Questions & Answers
Glossary of Terms
Download Guide
Get Adobe Reader

Note on Cosmic Distances

In astronomy, distances are measured in units of light years, where one light year is the distance that light travels in a year—10 trillion kilometers. For historical reasons having to do with measuring distances to nearby stars, professional astronomers use the unit of parsecs, with one parsec being equal to 3.26 light years.

Astronomers compute the distance to remote galaxies (ones that are more than about 20 million light years away) with Hubble's law. According to Hubble's law, the universe is expanding in such a way that distant galaxies are receding from one another with a speed which is proportional to their distance. The recession causes the radiation from a galaxy to shift to longer wavelengths—the red shift. From a measurement of the red shift and the constant of proportionality, called Hubble's constant, astronomers can determine the distance to a galaxy.

One of the central problems of modern astronomy is to accurately determine Hubble's constant, which is a measure of the rate of expansion of the universe. At present it is known to an accuracy of about 20 percent, so we usually modify distances by saying "about 100 million light years," for example. We assume throughout the Photo Album a value of the Hubble constant that corresponds to a recession velocity of 600 kilometers per second for a source at a distance of 30 million light years or 10 million parsecs (H0 = 60 km/s/Mpc).



Chandra Images by Date:  [ ' 24 | ' 23 | ' 22 | ' 21 | ' 20 | ' 19 | ' 18 | ' 17 | ' 16 | ' 15 | ' 14 | ' 13 | ' 12 | ' 11 | ' 10 | ' 09 | ' 08 | ' 07 | ' 06 | ' 05 | ' 04 | ' 03 | ' 02 | ' 01 | ' 00 | ' 99 ]