Tonight's Sky
Sun
Sun
Moon
Moon
Mercury
Mercury
Venus
Venus
Mars
Mars
Jupiter
Jupiter
Saturn
Saturn

Tonight's Sky — Change location

OR

Searching...

Tonight's Sky — Select location

Tonight's Sky — Enter coordinates

° '
° '

Web Extra: Gamma rays’ long road toward Fermi

Watch a gamma ray travel from its source in a faraway galaxy, through the extragalactic background, and finally to Earth, where the Fermi telescope can measure it.
RELATED TOPICS: GAMMA-RAY BURSTS
This NASA animation shows a gamma ray traveling from its blazar source, encountering the extragalactic background, and being absorbed by the Fermi telescope.
There are many ways to measure the extragalactic background (EBL), the universe’s glow from myriad sources. One of these measurement techniques uses the light from faraway blazars — active galaxies with supermassive black holes at their centers, blasting jets of high-speed particles and radiation out into space. Scientists study the gamma rays streaming from these high-powered jets. The gamma rays have a very long journey between their blazars and Earth, where astronomers can measure them. Along the way, some of these gamma rays will strike an intervening EBL photon and annihilate into a positron and an electron.
There is a special relationship between the energy of the gamma ray and the energy of the EBL photon. A higher-energy gamma ray annihilates only with a lower-energy EBL photon, and vice versa. Astronomers can therefore use the missing gamma rays to tell them about the EBL interference.

In this animation, watch a gamma ray complete its long journey from its blazar origins, through the EBL, to the Fermi telescope orbiting Earth that measures its final energy.
Video credit: NASA/Goddard Space Flight Center/Cruz deWilde
0

JOIN THE DISCUSSION

Read and share your comments on this article
Comment on this article
Want to leave a comment?
Only registered members of Astronomy.com are allowed to comment on this article. Registration is FREE and only takes a couple minutes.

Login or Register now.
0 comments
ADVERTISEMENT
FREE EMAIL NEWSLETTER

Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter. View our Privacy Policy.

ADVERTISEMENT
ADVERTISEMENT
Apollo_RightRail
A chronicle of the first steps on the Moon, and what it took to get there.
Find us on Facebook