Radio Astronomy from A to Z

Simply put, an active galactic nucleus (AGN) is the central region of a very bright and active galaxy. Scientists assume that at the center of every galaxy lies a supermassive black hole. When this black hole is supplied with matter (gas and dust) from its surroundings, it results in an active, luminous galactic core.

The Big Bang theory describes the origin and evolution of the universe. According to this theory, the universe began about 13.8 billion years ago from an extremely dense and hot state. Since then, it has been expanding and gradually cooling down. Numerous observations - such as the cosmic microwave background radiation and the redshift of distant galaxies - support this model.

In a binary system (double system), two astronomical objects orbit a common center. These astronomical objects can be, for example, stars, planets and moons, or galaxies. There are also binary star systems consisting of two neutron stars. In such a system, matter from one object is transferred to the other – a process known as accretion.

Through this transfer of matter in neutron stars, not only does the mass of one object increase, but so does its rotation speed. It can become so fast that it turns into a millisecond pulsar.

Binary systems that include pulsars allow astronomers to test Einstein’s general theory of relativity.

Further information:
Compact Binary Evoliution

A black body is a theoretical, idealized object that completely absorbs all incoming radiation — including light — and re-emits it fully. The radiation emitted by a black body depends solely on its temperature. Black bodies serve as a standard for studying thermal radiation. Real objects — for example, the Sun — behave in a similar way, but never represent a perfect black body.

Black holes are very dense objects in the universe. This means they concentrate a large amount of mass in a relatively small space. As a result, their gravitational force is so strong that even electromagnetic waves - such as light - cannot escape it. Since even light is “swallowed” by a black hole, it is not visible in the optical range. To the human eye, it would simply appear black.

In April 2019, the first image of a black hole - or rather its shadow - was released. This achievement relied on multiple ground-based radio telescopes observing the source, in this case the black hole, simultaneously and then correlating the collected data.

Further Information:
Astronomers Capture First Image of a Black Hole

Blazars are a type of active galactic nucleus. This means they are supermassive black holes located at the centers of galaxies that are being fed with matter. The black holes classified as blazars have an additional characteristic feature: they are associated with a magnetized jet that is directed toward Earth.

Further information:
Blazars (PDF document)

A star reaches the end of its main sequence phase when it has exhausted the hydrogen in its core. At that point, fusion into helium can no longer occur. Low-mass stars then expand into red giants, collapse into white dwarfs, and eventually become black dwarfs.

Before high-mass stars can become neutron stars or black holes following a supernova explosion, they go through additional burning phases. The rule is: The more massive a star is, the more burning phases it undergoes before a potential supernova.

Sequence of the maximum possible burning phases:

1. Hydrogen burning

2. Helium burning

3. Carbon burning

4. Neon burning

5. Oxygen burning

6. Silicon burning

The Chandrasekhar limit is a theoretical upper limit for the maximum mass of a white dwarf. It is approximately 1.44 solar masses. Once a white dwarf accumulates enough mass to exceed the Chandrasekhar limit, the pressure can no longer withstand gravity, and the star collapses. Depending on its mass and conditions, the star can then become either a neutron star or a black hole. If this collapse causes a massive explosion, it is called a Type Ia supernova.

The limit is named after the (astro)physicist and Nobel laureate Subrahmanyan Chandrasekhar. He demonstrated that stars of a certain size collapse into a compact, radiant star - a white dwarf - when their hydrogen fuel runs out.

Cherenkov radiation is a physical phenomenon that can be observed as bluish light. It occurs when charged particles move through a medium (such as water or ice) faster than the speed of light in that same medium. Electromagnetic radiation (the blue light) is emitted, which can be detected with highly sensitive cameras.

Cherenkov radiation is used in nuclear and (astro)particle physics to detect highly energetic charged particles (for example, neutrinos) and to measure their speed.

Magnetic fields permeate the entire universe. Their origin remains unclear to this day. What is certain, however, is that they can influence everything in the universe — from the scale of planets to entire galaxies.

Cosmic background background is a faint radiation coming from all directions in the universe. It is a remnant from the early universe — approximately 380,000 years after the Big Bang. Today, it can be measured in the microwave range and has a temperature of 2.7 K (about -270°C). It is considered the most important evidence for the Big Bang theory.

