Astronomy - Beyond The Visible Spectrum

Beyond The Visible Spectrum

The electromagnetic spectrum is the entire range of frequencies of electromagnetic radiation and their respective wavelengths and photon energies. 

Nearly all types of electromagnetic radiation can be used  to study and characterize matter.

The electromagnetic spectrum extends from below the low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometres down to a fraction of the size of an atom.

For most of history, visible light was the only known part of the electromagnetic spectrum. The ancient Greeks recognized that light travelled in straight lines and studied some of its properties, including reflection and refraction.

The study of light continued, and during the 16th and 17th centuries conflicting theories regarded light as either a wave or a particle.

The first discovery of electromagnetic radiation other than visible light came in 1800, when William Herschel discovered infrared radiation.

He was studying the temperature of different colours by moving a thermometer through light split by a prism.

He noticed that the highest temperature was beyond red. He theorized that this temperature change was due to "calorific rays" that were a type of light ray that could not be seen.

The next year, Johann Ritter, working at the other end of the spectrum, noticed what he called "chemical rays" (invisible light rays that induced certain chemical reactions).

These behaved similarly to visible violet light rays, but were beyond them in the spectrum. They were later renamed ultraviolet radiation.

Electromagnetic radiation was first linked to electromagnetism in 1845, when Michael Faraday noticed that the polarization of light traveling through a transparent material responded to a magnetic field.

During the 1860s James Maxwell developed four partial differential equations for the electromagnetic field.

Maxwell realized that they must travel at a speed that was about the known speed of light.

This startling coincidence in value led Maxwell to make the inference that light itself is a type of electromagnetic wave.

In 1886 the physicist Heinrich Hertz built an apparatus to generate and detect what are now called radio waves.

Hertz also demonstrated that the new radiation could be both reflected and refracted in the same manner as light.

In 1895 Wilhelm Röntgen noticed a new type of radiation emitted during an experiment with an evacuated tube subjected to a high voltage. He called these radiations x-rays.

The last portion of the electromagnetic spectrum was filled in with the discovery of gamma rays. In 1900 Paul Villard was studying the radioactive emissions of radium when he identified a new type of radiation.

In 1910, British physicist William Henry Bragg demonstrated that gamma rays are electromagnetic radiation.

Astronomers started to investigate portions of the electromagnetic spectrum outside the optical in the 1930s.

Advances in radar and rocket technology during World War II gave this new research a big push, and it has continued to grow ever since.

Non-optical telescopes examine light from the sky at wavelengths other than those of visible light.

Many different types exist to study incoming radio waves, microwaves, infrared and near-infrared rays, ultraviolet rays, X-rays and gamma rays.

The human eye can only see a tiny band of the electromagnetic spectrum.

That tiny band is enough for most day-to-day things you might want to do on Earth, but stars and other celestial objects radiate energy at wavelengths from the shortest (high-energy, high-frequency gamma rays) to the longest (low-energy, low-frequency radio waves).

 

Radio Telescopes

The 76 meter Jodrell Bank

At the far end of the electromagnetic spectrum we find the radio waves, with frequencies less than 1000 megahertz and wavelengths of a metre and more.

Radio waves penetrate the atmosphere easily, unlike higher-frequency radiation, so ground-based observatories can observe them.

Radio telescopes feature three main components that each play an important role in capturing and processing incoming radio signals.

The first is the antenna or ‘dish’ that faces the sky. The Parkes radio telescope in New South Wales, Australia, for instance, has a dish with a diameter of 64 metres, while the Aperture Spherical Telescope in southwest China is 500 metre diameter.

The dish is parabolic, directing radio waves collected over a large area to be focused to a receiver sitting in front of the dish.

The larger the antenna, the weaker the radio source that can be detected, allowing larger telescopes to see more distant and faint objects billions of light years away.

The receiver works with an amplifier to boost the very weak radio signal to make it strong enough for measurement.

Receivers today are so sensitive that they use powerful coolers to minimise thermal noise generated by the movement of atoms in the metal of the structure.

They may be used singly, or linked together electronically in an array. Unlike optical telescopes, radio telescopes can be used in the daytime as well as at night.

Radio telescopes are used to observe a wide array of subjects, including energetic pulsar and quasar systems, galaxies, nebulae, and of course to listen out for potential alien signals.

Microwave Telescope

Microwave radiation spans a range of wavelengths that can be produced by very cold astronomical sources, or by warm sources like protoplanetary disks and clouds of interstellar molecules.

Microwave telescopes must be able to act somewhat like infrared telescopes and somewhat like radio telescopes.

They therefore are built and operated using a fascinating blend of technologies.

Depending on the scientific goals, they can be put in space, in high-altitude balloons, or on the ground at mountaintop observatories.

Some of the most fascinating sources of microwaves lie well outside our solar system, across our galaxy and across the universe.

