Galaxy

(plural galaxies) specifically, the name given to the band of stars, numbering one hundred thousand million bodies, which includes the Sun, and is alternatively called the Milky Way. The galaxy has a spiral structure and is approximately one hundred thousand LIGHT YEARS across. In general usage it refers to a collection of stars held together by gravitational forces. They form different shapes including spiral, elliptical and irregular galaxies and range from dwarf galaxies with 100 000 stars to massive galaxies such as the giant elliptical M 87 which has three thousand billion stars.

Taken from Dictionary of Science

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A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference to the Milky Way galaxy. Typical galaxies range from dwarfs with as few as ten million (10⁷) stars up to giants with one trillion (10¹²) stars, all orbiting the galaxy's center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.

Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with dusty, curving arms. Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.

There are probably more than 100 billion (10¹¹) galaxies in the observable universe. Most galaxies are 1,000 to 100,000^[4] parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.

Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.

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Bacterium

(plural bacteria) a MICRO-ORGANISM (one that cannot be seen with the naked eye but only with the aid of a MICROSCOPE), which usually has a body made of only one cell (unicellular). Bacteria are believed by many scientists to be the first organisms to have existed on Earth and they have always been extremely important in all life processes. Bacteria occur in water, air, soil and rotting plant or animal debris, and even in extremely inhospitable environments such as hot springs full of chemicals. They are vitally important in the breakdown and decomposition of ORGANIC material--without bacteria, nothing would rot away. Most bacteria need oxygen in order to live and are called aerobic, but others do not and are termed anaerobic. The surrounding layer around a bacterial cell is called the cell wall. Two types of bacteria, which each have a different cell wall structure (Gram positive or Gram negative) are identified by a test known as Gram's stain. Some bacteria have a protective, slimy outer layer called a capsule, and others may have hairs called filaments which cause movement. Bacteria occur in a number of different shapes and forms which help to identify them. These are spiral (plural spirilli, singular, spirillus), spherical or round (plural cocci, singular coccus), rod-like (plural bacilli, singular, bacillus), comma-shaped (vibrio) and corkscrew-shaped (spirochaetae). Bacteria usually reproduce by asexual reproduction and a few are responsible for extremely serious diseases in plants, animals and man, e.g. typhoid, tuberculosis, diphtheria and cholera. These diseases can be treated with ANTIBIOTICS although there is a problem of resistance of some bacteria to certain antibiotic drugs.

Taken from Dictionary of Science

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The bacteria ([bækˈtɪərɪə]; singular: bacterium) are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, water, and deep in the Earth's crust, as well as in organic matter and the live bodies of plants and animals. There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water; in all, there are approximately five nonillion (5×10³⁰) bacteria on Earth, forming much of the world's biomass. Bacteria are vital in recycling nutrients, with many steps in nutrient cycles depending on these organisms, such as the fixation of nitrogen from the atmosphere and putrefaction. However, most bacteria have not been characterized, and only about half of the phyla of bacteria have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

There are approximately ten times as many bacterial cells in the human flora of bacteria as there are human cells in the body, with large numbers of bacteria on the skin and as gut flora. The vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system, and a few are beneficial. However, a few species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy and bubonic plague. The most common fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people a year, mostly in sub-Saharan Africa. In developed countries, antibiotics are used to treat bacterial infections and in agriculture, so antibiotic resistance is becoming common. In industry, bacteria are important in sewage treatment, the production of cheese and yoghurt through fermentation, as well as in biotechnology, and the manufacture of antibiotics and other chemicals.

Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved independently from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.

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Taxis

(plural taxes) the movement of a cell or organism in response to a stimulus in environment. This stimulus may be temperature (thermotaxis), light (phototaxis), gravity (geotaxis) or chemical (CHEMOTAXIS).

Taken from Dictionary of Science

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A taxis (plural taxes, pronounced /ˈtæksiːz/) is an innate behavioral response by an organism to a directional stimulus or gradient of stimulus intensity. A taxis differs from a tropism (turning response, often growth towards or away from a stimulus) in that the organism has motility and demonstrates guided movement towards or away from the stimulus source. It is sometimes distinguished from a kinesis, a non-directional change in activity in response to a stimulus that results in the migration toward or away from a stimulus.

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Quantum

(plural quanta) a small amount or unit of electromagnetic radiation which can be thought of as a particle of energy.

Taken from Dictionary of Science

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In physics, a quantum (plural: quanta) is the minimum unit of any physical entity involved in an interaction. An example of an entity that is quantized is the energy transfer of elementary particles of matter (called fermions) and of photons and other bosons. The word comes from the Latin "quantus", for "how much." Behind this, one finds the fundamental notion that a physical property may be "quantized", referred to as "quantization". This means that the magnitude can take on only certain discrete numerical values, rather than any value, at least within a range. There is a related term of quantum number.

A photon, for example, is a single quantum of light, and may thus be referred to as a "light quantum". The energy of an electron bound to an atom (at rest) is said to be quantized, which results in the stability of atoms, and of matter in general.

As incorporated into the theory of quantum mechanics, this is regarded by physicists as part of the fundamental framework for understanding and describing nature at the infinitesimal level, for the very practical reason that it works. It is "in the nature of things", not a more or less arbitrary human preference.

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Bacillus

(plural bacilli)

a BACTERIUM which has a rod-shaped form.

Taken from Dictionary of Science
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Bacillus is a genus of rod-shaped bacteria and a member of the division Firmicutes. Bacillus species are obligate aerobes, and test positive for the enzyme catalase. Ubiquitous in nature, Bacillus includes both free-living and pathogenic species. Under stressful environmental conditions, the cells produce oval endospores that can stay dormant for extended periods. These characteristics originally defined the genus, but not all such species are closely related, and many have been moved to other genera.

Taken from Wikipedia