NML Cygni

NML Cygni

H-Alpha light image of Cygnus OB2, the stellar association in which NML Cygni is located
Observation data Epoch J2000      Equinox J2000
Constellation	Cygnus
Right ascension	20h 46m 25.6s
Declination	+40° 06′ 59.4″
Apparent magnitude (V)	16.60
Characteristics
Spectral type	M6I[1]
B−V color index	2.0
Variable type	Semiregular
Astrometry
Proper motion (μ)	RA: -1.55[2] mas/yr Dec.: -4.59[2] mas/yr
Parallax (π)	0.620[2] ± 0.047[2] mas
Distance	1.61k[2] pc
Details
Mass	25–40[2] M☉
Radius	1,650[2] R☉
Luminosity	270,000[2] L☉
Other designations
V* V1489 Cyg, RAFGL 2650, IRC +40448
Database references
SIMBAD	data

NML Cygni or V1489 Cygni is a red hypergiant star^[2] and one of the largest stars currently known with a radius about 1,650 times the Sun's,^[3] equal to 7.67 astronomical units. Its distance from Earth is estimated to be around 1.6 kpc, or about 5,300 light-years.^[3]

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.^[4]

Characteristics

The diameter of NML Cygni, at 1,650 times that of the sun, is about 2,295,000,000 kilometres (1.426×10⁹ mi). If placed at the center of the Solar System, its surface would extend beyond the orbit of Jupiter, filling over half the gap between Jupiter and Saturn's orbit. It contains a volume approximately 4.5 billion times that of the Sun. The bolometric luminosity (L_bol) for this star is near 3 × 10⁵ L_☉. Its bolometric magnitude (M_bol) is around −9.0. It is one of the most luminous cool hypergiant stars.^[4] It is also a semiregular variable star with a period of around 940 days.^[4]

This star is evolved and a number of heavy elements and molecules have been detected in its atmosphere, particularly oxygen, hydroxyl, and water. It has a dusty environment surrounding it,^[2][4] and it exhibits a bean-shaped asymmetric nebula that is coincident with the distribution of its H2O|H₂O vapor masers.^[5]

NML Cygni has an estimated mass loss rate of 2×10⁻⁴ M_☉per year,^[6][7] one of the highest known for any star. The annual parallax of this star is measured to be around 0.62 milli-arc-seconds.^[2] 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.^[8] These dust envelopes are formed due to the strong post-main sequence wind velocity, 23 km/s, and the high mass loss.

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.^[4]

Observational history

NML Cygni was discovered in 1965, by Neugebauer, Martz, and Leighton which described two extremely red luminous stars, their colour being described as consistent with a black body temperature of 1,000K.^[9] The name NML comes from the names of these three discoverers.^[10] The second star was briefly referred to as NML Tauri^[11] but is now known as IK Tauri,^[12] an M9 Mira variable. NML Cygni has since also been given the designation V1489 Cygni on account of the small semi-regular brightness variations,^[13] 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 (Wilson and Barrett).^[14] The presence of molecules like H₂O, SiO, CO, HCN, CS, SO, SO₂, and H₂S was also revealed in the later years.^[6]

See also

• VY Canis Majoris

References