Jodrell Bank

Gas

While the stars are of interest to radio astronomers, by far the majority of the radio emission from the sky comes from gas lying between the stars.

Cosmic hydrogen

About three-quarters of the observable universe is made of hydrogen.It is the nuclear fuel for the stars and the main constituent of the gas that lies between the stars. Interstellar hydrogen atoms emit radio waves at a wavelength of 21 cm. When astronomers detect a `line' at this wavelength they know that it is coming from hydrogen atoms. Detailed studies of the 21-cm line can reveal not only the existence of hydrogen clouds in our Galaxy and elsewhere, but also their motions and their temperature. It was the discovery of this line that led to the Mark 1 being redesigned to work at 21 cm even after construction had begun.

A cloud of hydrogen gas enveloping the galaxies M81 and M82 revealed by the Lovell Telescope

The Lovell Telescope is currently used for a variety of studies which make particular use of its angular resolution and sensitivity at 21 cm. Many observations in the infrared, ultraviolet and x-ray regions of the spectrum only become meaningful when the amount of intervening hydrogen is known, and this can be measured from observations of the 21-cm line. The Lovell Telescope is often used to support other astronomical investigations in this way.

Motions of hydrogen clouds towards or away from earth cause the observed wavelength to vary slightly, and this gives very important clues to the dynamics of gas clouds in our Milky Way and even in more distant galaxies.

An important method of measuring the distance to galaxies depends on being able to measure the speed at which hydrogen clouds are moving within them. This in turn helps astronomers to map the movements of galaxies and measure the rate of expansion of the universe.

Assembling a new spectrometer at Jodrell Bank for the observation of line emission from molecules in space

Molecules and masers

Hydrogen is not the only substance that emits radio waves at distinct wavelengths. Line emission has also been detected from molecules such as water, hydroxyl, carbon monoxide, ammonia, formaldehyde, methanol and numerous more complex molecules. Each molecule radiates a characteristic set of wavelengths that serve to identify it. Hydroxyl (OH), for example, emits at four wavelengths near 18 cm, while water emits near 1.4 cm.

These molecules are found deep inside the dense, cold cores of interstellar dust clouds, while a few of them are also present in the warm outer atmospheres of red giant stars. By making careful measurements of the spectra of these molecules, astronomers can learn a great deal about the conditions of the environment in which they are found.

Positions and velocities of molecular clouds in the Galactic Centre are revealed in this Lovell Telescope map of two spectral lines of hydroxyl (OH).

Because the radio waves are emitted only in narrow lines, sensitive telescopes are needed for the observations. The Lovell Telescope regularly studies hydroxyl and other lines to learn more about the structure of our Galaxy.

Certain interstellar molecules, including hydroxyl, water, methanol, and silicon monoxide, are known to radiate as masers. A maser occurs where molecules can be 'pumped' with energy which is later released in a sudden avalanche of radio waves, all at the same precise wavelength. Large amounts of infrared radiation are needed to pump up the molecules, and this is available in the vicinity of hot, young stars still surrounded by the dust and gas from which they formed, and in the warm outer atmospheres of old, red giant stars. Powerful megamasers are also found in the centres of some active galaxies.

Masers are very small bright radio sources and interferometric methods are essential if the spots are to be mapped accurately. The Lovell Telescope, forming, part of MERLIN, is regularly used to make detailed maps of maser regions. These studies have shed light on the structure of magnetic fields in star-forming regions and the flow of gas from newly formed stars. Observing several maser lines at the same time has proved to be a powerful way of studying the atmospheres of red giants and showing gas clouds accelerating away from the surface.

How radio waves are made

Objects emit radio waves because of their temperature. The hotter the object, the more radio waves it emits. This is known as thermal emission and is a very common source of radio waves in the universe. Thermal emission comes from gas clouds stars and planets. it is most important at shorter wavelengths.

Sometimes high-speed electrons are whirled around by magnetic fields in space. They radiate away their kinetic energy as radio waves, a process known as non-thermal emission. This process is a powerful source of radio waves in many types of astronomical object, including supernova remnants, pulsars, the Milky Way, radio galaxies and quasars. It is most important at longer wavelengths.

An astronomical sources may generate radio waves over a broad band wavelengths. This is known as continuum emission. Or it may transmit at one or more precisely defined wave lengths, much like radio or television station. This is known as line emission. Many molecules can be identified in space by measuring the wavelengths of their line emission.

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