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O

Definition: O

O

Noun

1. The blood group whose red cells carry neither the A nor B antigens; "people with type O blood are universal donors".

2. A nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust.

3. The 15th letter of the Roman alphabet.

Source: WordNet 1.7.1 Copyright © 2001 by Princeton University. All rights reserved.
 

Date "O" was first used in popular English literature: sometime before 1010. (references)

Note: O \O\ ([=o]), noun; plural O'sor Oes([=o]z). 1. The letter O, or its sound. ``Mouthing out his hollow oes and aes.'' --Tennyson. 2. Something shaped like the letter O; a circle or oval. ``This wooden O [Globe Theater]''. --Shakespeare 3. A cipher; zero. [Rare]. (references)

 

Specialty Definition: O

DomainDefinition

Computing

O ASCII code 79, The letter of the alphabet, not to be confused with 0 (zero) the digit. (1999-02-07). Source: The Free On-line Dictionary of Computing.

Health

A drug that may improve the response of cancer cells to chemotherapy. (references)

Literature

O This letter represents an eye, and is called in Hebrew ain (an eye).
O The fifteen O's are fifteen prayers beginning with the letter O (See Hora Beatissima Virginis Mariae.)
The Christmas O's. For nine days before Christmas (at 7 o'clock p.m.) are seven antiphones (3 syl.), each beginning with O, as O Sapientia, O Radix, etc. Source: Brewer's Dictionary.

Tips from 1870

Usage: O, Oh. While good usage is far from uniform, many excellent authors employ O only in cases of direct address and oh when strong and sudden emotion is to be expressed. O is always written with a capital letter, and should be followed by the name of the person or thing addressed, and the exclamation or interrogation point placed at the end of the sentence; as, "O Death, where is thy sting? O Grave, where is thy victory?" "O the cold and cruel winter!"
Oh in the body of a sentence may begin with a small letter, and is immediately followed by the exclamation point; as, "Oh! how terrible was his fate!" "The sad intelligence was gently given, but oh! the shock was almost unbearable." Source: Slips of Speech.

Source: compiled by the editor from various references; see credits.

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Specialty Definition: Aozora Bunko: O

(From Wikipedia, the free Encyclopedia)

See Aozora Bunko

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Big O notation

(From Wikipedia, the free Encyclopedia)

Big O notation (with a capital letter O -- originally an omicron -- not a zero), also called Landau's symbol, is a symbolism used in complexity theory, computer science, and mathematics to describe the asymptotic behavior of functions. It indicates how fast a function grows or declines.

Landau's symbol comes from the name of the German number theorist Edmund Landau who invented the notation. The letter O is used because the rate of growth of a function is also called its order.

For example, when analyzing some algorithm, one might find that the time (or the number of steps) it takes to complete a problem of size n is given by T(n) = 4 n2 − 2 n + 2. If we ignore constants (which makes sense because those depend on the particular hardware the program is run on) and slower growing terms, we could say "T(n) grows at the order of n2" and write:T(n) = O(n2).

In mathematics, it is often important to get a handle on the error term of an approximation. For instance, one may write

to express the fact that the error is smaller in absolute value than some constant times x3 if x is close enough to 0.

For the formal definition, suppose f(x) and g(x) are two functions defined on some subset of the real numbers. We write

f(x) = O(g(x)) as x → ∞
if and only if there exist constants N and C such that
|f(x)| ≤ C |g(x)|    for all x > N.
Intuitively, this means that f does not grow faster than g.

If a is some real number, we write

f(x) = O(g(x))     for x -> a
if and only if there exist constants d > 0 and C such that
|f(x)| ≤ C |g(x)|    for all x with |x-a| < d.

The first definition is the only one used in computer science (where typically only positive functions with a natural number n as argument are considered; the absolute values can then be ignored), while both usages appear in mathematics.

