core
, in physicscore,in physics, the extremely dense central core of aatom.
The Nature of the Core
Composition
Atomic nuclei consist of two types of particles, protons and neutrons, which are collectively known as nucleons. ONEprotonit is simply the nucleus of an ordinary hydrogen atom, the lightest atom, and has a unit positive charge. ONEneutronis an uncharged particle of about the same mass as the proton. The number of protons in a given nucleus is the atomic number of that nucleus and determines which chemical substanceelementthe nucleus will consist of when surrounded by electrons.
The total number of protons and neutrons together in a nucleus is the atomic mass number of the nucleus. Two nuclei can have the same atomic number but different mass numbers, so they are different forms, orisotopes, of the same element. The mass number of a given isotope is the nearest whole number toindividual weightof that isotope and is approximately equal to the atomic weight (in the case of carbon-12, exactly equal).
Size and Density
The nucleus occupies only a tiny fraction of the volume of an atom (the radius of the nucleus is about 10,000 to 100,000 times smaller than the radius of the atom as a whole), but it contains almost all the mass. An idea of the extremedensityof the core is revealed by a simple calculation. The radius of the hydrogen nucleus is of the order of 10−13cm so that its volume is of the order of 10−39from3(cubic centimeter) its mass is about 10−24g (gram). Combining these to calculate the density, we get 10−24g/10−39from3≈ 1015g/cm3, or about one thousand trillion times the density of matter on ordinary scales (the density of water is 1 g/cm3).
Mass Defect, Binding Energy and Nuclear Reactions
When nuclear masses are measured, the mass is always found to be less than the sum of the masses of the individual nucleons bound to the nucleus. The difference between the nuclear mass and the sum of the individual masses is known as the mass defect and is due to the fact that part of the mass must be converted into energy to make the nucleus stable. This nuclear binding energy is related to the mass defect by the famous formula fromrelevance,m=mc2, wheremis energyMis mass, anddois the speed of light. The binding energy of a nucleus increases with increasing mass number.
A more interesting property of a nucleus is the binding energy per nucleon, found by dividing the binding energy by the mass number. The average binding energy per nucleon is observed to increase rapidly with increasing mass number up to a mass number of about 60, then to decrease rather slowly with higher mass numbers. Thus, nuclei with mass numbers around 60 are the most stable, and those with very small or very large mass numbers are the least stable.
Two important phenomena arise from this property of nuclei. Nuclear fission is the spontaneous splitting of a large-mass nucleus into two smaller-mass nuclei. Nuclear fusion, on the other hand, is the combination of two light nuclei to form a heavier single nucleus, again with an increase in the average binding energy per nucleon. In both cases, the change to a stable final state is accompanied by the release of a large amount of energy per unit mass of the reacting materials compared to the energy released in chemical reactions (seenuclear energy).
Models of the Core
Many models of the nucleus have been developed that fit some aspects of nuclear behavior, but no model has successfully described all aspects. One model is based on the fact that certain properties of a nucleus are similar to those of an incompressible liquid drop. The liquid drop model was particularly successful in explaining the details of the fission process and in developing a formula for the mass of a particular nucleus as a function of its atomic number and its mass number, the so-called semiempirical mass formula.
Another model is the Fermi gas model, which treats nucleons as if they were particles of a Pauli-confined gasprinciple of exclusion, which allows only two particles of opposite spin to occupy a specific energy level described byquantum theory. These pairs of particles will first fill the lowest energy levels and then successively higher ones, so that the "gas" is one of the lowest energy levels. There are actually two independent Fermi gases, one of protons and one of neutrons. The tendency of nucleons to occupy the lowest possible energy level explains why the numbers of protons and neutrons tend to be nearly equal in lighter nuclei. In heavier nuclei the effect of electrostatic repulsion between the greater number of charges than protons increases the energy of the protons, resulting in more neutrons than protons (for example, for uranium-235, there are 143 neutrons and only 92 protons). The combination of nucleons in energy levels also helps explain the tendency of nuclei to have even numbers of protons and neutrons.
