“The universe has turned out to be wicked. He’s not going to give us easy answers.” The astrophysicist John Huchra , from the Harvard-Smithsonian Center for Astrophysics , expressed himself two decades ago in an ironic and categorical way. The universe has proven to have an almost infinite capacity to amaze astronomers. As new and larger telescopes come online, more unknowns appear.
One of those mysteries is a star visible in the summer sky . Without straying too far from the constellation of the Eagle whose main star, Altair, is one of the three brightest stars in the summer sky, we can walk through the constellation of the Swan, the northern cross, located in the middle of that milky band that crosses the sky from part to part and that sometimes receives the name of “the way of Santiago”. Just where the two arms intersect, in an area where groups of stars and dark nebulae abound, is P Cygni, a faint star of fifth magnitude – to the naked eye and with good skies we can see up to the sixth -.
It is at a distance of about 5000 light-years from us and we may be surprised to discover that this faint star that we can barely see is one of the brightest in the Milky Way. Its brightness usually varies by 20% -0.2 magnitudes-, which makes it a variable star, the goal of many amateur astronomers interested in drawing what is called its light curve over time. But this star is very peculiar because sometimes its brightness goes out of control .
What is really striking is that P Cygni is a newcomer in the history of astronomy. No one knew about it until August 18, 1600, when a Dutch astronomer and cartographer named Willem Janszoon Blaeu discovered it. Surely the surprise was capitalized because there was nothing there before. In 1603 a German astronomer named Johann Bayer included it in his sky atlas, Uranometria, as a third magnitude star and labeled it with the letter P, and it has remained that way ever since. For six years the star remained bright, but then gradually it began to dim, until it disappeared from view in 1626 . But he did not leave forever, but was seen again in 1655 . During the following years its brightness fluctuated many times and it was not until 1725 when it stabilized at the fifth magnitude, which it still maintains today (exactly, it is magnitude 4.8).
And it is that our beloved little star P Cygni is a hypergiant blue luminous variable , a very exclusive group where both the most luminous and the most massive stars in the Universe are found: the temperature on its surface is about 100,000º C, almost 20 times higher than the Sun, and emits 700,000 times more energy.
However, P Cygni is not a typical example of this type of star. Typically, the brightness of a luminous blue variable star fluctuates over years or decades, with occasional outbursts in which the brightness increases dramatically. However, P Cygni has kept its luminosity practically the same since the abrupt changes that we could see in the 17th century. Of course, due to the analysis of the environment of P Cygni , it is thought that it has suffered a series of large eruptions about 900, 2100 and possibly 20,000 years ago. From observations, we know that since its brightness stabilized, it has been increasing by about 0.15 magnitudes per century, which is attributed to a slow decrease in temperature. Why does he do this? Astronomers believe that this star is heading for its next evolutionary phase, that of a red supergiant . We are apparently looking at something extremely difficult to witness: a short stage in the life of a very massive star, lasting only about 100,000 years. We do not know what new surprises P Cygni hides, but its end is clear: it will explode as a supernova and its remains will end up forming a black hole.