Here is a list of classes of functions that are commonly encountered when analyzing algorithms. The slower growing functions are listed first. c is some arbitrary constant.

notationname
O(1)constant
O(log(n))logarithmic
O((log(n))c)polylogarithmic
O(n)linear
O(n log(n))sometimes called "linearithmic"
O(n2)quadratic
O(nc)polynomial, sometimes "geometric"
O(cn)exponential
O(n!)factorial

Note that O(nc) and O(cn) are very different. The latter grows much, much faster, no matter how big the constant c is. A function that grows faster than any power of n is called superpolynomial. One that grows slower than an exponential function of the form cn is called subexponential. An algorithm can require time that is both superpolynomial and subexponential; examples of this include the fastest algorithms known for integer factorization.

Note, too, that O(log n) is exactly the same as O(log(nc)). The logarithms differ only by a constant factor, (since log(nc)=c log(n)) and thus the big O notation ignores that. Similarly, logs with different constant bases are equivalent.

The above list is useful because of the following fact: if a function f(n) is a sum of functions, one of which grows faster than the others, then the faster growing one determines the order of f(n). Example: If f(n) = 10 log(n) + 5 (log(n))3 + 7 n + 3 n2 + 6 n3, then f(n) = O(n3). One caveat here: the number of summands has to be constant and may not depend on n.

This notation can also be used with multiple variables and with other expressions on the right side of the equal sign. The notation:

f(n,m) = n2 + m3 + O(n+m)
represents the statement:
∃C ∃N ∀n,m>N : f(n,m)≤n2+m3+C(n+m)

Obviously, this notation is abusing the equality symbol, since it violates the axiom of equality: "things equal to the same thing are equal to each other". To be more formally correct, some people (mostly mathematicians, as opposed to computer scientists) prefer to define O(g(x)) as a set-valued function, whose value is all functions that do not grow faster then g(x), and use set membership notation to indicate that a specific function is a member of the set thus defined. Both forms are in common use, but the sloppier equality notation is more common at present.

Another point of sloppiness is that the parameter whose asymptotic behaviour is being examined is not clear. A statement such as f(x,y) = O(g(x,y)) requires some additional explanation to make clear what is meant. Still, this problem is rare in practice.

Related notations

In addition to the big O notations, another Landau symbol is used in mathematics: the little o. Informally, f(x) = o(g(x)) means that f grows much slower than g and is insignificant in comparison.

Formally, we write f(x) = o(g(x)) (for x -> ∞) if and only if for every C>0 there exists a real number N such that for all x > N we have |f(x)| < C |g(x)|; if g(x) ≠ 0, this is equivalent to limx→∞ f(x)/g(x) = 0.

Also, if a is some real number, we write f(x) = o(g(x)) for x -> a if and only if for every C>0 there exists a positive real number d such that for all x with |x - a| < d we have |f(x)| < C |g(x)|; if g(x) ≠ 0, this is equivalent to limx -> a f(x)/g(x) = 0.

Big O is the most commonly used of five notations for comparing functions:

Notation Definition Analogy
f(n) = O(g(n)) see above ≤
f(n) = o(g(n)) see above <
f(n) = Ω(g(n)) g(n)=O(f(n)) ≥
f(n) = ω(g(n)) g(n)=o(f(n)) >
f(n) = Θ(g(n)) f(n)=O(g(n)) and g(n)=O(f(n)) =

The notations Θ and Ω are often used in computer science; the lower-case o is common in mathematics but rare in computer science. The lower-case ω is rarely used.

A common error is to confuse these by using O when Θ is meant. For example, one might say "heapsort is O(n log n) in average case" when the intended meaning was "heapsort is Θ(n log n) in average case". Both statements are true, but the latter is a stronger claim.

Another notation sometimes used in computer science is Õ (read Soft-O).
f(n) = Õ(g(n)) is shorthand for f(n) = O(g(n) logkn) for some k. Essentially, it is Big-O, ignoring logarithmic factors.