However, neither the liquid drop model nor the Fermi gas model can explain the extraordinary stability of nuclei that have certain values of either the number of protons or the number of neutrons or both. These so-called magic numbers are 2, 8, 20, 28, 50, 82 and 126. Because of the similarity between this phenomenon and the stability of the noble gases, which have certain numbers of electrons bound in closed "shells", it was proposed a shell model for the nucleus. However, there are big differences between the electrons in an atom and the nucleons in a nucleus. First, the nucleus provides a center of force for an atom's electrons, while the nucleus itself does not have a single center of force. Second, there are two different types of nucleons. Third, the assumption of independent particle motion that is made in the case of electrons is not so easily made for nucleons. The liquid drop model is actually based on the assumption of strong forces between nucleons that significantly restrict their motion. However, these difficulties were resolved and a good explanation of the magic numbers was achieved based on the shell model, which included the assumption of strong coupling between the spin angular momentum of a nucleon and its orbital angular momentum. Various attempts have been made, with partial success, to construct a model that incorporates the best features of both the liquid drop model and the shell model.
Scientific Note on the Nucleus and Nuclear Reactions
A nucleus can be easily represented by the chemical symbol for the element along with a subscript and an exponent for the atomic number and mass number. (The subscript is often omitted, since the symbol of the element determines the atomic number.) The nucleus of ordinary hydrogen, i.e. the proton, is represented by1H1, an alpha particle (helium nucleus) is2He4, the most common isotope of chlorine is17Cl35, and the uranium isotope used in the atomic bomb is92U235.
Nuclear reactions involving changes in atomic number or mass number can be easily expressed using this notation. For example, when ErnestRutherfordproduced the first artificial nuclear reaction (1919), involved bombarding a nitrogen nucleus with alpha particles and resulted in an oxygen isotope with the release of a proton:2He4+7N14→8THE17+1H1. Note that the sum of the atomic numbers on the left is equal to the sum on the right (ie, 2+7=8+1), and likewise for the mass numbers (4+14=17+1).
Scientific Research of the Core
After its discoveryradioactivityby A. H. Becquerel in 1896, Ernest Rutherford identified two types of radiation emitted by naturally occurring radioactive substances and named them alpha and beta. A third, gamma, was identified later. In 1911 he bombarded a thin gold foil target with alpha rays (later identified as helium nuclei) and found that although most of the alpha particles passed directly through the foil, some were deflected by large amounts. By a quantitative analysis of his experimental results, he was able to suggest the existence of the nucleus and estimate its size and charge.
After the discovery of the neutron in 1932, physicists turned their attention to understandingstrong interactions, or strong nuclear force, which binds protons and neutrons together in nuclei. This force must be large enough to overcome the significant repulsive force that exists between many protons due to their electric charge. It must exist between nucleons regardless of their charge, as it acts equally on protons and neutrons, and must not extend too far from the nucleons (i.e. it must be a short-range force), as it has negligible effect on protons or neutrons outside from the core.
In 1935 Hideki Yukawa proposed a theory that this nuclear "glue" was produced by the exchange of a particle between nucleons, just as the electromagnetic force is produced by the exchange of aphotonbetween charged particles. The magnitude of a force depends on the mass of the particle carrying the force. The larger the mass of the particle, the smaller the range of the force. The range of the electromagnetic force is infinite because the mass of the photon is zero. From the known range of the nuclear force, Yukawa calculated that the mass of the hypothetical carrier of the nuclear force is about 200 times that of the electron. Given the namemesonbecause its mass is between that of the electron and those nucleons, this particle was finally observed in 1947 and is now called the pi meson ormore, to distinguish it from other mesons that have been discovered (seeelementary particles).
Both the proton and the neutron are surrounded by a cloud of ions that are emitted and reabsorbed within an incredibly short period of time. Some other mesons are supposed to be created and destroyed in this way as well, all these particles are called "virtual" because they exist in violation of the law of conservation of energy (seeconservation laws) for a very short time allowed by theprinciple of uncertainty. It is now known, however, that at a more fundamental level the real carrier of the strong force is a particle called a gluon.