Further information:
The Early Cooling of our Universe

This form of energy is purely hypothetical. The term was coined to explain the accelerated expansion of the universe. In science, there is a theory that this expansion is driven by an as-yet unproven energy - dark energy. It is believed to counteract gravity and thus cause the universe to expand increasingly faster. The proportion of dark energy in the universe is estimated to be about 70%.

Meanwhile, there are also theories that explain the accelerated expansion of the universe without dark energy.

Dark matter is a so far unproven, invisible form of matter. This hypothetical matter serves as an explanation for the gravitational cohesion of galaxies within galaxy clusters. The visible matter alone cannot explain why galaxies, despite their high speeds, do not drift apart but remain gravitationally bound.

Further evidence for dark matter comes from spiral galaxies: the uneven distribution of visible matter does not seem to affect the orbital speeds of the stars. This also suggests that there must be more matter in spiral galaxies than we can see. It is estimated that about 80% of the total matter in the universe is "dark.“

Cosmic dust - also called stardust - consists of tiny, solid particles of matter in interstellar space. They are much smaller than a grain of sand.

Cosmic dust is produced during supernova explosions or is ejected into space by older, dying stars. Additionally, cosmic dust can form in planetary systems like ours through collisions of asteroids or comets.

The electromagnetic spectrum includes all electromagnetic waves with their different wavelengths and frequencies. The part visible to us - the spectrum of visible light - is very small compared to the entire spectrum. All other electromagnetic waves are invisible to us but can still be observed instrumentally. The radio waves relevant to our research at the Max Planck Institute for Radio Astronomy have the longest wavelengths.

In general, radio waves can range in wavelength from just under a millimeter to over a kilometer. However, on Earth, due to absorption in the ionosphere, only wavelengths of up to about 10 meters can reach the surface.

The event horizon is the outer boundary of a black hole. Any matter or radiation that crosses this boundary is subject to the black hole’s gravitational pull and has no chance of escaping.
 

The expansion of the universe describes how space itself has been continuously stretching since the Big Bang, nearly 14 billion years ago. Galaxies are moving away from one another, and the farther they are, the faster they appear to recede. This discovery goes back to the observations of the astronomer Edwin Hubble and is considered one of the most important findings of modern cosmology.

Fast Radio Bursts (FRBs) are bright flashes of radio waves. These bursts last only a fraction of a second, and so far, most of these events have been observed only once. Their origin is unknown; however, it is known that the bursts come from very distant galaxies.

Further Information:
Lise Meitner Research Group on Fast Radio Bursts as Astrophysical Tools

Frequencies play an important role in astronomy because light and other forms of electromagnetic radiation have different frequencies (or wavelengths) depending on their energy. Radio waves have low frequencies, while X-rays and gamma rays have very high frequencies. By measuring these frequencies, astronomers can draw conclusions about the temperature, motion, and composition of celestial objects. The wavelength and frequency of electromagnetic radiation are related by the following equation: λ × µ = c (where λ represents the wavelength, µ the frequency, and c the speed of light).

Galaxies are regions in space that can span up to several hundred thousand light-years and consist of astronomical objects such as stars, dust, and planetary systems. Due to gravitational forces, these objects are bound together within the galaxy. Galaxies can vary greatly in type and shape. Our neighboring galaxy, the Andromeda Galaxy, is a spiral galaxy, while our home galaxy, the Milky Way, is classified as a barred spiral galaxy.

Further information:
Atlas of Galaxies
Galaxy Trail

Gamma-ray bursts – also known as gamma flashes – are extremely short but intense explosions in space. Since they emit in the gamma range, we cannot see them with the human eye. As for their origin, there are currently only theories. They likely occur, for example, when two neutron stars or a neutron star and a black hole merge. Another possible source could be a massive star exploding as a supernova.

From the material that originated with the Big Bang, stars and galaxies gradually formed over time. A large portion of this primordial material still exists today between stars and galaxies. This is known as intergalactic gas.

Supernovae also produce dust and gas. From this, vast clouds of interstellar material can form, from which new stars are born. These clouds are known as nebulae.

