West Virginia

125

Reber Radio Telescope, Green Bank, WV

38° 25′ 48.61″ N, 79° 49′ 4.45″ W

The Homemade Telescope

In 1937, an amateur astronomer and radio ham named Grote Reber hand-built his own radio telescope in his backyard in Wheaton, Illinois. The almost 10-meter parabolic dish was the only operating radio telescope in the world until after the end of the Second World War. Using the telescope, he drew radio maps of the sky and discovered that some apparently dark areas (with no stars) nevertheless were producing radio signals. Between 1937 and 1948, Reber and his home-built telescope revolutionized astronomy, and his telescope marked the birth of radio astronomy.

In 1948 Reber sold the telescope to the U.S. National Bureau of Standards, and it began a countrywide journey—first being moved from Illinois to Virginia, then to Colorado, and finally to the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia, where it sits today. It is still in working order.

Reber didn’t have enough money to build a fully steerable telescope—it took half a year’s salary just to get the telescope built—and so his backyard dish could only rotate in one direction, relying on the motion of the Earth to scan in the other direction. When the telescope was bought by the NRAO, it was mounted on a rotating platform to make it fully steerable; Reber helped supervise the work before moving to Tasmania to continue radio astronomy.

The NRAO has many other radio telescopes, including part of the horn antenna used by Harold Ewen and Edward Purcell to detect the 21-centimeter hydrogen line (see sidebar).

Happily, the general public is welcome at the observatory. The recently constructed Green Bank Science Center has many exhibits, mostly aimed at schoolchildren. For adults, there’s a bus tour of the observatory that takes in the historic telescopes and the Robert C. Byrd Green Bank Telescope, the largest fully steerable radio telescope in the world.

The NRAO is in a wonderfully quiet location, partly because of its remoteness in the Potomac Highlands, and partly because most electronic equipment is banned—no cell phones, no digital cameras, no MP3 players. Electronic items might interfere with the very sensitive radio receivers used for listening to the sky.

Practical Information

Find details about visiting the NRAO at Green Bank at http://www.gb.nrao.edu/.

21-Centimeter Hydrogen Line

About 240,000 years after the Big Bang, hydrogen and helium atoms began to form. Initially they were ionized, but after a period of cooling, the ionized atoms captured free electrons and became neutral. When this occurred, the universe became transparent and photons were free to travel, resulting in the cosmic background microwave radiation (see page 413). However, the hydrogen present also emitted radiation because of hyperfine splitting.

A neutral hydrogen atom has a single proton and a single electron. In 1913, the physicist Niels Bohr introduced a model of the hydrogen atom that explained why the spectrum of light (and other electromagnetic radiation) from excited hydrogen atoms consisted of specific lines of color. By applying thousands of volts to a glass tube containing hydrogen gas, it’s possible to obtain a spectrum of light from the gas (see Figure 125-1).

Figure 125-1. Hydrogen emission lines

Bohr’s model showed that the radius of the electron’s orbit around the proton in a hydrogen atom was constrained to certain fixed positions (called quanta), and that when the electron changed orbit it either absorbed energy from a photon, or emitted one (see Figure 125-2).

Figure 125-2. Bohr’s model of hydrogen

Bohr also suggested that the energy level associated with the electron depended on the orbit. The closer the orbit, the lower the energy, and when jumping from a far-to-near orbit, energy (in the form of a photon) is emitted. Bohr’s rule was that the frequency of the emitted photon, f, is determined by Planck’s constant and the change in energy level, ΔE: f = ΔE / h.

Bohr’s model was approximately correct, but was only useful for hydrogen. It was subsequently refined by quantum mechanics, which helped explain an anomaly in the emitted spectrum of hydrogen.

When at the lowest possible energy level, with the electron closest to the proton, there are actually two separate energy levels for the atom. These occur because not only does the electron orbit the proton (actually, “orbiting” is just an approximation, but close enough), both the electron and proton have a spin. The spin can be thought of as the particle rotating about its own axis.

This rotation leads to a magnetic field created by the proton and the electron. When the magnetic fields are in the same direction (i.e., both norths are in the same direction), the atom has a slightly higher energy than when they are opposed.

When the electron spin changes direction, there is a change in energy. If the energy decreases, then a photon is emitted, and the emitted wavelength depends on the difference in the energy level between the two possible electron spins—essentially as Bohr had predicted. This is called hyperfine splitting of the emission spectrum (because a single line of spectrum is actually made up of more than one closely spaced line), and in hydrogen the emitted photon has a wavelength of 21 centimeters.

This transition between spins occurs with a very, very small probability. However, the universe is filled with neutral hydrogen, and hence microwave radiation at 21 centimeters is detectable and is evidence of the Big Bang. It’s also useful to astronomers because the radiation can be used to map the presence of hydrogen throughout the universe, which can aid in the understanding of the structure of the universe after the Big Bang.