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A dark nebula or absorption nebula is a type of interstellar cloud, particularly molecular clouds, that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection nebulae.
The time following the emission of the cosmic microwave background—and before the observation of the first stars—is semi-humorously referred to by cosmologists as the Dark Age, and is a period which is under intense study by astronomers (see 21 centimeter radiation).
A faint blue galaxy (FBG) is an inconspicuous, often small galaxy with low surface luminosity.In addition to being dim, they show a remarkable preponderance of sparsely scattered blue stars, but comparatively few red stars, which in most galaxies are by far the most common.
The first luminous blue variable to be identified as a variable star was P Cygni, and these stars have been referred to as P Cygni type variables. The General Catalogue of Variable Stars decided there was a possibility of confusion with P Cygni profiles , which also occur in other types of stars, and chose the acronym SDOR for "variables of the ...
In a technique known as blue field entoptoscopy, the effect is used to estimate the blood flow in the retinal capillaries.The patient is alternatingly shown blue light and a computer generated picture of moving dots; by adjusting the speed and density of these dots, the patient tries to match the computer generated picture to the perceived entoptic dots.
The first one to address the problem of an infinite number of stars and the resulting heat in the Cosmos was Cosmas Indicopleustes, a 6th-century Greek monk from Alexandria, who states in his Topographia Christiana: "The crystal-made sky sustains the heat of the Sun, the moon, and the infinite number of stars; otherwise, it would have been full of fire, and it could melt or set on fire."
Star system Nebula Median distance Stars in system Spectral type Apparent magnitude (V) Comments and references P Cygni (34 Cygni) 5251±180: 1: B1-2 Ia-0ep: 4.82: The closest luminous blue variable star to Earth V4029 Sagittarii (HD 168607) 6,000 1 B9Ia + 8.12 to 8.29 [4] [5] near the Omega Nebula: V905 Scorpii (HD 160529) 6,100 1 LBV 6.66 [4] [5]
The Sun is composed primarily of the chemical elements hydrogen and helium; they account for 74.9% and 23.8%, respectively, of the mass of the Sun in the photosphere.All heavier elements, colloquially called metals in stellar astronomy, account for less than 2% of the mass, with oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant.