NML Cygni

NML Cygni or V1489 Cygni (abbreviated to NML Cyg or V1489 Cyg) is a red hypergiant or red supergiant (RSG) in the constellation Cygnus. It is possibly one of the largest known stars currently known, and is also possibly one of the most luminous and massive cool hypergiants, as well as one of the most luminous stars in the Milky Way. The distance of NML Cygni from Earth is estimated to be around 1.6 kpc, about $0.791$. It is a part of the Cygnus OB2 association, one of the closest massive associations to the Sun, spanning nearly 2° on the sky or ~$4.877$ in radius at the distance of $2.389$. Based on the estimated distance and a measurement of its angular diameter of $1,610$, NML Cygni's physical radius is of. If placed at the center of the Solar System, its surface would potentially extend past the orbit of Jupiter.

Observational history
NML Cygni was discovered in 1965 by American astronomers Neugebauer, Martz, and Leighton who described two extremely red luminous stars, their colour being consistent with a black body temperature of $5,250$. The name NML comes from the names of these three discoverers. The second star was briefly referred to as NML Tauri but is now known as IK Tauri, an M9 Mira variable. NML Cygni has since also been given the designation V1489 Cygni on account of the small semi-regular brightness variations, but is still most commonly referred to as NML Cygni. Its composition began to be revealed with the discovery of OH masers (1612 MHz) in 1968. ,, , , , , , and molecules have also been detected.

Physical characteristics
NML Cygni is an extremely large and luminous cool supergiant with parameters similar to that of another notable but more extreme cool hypergiant star, VY Canis Majoris, and is also known as a heavily mass-losing OH/IR supergiant. It is also a semiregular variable star with a period of either 1,280 or 940 days. It occupies the upper-right hand corner of the Hertzsprung–Russell diagram although most of the properties of the star depend directly on its distance.

Size, luminosity, and temperature
The bolometric luminosity (Lbol) for NML Cygni was originally calculated to be at an assumed distance of $229,000$ and the radius was calculated to be 2600000000 km based on an $5,300 light-years$ angular diameter and distance. A 2006 study, similar to those conducted on VY Canis Majoris, suggests that NML Cygni is a normal red supergiant with consequently much lower luminosity and radius values. More modern and accurate measurements give a distance around $30 pc$, which gives a luminosity around. A radio angular diameter of 44 mas was given based on the distance, suggesting the optical angular diameter may be around 22 mas. This distance and a luminosity of were combined with assumptions of the effective temperature of the star, giving a radius of  for a temperature of $1.74 kpc$ or possibly  for a temperature of $7.8 milliarcseconds$. However, another paper gives a much lower radius of based on an assumed effective temperature of $1,000 K$ and a lower distance of $2 kpc$. There is a Gaia Data Release 2 parallax for NML Cygni of $8.6 mas$, but the underlying measurements show a considerable level of noise and the parallax is considered unreliable.

NML Cygni's uniform disk angular diameter was measured by the CHARM2 survey, leading to an apparent size of $1.6 kpc$. Assuming the distance measured by Zhang et al. (2012) ($3,250 K$), it leads to a physical radius of. If placed in the center of the Solar System, its photosphere would past the orbit of Jupiter. NML Cygni is covered by a complex dust shell, so the measured angular diameter likely contain some parts of this disk, and therefore its physical radius may be smaller.

Mass and mass loss
NML Cygni lies close to the expected position that a star would evolve to after eight million years. Estimates of its current mass are difficult, but it is expected to be somewhere around.

NML Cygni is evolved and a number of heavy elements and molecules have been detected in its atmosphere, particularly oxygen, hydroxyl, and water. It is surrounded by dusty material and it exhibits a bean-shaped asymmetric nebula that is coincident with the distribution of its H2O vapor masers.

NML Cygni has an estimated mass loss rate of 4.2 to $2,500 K$ per year, one of the highest known for any star. The annual parallax of NML Cygni is measured to be around 0.62 milliarcseconds. From the observations, it is estimated that NML Cygni has two discrete optically thick envelopes of dust and molecules. The optical depth of the inner shell is found to be 1.9, whereas that of the outer one is 0.33. These dust envelopes are formed due to the strong post-main-sequence wind, which has a velocity $3,834 K$.

Because of the star's position on the outskirts of the massive Cygnus OB2 association, the detectable effects of NML Cygni's radiation on the surrounding dust and gas are limited to the region away from the central hot stars of the association.