Galactic Rotation

the rotation of the GALAXY, and all its gas, stars and dust, which increases towards its centre. The rotation near the SUN is approximately 250 kms^-1 and to make one galactic orbit takes the Sun in the region of 250 million years.

Taken from Dictionary of Science

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The rotation curve of a galaxy can be represented by a graph that plots the orbital velocity of the stars or gas in the galaxy on the y-axis against the distance from the center of the galaxy on the x-axis. Stars revolve around the center of galaxies at a constant speed over a large range of distances from the center of the galaxy. Thus they revolve much faster than would be expected if they were in a free Newtonian potential. The galaxy rotation problem is this discrepancy between the observed rotation speeds of matter in the disk portions of spiral galaxies and the predictions of Newtonian dynamics considering the visible mass. This discrepancy is currently thought to betray the presence of dark matter that permeates the galaxy and extends into the galaxy's halo. An alternative explanation is a modification of the laws of gravity, such as MOND (Modified Newtonian Dynamics).

Taken from Wikipedia

Dark Nebulae

clouds of thick gas and dust common throughout the MILKY WAY and other GALAXIES.

Taken from Dictionary of Science

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A dark nebula is a type of interstellar cloud that is so dense that it obscures the light from the background emission or reflection nebula (e.g., the Horsehead Nebula) or that it blocks out background stars (e.g., the Coalsack Nebula). The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of larger molecular clouds. Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds. Isolated small dark nebulae are called Bok globules. Like other interstellar dust/material, things it obscures are only visible using radio waves in radio astronomy.

Dark clouds appear so because of submicrometre-sized dust particles, coated with frozen carbon monoxide and nitrogen, which effectively block the passage of light at visible wavelengths. Also present are molecular hydrogen, atomic helium, C¹⁸O, CS, NH₃ (ammonia), H₂CO (formaldehyde), c-C₃H₂ (cyclopropenylidene) and a molecular ion N₂H⁺ (diazenylium), all of which are relatively transparent. These clouds are the spawning grounds of stars and planets, and understanding their development is essential to understanding star formation.

The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes. The largest dark nebulae are visible to the naked eye, appearing as dark patches against the brighter background of the Milky Way.

In the inner regions of dark nebulae important events take place, such as the formation of stars and masers.

Taken from Wikipedia

Galactic Halo

a collection of stars, gas and dust, nearly spherical in shape, which has the same center as the galaxy. It contains population II stars and creates a lot of the background radio emissions.

Taken from Dictionary of Science
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The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:

• the galactic spheroid (stars)

• the galactic corona (hot gas, i.e. a plasma)

• the dark matter halo

The distinction between the halo and the main body of the galaxy is clearest in spiral galaxies, where the spherical shape contrasts with the flat disc. In an elliptical galaxy, there is no sharp transition between the body of the galaxy and the halo.

Taken from Wikipedia