The general theory of relativity is an extension of the special theory of relativity. It no longer describes gravity as a force, but as a curvature of space and time caused by mass and energy. Massive objects, such as stars or black holes, noticeably distort the space-time continuum – causing other bodies to move along curved paths. This theory forms the foundation of our modern understanding of gravitation and cosmology.

Astronomers understand gravitational waves as tiny distortions in space-time. They are generated, for example, when two black holes merge and propagate at the speed of light. Figuratively speaking, the two black holes are like a stone thrown into still water, creating waves (gravitational waves) on the surface of the water (the universe), which continue to spread and carry energy away.

Einstein predicted the existence of gravitational waves in 1916 but doubted they would ever be detected, as they are extremely weak. The first indirect detection of gravitational waves was achieved in 1974 through observations of a pulsar in a binary system. In 2015—almost 100 years after Einstein’s prediction—gravitational waves were detected directly for the first time using ground-based observatories.

Scientists suspect that all gravitational waves in the universe add up to a background noise - called the gravitational wave background - and are searching for these very long-wavelength gravitational waves using pulsar timing arrays.

Hawking radiation is a theoretical concept formulated by Stephen Hawking in 1974. It refers to radiation emitted by a black hole. According to this concept, small particles and their antiparticles form near a black hole. They exist only for a very short time and usually annihilate each other again. However, near the event horizon, it can happen that one of the particles falls into the black hole while the other escapes. These escaping particles form what is known as Hawking radiation. This radiation is the reason why a black hole gradually loses mass over time.

The Hertzsprung–Russell diagram plots stars according to their absolute brightness and spectral type, or alternatively, their luminosity and surface temperature. The so-called main sequence stretches from the upper left (luminous, hot stars) to the lower right (cool stars with low luminosity), and it contains most stars – those in the phase of hydrogen burning. The diagram also reveals a star's current stage of evolution, as both temperature and luminosity change over the course of a star’s life.

Interferometry is a measurement technique that makes use of the superposition of waves. In radio astronomy, signals from multiple radio telescopes are combined by observing the same source simultaneously, and their data are then digitally combined (correlated). This creates a virtual radio telescope with extremely high resolution. For example, to capture the first image of a black hole, radio telescopes distributed across the entire globe were connected. In this way, a telescope was effectively created with the resolution of a virtual telescope the size of the Earth.

Jets are streams of particles that originate in the accretion disk of a supermassive black hole. They are ejected perpendicular to the disk in both directions into space and are guided by a spiral-shaped magnetic field.

Magnetars are pulsars that possess an exceptionally strong magnetic field.

In astronomy, matter refers to all forms of substance that make up stars, planets, gas clouds, and galaxies. It consists of atoms, molecules, dust, and plasma. Most of the visible matter in the universe is found in stars and gas clouds, while the majority of the universe’s total mass consists of so-called dark matter and dark energy, which cannot be directly observed.

The composition of the universe can be represented as follows:

• Visible matter: 5%

• Dark matter: 27%

• Dark energy: 68%

The Milky Way is our home galaxy. It consists of several hundred billion stars, vast clouds of gas and dust, and dark matter. At its center lies a supermassive black hole. Seen from the side, the Milky Way has the shape of a flat disk with a dense, bright core. The bright band we can see across the night sky is the inner view of our galaxy as seen from the position of the Sun.

Further information: 
The Milky Way

No element is more abundant in the universe than neutral hydrogen. The nucleus of a hydrogen atom consists of a single proton. Together with an electron, it forms neutral hydrogen, also known as HI.

Neutrinos are extremely light, electrically neutral particles that interact only very weakly with matter and can pass straight through the Earth. High-energy neutrinos have a very low probability of colliding with atomic nuclei on Earth. This makes it possible to detect neutrinos that arrive from the opposite hemisphere of the Earth — effectively allowing the planet to act like a telescope for these cosmic particles.

When a neutrino interacts with the nucleus of water or ice, it produces charged particles that travel faster than light does in that particular medium (water or ice). This results in what is known as Cherenkov radiation — a bluish light that can be captured using highly sensitive cameras.