The notations described here are used for approximating formulas (e.g. those in the sum article), for analysis of algorithms (e.g. those in the heapsort article), and for the definitions of terms in complexity theory (e.g. polynomial time).

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Big O notation."

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Casualties of the September 11, 2001 Terrorist Attacks

(From Wikipedia, the free Encyclopedia)

Any tributes to the individuals lost in this tragedy are welcome and encouraged at our memorial site. Some articles originally posted to wikipedia have been moved there - if you are looking for such an article, please check there.

See also Missing Persons, Foreign casualties, and Survivors.

Casualties

Planes - World Trade Center - Pentagon
A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

As of October 29, 2003, 2,995 people were presumed dead as a result of all four September 11 attacks. This includes the casualties at the World Trade Center, the Pentagon, on the airplanes and the hijackers.

Planes

265 people killed on four planes; 232 passengers, 25 flight attendants, 8 pilots. (Note that this total includes the 19 hijackers, who reportedly boarded the planes as passengers.)

See also: Memorial wiki tributes to the occupants of each plane

World Trade Center

By October 29, 2003, 2605 people were listed as confirmed dead and 1058 bodies had been identified. (Note: this total does not include the 127 passengers and 20 crew on the two aircraft or the 10 hijackers).

The listing and memorial.

See also:

Missing Persons

The number of missing people grew to estimates as high as over 6000 in the months following the attack, but steadily declined as stories were checked and duplicate entries removed. (See Timeline of WTC missing).

As of August 2002, there were approximately 90 people who were officially missing; that is, their remains had not been identified and no family members had requested a death certificate.

Detailed listing.

Survivors

The great majority of the over 40,000 people working at the World Trade Center at the time of the attack evacuated safely, including 18 who escaped from above the impact zone in the second tower hit. By 9/20/2001 6291 people, including rescue and recovery workers, had been treated for injuries.

Detailed listing.

Pentagon

The Pentagon reports 125 staffers killed or missing, with 121 remains recovered and identified, as of Sept. 11, 2002. At least one person died later as a result of wounds incurred.

The listing and memorial.

Missing Persons

The Pentagon reports 4 staffers missing. One passenger on the airliner which hit the Pentagon was also never identified.

Detailed listing.

Survivors

88 treated at hospital.

Detailed entry.

Victim legends

Due to the very large number of World Trade Center casualties and missing persons, victim legends were a common form of September 11, Terrorist Attack urban legends. These were tales of victims who did not exist, spread by word-of-mouth and the Internet. Official sites, such as http://www.september11victims.com, contain accurate entries and are trusted content. Because Wikipedia, and many other websites allowed freely adding victims, there were no doubt many obvious fake entries. Fake victims added to these lists were often simply missing at the time of the attacks, or actually survivors of the attacks.

See also

September 11, 2001 Terrorist Attack - Donations - Assistance - Memorials and Services

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Casualties of the September 11, 2001 Terrorist Attacks."

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List of airports: O

(From Wikipedia, the free Encyclopedia)

List of airports: A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

O

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List of Biblical names starting with O

(From Wikipedia, the free Encyclopedia)

List of Biblical names
A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - Y - Z

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List of books by title: O

(From Wikipedia, the free Encyclopedia)

List of books in alphabetical order by title:

A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

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List of cities in Germany starting with O

(From Wikipedia, the free Encyclopedia)

List of cities in Germany: A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

TownPopulationDistrictBundesland
Oberhausen223,400--North Rhine-Westphalia
Offenbach116,200--Hesse
Oldenburg153,500--Lower Saxony
Olsberg16,263HochsauerlandNorth Rhine-Westphalia
Oranienburg30,200OberhavelBrandenburg
Osnabrück259,800--Lower Saxony
Osterholz-Scharmbeck31,200OsterholzLower Saxony
Osterode26,500OsterodeLower Saxony

A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "List of cities in Germany starting with O."

