Hobbies And Interests
Features
O-class stars are massive and very luminous. They appear blue because they radiate in the ultraviolet and short wavelengths of visible light. Although they are large, at more than 10 times the mass of the sun, they are not as big as the red supergiants, such as Betelgeuse. Neither are they the hottest stars, as this distinction goes to white dwarfs and Wolf-Rayet stars. O-class stars do have one distinguishing feature, and that is their status as the only main sequence stars to contain ionized helium in their spectrum. While hydrogen and helium are their primary components, these stars burn hot enough to produce helium ions. There are only 17 known O-class stars.
Theories/Speculation
Astronomers have determined that the spectral classification of stars has a lot to do with whether they will be able to support a solar system, in particular a planet capable of sustaining life. O-class stars are so hot that they produce what is known as a photoevaporation effect. When a large O-class or B-class star burns hydrogen and helium, it throws radiation throughout the protoplanetary disk, which disperses the atmosphere of all nearby planets. Without an atmosphere, there can be no life.
Time Frame
O-class stars live a short life. Because of their high mass, they convert hydrogen into helium after just a few million years. After this time, they leave the main sequence and expand in size, by as much as 100 times. They become red supergiants and continue to burn helium until it too has been used up. The core begins to contract, but because of the high temperature, continues to fuse heavier elements together to use as fuel. Eventually, the star fuses elements into iron, which does not release energy, but demands more energy to keep the process going. Because of this, iron accumulates in the center of the star, until the core can no longer support its own weight and collapses, causing a Type I supernova, or in some cases, a hypernova. These huge explosions will form a central black hole that sucks in all nearby material. Just outside the influence of the black hole, a stellar nursery forms and new stars are born.
Misconceptions
It is often thought that O-class stars represent a particular stage in the evolution of stars. While it is true that most stars follow the main sequence throughout their lifetimes, going from hotter to cooler stars as they burn hydrogen, not all stars begin on the hot end of the spectral scale, and many do not follow the main sequence at all. Whether a star follows the main sequence depends on its initial mass and luminosity. As massive and bright as they are, O-class stars leave the main sequence relatively early and become supergiants, which are located above and to the right of the main sequence.
Geography
O-type stars are often found in groups, called "OB associations." Two of the most recognizable O-class stars are located in the constellation of Orion. The triple star system Zeta Orionis (Alnitak) and the multiple star Delta Orionis (Mintaka) are two of the three stars that make up the belt of Orion. Another well-known O-class star is located in the southern constellation of Puppis. Zeta Puppis, also known as Naos, is the brightest star in the constellation, and the only O-class star. It has been established that this star is not native to this constellation, but is a runaway star that has traveled more than 400 light years since its birth, which explains its lonely designation.
Considerations
As astronomers continue to study O-class stars, they find that even in such a small family of stars, there are big variations, and more ways to classify them. Wolf-Reyat stars are the same temperature as other O-class stars, but cannot be classified as such because they have very different emission lines than other O-type stars. Similarly, rare stars known as Luminous Blue Variables vary in brightness and mass, and can straddle the border between the O and B spectral classes, making it hard for astronomers to define them as one or the other.