A neutron star is – simply put – a giant atomic nucleus with a diameter of up to only 25 kilometers (about the size of Cologne), but a mass about 500,000 times that of Earth (typically between one and four solar masses). Such a neutron star can form when a massive star (more than eight times the mass of the Sun) explodes as a supernova. This process involves several steps:

1. The nuclear fuel is exhausted.
2. The core of the star collapses under its immense gravitational forces.
3. A tremendous amount of energy is released.
4. The outer layers of the star are ejected — this event is known as a supernova.
5. At the same time, the star's core is compressed by gravity to an extreme degree.
6. Protons and electrons combine into neutrons and neutrinos — meaning the atoms inside the star are effectively destroyed.

Very rapidly rotating neutron stars can be observed as pulsars.

Further information:
Neutron Stars

A nova — not to be confused with a supernova — can occur in a close binary star system where a white dwarf is present. As the white dwarf gains mass, hydrogen fusion is triggered on its surface. This process causes an extreme increase in temperature and ultimately leads to an explosive expansion of the star’s outer layers. This expansion is accompanied by a sudden outburst in brightness.

Nuclear fusion is a chemical reaction in which two atomic nuclei fuse to form a new nucleus, releasing energy in the process. In the cores of stars, fusion occurs under high pressure and high temperatures. Four hydrogen nuclei fuse into two helium nuclei. This process is also known as hydrogen burning. It is the reason why stars are able to shine and emit heat.

Photons are the smallest units of light when electromagnetic radiation is described as particles. They are massless particles that always move at the speed of light. Photons carry energy and, depending on their energy level, can represent visible light, X-rays, radio waves, or other forms of electromagnetic radiation.

Pulsars are extremely strongly magnetized, rotating neutron stars that formed in supernova explosions. Simply put, we can compare them to lighthouses, but instead of visible light, they primarily emit highly focused radio waves. When the geometry is right, this radio beam becomes briefly "visible" to us on Earth during each rotation and then disappears until it appears again. This happens with such regularity that pulsars can be as precise as atomic clocks. By observing pulsars, we can test general relativity and alternative theories of gravity, as the gravitational field near neutron stars—and thus pulsars—is particularly strong.

The first pulsar was discovered by Jocelyn Bell Burnell in 1967 with the Interplanetary Scintillation Array during her time as a doctoral student at Churchill College, Cambridge. The signals could not be assigned to any known object at the time, so the source was jokingly called "Little Green Man." Further analysis revealed that the source was a very rapidly rotating neutron star — the first detected radio pulsar. For this discovery, the 1974 Nobel Prize in Physics was awarded — not to Jocelyn Bell Burnell, but to her doctoral advisor Antony Hewish and Martin Ryle (for aperture synthesis technique).

A pulsar is also located in the Crab Nebula, one of the best-known supernova remnants. It is one of the few pulsars whose pulses can be detected across all regions of the electromagnetic spectrum. The supernova explosion that produced this pulsar, designated PSR B0531+21, was notably observed by Chinese astronomers on July 4, 1054, and due to its brightness was visible during the day for months.

Fascinating Facts:

• Pulsars can spin up to 716 times per second. That is 1.5 times faster than a typical kitchen blender.
• A teaspoon of pulsar material would weigh as much as Mount Everest.
• The diameter of pulsars is less than 25 kilometers, meaning they can be as small as a city.

A Pulsar Timing Array (PTA) is a network of millisecond pulsars observed with one or ideally multiple radio telescopes, used to search for and directly detect gravitational waves in the nanohertz frequency range (i.e., with wavelengths on the order of several light-years).

Pulsars rotate very steadily, and their pulses can be measured with microsecond accuracy. When gravitational waves pass through the network of pulsars (the PTA), their pulses arrive slightly earlier or later on Earth because gravitational waves stretch and compress spacetime.

Quasars (quasi-stellar radio sources) are a type of active galactic nucleus. This means they are supermassive black holes located at the centers of galaxies and fed by surrounding matter. Not only are they among the oldest objects in the universe - being millions to billions of light-years away from Earth - but they are also some of the most luminous.

