Black holes are among the most profound predictions of Einstein’s theory of general relativity. Originally studied as a mere mathematical consequence of the theory rather than as physically relevant objects, they soon became thought of as generic and sometimes inevitable outcomes of the gravitational collapse that initially forms a galaxy.

In fact, most physicists have suspected that our own galaxy revolves around a supermassive black hole at its center. There are other ideas too — such as “dark matter” (an invisible substance thought to make up most of the matter in the universe). But now an international team of astronomers, including a team that I led from the University of Central Lancashire, has unveiled the first image of the object lurking at the center of the Milky Way — and it is a supermassive black hole.

This means there is now overwhelming evidence for the black hole, dubbed Sagittarius A*. While it might seem a little scary to be so close to such a beast, it is in fact some 26,000 light-years away, which is reassuringly far. In fact, because the black hole is so far away from Earth, it appears to us to have about the same size in the sky as a donut would have on the Moon. Sagittarius A* also seems rather inactive — it is not devouring a lot of matter from its surroundings.

What is SAGITTARIUS A*?

Sagittarius A*, often abbreviated to Sgr A* and pronounced “Sagittarius A star”, it is a Supermassive Black Hole at the center of the Milky Way Galaxy, located in the constellation Sagittarius. It is a strong source of radio waves and is embedded in the larger Sagittarius A complex. Most of the radio radiation is from a synchrotron mechanism, indicating the presence of free electrons and magnetic fields. Sagittarius A* is a compact, extremely bright point source. X-ray, infrared, spectroscopic, and radio interferometric investigations have indicated the very small dimensions of this region. Infrared observations of stars orbiting the position of Sagittarius A* demonstrate the presence of a black hole with a mass equivalent to 4,310,000 Suns. (For these infrared observations, American astronomer Andrea Ghez and German astronomer Reinhard Genzel were awarded the 2020 Nobel Prize for Physics.) These properties are similar to those of other galaxies with active nuclei (e.g., Seyfert galaxies) but on a smaller scale.

A gas cloud, G2, passed through the Sagittarius A* region in 2014 and managed to do so without disappearing beyond the event horizon, as theorists predicted would happen. Rather, it disintegrated, suggesting that G2 and a previous gas cloud, G1, were star remnants with larger gravitational fields than gas clouds.

QUICK FACTS: SAGITTARIUS A*

Also known as: Sgr A*.

Distance from Earth: 26,000 light-years.

Size: 4.6 million times the mass of the Sun.

Mass: 8.26×1036 kg (4.154±0.014)×106M☉

Type of object: Supermassive black hole.

Location in the sky: Sagittarius Constellation.

Location in the universe: Center of our Milky Way galaxy.

Did you know: In 2018, researchers found evidence for thousands of stellar-mass black holes located within 3 light-years of Sagittarius A* at the center of our Milky Way galaxy..

Observation and Description

Black holes are notoriously difficult to spot, usually only inferred by the effects they have on their environment. This is because not only do they not emit light, but black holes also trap photons behind a boundary called the event horizon, making studying them directly in optical light near impossible. Observing Sagittarius A* from Earth is made even more difficult due to the fact that it is shrouded by a thick screen of intervening dust.

On May 12, 2022, the first image of Sagittarius A* was released by the Event Horizon Telescope Collaboration. The image, which is based on radio interferometer data taken in 2017, confirms that the object contains a black hole. This is the second image of a black hole. This image took five years of calculations to process. The data was collected by eight radio observatories at six geographical sites. Radio images are produced from data by aperture synthesis, usually from night long observations of stable sources. The radio emission from Sgr A* varies on the order of minutes, complicating the analysis.

Their result gives an overall angular size for the source of 51.8±2.3 μas). At a distance of 26,000 light-years (8,000 parsecs), this yields a diameter of 51.8 million kilometers (32.2 million miles). For comparison, Earth is 150 million kilometers(1.0 astronomical unit; 93 million miles) from the Sun, and Mercury is 46 million km (0.31 AU; 29 million mi) from the Sun at perihelion. The proper motion of Sgr A* is approximately −2.70 mas per year for the right ascension and −5.6 mas per year for the declination. The telescope’s measurement of these black holes tested Einstein’s theory of relativity more rigorously than has previously been done, and the results match perfectly.