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List of colleges and universities starting with O

(From Wikipedia, the free Encyclopedia)

A -- B -- C -- D -- E -- F -- G -- H -- I -- J -- K -- L -- M -- N -- O -- P -- Q -- R -- S -- T -- U -- V -- W -- X -- Y -- Z
  1. ORT Uruguay
  2. Oakland University
  3. Oberlin College
  4. Occidental College
  5. Odense University
  6. Odessa College
  7. Odessa State Politechnic University
  8. Oglethorpe University
  9. Ohio College of Podiatric Medicine
  10. Ohio Northern University
  11. Ohio State University (Columbus, Ohio)
  12. Ohio University
  13. Ohio Wesleyan University
  14. Oita University
  15. Okanagan University College
  16. Okayama Prefectural University
  17. Oklahoma Baptist University
  18. Oklahoma City University
  19. Oklahoma State University
  20. Old Dominion University
  21. Olin College of Engineering
  22. Olivet Nazarene University
  23. Omega School of Theology
  24. Onondaga Community College
  25. Ontario Institute for Studies in Education
  26. Open Learning Agency
  27. Open University
  28. Open University (UK)
  29. Open University of Israel
  30. Open University of the Netherlands (Heerlen, The Netherlands)
  31. Oppland College
  32. Oral Roberts University
  33. Orange Coast College
  34. Oregon Graduate Institute of Science and Technology
  35. Oregon Health and Science University
  36. Oregon Institute of Technology
  37. Oregon State University
  38. Oriental Institute of Technology
  39. Osaka Kyoiku University
  40. Osaka Medical College
  41. Osaka Prefecture University
  42. Osaka University
  43. Osaka University of Foreign Studies
  44. Osmania University
  45. Ostfold College
  46. Otterbein College
  47. Otto von Guericke Universitat, Magdeburg
  48. Oulu Institute of Technology
  49. Our Lady of the Lake University
  50. Owensboro Community College
  51. Oxford Brookes University
  52. Oxford University

See also : Colleges and universities

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "List of colleges and universities starting with O."

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List of Japanese authors:O

(From Wikipedia, the free Encyclopedia)

List of Japanese authors

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List of people by name: O

(From Wikipedia, the free Encyclopedia)

List of people by name: A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z Oa-Ok - Ol-Oo - Op-Oz

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "List of people by name: O."

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List of people by name: Oa-Ok

(From Wikipedia, the free Encyclopedia)

List of people by name: A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z Oa-Ok - Ol-Oo - Op-Oz

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List of people by name: Ol-Oo

(From Wikipedia, the free Encyclopedia)

List of people by name: A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z Oa-Ok - Ol-Oo - Op-Oz

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List of people by name: Op-Oz

(From Wikipedia, the free Encyclopedia)

List of people by name: A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z Oa-Ok - Ol-Oo - Op-Oz

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List of rare diseases starting with O

(From Wikipedia, the free Encyclopedia)

This list of rare diseases was originally taken from the NIH public domain resource at http://ord.aspensys.com/asp/diseases/diseases.asp .

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

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List of songs by name: O

(From Wikipedia, the free Encyclopedia)

List of songs by name: 0 - A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

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O

(From Wikipedia, the free Encyclopedia)

O is the fifteenth letter of the Latin alphabet.

In Greek (Omikron), Etruscan and Latin O stood for the vowel /o/. Although Semitic 'Ajin was used in some alphabets to transcribe [o], the sound value was usually consonantic: [?/] (as the Arabic letter ع called 'Ajn').

A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z

Oscar represents the letter O in the NATO phonetic alphabet.

O is also:

See also: º, Ò, Ó, Ô, Ŏ, Õ, Ö (Oe), Ø, Œ (OE)

Two-letter combinations starting with O:

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Oxide

(From Wikipedia, the free Encyclopedia)

An oxide is a chemical compound of oxygen with other chemical elements, e.g. (rust) iron oxide or bauxite (aluminum oxide). Oxides are extremely common in Earth's crust, and indeed in solid matter throughout the universe.

