NML Cygni
Spectral class: M
NML Cygni or V1489 Cygni is a red hypergiant star and one of the largest stars currently known, with a radius equal to 7.67 astronomical units), thus a diameter of about Template:Convert.
It is one of the extreme luminous hypergiant stars. NML Cygni is a part of the Cygnus OB2 association.
Characteristics
This star has a dusty environment surrounding it, and it exhibits a bean-shaped asymmetric nebula which is coincident with the distribution of its H2O vapor masers. It is a semiregular variable star with a period of around 940 standard days. It is one of the most luminous cool hypergiant stars.
From the observations, it is estimated that this star has two discrete optically thick envelopes of dust and molecules. The optical depth of the inner shell is found to be 1.9 while that of the outer one is 0.33. These dust envelopes are formed due to the strong post-main sequence wind velocity and the high mass emission rate. The post-MS wind velocity is 23 km/s. Because of its unique location within the Milky Way the star does not dominate its local interstellar environment.
It has also been given the designation V1489 on account of the small semi-regular brightness variations caused by pulsations and mass loss.
NML Cyg is a massive oxygen-rich star. The presence of molecules like H2O, SiO, CO, HCN, CS, SO, SO2 and H2S has also been revealed.
