Epoch J2000 Equinox J2000
|Right ascension||20h 46m 25.54s|
|Declination||+40° 06′ 59.4″|
|Apparent magnitude (V)||16.60|
|Apparent magnitude (K)||12.3|
|B−V color index||+2.04|
|Proper motion (μ)||RA: −1.55 mas/yr
Dec.: −4.59 mas/yr
|Parallax (π)||0.620 ± 0.047 mas|
|Radius||1,183 (1,640[a]-2,770[b]) R☉|
|Temperature||3,834 (2,500-3,250) K|
NML Cygni or V1489 Cygni is a red hypergiant and one of the largest stars currently known with a radius of 1,640 solar radii (1.14×109 kilometres; 7.6 astronomical units) or between 1,183–2,770 solar radii (823,000,000–1.927×109 kilometres; 5.50–12.88 astronomical units). Its distance from Earth is estimated to be around 1.6 kpc, about 5,300 light-years.
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. H
2O, SiO, CO, HCN, CS, SO, SO
2, and H
2S molecules have also been detected.
The radius of NML Cygni has recently been calculated to be 1,183 R☉ with an effective temperature of 3,834 K. An accurate measure of its distance and luminosity combined with assumptions of its effective temperature give a radius of 1,640 R☉[a] for a temperature of 3,250 K or 2,770 R☉[b] for a temperature of 2,500 K. In 2004, Zubko et al estimated the radius to be much larger around 3,740 R☉,[c] based on an assumed distance of 2,000 pc and an 8.6 mas angular diameter. More modern measurements give a radio angular diameter of 44 mas at 1,600 pc, suggesting the optical angular diameter may be around 22 mas.
If placed at the center of the Solar System, its surface would extend past the orbit of Jupiter or Saturn. It contains a volume between 1.6 and 21.4 billion times that of the Sun. The bolometric luminosity (Lbol) for NML Cygni is near ×105 L☉. Its 3bolometric 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. An earlier calculation of the luminosity gave a luminosity of ×105 L☉. 5 Massey, Levesque, and Plez's study, like VY CMa, think that NML Cygni is a normal red supergiant with a much smaller luminosity, and consequently a smaller value for the radius. NML Cyg is also a semiregular variable star with a period of around 940 days.
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.
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