In radio astronomy, celestial objects are studied by detecting the radio waves they emit. The Earth’s atmosphere is transparent to a large portion of these radio frequencies, allowing the use of ground-based radio telescopes. Such telescopes require as large a collecting area as possible because the sources being observed are extremely distant and the signals arriving on Earth are correspondingly weak.

Radio galaxies have Active Galactic Nuclei (AGN) at their centers, which are characterized by high radio luminosity. In astronomical jargon, radio galaxies are referred to as radio-loud sources. The radiation they emit does not primarily come from starlight, but rather from enormous, invisible streams of gas and magnetic fields located outside the visible part of the galaxy.

Typically, radio galaxies host a supermassive black hole at their core, which draws in surrounding material. This process releases tremendous amounts of energy, giving rise to what are known as jets  - near-light-speed plasma beams that stream out of the galaxy. These jets can extend for millions of light-years and produce synchrotron radiation, which we can detect as radio waves.

Red giants are stars with masses ranging from 0.3 to 8 times that of the Sun, which are nearing the end of their life cycle. The hydrogen fuel in their cores has already been depleted. As a result, the stars begin to contract under the force of gravity. This causes their density, pressure, and temperature to rise until a fusion process is triggered in the shell surrounding the core.

To restore the balance between the outward and inward pressures, the stars expand significantly—reaching up to about one hundred times their original size.

A redshift in the spectrum of a signal occurs when the wavelength of electromagnetic radiation becomes longer compared to the wavelength at the time of its emission. This means there is a shift toward the long-wavelength, or red, end of the electromagnetic spectrum. There are different types of redshift:

• Doppler Effect: The source of the electromagnetic radiation is moving away from the observer.

• Gravitational Redshift: Light escapes the gravity of curved spacetime and loses energy in the form of radiation.

• Cosmological Redshift: Caused by the accelerated expansion of the universe.

The Schwarzschild radius refers to the distance from the center of a non-rotating black hole to its event horizon. Within this radius, the gravitational force is so strong that not even light can escape. The Schwarzschild radius is directly proportional to the mass of the black hole - the greater the mass, the larger the Schwarzschild radius.

If matter is compressed into a spherical shape such that its radius becomes smaller than the Schwarzschild radius, a black hole forms. The radius can be calculated using the following formula: Rₛ = 2GM / c²

The Schwarzschild radius for Earth is 9 mm. For a human, the Schwarzschild radius would be much smaller than the nucleus of an atom.

The special theory of relativity was developed by Albert Einstein in 1905. It describes how space and time are interconnected when objects move at very high speeds — particularly close to the speed of light. A central result is that time and distances are relative: for moving observers, time passes more slowly and distances appear contracted.

Using spectral lines, researchers can analyze the chemical composition as well as the physical properties of celestial bodies. These lines are created when atoms or molecules absorb or emit light at specific frequencies or wavelengths, thereby taking in or releasing energy in the form of photons. In the spectrum, such lines appear as dark (absorption) or bright (emission) stripes. Each element has a unique spectrum, which scientists use to study the following:

• Chemical Analysis: Astronomers can determine which elements and molecules are present in stars, galaxies, and nebulae by examining emission and absorption lines, such as those from hydrogen and carbon dioxide.

• Doppler Effect: When an object moves, the frequencies of the radio waves it emits change. This frequency shift can reveal whether the object is moving toward or away from us. Redshift means the object is moving away (mnemonic: the rear lights of a car are red, which we see when the car moves away), while blueshift occurs when an object moves closer.

• Temperature and Density: The width and intensity of spectral lines provide information about the physical conditions of the celestial body.

• The 21-cm Spectral Line: Neutral hydrogen (HI) emits a faint but characteristic radio line at a wavelength of 21 cm (1.4 GHz), which is not blocked by dust. This allows for the detection of very weak signals from distant galaxies. Among other things, this enables extremely precise measurements of galaxy movements relative to Earth. Since neutral hydrogen exists throughout the universe, a map of the entire sky in HI light (captured with the Parkes and Effelsberg radio telescopes) has been created. This map serves as a reference source for researchers during observations across various wavelengths.

Further Information:
The Effelsberg-Bonn HI Survey
HI4PI: A new all-sky survey of neutral hydrogen

Spectroscopy is a physical method used to decompose light into its color components (wavelengths). For example, white sunlight can be split into a “rainbow.”

