P Cygni (34 Cygni) is a variable star in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

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, a ...

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 1012 ...

s (1,560 parsecs) from Earth, it is a hypergiant

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 B1-2 Ia-0ep that is one of the

most luminous stars This is a list of stars arranged by their absolute magnitude – their intrinsic stellar luminosity. This cannot be observed directly, so instead must be calculated from the apparent magnitude (the brightness as seen from Earth), the distance t ...

in the Milky Way.

Visibility

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

s (1,500–1,800 parsecs) 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, 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 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 ...

; 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 since a series of large outbursts in the 17th century. Similar events have been seen in Eta Carinae 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 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 A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...

. The recent supernova

SN 2006gy SN 2006gy was an extremely energetic supernova, also referred to as a hypernova or quark-nova, that was discovered on September 18, 2006. It was first observed by Robert Quimby and P. Mondol,

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 A Type II supernova (plural: ''supernovae'' or ''supernovas'') results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, but no more than 40 to 50 times, the mass of the Sun () to undergo this ...

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 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 blueshifted 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. D ...

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

of spectral type B that would have a mass between 3 and 6 times the mass of the Sun and would orbit P Cygni each 7 years in a high eccentricity orbit. Infall of matter into the secondary star would produce the release of gravitational energy, 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