Bibliography
See G. Gamow,The atom and its core(1961); R. K. Adair,The Great Design: Particles, Fields and Creation(1987).
The Columbia Electronic Encyclopedia™ Copyright © 2022, Columbia University Press. With permission from Columbia University Press. All rights reserved.
core
(young-klee-ŭs)1.The kilometer-sized dark body that is the permanent part of acometand is thought by most researchers to be the source of all comet activity. The density is about 0.2 g cm–3. Contains about 75% by mass ice (mostly water ice, 85% but with free amounts of CO2, CO., HOUSE2CO, ONLY3OH, and NH3) and 25% by mass of dust believed to have a composition similar to carbonate chondrite meteorites (cf.carbonaceous chondrite). Often described as adirty snowball. A comet of mass 1018grams would have a core about 12 km in diameter. A positive identification of the cometary nucleus using radar has recently been achieved. The nucleus of the cometHalleyillustrated by theGiottospacecraft in March 1986, and it was found to be a huge potato-shaped object, 16 km long and 8 km wide. It was only active over 10% of its surface and rotated every 54 hours, advancing every 7.4 days. A comet's dirty ice core is surrounded by a thin layer (a few centimeters) of insulating dust from which the ice has sublimated. This enables it to survive over 2000 perihelion passes. On each pass the core loses an average of one meter thick layer of material. In Halley's Comet this amounted to 3 × 1014G. This material forms thecomaandcomet tailsand also any relatedmeteoric stream.
2.The small core of oneatom, which consists ofprotonsandneutronslinked together by strong nuclear forces. The nucleus has a positive charge equal toShe, wheremis the magnitude of the electron charge andGthe number of protons present – the atomic number. The total number of protons plus neutrons is the mass number,ONE. A given element is characterized by its atomic number, but may, within limits, have different numbers of neutrons in its nuclei, giving rise to differentisotopesof the element. The total mass of protons and neutrons bound together in a nucleus is less than when the particles are unbound. This mass difference is equivalent to the energy required to bind the particles together. Nuclei are represented by their chemical symbols to which numbers are attached. usually the mass number is added as a left superscript to indicate a particular isotope, as in4He. Nuclei radii are usually measured in femtometers (1 fm = 10–15M). The femtometer is sometimes referred to as "fermi".
3.The central region of a galaxy.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is fromThe Great Soviet Encyclopedia(1979). It may be outdated or ideologically biased.
Core
in beekeeping, a small colony of bees with a young virgin queen or spare mated queen.
Cores are widely used in beekeeping operations engaged in breeding queens. Kernels are also used to maintain spare mated queens to replace those that die in the spring. To form a nucleus, frames with honey, bee bread and brood are placed in hives divided into three or four compartments by dividers or in side compartments of hives with ordinary bee colonies. Flats are inhabited by young flightless bees (small numbers come from strong colonies) and a virgin queen is presented. After the queen has mated with a drone and begins to lay eggs, she is removed and another virgin queen is placed in her place to be fertilized by the drones, or a new batch of mature queen cells is placed in the nucleus. In this way, two to four mated queens can be obtained from one nucleus in one summer season.
smallH. GASANOV
Core
a body within a cell that, along with the cytoplasm, is an essential cellular component in protozoa and multicellular animals and plants.
The nucleus contains chromosomes and the products of chromosomal activity. All organisms are divided into eukaryotes and prokaryotes, depending on whether their cells are nucleated or not, respectively. Although prokaryotes do not have a formed nucleus (the membrane is absent), deoxyribonucleic acid (DNA) is present. The nucleus stores most of the cell's genetic information. Genes contained in chromosomes play an important role in the transmission of hereditary characters in certain cells and organisms. The nucleus interacts closely and continuously with the cytoplasm. The messenger molecules that carry genetic information to the protein synthesis centers in the cytoplasm are synthesized in the nucleus. Thus, the nucleus controls the synthesis of all proteins and, through them, all physiological processes in the cell. Therefore, non-nucleated cells and cell fragments obtained experimentally always die. nuclear transplantation into such cells restores viability. The Czech scientist J. Purkinje was the first to observe the nucleus in a chicken egg (1825). The British scientist R. Brown was the first to describe the nucleus in plant cells (1831-33), and the German scientist T. Schwann was the first to describe the nucleus in animal cells (1838-39).