In 2019, measurements made with the High-resolution Airborne Wideband Camera-Plus (HAWC+) mounted in the SOFIA aircraft revealed that magnetic fields cause the surrounding ring of gas and dust, temperatures of which range from −280 to 17,500 °F (99.8 to 9,977.6 K; −173.3 to 9,704.4 °C), to flow into an orbit around Sagittarius A*, keeping black hole emissions low.

Astronomers have been unable to observe Sgr A* in the optical spectrum because of the effect of 25 magnitudes of extinction by dust and gas between the source and Earth.

SAGITTARIUS A* FIRST IMAGE

An image of the supermassive black hole at the center of the Milky Way, a behemoth dubbed Sagittarius A*, was revealed by the Event Horizon Telescope on May 12, 2022.

The image was captured using observations of light — the light emitted by matter that’s heated up as it hurtles toward the center of Sag A*. This technique gives scientists a view of essentially the shadow of the black hole.

Capturing an image of a black hole is no easy task and requires a global network of observatories that coordinates to act like a telescope the size of Earth — the Event Horizon Telescope (EHT).

There is still a great deal to learn about Sagittarius A* but the first image of the Milky Way’s central black hole could reveal further secrets held by the cosmic object that has shaped our galaxy.

The History of Discovery

Theories surrounding Sagittarius A* and its massive occupant date back to the early 1930s when Karl Jansky found a radio signal emitted from a location in the direction of the Sagittarius constellation directed towards the center of the Milky Way.

The galactic center compact radio source Sagittarius A* was then identified in February 1974 by astronomers Bruce Balick and Robert L. Brown. It was during the 1980s that astronomers formulated the idea that the central compact object was likely to be a black hole of a size — until then — unimaginable.

Karl Jansky, considered a father of radio astronomy, discovered in April 1933 that a radio signal was coming from a location in the direction of the constellation of Sagittarius, towards the center of the Milky Way. The radio source later became known as Sagittarius A. His observations did not extend quite as far south as we now know to be the Galactic Center. Observations by Jack Piddington and Harry Minnett using the CSIRO radio telescope at Potts Hill Reservoir, in Sydney discovered a discrete and bright “Sagittarius-Scorpius” radio source, which after further observation with the 80-foot (24-metre) CSIRO radio telescope at Dover Heights was identified in a letter to Nature as the probable Galactic Center.

Later observations showed that Sagittarius A actually consists of several overlapping sub-components; a bright and very compact component, Sgr A*, was discovered on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the baseline interferometer of the National Radio Astronomy Observatory. The name Sgr A* was coined by Brown in a 1982 paper because the radio source was “exciting”, and excited states of atoms are denoted with asterisks.

Since the 1980s, it has been evident that the central component of Sgr A* is likely a black hole. In 1994, infrared and submillimetre spectroscopy studies by a Berkeley team involving Nobel Laureate Charles H. Townes and future Nobel Prize Winner Reinhard Genzel showed that the mass of Sgr A* was tightly concentrated and of the order 3 million Suns.

On October 16, 2002, an international team led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics reported the observation of the motion of the star S2 near Sagittarius A* throughout a period of ten years. According to the team’s analysis, the data ruled out the possibility that Sgr A* contains a cluster of dark stellar objects or a mass of degenerate fermions, strengthening the evidence for a massive black hole. The observations of S2 used near-infrared (NIR) interferometry (in the Ks-band, i.e. 2.1 μm) because of reduced interstellar extinction in this band. SiO masers were used to align NIR images with radio observations, as they can be observed in both NIR and radio bands. The rapid motion of S2 (and other nearby stars) easily stood out against slower-moving stars along the line-of-sight so these could be subtracted from the images.

The VLBI radio observations of Sagittarius A* could also be aligned centrally with the NIR images, so the focus of S2’s elliptical orbit was found to coincide with the position of Sagittarius A*. From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be 4.1±0.6 million solar masses, confined in a volume with a radius no more than 17 light-hours (120 AU [18 billion km; 11 billion mi]). Later observations of the star S14 showed the mass of the object to be about 4.1 million solar masses within a volume with radius no larger than 6.25 light-hours (45 AU [6.7 billion km; 4.2 billion mi]). S175 passed within a similar distance. For comparison, the Schwarzschild radius is 0.08 AU (12 million km; 7.4 million mi). They also determined the distance from Earth to the Galactic Center (the rotational center of the Milky Way), which is important in calibrating astronomical distance scales, as 8,000 ± 600 parsecs (30,000 ± 2,000 light-years). In November 2004, a team of astronomers reported the discovery of a potential intermediate-mass black hole, referred to as GCIRS 13E, orbiting 3 light-years from Sagittarius A*. This black hole of 1,300 solar masses is within a cluster of seven stars. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.[citation needed]

