Physics Curiosities

Black Holes

What is a black hole?

 black hole is considered to be an object whose gravitational field is large enough not to allow anything to escape, not even light. At present, combining scientific discoveries and speculations, it can be theorized that there are 4 types of black holes in the universe . Go to the next page to know them.

In an approach of classical physics, black holes are celestial objects with very large mass – some of them hundreds of times the mass of the Sun – that occupy a very small space.

Its gravitational field is so intense that not even the speed of light is greater than its escape velocity.

With this, the light that enters a black hole can no longer come out, making it unobservable by the usual techniques that analyze the light emitted or reflected by celestial objects.

And what is escape velocity?

We call escape velocity one whose intensity is sufficient for an object to “escape” from the gravitational field. The escape velocity on the Earth’s surface is approximately 11.2 km / s; for an object to free itself from the gravity of our planet, it needs to be launched at a higher speed than this.

If a black hole cannot be seen, how is it detected?

The observation of a black hole happens indirectly, because what you can see are the effects it causes in the nearby regions. Due to its immense gravitational field, other bodies tend to be attracted to it. By measuring the speed with which objects move towards you in neighboring regions, it is possible to discover their mass.

When a black hole absorbs matter from nearby bodies, this matter gets compressed, heats up significantly, and emits a large amount of X-ray radiation. The first black hole detections were made with sensors that captured this X-ray emission.

Strong evidence has already been observed that there are supermassive black holes at the center of some spiral galaxies, including some scientists believe that there is one of these black holes at the center of our galaxy, the Milky Way.

Who discovered black holes in space?

(I think it was in 1958). In 1916 the German astronomer Karl Schwarzschild developed the concept of a black hole based on Einstein’s theory of relativity, which later describes black holes without charge and without rotation.

In 1963 Roy Kerr described rotating black holes. From 1965 to 1970, Stephen Hawking and Roger Penrose defined a black hole as the set of events from which nothing is possible to escape at great distances.

In 1967 the scientist Werner Israel demonstrated, based on the Theory of Relativity, that black holes without rotation were perfectly spherical and their mass determined their size.

In 1969 the scientist John Wheeler gave the term black hole as the graphic description of a time when there were two theories of light: twilight (light was made up of particles) and wave.

In 1970 Brandon Carter proved that if a black hole rotating in a stationary way had an axis of symmetry, its size and shape would depend on the mass and the speed of rotation.

In 1971 Stephen Hawking showed that any stationary rotating black hole would have an axis of symmetry.

In 1973 David Robinson used the Carter and Hawking result to prove the Kerr hypothesis.

Characteristics of black holes

  • Great gravitational attractive force – so strong that light cannot escape.
  • They have a mass of at least thousands of suns.
  • They do not have rigid surfaces.
  • They emit a form of radiation known as Hawking radiation.
  • They have no external attributes except mass, angular momentum, and charge.
  • It contains a singularity within.
  • They are potentially at the center of every galaxy

Radio Schwarzschild

The German astronomer Karl Schwarzschild calculated what is now known as the Schwarzschild radius or the event or event horizon . This radius defines a sphere surrounding a body of a particular mass, known as the singularity, of which its properties are not known.

How is a black hole formed?

The origin of a black hole can only be established theoretically. The gravitational collapse of a massive star to form a black hole was first described in 1939 by J. Robert Oppenheimer and Hartland Snyder.

A black hole forms when a body of a certain mass (a star) contracts to a size smaller than the gravitational radius. This contraction is due to the great force of gravity that attracts objects to the body’s center of mass.

For example, if the Sun were to shrink to less than 3 km, it would transform into a black hole with a gravitational force of 1.5 x 10 12 times the gravity of Earth.

Most black holes are formed from the remnants of large stars that explode as supernovae. They could also be formed by the collision between two stars or a black hole with a neutron star.

Black hole types

Modern astrophysicists consider the existence of three types of black holes in the Universe:

  • stellar black holes , that is, black holes that were born when massive stars died;
  • supermassive black holes : those in the centers of galaxies that have masses greater than 10 billion suns;
  • primordial black holes : they were formed at the beginning of the expansion of the Universe from inhomogeneity.

Parts of a black hole

Event horizon

The event horizon or Schwarzschild radius is the sphere that surrounds the singularity of the black hole from which nothing escapes. That is, it is the dark area of ​​the black hole.

The event or event horizon divides two areas that cannot communicate with each other, that is, it is the point where the escape velocity is the same as the speed of light. It acts as a one-way membrane, where particles and radiation can enter but cannot return.

Ergosphere

This is the region around a rotating black hole. In this region the same space-time creeps in the direction of rotation at a speed greater than the local speed of light in relation to the rest of the universe.

ISCO

The ISCO ( Innermost Stable Circular Orbit ) or marginally stable orbit is the smallest radius circle outside of which free particles can orbit stably around a black hole.

Accretion disk

The accretion disk is the group of circles of gaseous elements that rotate in orbits around the black hole, forming a disk that surrounds it. The ISCO is the region of the last stable circular orbit.

The main source of luminosity for the accretion disk is the gravitational energy released when gases in the disk spiral down toward the event horizon.

Jets are long, fast, narrow streams of matter that emerge from very compact regions around the black hole, usually in opposite directions, presumably normal to the plane of the accretion disk.

First image of a black hole

The first image of a black hole was published on April 10, 2019 by the multinational event horizon telescope group EHT ( Event Horizon Telescope ). Using eight radio telescopes distributed in different points of the Earth they managed to form a virtual telescope the size of the Earth. The observatories that participated in this stage were:

  • Atacama Pathfinder Experiment Observatory (APEX): altitude 5 100 m, Cerro Chajnantor, Chile.
  • Atacama Large Millimeter / submillimeter Array Observatory (ALMA): altitude 5,000 m, Cerro Chajnantor, Chile.
  • IRAM 30-meter telescope: altitude 2,850 m, Pico Veleta, Spain.
  • Telescope “James Clerk Maxwell”: altitude 4 100 m, Maunakea, Hawaii, USA
  • Telescope “Alfonso Serrano”: altitude 4 600 m, Sierra Negra, Mexico.
  • Submillimeter Array Observatory (SMA): altitude 4 100 m, Maunakea, Hawaii, USA
  • Submillimeter Telescope Observatory (SMT): Altitude 3 100 m, Mount Graham, Arizona, USA.
  • Observatory South Pole Telescope (SPT): altitude 2 800 m, South Pole Station, Antarctica.

The EHT project aims to observe the immediate environment of black holes. In this sense they chose two black holes:

  1. at the center of the Messier M87 galaxy, a giant elliptical galaxy located in the constellation Virgo.
  2. In the Milky Way the black hole Sagittarius A * (pronounced sagittarius A star).

How many black holes are there?

Black holes can be found in the centers of galaxies and in binary star systems , that is, systems of two stars orbiting in the same center.

Astronomers can detect black holes through disturbances in the X-ray emissions they produce in nearby stars. It is estimated that in our galaxy alone there may be more than ten million black holes.

For example, in the center of the Milky Way there is a black hole known as Sagittarius A *. It has a mass 4.3 million times that of our Sun and is 25,000 light years from Earth.

Why a black hole?

The existence of black holes introduced the concept of “invisibility” into physics . That is, for any observer outside the black hole, there is this region in which nothing is observed, an “invisible” region in space. However, it does not mean that there is nothing inside, or that it is a “hole” in space-time.

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