Magnitude 16.6 Apparent magnitude (V) 16.6 | ||
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Radius 823.4 million km (1,183 R☉) Mass 6.464 × 10^31 kg (32.5 M☉) Similar WOH G64, UY Scuti, VY Canis Majoris, KY Cygni, VX Sagittarii | ||
NML Cygni or V1489 Cygni is a red hypergiant and one of the largest stars currently known with a radius about 1,183 times the Sun's, equal to 5.5 astronomical units. Its distance from Earth is estimated to be around 1.6 kpc, about 5,300 light-years.
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NML Cygni is a part of the Cygnus OB2 association, one of the closest massive associations to the Sun, spanning nearly 2° on the sky or ∼30 pc in radius at the distance of 1.74±0.2 kpc.
Observational history
NML Cygni was discovered in 1965 by Neugebauer, Martz, and Leighton who described two extremely red luminous stars, their colour being consistent with a black body temperature of 1,000 K. 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. Molecules like H
2O, SiO, CO, HCN, CS, SO, SO
2, and H
2S have also been detected.
Characteristics
The radius of NML Cygni is about 1,183 solar radii (823,000,000 km; 5.50 AU). If placed at the center of the Solar System, its surface would extend to about the orbit of Jupiter. It contains a volume approximately 1.6 billion times that of the Sun. The bolometric luminosity (Lbol) for NML Cygni is near 3 × 105 L☉. Its bolometric magnitude (Mbol) is around −9.0. It is one of the most luminous cool hypergiants, as well as one of the most luminous stars in the Milky Way. It is also a semiregular variable star with a period of around 940 days. NML Cygni lies close to the expected position that a 25 M☉ star would evolve to after eight million years.
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 2×10−4 M☉ 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 23 km/s.
Because of its 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.