For example, active galaxies, powered by supermassive black holes at their cores are some of the strongest microwave emitters.

Additionally, these black hole engines can create massive jets of plasma that also glow brightly in the microwave.

Some of these microwave-emitting structures can be larger than the entire galaxy that contains the black hole. 

The center of our own Milky Way galaxy is a microwave source, although it's not so extensive as in other, more active galaxies.

Pulsars (rotating neutron stars) are also strong sources of microwave radiation. These powerful, compact objects are second only to black holes in terms of ultimate density. With powerful magnetic fields and fast rotation rates broad spectrum radiation is produced, with the microwave emission being particularly strong. In fact, most pulsars are usually referred to as "radio pulsars" because of their strong radio emissions, but they can also be "microwave-bright".

When a microwave telescope is pointed around the sky, it detects a faint microwave glow.

The Cosmic Microwave Background Explorer (COBE) satellite made a detailed study of this cosmic microwave background (CMB) beginning in 1989.

Astronomers use the minor fluctuations in the CMB to learn more about the origins and evolution of the universe.

 

The Cosmic Microwave Background

Spitzer Space Infrared Telescope

Sitting just below visible light on the electromagnetic spectrum is infrared light, with wavelengths between 700 nanometres(one billionth of a metre) and 1 millimetre.

Much infrared radiation is absorbed by water vapour in the atmosphere, so infrared telescopes are usually at high altitudes in dry places or even in space, like the Spitzer Space Telescope.

Infrared telescopes are often very similar to optical ones. Mirrors and reflectors are used to direct the infrared light to a detector at the focal point.

The detector registers the incoming radiation, which a computer then converts into a digital image.

Ultraviolet Telescopes

Ultraviolet light is radiation with wavelengths just too short to be visible to human eyes, between 400 nanometres and 0.01 nanometres.

It has less energy than X-rays and gamma rays, and ultraviolet telescopes are more like optical ones.

Mirrors coated in materials that reflect UV radiation, such as silicon carbide, can be used to redirect and focus incoming light.

As redirected light reaches the focal point, a central point where all light beams converge, they are detected using a spectrogram.

This specialised device can separate the UV light into individual wavelength bands in a way akin to splitting visible light into a rainbow.

Analysis of this spectrogram can indicate what the observation target is made of.

This allows astronomers to analyse the composition of interstellar gas clouds, galactic centres and planets in our solar system. This can be particularly useful when looking for elements essential to carbon-based life such as oxygen and carbon.

X-rays are radiation with wavelengths between 10 nanometres(one billionth of a metre). and 0.01 nanometres.

They are used every day to image broken bones and scan suitcases in airports and can also be used to image hot gases floating in space.

Celestial gas clouds and remnants of the explosive deaths of large stars, known as supernovas, are the focus of X-ray telescopes.

X-ray telescopes often use highly reflective mirrors that are coated with dense metals such as gold, nickel or iridium.

Unlike optical mirrors, which can bounce light in any direction, these mirrors can only slightly deflect the path of the X-ray. The mirror is orientated almost parallel to the direction of the incoming X-rays.

The X-rays lightly graze the mirror before moving on, a little like a stone skipping on a pond. By using lots of mirrors, each changing the direction of the radiation by a small amount, enough X-rays can be collected at the detector to produce an image.

To maximise image quality the mirrors are loosely stacked, creating an internal structure resembling the layers of an onion.

In order to get above the Earth's atmosphere, which is opaque to X-rays, X-ray telescopes must be mounted on high altitude rockets, balloons or artificial satellites.

The first X-ray telescope employing grazing-incidence optics was employed in a rocket-borne experiment in 1965 to obtain X-ray images of the Sun.

 

Gamma-ray Telescopes

Gamma radiation is generally defined as radiation of wavelengths less than a hundredth of a nanometre(one billionth of a metre).

Gamma-ray telescopes focus on the highest-energy phenomena in the universe, such as black holes and exploding stars. A high-energy gamma ray may contain a billion times as much energy as a photon of visible light, which can make them difficult to study.

Unlike photons of visible light, that can be redirected using mirrors and reflectors, gamma rays simply pass through most materials.

This means that gamma-ray telescopes must use sophisticated techniques that track the movement of individual gamma rays to construct an image.

One technology that does this, in use in the Fermi Gamma-ray Space Telescope among other places, is called a pair production telescope.

It uses a multi-layer sandwich of converter and detector materials. When a gamma ray enters the front of the detector it hits a converter layer, made of dense material such as lead, which causes the gamma-ray to produce an electron and a positron (known as a particle-antiparticle pair).

The electron and the positron then continue to traverse the telescope, passing through layers of detector material. These layers track the movement of each particle by recording slight bursts of electrical charge along the layer. This trail of bursts allows astronomers to reconstruct the energy and direction of the original gamma ray. Tracing back along that path points to the source of the ray out in space. This data can then be used to create an image.