Strömgren sphereIn theoretical astrophysics, there can be a sphere of ionized hydrogen (H II) around a young star of the spectral classesO or B. The theory was derived by Bengt Strömgren in 1937 and later named Strömgren sphere after him. The Rosette Nebula is the most prominent example of this type of emission nebula from the H II-regions.
H II regionAn H II region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic centimetre. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.
OB associationAn OB association is a loosely organized, gravitationally unbound group of young, massive, main sequence stars primarily of spectral types O and B, characterized by their high temperatures, blue color, and luminosity, and by the presence of massive stars with masses between 10 and 90 times that of the Sun. Sometimes these late O and early B stars are referred to as OB stars.
Stellar classificationClassification of stars based on spectral properties In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element.
UltravioletUltraviolet radiation (UV; sometimes called ultraviolet light) is electromagnetic radiation of wavelengths of 100–400 nanometers, shorter than that of visible light, but longer than vacuum ultraviolet and extreme ultraviolet (radiation bands that overlap UV but share some properties with soft X-rays). UV radiation is present in sunlight and constitutes about 10% of the total electromagnetic radiation output from the Sun.
Interstellar mediumThe interstellar medium (ISM) is the matter and radiation that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic medium. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.
Molecular cloudA molecular cloud—sometimes called a stellar nursery if star formation is occurring within—is a type of interstellar cloud of which the density and size permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, H2), and the formation of H II regions. This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.
StarLarge self-illuminated object in spaceThe Sun, a G-type main-sequence star, the closest to EarthA star-forming region in the Large Magellanic CloudA star is a luminous spheroid of plasma held together by self-gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night; their immense distances from Earth make them appear as fixedpoints of light.*As an Amazon Associate I earn from qualifying purchases.