A cell usually has a nucleus, which is located near the center and usually has the appearance of a spherical or ellipsoidal vesicle. Sometimes the nucleus has an irregular or complex shape, for example, the nuclei of leukocytes and the macronuclei of injectors. Binucleate and multinucleate cells are common. they are usually formed by nuclear division without accompanying cytoplasmic division or by a symptom, that is, the fusion of many mononuclear cells (for example, striated muscle fibers). Nuclei vary in size from about 1 micrometer (μm) in some protozoa to about 1 mm in some eggs.
The nucleus is separated from the cytoplasm by a nuclear membrane consisting of two parallel membranes of lipoprotein units that are 7–8 nanometers (nm) thick and have a narrow perinuclear space between them. The nuclear membrane is perforated by pores 60–100 nm in diameter. the outer membrane is continuous with the inner membrane at the edges of the pores. The number of pores varies from cell to cell: from a few to 100–200 per square µm of nuclear surface. Along the edge of a pore is a ring of solid material, the annulus. The lumen of a pore often contains a central granule that is 15–20 nm in diameter and is joined to the annulus by radial fibers. The fibers and the pore form the pore complex, which apparently regulates the passage of macromolecules through the nuclear membrane, for example, the entry of protein molecules into the nucleus and the exit of ribonucleoprotein particles from the nucleus. The outer membrane is continuous in some places with the membranes of the endoplasmic reticulum. It usually contains protein-synthesizing particles known as ribosomes. The inner membrane sometimes forms vacuoles deep in the nucleus.
The nucleus consists of nuclear fluid (karyolymph, karyoplasm), which contains formed elements, including nucleoli and chromatin. Chromatin is a more or less irregular clump of chromosomes found in a non-dividing nucleus. Composed of DNA and proteins linked together in a complex called deoxyribonucleoprotein (DNP), chromatin can be detected by the Feulgen test. When a nucleus divides, all the chromatin is condensed into chromosomes. When mitosis ends, many parts of the chromosomes relax again. these segments (euchromatin) contain mostly unique (non-repetitive) genes. The other parts of the chromosomes remain compact (heterochromatin). This is where the repetitive DNA sequences are arranged. In a nondividing nucleus much of the euchromatin consists of a loose network of DNP fibers 10–30 nm thick. heterochromatin contains dense masses, or chromocenters, in which DNP fibers are tightly packed. Some of the euchromatin may also become compact. such euchromatin is considered inactive with respect to DNA synthesis. Chromocenters usually border the nuclear membrane or nucleus. Some data indicate that DNP fibers are attached to the inner nuclear membrane.
DNA synthesis or replication takes place in a non-dividing nucleus. The process can be studied by labeling the DNA precursors (usually thymidine) contained in the nucleus with radioisotopes. Many segments, or transcripts, have been shown to exist along chromatin fibers. Each copy has its own starting point for DNA synthesis from which replication spreads in both directions. Chromosomes themselves are duplicated as a result of DNA replication.
Sensing of the genetic information encoded in DNA takes place in nuclear chromatin by the synthesis from DNA of messenger ribonucleic acid (mRNA) molecules and other types of RNA molecules involved in protein synthesis. Specific regions of chromosomes and, consequently, chromatin contain repetitive genes that code for ribosomal RNA. Nuclei rich in ribonucleoproteins (RNPs) form in these regions. The main function of RNP is to synthesize the RNA that makes up ribosomes.