After monitoring stellar orbits around Sagittarius A* for 16 years, Gillessen et al. estimated the object’s mass at 4.31±0.38 million solar masses. The result was announced in 2008 and published in The Astrophysical Journal in 2009. Reinhard Genzel, team leader of the research, said the study has delivered “what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the Galactic Center show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt.”

On January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sgr A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sgr A*, according to astronomers.

On 13 May 2019, astronomers using the Keck Observatory witnessed a sudden brightening of Sgr A*, which became 75 times brighter than usual, suggesting that the supermassive black hole may have encountered another object.

Conclusive evidence that the compact object Sagittarius A* is a supermassive black hole was delivered in 2018 when emissions caused by magnetic interactions from hot gas clumps close to the black hole moving at around 30% the speed of light were observed by astronomers using the European Southern Observatory (ESO)’s Very Large Telescope (VLT).

Over the next decade, astronomers continued to rule out other possible candidates for this object — including tightly clustered stars — which strengthened the idea that Sagittarius A* is a supermassive black hole.

The publication of the picture of the Sagittarius A* black hole is a tremendously exciting achievement by the collaboration.

This work should impact efforts using radio telescopes to observe and understand the “shadow” cast by the event horizon of Sgr A* against the background of surrounding, glowing matter. It will also be useful for understanding the impact that orbiting stars and gas clouds might make with the matter flowing towards and away from the black hole.

Hope you Learned more!

Topics:

• is Light a Matter or Energy:

— — |Light is a Particle

— — |Light is an Electromagnetic Wave

• Light as Wave
• Light as Electromagnetic Radiation
• The Speed of Light
• Quantum theory of Light

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• Light is a form of electromagnetic radiation that shows properties of both waves and particles. It is a form of energy. Light also keeps the Earth warm. Light exists in tiny energy packets called photons.
• Light waves are made of a mixture of electricity and magnetism so they are called electromagnetic waves. These waves travel very quickly, about 186,000 miles (300,000kilometers) per second. This means a beam of light could go 7 ½ times around the world in one second.

Light is electromagnetic radiation within the portion of the electromagnetic spectrum that is perceived by the human eye.

Also, when we say light, we actually mean visible light which is a tiny part of the electromagnetic spectrum:

• Energy in form of electromagnetic radiation. Electromagnetic radiation consist of an enormous range of wavelengths and frequencies. Gamma rays have the smallest wave lengths because they are the highest energy photons. But most gamma rays are just under ten picometers, which is still way smaller than a hydrogen atom. For reference, a hydrogen atom compared to a cent is about as big as a cent compared to the Moon.

is Light a Matter Or Energy?

Light is both a particle and a wave. Light has properties of both a particle and an electromagnetic wave but not all the properties of either. It consists of photons that travel in a wave like pattern.

The debate has raged for generations amongst the giants of the physics community regarding the nature of light, namely whether it is a particle or an electromagnetic wave. For centuries, this mysterious and elusive phenomenon left scientists baffled because with each experiment conducted to define its nature, it seemed to change the way it behaved.

In simple terms, light is one of nature’s freaky exceptions, and is considered to be both a wave and a particle. This variability is also one of the fundamental tenets of the theory of Quantum Mechanics. Let’s look at what happened over the years as people came to this important conclusion.

Light is a Particle:

The idea that light may be a particle was first advocated by Sir Issac Newton, but the idea didn’t catch on particularly well until the 19th century, when Albert Einstein revived the view. He argued that properties such as the reflection and refraction of light could only be explained if light was made up of particles.

Waves do not travel in straight lines and cannot exhibit those properties outlined by Newton and Einstein. However, if that’s true, then why was light rejected as a particle? The partial answer is that it did not fulfill or have all the properties that define a particle. A particle is a minute fragment or a quantity of matter with certain properties, such as mass and volume. The smallest unit of light is considered to be a photon, which does not have mass. Also, results of experiments by other researchers during the period between Newton and Einstein showed light having wave-like properties, which made them conclude that light was energy, instead of matter.