Light is energy that propagates through space as electromagnetic waves. The wavelength (or frequency) of these waves determines their color. Atoms and molecules can emit (release) or absorb (take in) light of very specific wavelengths. Since these wavelengths are unique for each element, the composition of the matter in question can be derived from its spectrum.

A spectrum is formed when white light passes through a prism and is split into its individual color components (i.e., wavelengths).

There are three types of spectra:

• Continuous Spectrum (also called continuum): Emitted by bodies that radiate heat. Light of all wavelengths is emitted, making the spectrum appear like a rainbow.

• Emission Lines: Occur when atoms or molecules emit energy at specific wavelengths. In a spectrum, these appear as bright or colored lines.

• Absorption Lines: Occur when atoms or molecules absorb light at specific wavelengths. In a spectrum, these appear as dark lines.

The speed of light represents the propagation speed of electromagnetic waves in a vacuum. Light travels 300,000 km in one second. It thus takes about 1.3 seconds for light to travel from the Earth to the Moon, and about 8 minutes from the Sun to the Earth. This is the maximum possible speed in the universe.

Nothing can move faster than light in a vacuum. In materials (such as water or glass), light travels more slowly than in a vacuum. Particles in such a medium can, under certain circumstances, move faster than light in that medium. If an electron in water moves faster than light, a bluish radiation called Cherenkov radiation is produced.

Stars are massive, self-luminous celestial bodies. They consist of hot gas and plasma and produce helium from hydrogen through a nuclear fusion process, releasing a tremendous amount of energy.

Like humans, stars are born, go through different developmental stages, and eventually die.

• Formation: Stars form when the matter in a molecular cloud condenses, eventually causing the core of this cloud to collapse. During several phases of collapse, a prestellar core forms, followed by a protostar, and finally a pre-main-sequence star. Massive stars particularly form in star clusters.

 

• Death.

The more massive a star is, the shorter its lifespan. The end of a star’s life is reached when it has exhausted its fuel. For low-mass stars, this occurs when no hydrogen remains to fuse into helium. These low-mass stars then expand into red giants, collapse into white dwarfs, and eventually end as black dwarfs. Although massive stars have overall shorter lifespans, they go through up to six fusion phases. After multiple fusion phases and a supernova explosion, massive stars can become neutron stars or black holes.

When the nuclear fuel of a massive star is exhausted, the core of the star collapses due to its strong gravitational forces. Additionally, the outer envelope of the star is expelled, an event known as a supernova.

Scientists distinguish between two types of supernovae:

• Type I: In a Type I supernova, stars with a maximum of eight solar masses can explode. This typically occurs in a binary system where two stars orbit each other closely. The two objects in such a binary system are usually a white dwarf and a red giant. The white dwarf accretes material from its companion - the red giant - until it can trigger a new fusion process. This extremely energetic process leads to the complete explosion of the white dwarf.
• Type II: When a star with a mass between 8 and 40 to 50 solar masses has exhausted its fuel, it becomes unstable and collapses due to gravity. At the same time, its outer envelope is expelled - the star explodes in a Type II supernova. This releases so much energy that the star can shine brighter than an entire galaxy for a certain period. Besides the supernova remnant, a Type II supernova can result in the formation of a neutron star or a black hole.

Synchrotron radiation occurs when electrons or other charged particles moving at relativistic speeds - speeds close to the speed of light - are accelerated and deflected by a magnetic field. In doing so, they emit light - this is synchrotron radiation.

Synchrotron radiation also exists in space. In astronomy, it occurs whenever hot plasma is present in a magnetic field. Examples of cosmic synchrotron sources are pulsars, radio galaxies, and quasars. A natural source of synchrotron radiation in space is Jupiter, which continuously irradiates its moons with this type of radiation.

A white dwarf is a low-mass star in the final phase of its life. More precisely, it is the burnt-out carbon core of a red giant that has shed its outer layers. White dwarfs are about the size of Earth but as heavy as the Sun. Since the energy source of these stars has dried up, they cool down over several billion years and eventually end as black dwarfs. Our Sun will also go through this process in the distant future.