Oxides are usually created through the process of oxidation.

See Also

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Oxygen

(From Wikipedia, the free Encyclopedia)

Nitrogen - Oxygen - Fluorine
 
O
S  
 
 

Full table
General
Name, Symbol, NumberOxygen, O, 8
Chemical series nonmetals
Group, Period, Block16 (VIA), 2 , p
Density, Hardness 1.429 kg/m3 (273K), NA
Appearance colorless
Atomic Properties
Atomic weight 15.9994 amu
Atomic radius (calc.) 60 (48) pm
Covalent radius 73 pm
van der Waals radius 152 pm
Electron configuration [He]2s2s22p4
e- 's per energy level2, 6
Oxidation states (Oxide) -2,-1 (neutral)
Crystal structure cubic
Physical Properties
State of matter gas (paramagnetic)
Melting point 50.35 K (-368.77 °F)
Boiling point 90.18 K (-297.08 °F)
Molar volume 17.36 ×1010-3 m3/mol
Heat of vaporization 3.4099 kJ/mol
Heat of fusion 0.22259 kJ/mol
Vapor pressure __ Pa at __ K
Speed of sound 317.5 m/s at 293 K
Miscellaneous
Electronegativity 3.44 (Pauling scale)
Specific heat capacity 920 J/(kg*K)
Electrical conductivity __ 106/m ohm
Thermal conductivity 0.02674 W/(m*K)
1st ionization potential 1313.9 kJ/mol
2nd ionization potential 3388.3 kJ/mol
3rd ionization potential 5300.5 kJ/mol
4th ionization potential 7469.2 kJ/mol
Most Stable Isotopes
isoNAhalf-life DMDE MeVDP
16O99.762%O is stable with 8 neutrons
17O0.038%O is stable with 9 neutrons
18O0.2%O is stable with 10 neutrons
SI units & STP are used except where noted.
Oxygen is a chemical element in the periodic table that has the symbol O and atomic number 8. The element is common and ubiquitous, found not only on Earth but throughout the universe. Free oxygen, as on Earth, is thermodynamically unstable, but exists through the action, first, of photosynthetic anaerobes and, in later epochs, of photosynthetic terrestrial plants.

Notable Characteristics

At standard temperature and pressure, oxygen is found as a gas consisting of two oxygen atoms, chemical formula O2. This oxygen is an important component of air, produced by plants during photosynthesis and is necessary for animals' respiration. The word oxygen derives from two words in Greek, the Greek oxus (acid) and gennan (generate).

Liquid oxygen and solid oxygen have a light blue color and both are highly paramagnetic. Liquid oxygen is usually obtained by the fractional distillation of liquid air.

Applications

Oxygen finds considerable use as an oxidizer, with only fluorine having a higher electronegativity. Liquid oxygen finds use as an oxidizer in rocket propulsion. Oxygen is essential to respiration, so oxygen supplementation has found use in medicine. People who climb mountains or fly in airplanes generally have supplemental oxygen supplies. Oxygen is used in welding, and in the making of steel and methanol.

Oxygen, as a mild euphoric, has a history of recreational use that extends into modern times. Oxygen bars can be seen at parties to this day. In the 19th century, oxygen was often mixed with nitrous oxide to promote a kind of analgesic effect.

History

Oxygen was discovered by the Swedish pharmacist Karl Wilhelm Scheele in 1771, but this discovery was not immediately recognized, and the independent discovery by Joseph Priestley was more widely known. It was named by Antoine Laurent Lavoisier in 1774.

Occurrence

Oxygen is the most abundant element in the Earth's crust, estimated to comprise 46.7% of the crust. Oxygen comprises about 87% of the oceans (as H2O, water) and 20% of the atmosphere of Earth (as O2, molecular oxygen, or O3, ozone). Oxygen compounds, particularly metal oxides, silicates (SiO44-) and carbonates (CO32-), are commonly found in rocks and soil. Frozen water is a common solid on the outer planets and comets. The ice caps of Mars are made of frozen carbon dioxide. Oxygen compounds are found throughout the universe and the spectrum of oxygen is often seen in stars. In fact stars wouldn't produce light without oxygen.