No!, Light is an Electromagnetic Wave:

A number of scientists, including Fresnel, Young and Maxwell, are credited with investigating the wave-like properties of light. A wave is a transfer of energy from one point to another without the transfer of material between the two points. Young performed the single-slit experiment, which was instrumental in establishing the wave-like properties of light, such as interference and diffraction. He passed a beam of light through a slit and observed the image it formed on the screen placed behind the slit screen.

If the corpuscular theory of light (light is a particle) proposed by Newton was true, then the pattern on the screen should have been light in the shape and size of the slit. However, the light pattern on the screen was more diffused/ diffracted, which indicated that light has an interference property, just like those exhibited by energy waves. Interference is a phenomenon in which two waves (considered to be linear systems) either have an additive or subtractive effect on each other’s intensity, which make the resultant wave either greater or lower in amplitude.

Light as a wave

Isaac Newton’s corpuscular model of light (see Early particle and wave theories) was championed by most of the European scientific community throughout the 1700s, but by the start of the 19th century it was facing challenges. About 1802 Thomas Young, an English physician and physicist, showed that an interference pattern is produced when light from two sources overlaps. Though it took some time for Young’s contemporaries fully to accept the implications of his landmark discovery, it conclusively demonstrated that light has wavelike characteristics. Young’s work ushered in a period of intense experimental and theoretical activity that culminated 60 years later in a fully developed wave theory of light. By the latter years of the 19th century, corpuscular theories were abandoned. Before describing Young’s work, an introduction to the relevant features of waves is in order.

Light as electromagnetic radiation

In spite of theoretical and experimental advances in the first half of the 19th century that established the wave properties of light, the nature of light was not yet revealed — the identity of the wave oscillations remained a mystery. This situation dramatically changed in the 1860s when the Scottish physicist James Clerk Maxwell, in a watershed theoretical treatment, unified the fields of electricity, magnetism, and optics. In his formulation of electromagnetism, Maxwell described light as a propagating wave of electric and magnetic fields. More generally, he predicted the existence of electromagnetic radiation: coupled electric and magnetic fields traveling as waves at a speed equal to the known speed of light. In 1888 German physicist Heinrich Hertz succeeded in demonstrating the existence of long-wavelength electromagnetic waves and showed that their properties are consistent with those of the shorter-wavelength visible light

The Speed of light

The speed of light in a vacuum is defined to be exactly 299 792 458 m/s (approx. 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum.

Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in the apparent period of Io’s orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth’s orbit.[15] However, its size was not known at that time. If Rømer had known the diameter of the Earth’s orbit, he would have calculated a speed of 227 000 000 m/s

According to physicist Albert Einstein’s theory of special relativity, on which much of modern physics is based, nothing in the universe can travel faster than light. The theory states that as matter approaches the speed of light, the matter’s mass becomes infinite. That means the speed of light functions as a speed limit on the whole universe. The speed of light is so immutable that, according to the U.S. National Institute of Standards and Technology, it is used to define international standard measurements like the meter (and by extension, the mile, the foot and the inch). Through some crafty equations, it also helps define the kilogram and the temperature unit Kelvin.

Quantum theory of light

By the end of the 19th century, the battle over the nature of light as a wave or a collection of particles seemed over. James Clerk Maxwell’s synthesis of electric, magnetic, and optical phenomena and the discovery by Heinrich Hertz of electromagnetic waves were theoretical and experimental triumphs of the first order. Along with Newtonian mechanics and thermodynamics, Maxwell’s electromagnetism took its place as a foundational element of physics.

However, just when everything seemed to be settled, a period of revolutionary change was ushered in at the beginning of the 20th century. A new interpretation of the emission of light by heated objects and new experimental methods that opened the atomic world for study led to a radical departure from the classical theories of Newton and Maxwell — quantum mechanics was born. Once again the question of the nature of light was reopened.

Thanks For Reading

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Source and References:

Source from Britannica:

• Light
• Light as a Wave
• Light as Electromagnetic Radiation
• The Speed of Light

Source from Wikipedia:

• Light

Source from Scienceabc:

• What Is Light? Matter Or Energy?
• Light is a Particle
• No!, Light is an Electromagnetic Wave

Source from Kurzgesagt:

• What is Light?

Source from Wonder Physics:

• What is light

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