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Index (A)

AA - AB

AC - AC

AD - AF

AG - AJ

AK - AM

AN - AN

AO - AR

AS - AV

AW - AZ

Moment of truth
The Sun, with a diameter of 1,392,000 km, is made of 94% hydrogen atoms.


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Abel flash-point apparatus Min. Ext.
A petroleum-testing apparatus for determining the flash-point. The basic apparatus is covered and fitted with a stirrer, a thermometer and a heater.

ablation Space, Astron.
A process in which heat is carried away from an object (e.g. falling from space) as the surrounding air blows away the vaporized outer layers of the object. As a result, the object reduces in size and slows down due to air resistance. For instance, fiery entry of a falling meteorite through the Earth's atmosphere. The deceleration module from the Gelileo space probe consists of a forward heat shield made of carbon phenolic. Much of the heat was dissipated via ablation as the probe was dropped towards the planet Jupiter in December 1995. The mass of the shield was reduced from 152 kg to 65 kg as the probe slowed from 170 000 km per hour to 430 km per hour in four minutes.

absolute alcohol Chem.
Ethanol (ethyl alcohol) almost free from water (not more then 1%). It has a boiling point of 78.4C and very hygroscopic. It can be prepared by fractionating distillation of an azeoptropic mixture of the alcohol and water (rectified spirit), with the addition of benzene as a drying agent.

absolute brightness Astron.
The apparent brightness, measures in magnitude, a celestial object would have if it were placed at a distance of 10 parsecs (32.6 light years) from Earth. It is also called intrinsic brightness. An object that is one absolute magnitude brighter than another is 2.5 times intrinsically brighter. The smaller the value of absolute magnitude the brighter the object. For example, the Sun has an absolute brightness lower than that of Sirius (absolute magnitudes of +4.43 and +1.43, respectively).

absolute magnitude Astron.
See absolute brightness.

absolute pressure Chem., Phys.
Pressure measured on a guage instrument that reads zero at zero pressure. See also gauge pressure.

absolute temperature Phys.
A temperature measured relative to absolute zero. It is the thermodynamic temperature based on the scale kelvin (K) and is the same size as the degree Celcius (C). The smallest value is 0 K (absolute zero), which is equivalent to -273.15 C.

absolute zero Phys.
Minimum attainable temperature, equivalent to -273.15 C. At this temperature, the atoms or molecules of any substances possess no heat energy and is impossible to cool it below this temperature.

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