68 Cygni is the Flamsteed designation for a star in the constellation Cygnus. Located approximately distant, the star is a hot blue giant of spectral type O7.5IIIn((f)), a massive star that is likely currently expanding to become a supergiant. The star is surrounded by a ring-shaped nebula (likely a Strömgren sphere) named S 119.
68 Cygni is currently classified as a rotating ellipsoidal variable, varying between apparent magnitudes 4.98 and 5.09, although the classification as a rotating ellipsoidal variable is subject to controversy. Barely visible to the naked eye, the star is likely to have a mass of around 26 solar masses and a temperature of approximately 34,000 kelvins, although many of the star's physical parameters are subject to uncertainties due to the unclear nature of the system.
Naming
Being visible to the naked eye, 68 Cygni would have been observable since ancient times, but the first known cataloging and designation of the star was by Johann Bayer, who assigned it the Bayer designation A Cygni. Bayer grouped stars into six magnitude groupings (1st- through 6th-magnitude) and assigned Greek letters to the brightest 24 stars in these groupings, sorted by right ascension within groups. When he ran out of Greek letters, Bayer then moved on to Latin letters; thus, 68 Cygni was the 25th star Bayer designated in Cygnus.
The star was later observed by John Flamsteed, who cataloged naked-eye stars by constellation. Contrary to popular belief, Flamsteed did not assign the stars Flamsteed designations; rather, the French astronomer Joseph Jérôme de Lalande assigned Flamsteed numbers to stars in each constellation in order of right ascension. Since this star was the 68th star by right ascention in Cygnus, it was numbered 68 Cygni, the designation by which this star is most commonly known today, as most Latin-letter Bayer designations are no longer in common use.
After the star was discovered to be variable, it was assigned a variable star designation. As the 1809th variable star without a Bayer designation to be discovered in Cygnus, 68 Cygni was assigned the designation V1809 Cygni in 1984.
Observation
68 Cygni has an apparent magnitude of approximately 5.0, making the star a fairly inconspicuous star in the night sky, and nearly invisible when the moon is full. With a new moon, according to the Bortle scale, the star cannot be easily seen with the naked eye from polluted areas or with a full moon, but can be seen from suburban areas when overhead.
In the night sky, 68 Cygni is located west of the main asterism of Cygnus, approximately seven degrees east-southeast of the first-magnitude star Deneb. This is near the midpoint of the imaginary line between the 3rd-magnitude stars Xi Cygni and Rho Cygni. The star can also be found by continuing north along the line between the stars Zeta Cygni, Upsilon Cygni, Tau Cygni, and Sigma Cygni, as 68 Cygni is the first easily visible naked-eye star one will come to.
Properties
According to the star's measured parallax of 0.70 milliarcseconds, it is located approximately distant, although such low parallax values are subject to low precision. With taking into account the error estimate of 0.23 milliarcseconds, the star's distance could be anywhere between and distant, although values close to the mean value are more likely.
68 Cygni is a massive blue giant of spectral type O7.5IIIn((f)). Such massive stars only remain in the main sequence phase for a few million years, less than a thousandth of the expected main sequence lifetime of the sun. 68 Cygni has already expanded off the main sequence to become a blue giant, and is likely expanding to become a blue supergiant, although for very hot stars such as 68 Cygni evolution to the giant and supergiant classes does not indicate exhaustion of core hydrogen, nor is it accompanied by a large increase in luminosity. Instead the products of fusion burning are distributed throughout the star by convection and rotational mixing, producing helium and nitrogen lines in the spectrum, leading to an increase in the thickness of the stellar wind, and accompanied by a modest expansion and cooling of the star. The "n" in the spectral type indicates the presence of broad absorption lines due to how rapidly the star is spinning. The "((f))" indicates strong absorption in the He II spectral lines and weak N III emission lines.