OB Stars
OB stars are a collective term for two classifications of stars—O and B stars—recognized for their extreme temperatures and brightness. O stars have surface temperatures exceeding 44,500 degrees Fahrenheit, while B stars range from 17,500 to 44,500 degrees Fahrenheit. Both types emit significant ultraviolet (UV) radiation, which plays a crucial role in ionizing surrounding hydrogen gas, creating regions known as HII regions, essential for star formation. These stars are typically found in groups called OB associations, often forming within giant molecular clouds. Notably, one of the best-known OB associations is Orion OB1, situated in the Orion constellation. Despite their brilliance, OB stars have relatively short lifespans, lasting only about ten million years, after which they may explode as supernovae, contributing to the creation of heavier elements in the universe. The formation and lifecycle of OB stars are integral to understanding cosmic evolution and the dynamics of our galaxy.
OB Stars
FIELDS OF STUDY: Astronomy; Astrophysics; Historical Astronomy
ABSTRACT: Astronomers classify stars using a letter system based on spectral type. OB stars, which fit the O and B spectral classifications, are the hottest stars on the spectral scale. The ultraviolet radiation emitted by OB stars helps form interstellar gases, which in turn create conditions that lead to the formation of new stars. Groups of OB stars are often found in clusters, or star associations. Scientists study OB stars because their presence shows areas of new star birth and also provides a record of past star creation.
Star Classifications
In the early twentieth century, astronomers at the Harvard Observatory developed a method of classifying stars based on their surface temperatures. This method was called spectral classification because they used the stars’ emission spectra to determine their temperature. The Harvard spectral classification divided stars into seven classes: O, B, A, F, G, K, and M. O stars have the highest surface temperature, and M stars have the lowest. Later, each class was divided into numbered subclasses. "OB stars" refers collectively to the short-lived O and B stars. These two types are typically found together.
In 1943, American astronomers William Morgan (1906–94), Philip Keenan (1908–2000), and Edith Kellman (1911–2007) published the Yerkes spectral classification. It was based on the Harvard method, but it measured luminosity as well as temperature. The name was a reference to Yerkes Observatory, where the three were working at the time. Morgan and Keenan later refined the system, which came to be known as the Morgan-Keenan (MK) classification. Astronomers still use the MK system today.
Characteristics of OB Stars
O and B stars are the two hottest classes of stars. B stars have a surface temperature between 17,500 and 44,500 Fahrenheit (F), and O stars have a surface temperature above 44,500 F. They are on the blue end of the visible light spectrum, close to ultraviolet light, which is invisible to humans. As a result, O and B stars produce a great deal of ultraviolet (UV) radiation.
O stars are massive and emit such strong radiation that they can ionize any hydrogen within a thousand light-years. B stars are also high mass and emit large quantities of UV radiation, but they contain more helium than O stars. O and B stars are often found clustered together in what astronomers call OB star associations. These associations are believed to form in giant molecular clouds. One of the most studied OB associations is Orion OB1, which includes the young stars of the Orion Nebula and other stars of the Orion constellation.
O and B stars are very bright and strong, but they burn through their energy quickly. They live for a relatively short time, perhaps ten million years. In contrast, Earth’s sun has been burning for about five billion years. OB stars are more likely to undergo a massive stellar explosion known as a supernova, a process that creates many of the heavier elements found in the universe.
HII Regions
In the mid-1900s, Swedish astronomer Bengt Georg Daniel Strömgren (1908–87) discovered a relationship between the density of the interstellar gas surrounding a star, the star’s temperature, and the range of the area where hydrogen was ionized by the star’s UV radiation. The spheres of ionized atomic hydrogen gas surrounding young OB stars became known as Strömgren spheres. These spheres are a type of HII region, which refers to any interstellar gas cloud ionized by UV radiation. The "HII" designation refers to the fact that the ionized gas is primarily atomic hydrogen; this is to distinguish it from either neutral, un-ionized atomic hydrogen (HI) or molecular hydrogen (H2).
HII regions are commonly found in conjunction with OB star associations, because the massive OB stars release large amounts of the UV radiation that creates these regions. The stellar winds generated by OB stars create holes in the molecular cloud that surrounds the region, and ionized gases rush into these holes. The resulting shock waves compact the gas and lead to the formation of a new cluster of stars. This star-formation process repeats when some of the new stars become massive enough to create HII regions of their own. While only a small percentage of ionized gas is actually compressed into stars, most of the stars in the Milky Way are thought to have formed via OB star associations and HII regions.
PRINCIPAL TERMS
- HII region: an interstellar region formed when gas clouds consisting primarily of atomic hydrogen are ionized by the ultraviolet radiation given off by OB stars.
- OB star association: a loose collection of O and B stars. One of the best-known OB star associations is Orion B1 in the Milky Way galaxy.
- spectral classification: a method of classifying stars based on various elements of their emission spectra, as determined by such characteristics as surface temperature, luminosity, and size.
- Strömgren sphere: a spherical region of ionized atomic hydrogen (HII) surrounding a young OB star in an HII region.
- ultraviolet radiation: short-wavelength electromagnetic radiation just beyond the visible light spectrum.
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