Many Wolf Rayet stars are found in close binary systems that consist of a massive, luminous, main-sequence 0 star and a less massive, smaller, yet even more luminous companion. Because it is so luminous, the small star expels the outer parts of its atmosphere in the form of a dense stellar wind, winch produces a complicated but characteristic spectrum. It seems probable that such stars are the result of mass exchange. For example V444 Cygni has an orbital period of 4.2 days. The bright star has a mass of about ten solar masses and is probably a helium star which has transferred most of its original mass to its companion. The O star has a mass of about 25 solar masses and is presumably now brighter than it was originally because of its increase in mass. A number of X-ray stars are found to be binary stars consisting of an 0-type supergiant and a compact object – either a neutron star or a black hole. Astronomers speculate that when the helium star in V444 Cygni runs out of nuclear fuel and collapses, and when the 0-type main-sequence star evolves and expands to become a supergiant, V444 Cygni could become an X-ray star.

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