Since prehistoric times, people have looked to the sky for answers and for inspiration.

Only in the twentieth century, though, have people been able to look without using light — that is, without using visible light.

Astronomers and astrophysicists today have the entire electromagnetic radiation spectrum at their disposal for probing the cosmos.

Iowa State astronomers, astrophysicists, physicists and electrical engineers who observe and study various cosmic phenomena use everything from radio waves to gamma waves to unmask the heavens.

The prospect of using nonelectromagnetic radiation — in particular, tiny elementary particles called neutrinos — to uncover hard-to-detect objects looms ever nearer on the astrophysics horizon.

The electromagnetic radiation spectrum consists of all frequencies and wavelengths of energy that is comprised of individual massless bundles called photons, which travel together as waves and may interact with anything carrying electrical charge. Visible light lies roughly in the middle of the spectrum.

Marconi’s Predecessors

Radio waves, which have photons that may be a billion times less energetic than photons of visible light, are ubiquitous in the heavens.

Phil Appleton, professor of astronomy, said cosmic radio waves are often generated by electrically charged particles which are accelerated by cosmic magnetic fields.

Appleton has performed radio frequency studies of galaxies such as Arp 10, which has a turbulent ring layer which emits radio waves due to a comparatively rare electronic process called spin-flip in hydrogen atoms.

John Basart, professor of electrical engineering, has long held an interest in radio astronomy.

“I look mainly at galaxies and quasars, many of them classical double radio sources — objects that have two diametrically-opposed plasma jets shooting out from them, which radiate radio waves,” Basart said.

Moving up the scale in energy or down the scale in wavelength, is the microwave region, where the ultra-cold cosmic microwave background radiation, three degrees hotter than absolute cold, permeates the universe as a relic of the Big Bang.

This radiation has been clearly identified and accurately measured in recent years by the Cosmic Background Explorer satellite (COBE).

Hot and Bothered

Heavenly bodies also expose themselves to the critical observer by thermal agitation of charged particles and by collisions of particles.

For frequencies of high energy microwaves and above, thermal processes begin to play a larger role.

Just above the background radiation in energy is the millimeter and sub-millimeter wave range, still considered microwave but relatively unexplored in astronomy. Lee Anne Willson, university professor of astronomy, said “sub-millimeter waves are an area of increasing interest, because it’s a good way of investigating molecules found in interstellar space.”

Stars, and in fact most cosmic objects, emit thermal radiation of the next higher energy level — infrared. Astronomers indeed observe infrared radiation from dust clouds, such as the dust cocoons which surround quasars, Appleton said.

Beyond the Milky Way

Russ Lavery, assistant professor of astronomy, considers himself an extra-galactic astronomer. He performs most of his work in the optical (visible) wave range — where he uses images from the Hubble Space Telescope — and in the X-ray regime.

The galaxies also produce ultraviolet radiation, the wave band between visible and X-ray, but at relatively low intensities, Lavery said.

Lavery said he uses the optical range to study the structure of individual galaxies and the X-ray region to study clusters of galaxies.

“The particles of the hot gas that fill the sky between galaxies in large groupings radiate X-rays as a result of ‘Bremstrahlung’ collisions (a sort of side-swiping) among them,” Lavery said.

Most Rare

At the high frequency end of the electromagnetic spectrum are elusive gamma rays, the highest energy electromagnetic radiation, with photons as much as 10,000 billion times as energetic as those of visible light.

David Carter-Lewis, professor of physics, leads the gamma ray astronomy group. One of his graduate students is Frank Samuelson.

Samuelson said the primary heavenly bodies they study are Active Galactic Nuclei — objects which, at their core, contain a black hole with a mass at least a million times larger than that of the sun — and supernova remnants, which generally include a pulsar and a nebula, such as the Crab nebula.

Gamma rays can be detected directly, but this must be done above the atmosphere, Samuelson said.

ISU’s group uses the “Gamma Ray Telescope,” a 10-meter wide optical telescope located on Mt. Hopkins in Arizona.

The telescope detects visible light resulting from Cherenkov radiation, a radiation produced when electrically charged particles travel through a medium faster than light itself travels through the same medium, Samuelson said.

The cosmic gamma rays strike molecules at the top of the atmosphere producing a cascade of elementary particles, including many electrons. These electrons travel at speeds which are within the very small range of speeds that are faster than light travels in air, yet slower than light travels in interstellar space, and consequently emit the visible radiation which is detected by the telescope.

Next Frontier

Finally, for those not prone to convention, is the possibility of doing astronomy with the high-energy neutrinos. John Hauptman, professor of physics, said there are attempts across the world to detect cosmic neutrinos.

“Neutrinos come from the decays of elementary particles, and being neutral, they will travel in a straight line from wherever they are produced to earth,” Hauptman said.

“Any cosmic entity that has a lot of “dust” or debris around it, like accretion disks around binary stars, will block the visible light, but the neutrinos will go right through (the dust), so there’s the possibility that neutrinos can image the objects that are very difficult to image with photons,” Hauptman said.

Evidently, running the gamut of techniques pays off when browsing through the universe.