Compounds

Due to its electronegativity, oxygen forms chemical bonds with almost all other elements (which is the origin of the original definition of oxidation). The only elements to escape the possibility of oxidation are a few of the noble gases. The most famous of these oxides is of course hydrogen oxide, or water (H2O). Other well known examples include compounds of carbon and oxygen, such as carbon dioxide (CO2), alcohols (R-OH), aldehydes, (R-CHO), and carboxylic acids (R-COOH). Oxygenated radicals such as chlorates (ClO3-), perchlorates (ClO4-), chromates (CrO42-), dichromates (Cr2O72-), permanganates (MnO4-), and nitrates (NO3-)are strong oxidizing agents in and of themselves. Many metals such as Iron bond with oxygen atoms, Ferric Oxide (Fe2O3). Ozone (O3) is formed by electrostatic discharge in the presence of molecular oxygen. A double oxygen molecule (O2)2 is known, found as a minor component of liquid oxygen. Epoxides are ethers in which the oxygen atom is part of a ring of three atoms.

Isotopes

Oxygen has three stable isotopes and ten known radioactive isotopes. The radioisotopes all have half lives of less than three minutes.

Precautions

Prolonged exposure to pure oxygen at higher pressures can be toxic, having both pulmonary and neurological effects. Pulmonary effects include edema, loss of lung capacity and damage to lung tissues. Neurological effects can include loss of vision, convulsions and coma.

Certain derivatives of oxygen, such as ozone (O3), hydrogen peroxide, hydroxyl radicals and superoxide, are also highly toxic. The body has developed mechanisms to protect against these toxic species. For instance, the naturally-occurring glutathione can act as an antioxidant as can bilirubin which is normally a breakdown product of hemoglobin. Highly concentrated sources of oxygen promote rapid combustion and therefore are fire and explosion hazards in the presence of fuels. This is true as well of compounds of oxygen such as chlorates, perchlorates, dichromates, etc. Compounds with a high oxidative potential can often cause chemical burns.

The fire that killed the Apollo 1 crew on a test lauchpad spread so rapidly because the pure oxygen atmosphere was at normal atmospheric pressure instead of the one third pressure that would be used during an actual launch. (see partial pressure)

See also

External Links

Oxygen is also the name of a Cable TV channel in the US. Its programming targets women audience.

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Oxygen."

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Poker jargon starting with O

(From Wikipedia, the free Encyclopedia)

Poker jargon:

A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

No jargon listed at this time

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Poker jargon starting with O."

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Stellar classification

(From Wikipedia, the free Encyclopedia)

Stars can be classified by using Wien's Displacement Law; but, this poses difficulties for distant stars. Stellar spectroscopy offers a way to classify stars according to their absorption lines; particular absorption lines can be observed only for a certain range of temperatures because only in that range are the involved atomic energy levels populated. An early schema (from the 1800s) ranked stars from A - P; the modern classes are:

Spectral Types by Surface Temperature

A popular mnemonic for remembering the order is "Oh Be A Fine Girl, Kiss Me" (there are many variants of this mnemonic). This scheme was developed in the 1900s, by Cannon and the Harvard College Observatory. The Hertzsprung-Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature.

The reason for the odd arrangement of letters is historical. When people first started taking spectra of stars, they noticed that stars had very different hydrogen spectral lines strengths, and so they classified stars based on the strength of the hydrogen balmer series lines from A (strongest) to Q (weakest). Other lines of neutral and ionized species then came into play (H&K lines of calcium, sodium D lines etc). Later it was found that some of the classes were actually duplicates and those classes were removed. It was only much later that it was discovered that the strength of the hydrogen line was connected with the surface temperature of the star. The basic work was done by the "girls" of Harvard College Observatory, primarily Annie J. Cannon and Antonia Maury, based on the work of Williamina Fleming. These classes are further subdivided by arabic numbers (0-9). A0 denotes the hottest stars in the A class and A9 denotes the coolest ones.

