The substance of our Universe is structurally organized and forms a wide variety of phenomena of various scales with very different physical properties. One of the most important of these properties is temperature. Knowing this indicator and using theoretical models, one can judge many characteristics of a particular body - its condition, structure, age.
The scatter of temperature values among the various observable components of the Universe is very large. So, its lowest value in nature was recorded for the Boomerang nebula and is only 1 K. And what are the highest temperatures in the Universe known to date, and what features of various objects testify to? First, let's see how scientists determine the temperature of distant cosmic bodies.
Spectra and temperature
Scientists receive all information about distant stars, nebulae, and galaxies by examining their radiation. According to the frequency range of the spectrum, the maximum radiation occurs, the temperature is determined as an indicator of the average kinetic energy possessed by the particles of the body, because the radiation frequency is directly related to energy. So the highest temperature in the Universe should reflect, respectively, the highest energy.
The higher frequencies the maximum radiation intensity is characterized, the hotter the body under investigation. However, the full spectrum of radiation is distributed over a very wide range, and according to the features of its visible region (“color”), certain general conclusions can be drawn about the temperature of, for example, stars. The final assessment is based on the study of the entire spectrum, taking into account emission and absorption bands.
Spectral classes of stars
On the basis of spectral features, including color, the so-called Harvard classification of stars was developed. It includes seven main classes, denoted by the letters O, B, A, F, G, K, M and several additional ones. The Harvard classification reflects the surface temperature of stars. The sun, whose photosphere is heated to 5780 K, belongs to the class of yellow stars G2. The hottest blue stars of class O, the coldest - red - belong to class M.
The Harvard classification is supplemented by the Yerksky classification, or the Morgan-Kinan-Kellman classification (MKK according to the names of the developers), dividing the stars into eight luminosity classes from 0 to VII, closely related to the mass of the star - from hypergiants to white dwarfs. Our Sun is a class V dwarf.
Used together as axes along which color - temperature and absolute value - luminosity (indicating mass) are plotted, they made it possible to construct a graph, commonly known as the Hertzsprung-Russell diagram, which shows the main characteristics of the stars in their relationship.
The hottest stars
From the diagram it appears that the hottest are the blue giants, supergiants and hypergiants. These are extremely massive, bright and short-lived stars. Thermonuclear reactions in their bowels proceed very intensively, giving rise to monstrous luminosity and the highest temperatures. Such stars belong to classes B and O, or to a special class W (differs in broad emission lines in the spectrum).
For example, This Ursa Major (located at the “end of the handle” of the bucket) with a mass 6 times higher than the sun shines 700 times more powerful and has a surface temperature of about 22,000 K. The Zeta Orion has an Alnitak star, which is 28 times more massive than the Sun times, the outer layers are heated to 33,500 K. And the temperature of the hypergiant with the highest known mass and luminosity (at least 8.7 million times more powerful than our Sun) - R136a1 in the Large Magellanic Cloud - is estimated at 53,000 K.
However, the photospheres of stars, no matter how hot they are, will not give us an idea of the highest temperature in the Universe. In search of hotter areas you need to look into the bowels of the stars.
Space fusion furnaces
Really high temperatures, sufficient for nucleosynthesis of elements up to iron and nickel, develop in the nuclei of massive stars squeezed by colossal pressure. Thus, calculations for blue giants, supergiants, and very rare hypergiants give for this parameter an order of magnitude 10 9 K - a billion degrees by the end of a star’s life.
The structure and evolution of such objects is still not well studied, respectively, and their models are far from complete. It is clear, however, that all large-mass stars must possess very hot nuclei, no matter what spectral class they belong to, for example, red supergiants. Despite the undoubted differences in the processes occurring in the bowels of stars, mass is the key parameter determining the temperature of the nucleus.
Star remnants
In general, the fate of a star also depends on mass - how it will end its life path. Low-mass stars like the Sun, having exhausted the supply of hydrogen, lose their outer layers, after which a degenerate core remains from the luminary, in which thermonuclear fusion can no longer occur — a white dwarf. The outer thin layer of a young white dwarf usually has a temperature of up to 200,000 K, and the isothermal core, heated to tens of millions of degrees, is located deeper. The further evolution of the dwarf consists in its gradual cooling.
Giant stars will have a different fate - a supernova explosion, accompanied by an increase in temperature already to values of the order of 10 11 K. During the explosion, nucleosynthesis of heavy elements becomes possible. One of the results of this phenomenon is a neutron star - a very compact, superdense, with a complex structure, the remainder of a dead star. At birth, it is just as hot - up to hundreds of billions of degrees, but it cools rapidly due to intense neutrino emission. But, as we will see later, even a newborn neutron star is not the place where the temperature is the highest in the Universe.
Distant exotic objects
There is a class of space objects that are quite distant (and therefore ancient), characterized by completely extreme temperatures. These are quasars. According to modern views, a quasar is a supermassive black hole with a powerful accretion disk formed by a substance falling on it in a spiral - gas or, more precisely, plasma. Actually, this is an active galactic nucleus in the formation stage.
The plasma velocity in the disk is so high that due to friction it is heated to ultra-high temperatures. Magnetic fields collect radiation and part of the disk matter into two polar beams - jets, emitted by the quasar into space. This is an extremely high energy process. The luminosity of a quasar is on average six orders of magnitude higher than the luminosity of the most powerful star R136a1.
Theoretical models allow an effective temperature for quasars (that is, inherent in a completely black body radiating with the same brightness) to be no more than 500 billion degrees (5 × 10 11 K). However, recent studies of the nearest quasar 3C 273 led to an unexpected result: from 2 × 10 13 to 4 × 10 13 K - tens of trillions of Kelvin. This value is comparable with temperatures achieved in phenomena with the highest known energy release - in gamma-ray bursts. Today it is the highest temperature in the universe that has ever been recorded.
Hotter than all
It should be borne in mind that we see the quasar 3C 273 as it was about 2.5 billion years ago. So, given that the further we look into space, the more distant eras of the past we observe, in search of the hottest object, we have the right to look around the Universe not only in space, but also in time.
If we return to the very moment of her birth - about 13.77 billion years ago, which is impossible to observe - we will find a completely exotic Universe, in the description of which cosmology comes to the limit of its theoretical capabilities, associated with the limits of applicability of modern physical theories.
The description of the Universe becomes possible, starting from the age corresponding to the Planck time of 10 -43 seconds. The hottest object in this era is our Universe itself, with a Planck temperature of 1.4 × 10 32 K. And this, according to the modern model of its birth and evolution, is the maximum temperature in the Universe of all ever achieved and possible.