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Most stars are actually relatively cool objects emitting much of their electromagnetic radiation in the visible or near-infrared part of the spectrum. Ultraviolet radiation is the signature of hotter objects, typically in the early and late stages of their evolution. In the Earth's sky seen in ultraviolet light, most stars would fade in prominence.
Extremely hot stars (such as O- and B-type) emit proportionally more UV radiation than the Sun. Sunlight in space at the top of Earth's atmosphere (see solar constant) is composed of about 50% infrared light, 40% visible light, and 10% ultraviolet light, for a total intensity of about 1400 W/m 2 in vacuum.
The first list shows a few of the known stars with an estimated luminosity of 1 million L ☉ or greater, including the stars in open cluster, OB association and H II region. The majority of stars thought to be more than 1 million L ☉ are shown, but the list is incomplete. The second list gives some notable stars for the purpose of comparison.
The youngest and brightest stars we now see in the Orion Nebula are thought to be less than 300,000 years old, [51] and the brightest may be only 10,000 years in age. Some of these collapsing stars can be particularly massive, and can emit large quantities of ionizing ultraviolet radiation. An example of this is seen with the Trapezium cluster.
Binary stars provide the only direct way to measure the mass of stars, from observations of their orbital motions. Thus, observations of binary stars where the two components are at such different stages of stellar evolution can be used to determine the relationship between the mass of stars and how they evolve.
The UBV photometric system (from Ultraviolet, Blue, Visual), also called the Johnson system (or Johnson-Morgan system), is a photometric system usually employed for classifying stars according to their colors. It was the first standardized photometric system.
Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys.
Main-sequence stars vary in surface temperature from approximately 2,000 to 50,000 K, whereas more-evolved stars – in particular, newly-formed white dwarfs – can have surface temperatures above 100,000 K. [3] Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest.