Radio astronomy has led to a significant increase in astronomical knowledge, in particular through the discovery of several classes of new objects, including pulsars, quasars, and radio galaxies and, for a different kind of example, Sagittarius A*, the black hole at the center of the Milky Way. This is because radio astronomy allows us to see things that are undetectable in optical astronomy. Such objects represent some of the most extreme and energetic physical processes in the universe.
The cosmic microwave background radiation was first discovered with radio telescopes, and radio telescopes have also been used to study objects much closer to us, such as the sun and its activity, and radar mapping of the planets.
Ultraviolet Astronomy
Ultraviolet astronomy is the observation of electromagnetic radiation in the ultraviolet wavelength range between about 10 and 320 nanometers. Ultraviolet light (infamous for the sunburns it causes on our skin) is not visible to the human eye. Although many a cancer-risking red arm or back would seem to say otherwise, the Earth’s atmosphere absorbs most of the light at these wavelengths, so astronomical observations must be made from the upper atmosphere or space.
Measurements of the ultraviolet light spectrum (UV spectroscopy) reveal the chemical composition, density, and temperature of the interstellar medium, as well as the temperature and composition of young stars. UV observations also provide essential information about the evolution of galaxies. The ultraviolet universe looks very different from the familiar stars and galaxies seen in visible light. Most stars are relatively cool objects that emit much of their electromagnetic radiation in the visible or near-infrared part of the spectrum.
Ultraviolet radiation is the signature of hotter objects, typically in the early and late stages of their evolution. When viewing ultraviolet light emissions from the terrestrial sky, most stars would fade. Instead, the most visible stars would be a few very young and massive stars, and some very old stars and galaxies that are becoming hotter and producing high-energy radiation shortly before they die. However, gas and dust clouds could also block the view in many directions along the Milky Way.
With the help of ultraviolet astronomy, it was possible to learn significantly more about gas flows around hot stars and in binary systems. But researchers also gain new insights within our solar system with data from ultraviolet observations. For example, by studying the gases ionized in the tails of comets by the solar wind, it is possible to determine their composition. In addition, UV light has provided data on the composition of the atmospheres of planets such as Venus.
Infrared astronomy
Infrared astronomy studies objects visible in the infrared (IR) range, but invisible to the human eye. The wavelength of infrared light, also called thermal radiation, ranges from 0.75 to 300 micrometers.
Infrared astronomy had its early beginnings in the 1830s, a few decades after William Herschel discovered infrared light in 1800. Only after radio astronomy provided essential discoveries in the 1950s and 1960s, however, and astronomers then realized the true value of the information available outside the visible wavelength range, was modern infrared astronomy founded.
Practically no new techniques had to be developed for this purpose. Infrared and optical astronomy are often performed with the same telescopes because the same mirrors or lenses are usually effective over a wavelength range that included both visible and infrared light.
However, the water vapor in the Earth’s atmosphere absorbs some of the infrared light. Therefore, like radio telescopes, most infrared telescopes are located at high altitudes in dry locations, i.e., above as much of the atmosphere as possible. There are also infrared observatories in space, including the Spitzer and Herschel space telescopes. The James Webb Space Telescope (JWST), scheduled for launch in late 2021, also observes primarily in the infrared.
Infrared telescopes have helped find just-forming stars, nebulae, and stellar nurseries. They are also useful for observing extremely distant objects such as quasars. This is because quasars are moving away from Earth as the universe expands. The resulting large redshift makes them challenging targets for an optical telescope, and an infrared telescope provides much more information.
In May 2008, an international group of infrared astronomers demonstrated that intergalactic dust greatly dims the light from distant galaxies. In reality, the galaxies are almost twice as bright as they look. The dust absorbs much of the visible light and emits it as infrared light.
To achieve a higher-angular resolution, infrared telescopes can be combined to form interferometers. As in the radio domain, the effective resolution of an interferometer is determined by the distance between the telescopes and not by the size of any of the individual telescopes. When combined with adaptive optics that compensate for the effects of the atmosphere, infrared interferometers can achieve particularly high angular resolution.
A special problem of infrared astronomy is that these telescopes require cooling. Infrared light is heat radiation. Every heat source is, therefore, a source of interference. Space telescopes are, in particular, continuously heated by the sun and must be extensively shielded and cooled. The low temperature is often achieved by using a coolant that will eventually run out. Several times, space missions have been either terminated or switched to observations in shorter wavelengths when coolant supplies were depleted. For example, the WISE space telescope ran out of coolant in October 2010, approximately ten months after launch.
Gravitational wave astronomy
Gravitational wave astronomy aims to use tiny distortions of space-time, as predicted by Albert Einstein’s general theory of relativity, to collect data on massive objects and any processes that trigger such distortions. These may include neutron stars and black holes, events such as supernovae, and the early universe shortly after the Big Bang.
Einstein first predicted the existence of gravitational waves in 1916. For a long time, however, researchers were unsure whether they actually existed or were just artifacts of the theory. Indirect evidence of their existence was first provided in the late 1980s by studying a binary system consisting of a neutron star and a pulsar, with the pulsar moving just as it would have to if gravitational wave emission were present. The discoverers, Hulse and Taylor, were awarded the Nobel Prize in Physics in 1993.
On February 11, 2016, it was announced that the LIGO collaboration had directly detected gravitational waves for the first time in September 2015. Barry Barish, Kip Thorne and Rainer Weiss were awarded the 2017 Nobel Prize in Physics for this achievement. LIGO works with two perpendicular arms over which laser pulses are sent. When a gravitational wave arrives, these arms are distorted differently. The travel time of the laser light changed minimally, which is visible through a changed interference pattern.
The fact that the measuring instruments have so far recorded mainly catastrophic events has a clear reason: The frequencies of ordinary gravitational waves are very low—their wavelengths are therefore very high. Thus they are much more difficult to detect by LIGO. Higher frequencies (shorter wavelengths) occur in more dramatic events. They distort the two arms of the measurement construction more strongly and could therefore be observed first. For the future, therefore, astronomers are looking to space-based detectors for which longer arms are possible. ESA, for example, is planning a gravitational wave mission to be launched in 2034 called the evolved Laser Interferometer Space Antenna (eLISA).
Another project is to measure pulsars precisely. If they are hit by gravitational waves, there should be transit time changes in their signals. Researchers worldwide are working together in so-called ‘pulsar timing arrays’ to track down gravitational waves of low frequencies. However, results are not expected for a few years.
Neutrino Astronomy
Neutrino astronomy observes astronomical objects with neutrino detectors. Neutrinos are produced during certain types of radioactive decay or nuclear reactions, such as those that occur in the sun, in nuclear reactors, or when cosmic rays strike atoms. Because
of their weak interaction with matter, neutrinos offer a unique opportunity to observe processes inaccessible to optical telescopes.
Neutrinos continuously pass through the Earth in huge numbers. However, they interact so rarely with ordinary matter that only one interaction is registered per 1036 target atoms. And each interaction produces only a few photons or a single changed atom. Observation therefore requires a huge detector mass consisting of many atoms as well as a sensitive amplification system.
Given the very weak signal, sources of background noise must be reduced as much as possible. The detectors must be well shielded. Therefore, they are built deep underground or underwater. They detect particles flying upwards (i.e., coming from below them). ‘Upwards,’ because no other known particle can pass through the whole Earth, so that one is safe from interferences. The detectors must be located at a depth of at least one kilometer, and yet there is still an unavoidable background of extraterrestrial neutrinos interacting in the Earth’s atmosphere. The detectors consist of an array of light multiplier tubes housed in transparent pressure spheres, which in turn are suspended in a large volume of water or ice.
Why are neutrino detectors needed? If you look at celestial bodies like the sun in light of any wavelength, only the surface can be seen directly. Any light produced in the core of a star interacts with gas particles in the outer layers of the star and takes hundreds of thousands of years to reach the surface, making it impossible to observe the core directly. However, since neutrinos are also produced in the core of stars (as a result of nuclear fusion), the core is visible to neutrino astronomy.
Researchers have also discovered other sources of neutrinos, such as those released by supernovae. Several neutrino experiments have joined together to form the Supernova Early Warning System (SNEWS), looking for an increase in neutrino flux that could indicate a supernova event. Currently, astronomers are trying to detect neutrinos from other sources such as active galactic nuclei or gamma-ray bursts. Neutrino astronomy should also be able to indirectly detect dark matter.
So that you can also get something practical out of this, and perhaps give Peter a hand with his observations, I will conclude with a completely different but still related topic. You already own a telescope: your eyes.
Astronomy with the naked eye
It’s a common myth that you need a telescope to be an astronomer. Beginners often hear it when they ask for advice on how to approach their newly discovered hobby. You can indeed see much more with a telescope than with the naked eye or binoculars. But basically, you don’t need a telescope to study the sky. In other words, getting started in astronomy is actually free.
Beginners are best advised to simply use their eyes to familiarize themselves with the firmament—preferably with a star chart. Even if you buy a telescope later, it is essential to roughly know the sky. Either way, once you get started with the instruments Mother Nature has given you, you’ll be delighted to discover what you can detect with the naked eye.
Now, before you immediately run outdoors, here are a few things to consider if you want to enjoy stargazing. First, plan time for your eyes to adjust to the darkness, and second, choose a location that is as dark as possible. Any light pollution is more than annoying when stargazing. For example, unless you want to target the moon itself, it can be quite distracting. Viewing is better during a new moon, or if Luna shows up only as a narrow crescent.
A starry night is a breathtaking experience, and you will be amazed at what is presented to the unaided eye. Star clusters like the Pleiades are easy to spot, as is the star-forming region of the Orion Nebula. The band of the Milky Way can be seen from any dark location. With a simple star guide or planisphere, you’ll know right away where to look, but the objects are so conspicuous that they can be found without any accessories. Up to this point, astronomy doesn’t have to cost you a dime.
