The light we can see with our eyes is just a tiny portion of the radiation along the electromagnetic spectrum. What makes it a spectrum is that it spans a wide range of energy, from very low-energy radio waves to very high-energy gamma rays. The bands of the spectrum, in order of increasing energy, are: radio, microwaves, infrared, visible, ultraviolet, X rays, and gamma rays. Though each type of light differs in its energy (and therefore frequency), they share some basic properties. They are transverse waves and, unlike sound waves, don’t need a medium. It can travel through a vacuum – and can travel through the “emptiness” of space. Sound can’t, because it requires a medium. It’s true what they say: In space, no one can hear you scream.
The highest-energy light, gamma rays, can have about a billion times the energy of the light we can see with our eyes. That level of energy could cause great harm. Our atmosphere absorbs high-energy radiation, so we’re protected. One downside of our atmosphere absorbing gamma rays is that, in order to see the universe in the gamma ray section of the spectrum, we need to get above the atmosphere. NASA’s GLAST (Gamma-ray Large Area Space Telescope) satellite does just that.
Renamed the Fermi Gamma Ray Space Telescope after launch, the satellite loops in orbit around Earth and views the entire sky every day in the gamma ray band of the spectrum. Since gamma rays have the most energy of any type of electromagnetic radiation, the natural phenomena that create them must be very powerful. These include black holes, exploding stars, and other extreme phenomena.
Classroom Activity: Energy Spectrum Ratios
Students work in small groups to calculate the energy ratios between selected spots on the EM spectrum. (The instructor selects several EM-energy pairs for each group, perhaps three or four.) Then, the student groups conduct some quick research to come up with other scenarios that match the scaling difference. An example might relating the ratio of two energy bands to the different heights of, say, an ant and Mt Everest.
Phil Plait: GLAST is designed to look at gamma rays. And gamma rays are the highest energy form of light.
Dave Thompson: There’s the light we see with our eyes but there are lots of other types of light. Gamma rays are the most energetic form of light, the most powerful.
Valerie Connaughton: Gamma rays are the part of what we call the electromagnetic spectrum which starts in radio at very long wavelengths, goes through optical, then through x-rays, and then gamma rays are the very highest energy form of that type of radiation.
Neil Gehrels: The reason it’s important to look at the high-energy gamma rays is that many objects, the most violent and some of the most interesting objects in the universe, emit most of their light in this high energy gamma ray part.
Phil Plait: and the only things that can generate gamma rays are incredibly violent events, incredibly energetic events. We’re talking stars exploding, and neutron stars with really strong magnetic fields, and really exotic and strange objects like that.
Isabelle Grenier: It’s like a Christmas tree, shining and it’s flaring, and there are eruptions every day.
Peter Michaelson: Gamma ray bursts being an example, something that for a brief instant of time outshines the entire rest of the universe.
Chip Meegan: These are the biggest explosions in the universe.
Neil Gehrels: We think that they’re the signals that happen when a black hole is born but we don’t know in detail how it works. And by looking with GLAST, we’ll be able to study the physics of what causes a gamma ray burst.
Martin Pohl: The thing is that most of the gamma rays we look at in terms of gamma ray astronomy, never reach people and the atmosphere essentially absorbs all of those gamma rays, which is the reason why GLAST has to fly on a satellite. None of the gamma rays we want to see actually make it to the ground.
Neil Gehrels: GLAST is going to open-up that part of the electromagnetic spectrum to better understand the universe.
Valerie Connaughton: It provides the widest energy coverage for gamma ray bursts that has ever been put into space.
Isabelle Grenier: It’s going to see the frontiers of many objects, high-energy objects.
Steve Ritz: And history shows that when you open-up a new band in the electromagnetic spectrum, you can expect some surprises, some great surprises.
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