Gravitational Lensing in the Galaxy Cluster Abell 2218
Click on image for full size
NASA / A. Fruchter / STScI

Gravitational Lenses - Nature's Telescope

The possibility that the path of light could be bent by the gravity of a large object was predicted by Einstein's Theory of General Relativity, and this effect was observed soon after the theory was published. Because people normally think of glass or plastic lenses as bending light, we call any massive object that bends light rays a "gravitational lens." By measuring the amount of bending, we can then determine the mass of whatever is doing the bending.

In the cluster Abell 2218 (pictured here), distant blue galaxies behind the large cluster of galaxies are "squished" into a circular shape around the middle of the foreground cluster. By measuring the amount of distortion in the more distant blue galaxies, we can determine the mass of the cluster. In fact, we can even measure how much mass there is that we can't see -- this galaxy cluster happens to have nearly 400 trillion times the sun's mass in "dark" matter.

Last modified July 23, 2008 by Randy Russell.

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

A Long Time Ago in A Galaxy Far, Far Away...

A NASA Hubble Space Telescope image revealed the most distant galaxy found in the universe to date. The image the telescope took was of a galaxy cluster located 5 billion light years away. The discovered...more

Pretty Lenses are Caught by Hubble

The NASA Hubble Space Telescope has recently found some awesome mirages produced by lenses in space. These lenses act as giant magnifying glasses for the Universe. A lens is produced when a large object...more

Small Planet, Small Star

If there are other planets in the Universe that are like Earth, then they might be places where life could survive. However, the search for planets like Earth is not easy. Most of the planets found outside...more

Gamma Ray Bursts - The Most Powerful Objects in the Universe?

In the 1960's, the United States launched some satellites to look for very high energy light, called Gamma Rays. Gamma Rays are produced whenever a nuclear bomb explodes. The satellites found many bursts...more


During the early 1900's, which is not very long ago, astronomers were unaware that there were other galaxies outside our own Milky Way Galaxy. When they saw a small fuzzy patch in the sky through their...more

Neutron Stars

Neutron Stars are the end point of a massive star's life. When a really massive star runs out of nuclear fuel in its core the core begins to collapse under gravity. When the core collapses the entire star...more

Spiral Galaxies

Spiral galaxies may remind you of a pinwheel. They are rotating disks of mostly hydrogen gas, dust and stars. Through a telescope or binoculars, the bright nucleus of the galaxy may be visible but the...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA