P Cygni is an explosive variable star of spectral type B, with broad emission lines and strong absorption of violet light. It is a blue supergiant star located in the constellation Cygnus. It is a luminous blue variable, and the name is fully descriptive of its nature. It is 76 times larger than the Sun and 30 times more massive. P Cygni is a luminous blue variable (or LBV for short) star, maybe even the first one discovered. P Cygni stars lines show a profile type called “P Cyg”: an emission component to the red side of an absorption line. They have large masses and high intrinsic luminosities.
The star is located about 5,000 to 6,000 light-years (1,500–1,800 parsecs) from Earth. A P Cygni profile is a combination of features in a star’s spectrum that points to an outflow of material in the form of either an expanding shell of gas or powerful stellar wind. Located about 5,100 light-years (1,560 parsecs) from Earth, it is a hypergiant luminous blue variable (LBV) star of spectral type B1-2 Ia-0ep that is one of the most luminous stars in the Milky Way. The observations have been made with the middle resolution (R=3000) spectrograph mounted at the focus of a 190 mm flat-field telescope and an Audine camera.
Despite this vast distance, it is visible to the naked eye in suitable dark sky locations. P Cygni itself is a Be star, one of the most luminous stars known and only the third variable star to be discovered; it lies about 7,000 light-years away. It was unknown until the end of the 16th century when it suddenly brightened to 3rd magnitude. The first well-documented observations were made by Willem Blaeu. It was first observed on 18 August (Gregorian) 1600 by Willem Janszoon Blaeu, a Dutch astronomer, mathematician, and globe-maker.
P Cygni has been called a “permanent nova” because of spectral similarities and the obvious outflow of material, and was once treated with novae as an eruptive variable; however, its behavior is no longer thought to involve the same processes associated with true novae. The recent detection of a P Cygni profile in the X-ray emission from Circinus X-1 means that this powerful tool for probing stellar winds and other stellar outflows is now available in an entirely new region of the spectrum. It is widely considered to be the earliest known example of a luminous blue variable. However, it is far from a typical example.
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