More recently, the classification was extended into O B A F G K M L T, where L and T are extremely cool stars or brown dwarves.

Hertzsprung-Russell diagram

Click here for a larger image

Class O stars are very hot and very luminous, being strongly blue in colour. Naos (in Puppis) shines with a power close to a million times solar. These stars have prominent ionized and neutral helium lines and only weak hydrogen lines. Class O stars emit most of their radiation in ultra-violet.

B stars are again extremely luminous, Rigel (in Orion) is a prominent B class blue supergiant. Their spectra have neutral helium and moderate hydrogen lines. As O and B stars are so powerful, they live for a very short time. They do not stray far from the area which they were formed in as they don't have the time. They, then, tend to cluster together in what we call OB1 associations, which are associated with giant molecular clouds. The Orion OB1 association is an entire spiral arm of our Galaxy (brighter stars make the spiral arms look brighter, there aren't more stars there) and contains all of the constellation of Orion.

Class A stars are amongst the more common naked eye stars. Deneb in Cygnus is another star of formidable power, while Sirius is also an A class star, but not nearly as powerful. As with all class A stars, they are white. Many white dwarves are also A. They have strong hydrogen lines and also ionized metals.

F stars are still quite powerful but they tend to be main sequence stars, such as Fomalhaut in Pisces Australis. Their spectra is characterized by the weaker hydrogen lines and ionized metals, their colour is white with a slight tinge of yellow.

Class G stars are probably the most well known for only the reason that our Sun is of this class. They have even weaker hydrogen lines than F but along with the ionized metals, they have neutral metals. G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be.

Class K is slightly cooler than our Sun, they're orange stars. Some K stars are giants and supergiants, such as Antares while others like Alpha Centauri B are main sequence stars. They have extremely weak hydrogen lines, if it's present at all, and mostly neutral metals.

Class M is by far the most common class if we go by the number of stars. All our red dwarves go in here and they are plentiful; more than 90% of stars are red dwarfs, such as Proxima Centauri. M is also host to most giants and some supergiants such as Arcturus and Betelgeuse, as well as Mira variables. The spectrum of an M star shows lines belonging to molecules and neutral metals but hydrogen is usually absent. Titanium oxide can be strong in M stars.

The new class L are stars that are a very dark red in colour; they are brightest in infra red. Their gas is cool enough to allow metal hydrides and alkali metals to be prominent in the spectrum.

Right at the bottom of the scale is T. These are stars barely big enough to be stars and others that are substellar, being of the brown dwarf variety. They are black, emitting little or no visible light but being strongest in infrared. Their surface temperature is a stark contrast to the fifty thousand degrees or more for O stars, being a cool 700 degrees Celsius. Complex molecules can form, evidenced by the strong methane lines in their spectra.

T and L could be more common than all the other classes combined, if recent research is accurate. From studying the number of propylids (clumps of gas in nebulae from which stars are formed) then the number of stars in the galaxy should be several orders of magnitude higher than what we know about. It's theorised that these propylids are in a race with each other. The first one to form will become a proto-star, which are very violent objects and will disrupt other propylids in the vicinity, stripping them of their gas. The victim propylids will then probably go on to become main sequence stars or brown dwarf stars of the L and T classes, but quite invisible to us. Since they live so long (no star below 0.8 solar masses has ever died in the history of the galaxy) then these smaller stars will accumulate over time.

