coma

a state of unconsciousness from which a person cannot be aroused, caused by injury to the head, rupture of cerebral blood vessels, narcotics, poisons, etc.

coma

1. Astronomy the luminous cloud surrounding the frozen solid nucleus in the head of a comet, formed by vaporization of part of the nucleus when the comet is close to the sun

2. Botany

a. a tuft of hairs attached to the seed coat of some seeds

b. the terminal crown of leaves of palms and moss stems

3. Optics a type of lens defect characterized by the formation of a diffuse pear-shaped image from a point object

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

Coma due to concave mirror

coma

(koh -mă)

1. The most obvious part of a comet when it is close to the Sun. It consists of a diffuse luminous nebulous cloud of gas and dust that surrounds the comet's nucleus and is formed by the sublimation of ices and ejection of dust particles from the nucleus. The brightest part of the coma is the central region nearest the surface of the nucleus, where the density of material being released is at its highest; the gas density in this region is typically 10¹³ molecules per cm³. The visual boundary of the coma merges with the sky background. A coma's size varies with the comet's distance from the Sun and with the amount of material being released from the nucleus but can grow to typically 10⁴ to 10⁵ km in diameter. The luminosity of the coma is mainly produced by fluorescence from a variety of carbon, nitrogen, hydrogen, and oxygen radicals and reflection of sunlight by dust particles. The shape seems roughly circular but fan-shaped comas have been reported. The material comprising the coma is extremely rarefied: even faint stars shine through.

coma

[′kō·mə]

(astronomy)

The gaseous envelope that surrounds the nucleus of a comet. Also known as head.

(electronics)

A cathode-ray tube image defect that makes the spot on the screen appear comet-shaped when away from the center of the screen.

(medicine)

Unconsciousness from which the patient cannot be aroused.

(optics)

A manifestation of errors in an optical system, so that a point has an asymmetrical image (that is, appears as a pear-shaped spot).

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Coma

a life-threatening condition characterized by loss of consciousness, severe decrease or absence of reaction to external stimuli, extinction of reflexes (to the point of complete disappearance), disturbance of the depth and frequency of respiration, change in vascular tonus, acceleration or deceleration of the pulse, and upset of temperature regulation.

Coma is a consequence of deep inhibition in the cerebral cortex extending to the subcortex and lower sections of the central nervous system, as a result of acute disruption of blood circulation in the brain, head injury, inflammation (encephalitis, meningitis, malaria), intoxication (barbiturates, carbon monoxide), diabetes mellitus, uremia (urémic coma), and hepatitis (hepatic coma). In addition, there is disruption of the acid-base equilibrium in nervous tissue, oxygen starvation, disturbances of ion exchange, and energy starvation in the nerve cells. A precomatose state, during which the symptoms indicated develop, precedes coma. The condition can be treated by eliminating the primary causes of the coma and taking measures aimed at restoring the acid-base balance, reversing collapse, restoring respiration, and combating oxygen starvation.

Coma

an aberration of optical systems in which each part of the system at a distance d (circular zone) from the axis gives the image of a luminous point as a ring with a radius that increases as d increases; the centers of the rings are not coincident, so that their superposition, that is, the image of the point produced by the system as a whole, takes the form of an asymmetrical, diffuse spot (see Figure 1).

Figure 1. Coma in a simple optical system passing a beam of parallel rays: (a) beam, passing through the center of the system at angle w, intersects the focal plane FF’ at point O; rays traversing the system at points A₁, A₁, A₂, A₂, . . . , which are symmetrical with respect to the axis, are not collected by the system at point O in plane FF’, but rather at points O₁ O₂, . . . , at a distance from O proportional to dP (d being the distance from the axis at which a ray intersects the system); (b) the diffuse spot observed in the focal plane instead of point image O

The size of this spot is proportional to the square of the aperture angle of the system and the distance of the point-object from the system’s axis. The coma is very large with parabolic mirrors; it is, in fact, the principal factor limiting their visual field (see Figure 2). In complicated optical systems, coma is usually corrected together with spherical aberration by matching lenses. If one of the lens surfaces is off-center when a system is assembled, coma will also distort the images of points located on the system’s axis.

Figure 2. Coma effect with a parabolic mirror

Coma

a tuft of simple or pinnate hairs at the apex or base of the seeds of certain plants, for example, the willow herb. The coma is an outgrowth of the seed coat, or testa. It promotes distribution of seeds and fruits by means of wind, water, and animals.

Mentioned in

References in periodicals archive

When mortality in the present study was analysed with respect to Glasgow coma scale, the results were as follows:

STUDY OF AETIOLOGY AND CLINICAL OUTCOMES IN COMA PATIENTS WITH NORMAL BRAIN CT SCAN

Further research is needed before these results might be used to help guide care for coma patients, study author Dr Stein Silva, of Imserm (French National Research Institute) in Toulouse, said in a journal news release.

Brain Scans May Reveal Which Coma Patients Will Recover

He measured minus four on the Glasgow Coma Scale and it's very rare that people come back from being that low on the coma scale."

'Impact of car crash tragedy still hasn't sunk in' - survivor

Caption: Eight dark galaxies (circled) are among more than 800 in the Coma cluster (shown).

More dark galaxies reveal themselves: hundreds of nearly starless suspects found in nearby cluster

Most of the pieces in Induced Coma are organized thematically, which helps carry one impression over into another, all the while inverting, reverting, and disorienting the reader's expectations about the given topic.

Harold Jaffe, Induced Coma: 50 and 100 Word Stories

Aziz said doctors typically try every few days to bring someone out of a coma.

Schumacher full recovery unlikely, some say

2013 Feb 2.82, in 30cmT, f/10 x95, bright and easy shining with an overwhelming appearance, outer coma is very hazy with a misty and washed-out effect, small very bright core, but not quite star-like, size of the core estimated as 0.8' perhaps slightly uneven in shape.

Observations of Comet C/2012 F6 (Lemmon)

The heroine remains Susan Wheeler (Lauren Ambrose), a medical student who begins to grow suspicious about an unexpected number of patients slipping into comas at the hospital where she's working.

Coma

is fifty years old and has been in a coma for eighteen years, ten months and seventeen days.

The Weather in Fritz Bemelmans Park

To signal the series' repetition-with-difference, the works arc all titled COMA but each is given a unique subtitle that emulates the language of screenplays.

Bobbi Woods