The nucleus contains other types of RNA particles besides those in the nucleus. For example, perichromatin fibers 3–5 nm thick and perichromatin granules 40–50 nm in diameter are found at the boundaries of loose and compact chromatin. Both probably contain messenger RNA in combination with proteins. perichromatin granules are its inactive form. Perichromatin granules have been observed to leave the nucleus and enter the cytoplasm through nuclear membrane pores. Diachromatin granules (20–25 nm) and, sometimes, thick (40–60 nm) coiled RNP filaments are also present. Amoeba nuclei contain helical RNP filaments (30–35 nm × 300 nm). helices may enter the cytoplasm and possibly contain messenger RNA. In addition to structures containing DNA and RNA, some nuclei have purely proteinaceous inclusions in the form of spheres (for example, in the nuclei of developing eggs of many animals and the nuclei of many protozoa) or bundles of fibers or crystalloids (for example, in the nuclei of many cells in animal and plant tissues and in the macronuclei of some injectors). The nucleus may also contain phospholipids, lipoproteins, and enzymes (DNA polymerase, RNA polymerase, and the nuclear membrane enzyme complex, including adenosine triphosphatase).
A variety of specific types of nuclei are found in nature: giant nuclei in developing eggs (especially of fish and amphibians), nuclei containing giant polytene chromosomes (for example, those in dipteran salivary gland cells), compact nuclei of enucleated spermatozoa, macronuclei of injectors that are solids filled with chromatin and not synthesizing RNA, nuclei whose chromosomes condense continuously although nuclei form (as in some protozoa and some insect cells), and nuclei whose number of sets of chromosomes doubles or more (polyploidy).
The main method by which a nucleus divides is mitosis, which is characterized by duplication and condensation of chromosomes, destruction of the nuclear membrane (except in many protozoa and fungi), and proper separation of sister chromosomes in daughter cells. However, the nuclei of some specialized cells, especially polyploid cells, may divide by simple amitosis. Highly polyploid nuclei can divide not only into two but into many parts and can also divide by budding. This can result in division of whole sets of chromosomes or separation of genomes.
BIBLIOGRAPHICAL REFERENCES
Rukovodstvo po tsitologii, vol. 1. Moscow-Leningrad, 1965.
Raikov, Ι.Β.Kariologiia prosteishikh. Leningrad, 1967.
DeRobertis, Ε., W. Nowinsky, and F. Saez.Biologiia kletki. Moscow, 1973. (Translated from English.)
Chentsov, Iu. S., and V. Iu. Poliakov.Ul'trastruktura kletochnogo iadra. Moscow, 1974.
The core. Edited by A. J. Dalton and F. Haguenau. New York-London, 1968.
The nucleus of the cell, volumes. 1–3. Edited by H. Busch. New York-London, 1974.
I. B. RAIKOV
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
core
[′nü·kle·as](astronomy)
The small permanent body of a comet, believed to be one to several tens of kilometers in diameter and composed of water and volatile hydrocarbons.
(cell and molecular biology)
A small mass of differentiated protoplasm rich in nucleoproteins and surrounded by a membrane. found in most animal and plant cells, contains chromosomes, and functions in metabolism, growth, and reproduction.
(computer science)
This part of the control program that must always be present in main storage.
The main storage area used by the kernel (first definition) and other transient control program routines.
(hydrology)
A particle of any nature on which, or a place in which, water or ice molecules accumulate as a result of a phase change to a more condensed state.
(neuroscience)
A mass of nerve cells in the central nervous system.
(nuclear physics)
The central, positively charged, dense part of an atom. Also known as atomic nucleus.
(science and technology)
A central mass for which accretion takes place.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
core
In ancient construction, the inner part of the floor, consisting of strong cement, over which the pavement was laid, bound with mortar.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by The McGraw-Hill Companies, Inc.
core
1.Biology(in eukaryotic cells) a large compartment, bounded by a double membrane, that contains the chromosomes and associated molecules and controls the characteristics and growth of the cell
2.Anatomyany of the various groups of nerve cells in the central nervous system
3.Astronomythe central part at the head of a comet, consisting of small solid particles of ice and frozen gases, which evaporate as they approach the sun to form the coma and tail
4.Physicsthe positively charged dense region in the center of an atom, made up of protons and neutrons, around which the electrons revolve
5.Chema fundamental group of atoms in a molecule that serves as the basic structure for related compounds and remains unchanged during most chemical reactions
6.Botanythe central point of a starch grain
7.Logicthe largest individual that is a mere part of any member of a given class
Collins Discovery Encyclopedia, 1η έκδοση © HarperCollins Publishers 2005
FAQs
What is the function of the anterior thalamic nucleus? ›
The anterior thalamic nuclei are a vital node within hippocampal-diencephalic-cingulate circuits that support spatial learning and memory. Reflecting this interconnectivity, the overwhelming focus of research into the cognitive functions of the anterior thalamic nuclei has been spatial processing.