Once you have acquired a taste for astronomy, the desire for a pair of binoculars or a telescope will probably arise, and these instruments do not have to be expensive. If you choose carefully, your first piece of equipment can give you many years of pleasure and later serve as a reserve.
The night sky holds many wonders that can be perceived even without any instrument. According to theory, under perfect conditions, humans can recognize everything from a magnitude of +6.0 in the firmament—assuming good eyesight, of course. This means that you can capture 9,000 stars at once with a single glance. But if it’s dark enough all around, much more will show up—there are galaxies, planets, and star clusters, if you only know where to look. If you have a telescope at your disposal, possibly equipped with suitable filters, you can look thousands of light-years away—and just as far back into the past. A fascinating idea.
You need to give your eyes time to get used to the darkness, as I have already mentioned. Likewise, I’ve said you should avoid light pollution and cloudy skies. But you should also avoid alcoholic beverages, for best stargazing results, no matter how cold it may be outside—alcohol impairs the ability of your eyes to adapt, as does nicotine.
The best time to start observing is around the time of a new moon, when the satellite does not outshine all the treasures around it. I also recommend an elevated location, or at least a point where there are no houses, mountains, or trees blocking the horizon.
Once you have arrived at the best possible location, you will naturally want to perceive as much as conditions allow. Keep in mind that stars and planets are much easier to detect than galaxies and nebulae, even if they are of the same magnitude—the latter objects are scattered over a larger area of the sky and are thus much more diffuse than the concentrated points of light.
As the seasons progress, the constellations will gradually become familiar to you. Depending on whether it’s winter, spring, summer, or fall, constellations and asterisms such as Orion, the Big Dipper, the Summer Triangle, or the Great Square of Pegasus will wander through your field of vision. They also always provide useful reference points from which to make your way around the firmament.
The top three destinations
Orion Nebula (M42)
Right ascension: 05h 35m 17s
Declination: -05° 23′ 28″
Best season: Winter
Constellation: Orion
Magnitude: +4.0
If you look below the three stars of Orion’s belt, you will recognize the Orion Nebula as a pale spot in a clear sky without light pollution. It is a powerful gas cloud in which new stars are formed. It is 1,344 light-years away and can be seen even from the outskirts of a city.
Pleiades (M45)
Right ascension: 03h 47m 24s
Declination: +24° 07′ 00”
Best season: Winter
Constellation: Taurus
Magnitude: +1.6
In winter and spring, the Pleiades star cluster is impossible to miss. One can distinguish six or seven stars arranged in a pattern similar to that of the Big Dipper. The group is easily visible to the naked eye even with moderate light pollution.
Andromeda Galaxy (M31)
Right ascension: 00h 42m 44s
Declination: +41° 16′ 09″
Best season: Autumn
Constellation: Andromeda
Magnitude: +3.5
With a distance of 2.5 million light-years, the Andromeda Galaxy—also called Messier 31—is the most distant object we can perceive with unaided eyes. The environment should be sufficiently dark. Then you can see the galaxy on moonless nights in the constellation of the same name.
Technical jargon
Right ascension (RA)
In the sky, RA corresponds to the longitude on Earth, with the directions west and east. It is given in hours, minutes, and seconds because we see different parts of the sky during the night due to the Earth’s rotation.
Declination (Dec)
Dec indicates how high an object will rise in the firmament. Just like latitude, declination has north and south. The units are degrees, minutes of arc, and seconds of ar
c, each subdivided into increments of 60.
Magnitude
Magnitude is also called apparent brightness and indicates how bright an object appears when seen from Earth. The scale often causes confusion at first because the lower the number, the brighter an object is. For example, a star with magnitude -1 (mag -1) is more brilliant than one with +2.
Glossary of Acronyms
5G – 5th Generation (Telecommunications)
ADC – Atmospheric Dispersion Corrector
AGILE – Astro-rivelatore Gamma a Immagini LEggero
AGN – Active Galactic Nucleus
ASI – Agenzia Spaziale Italiana (Italian Space Agency)
ASTRON – ASTRonomisch Onderzoek in Nederland
AI – Artificial Intelligence
API – Application Program Interface
CCM – Command & Control Module
CNSA – Chinese National Space Administration
CORONAS-Photon – Complex ORbital Observations Near-earth of Activity of the Sun-Photon
DEO – DEOrbiting device
DLR – Deutsches Zentrum für Luft- und Raumfahrt (German Aerospace Center)
DRAMA – Debris Risk Assessment and Mitigation Analysis
eLISA – evolved Laser Interferometer Space Antenna
ESA – European Space Agency
ESO – European Southern Observatory
EU – European Union
GOES – Geostationary Operational Environmental Satellites
GRB – Gamma-Ray Burst
ISRO – Indian Space Research Organisation
HXMT – Hard X-ray Modulation Telescope (Chinese space observatory)
IPO – Initial Public Offering (stock market term)
IR – InfraRed
JPL – Jet Propulsion Laboratory
The Beacon: Hard Science Fiction Page 23