Also occasionally used are the stellar classifications R, N and S. R and N stars are carbon stars (that is, giants) which run parallel to the normal classification system from roughly mid G to late M. These have more recently been remapped into a unified carbon classifier C, with N0 starting at roughly C6. S stars have ZrO lines rather than TiO, and are in between the M stars and the carbon stars. S stars have carbon and oxygen abundances are almost exactly equal, and both elements are locked up almost entirely in CO molecules. For stars cool enough for CO to form that molecule tends to "eat up" all of whichever element is less abundant, resulting in "leftover oxygen" on the normal main sequence, "leftover carbon" on the C sequence, and "leftover nothing" on the S sequence.

In reality the relation between these stars and the traditional main sequence suggest a rather large continuum of carbon abundance and if fully explored would add another dimension to the stellar classification system.

Yerkes spectral classification

The Yerkes spectral classification, also called the MKK system, is a system of stellar spectral classification introduced in 1943 by William W. Morgen, Phillip C. Keenan and Edith Kellman of Yerkes Observatory.

This classification is based on spectral lines sensitive to stellar surface gravity which is related to luminosity, as opposed to the Harvard classification which is based on surface temperature. Since the radius of a giant star is much larger than a dwarf star while their masses are roughly comparable, the gravity and thus the gas density and pressure on the surface of a giant star are much lower than for a dwarf. These differences manifest themselves in the form of luminosity effects which affect both the width and the intensity of spectral lines which can then be measured.

Six different luminosity classes are distinguished:

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Abbreviations & Acronyms: O

The following table is compiled from various sources, across various languages. When English abbreviations or acronyms come from a non-English source, this is noted.
EntrySourceExpressionField

O

DanishFlydegraensenEuropean Union

O

DutchOostN/A

O

EnglishOerlikonN/A

O

FinnishHappiPublic Administration

o

FrenchCours offertFinance

O

GermanOszillatorN/A

O

GreekυποχρεωτικόςComputing

O

ItalianOvestGeography

O

PortugueseOxidanteN/A

O

SpanishOesteGeography
O Art XVIIDutchMemorandum van Overeenstemming betreffende de interpretatie van artikel XVII van de Algemene Overeenkomst inzake Tarieven en Handel 1994International Organizations
B and OEnglishBelladonna and OpiumN/A
T k oFrenchTonne-kilomètre offerteMeteorology & Standards, Transportation
Núm. oSpanishNúmeroMeteorology & Standards
O-AGEnglishO antigenMedicine

Source: compiled by the editor, based on several corpora (additional references).

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Synonyms: O

Synonyms: atomic number 8 (n), group O (n), oxygen (n), type O (n). (additional references)

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Synonyms within Context: O

ContextSynonyms within Context (source: adapted from Roget's Thesaurus).

Attention

Interjection: see! look, look here, look you, look to it! mark! lo! behold! soho! hark, hark ye! mind! halloo! observe! lo and behold! attention! nota bene, "N.B.", note well; I'd have you to know; notice! O yes! Oyez! dekko! ecco! yoho!

Desire

Interjection: would that, would that it were! O for! esto perpetual

Disapprobation

Forbid it Heaven! God forbid, Heaven forbid! out upon, fie upon it! away with! tut! O tempora! O mores! shame! fie, fie for shame! out on you!

Disrepute

Interjection: fie! shame! for shame! proh pudor! O tempora! O mores! ough! sic transit gloria mundi!

Improbity

Interjection: O tempora! O mores!.

Lamentation

Interjection: heigh-ho! alas! alack! O dear! ah me! woe is me! lackadaisy! well a day! lack a day! alack a day! wellaway! alas the day! O tempora O mores! what a pity! miserabile dictu! O lud lud! too true!

Money

Note, note of hand; promissory note, I O U; draft, check, cheque, back-dated check; negotiable order of withdrawal, NOW.

Publication

Interjection: Oyez! O yes! notice!

Seclusion Exclusion

Among them but not of them ; " and homeless near a thousand homes I stood "; far from the madding crowd's ignoble strife; " makes a solitude and calls it peace "; magna civitas magna solitudo; " never less alone than when alon