What happens if thalamus is damaged? ›Damage to your thalamus can result in: Unconsciousness and even coma. Sleep disorders, such as insomnia and fatal familial insomnia (inability to sleep, leading to death). Thalamic aphasia (jumbled words, meaningless speech).
Which thalamic nuclei is most important for vision? ›In the visual system, the lateral geniculate nucleus (LGN) of the dorsal thalamus is the gateway through which visual information reaches the cerebral cortex.
What does the anterior thalamus do for emotion? ›Human anterior thalamic nuclei (ANT) are involved in emotion–attention interaction. ANT–DBS has an effect on emotion–attention interaction. ANT–DBS leads to greater attentional capture by threat-related emotional stimuli. Emotional modulation of N2–P3 provides a biomarker for emotion–attention interplay.
What information does the anterior nucleus of the thalamus receive? ›Anterior nuclei of thalamus
These nuclei receive information from the limbic system, thus having important functions and influence upon emotional states, such as attention and alertness and memory acquisition.
The thalamus is composed of different nuclei that each serve a unique role, ranging from relaying sensory and motor signals, as well as regulation of consciousness and alertness.
Can you live without your thalamus? ›1 The precise functions of these nuclei have been elusive, although it is clear that they must be very important given the dire consequences of damage to them. To destroy the thalamus is to kill; a person cannot live without a thalamus although people and other animals can do quite well without major chunks of cortex.
Can you recover from thalamus damage? ›Most patients recover to a significant degree within 6 months of the ictus [8],[38], although several patients with persistent aphasic deficits after focal thalamic lesions have been described [9],[35],[36].
Which part of thalamus is related to memory? ›Two subregions of the human thalamus most often implicated in long-term memory are the anterior thalamic nucleus (AT) and mediodorsal nucleus (MD) [1].
Is the thalamus responsible for memory? ›The thalamus, with its cortical, subcortical, and cerebellar connections, is a critical node in networks supporting cognitive functions known to decline in normal aging, including component processes of memory and executive functions of attention and information processing.
What are the 3 main categories of thalamic nuclei? ›
The thalamus is a paired structure located in the center of the brain. Each side can divide into three groups of thalamic nuclei: a lateral nuclear group, a medial nuclear group, and an anterior nuclear group.
What are the thalamic disorders? ›Disorders of the centrally located thalamus, which integrates a wide range of cortical and subcortical information. Manifestations include sensory loss, MOVEMENT DISORDERS; ATAXIA, pain syndromes, visual disorders, a variety of neuropsychological conditions, and COMA.
Which thalamic nucleus is for hearing? ›The main auditory-responsive portion of the thalamus is called the medial geniculate body (MGB), and it is the information bottleneck for neural representations of sounds being sent to auditory cortex.
What are thalamic nuclei and functions? ›The thalamus is a mostly gray matter structure of the diencephalon that has many essential roles in human physiology. The thalamus is composed of different nuclei that each serve a unique role, ranging from relaying sensory and motor signals, as well as regulation of consciousness and alertness.
Where is the anterior nucleus of the thalamus? ›The anterior nuclear group (ANT) occupies the superior region of the thalamus and is separated from the rest of the thalamus by the anterior internal medullary lamina, resulting in a Y-shaped lamina.
Is the anterior nucleus of the thalamus in the limbic system? ›The anterior nuclear group (ANT) located in the rostral one-third of the thalamus is considered to be a significant part of the limbic system and a component of the circuit of Papez10 as it has extensive hippocampal–anterior thalamic interconnections11,12,13. The ANT is easy to identify (s. Fig.