P Cygni (34 Cygni) is a

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as e ...

in the constellation Cygnus. The

designation Designation may refer to: * Designation (law), the process of determining an incumbent's successor * Professional certification * Designation (landmarks), an official classification determined by a government agency or historical society * Designa ...

"P" was originally assigned by

Johann Bayer Johann Bayer (1572 – 7 March 1625) was a German lawyer and uranographer (celestial cartographer). He was born in Rain, Lower Bavaria, in 1572. At twenty, in 1592 he began his study of philosophy and law at the University of Ingolstadt, ...

in '' Uranometria'' as a

nova A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramati ...

. Located about 5,300

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 101 ...

s (1,560

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, a ...

s) from Earth, it is a

hypergiant A hypergiant (luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term ''hypergiant'' is defined as luminosity class 0 (zero) in the MKK ...

luminous blue variable Luminous blue variables (LBVs) are massive evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of the Larg ...

(LBV) star of

spectral type 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 th ...

B1-2 Ia-0ep that is one of the most luminous stars in the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked ey ...

Visibility

The star is located about 5,000 to 6,000

s (1,500–1,800

s) from Earth. Despite this vast distance, it is visible to the naked eye in suitable dark sky locations. It was unknown until the end of the 16th century, when it suddenly brightened to 3rd magnitude. It was first observed on 18 August (Gregorian) 1600 by

Willem Janszoon Blaeu Willem Janszoon Blaeu (; 157121 October 1638), also abbreviated to Willem Jansz. Blaeu, was a Dutch cartographer, atlas maker and publisher. Along with his son Johannes Blaeu, Willem is considered one of the notable figures of the Netherland ...

, a Dutch astronomer, mathematician and globe-maker. Bayer's atlas of 1603 assigned it the miscellaneous label P and the name has stuck ever since. After six years the star faded slowly, dropping below naked-eye visibility in 1626. It brightened again in 1655, but had faded by 1662. Another outburst took place in 1665; this was followed by numerous fluctuations. Since 1715 P Cygni has been a fifth magnitude star, with only minor fluctuations in brightness. Today it has a magnitude of 4.8, irregularly variable by a few hundredths of a magnitude on a scale of days. The visual brightness is increasing by about 0.15 magnitude per century, attributed to a slow decrease in temperature at constant luminosity. P Cygni has been called a "permanent nova" because of spectral similarities and the obvious outflow of material, and was once treated with

e as an eruptive variable; however, its behaviour is no longer thought to involve the same processes associated with true novae.

Luminous blue variable

P Cygni is widely considered to be the earliest known example of a

. However, it is far from a typical example. Typically, LBVs change in brightness with a period of years to decades, occasionally hosting outbursts where the brightness of the star increases dramatically. P Cygni has been largely unvarying both in brightness and

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of color ...

since a series of large outbursts in the 17th century. Similar events have been seen in

Eta Carinae Eta Carinae (η Carinae, abbreviated to η Car), formerly known as Eta Argus, is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around distant in th ...

and possibly a handful of extra-galactic objects. P Cygni does show evidence for previous large eruptions around 900, 2,100, and possibly 20,000 years ago. In more recent centuries, it has been very slowly increasing in visual magnitude and decreasing in temperature, which has been interpreted as the expected evolutionary trend of a massive star towards a

red supergiant Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antar ...

stage.

Evolution

Luminous blue variables like P Cygni are very rare and short lived, and only form in regions of galaxies where intense star formation is happening. LBV stars are so massive and energetic (typically 50 times the mass of the Sun and tens of thousands of times more luminous) that they exhaust their nuclear fuel very quickly. After shining for only a few million years (compared to several billion years for the Sun) they erupt in a supernova. The recent supernova SN 2006gy was likely the end of an LBV star similar to P Cygni but located in a distant galaxy. P Cygni is thought to be in the hydrogen shell burning phase immediately after leaving the main sequence. It has been identified as a possible type IIb supernova candidate in modelling of the fate of stars 20 to 25 times the mass of the Sun (with LBV status as the predicted final stage beforehand).

P Cygni profile

P Cygni gives its name to a type of spectroscopic feature called a P Cygni profile, where the presence of both absorption and emission in the profile of the same

spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

indicates the existence of a gaseous envelope expanding away from the star. The emission line arises from a dense stellar wind near to the star, while the

blueshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in fr ...

ed absorption lobe is created where the radiation passes through circumstellar material rapidly expanding in the direction of the observer. These profiles are useful in the study of

stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. ...

s in many types of stars. They are often cited as an indicator of a

star, although they also occur in other types of star. In P Cygni itself, the size of the stellar wind

H-alpha H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air and 656.46 nm in vacuum; it occurs when a hydrogen electron falls from its third to second lowest energy level. H-alph ...

emission region is milli-arcseconds. At the distance of 1,600

s this is a physical size of approximately 25 stellar radii.

Companion

It has been proposed P Cygni's eruptions could be caused by

mass transfer Mass transfer is the net movement of mass from one location (usually meaning stream, phase, fraction or component) to another. Mass transfer occurs in many processes, such as absorption, evaporation, drying, precipitation, membrane filtra ...

to a hypothetical

companion star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in ...

of spectral type B that would have a

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different element ...

between 3 and 6 times the mass of the Sun and would orbit P Cygni each 7 years in a high

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off- center, in geometry * Eccentricity (graph theory) of a ...

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such a ...

. Infall of matter into the secondary star would produce the release of

gravitational energy Gravitational energy or gravitational potential energy is the potential energy a massive object has in relation to another massive object due to gravity. It is the potential energy associated with the gravitational field, which is released (conve ...

, part of which would cause an increase of the luminosity of the system.

References

External links

Spectrum

AAVSO Variable Star of the Month. P Cygni: September 2009
{{Stars of Cygnus Cygnus (constellation) Cygni, P Cygni, 34 193237 100044 Luminous blue variables B-type hypergiants B-type supergiants 7763 16000818 BD+37 3871