Messier 43 or M43, also known as De Mairan's Nebula and NGC 1982, is a star-forming nebula with a prominent H II region in the equatorial constellation of Orion. It was discovered by the French scientist Jean-Jacques Dortous de Mairan some time before 1731,[3] then catalogued by Charles Messier in 1769.[a] It is physically part of the Orion Nebula (Messier 42), separate from that main nebula by a dense lane of dust known as the northeast dark lane.[5] It is part of the much larger Orion molecular cloud complex.
The main ionizing star in this nebula is HD 37061 (variable star designation NU Ori[b]), the focus of the H II region, 1,300 ± 160 ly (400 ± 50 pc) away.[c] This is a triple star system with the brighter component being a single-lined spectroscopic binary. The main component is a blue-white hued B-type main-sequence star with a stellar classification of B0.5V or B1V. It has 19±7 times the mass of the Sun (M☉) and 5.7±0.8 times the Sun's radius (R☉). It is radiating over 26,000 times the Sun's luminosity (L☉) from its photosphere at an effective temperature of 31,000 K. It is spinning rapidly with a projected rotational velocity of around 200 km/s.[5]
The H II region is a roundish volume of ionized hydrogen. It has a diameter of about 4.5′, at its distance meaning it measures 2.1 ly (0.65 pc). The net (meaning omitting the star) hydrogen alpha luminosity of this region is (3.0±1.1)×1035 erg s−1; equivalent to 78 L☉. There is a dark lane crossing the whole west-centre strip from north to south, known as the M43 dark lane, which forming a swirling belt extension to the south links to Orion's northeast dark lane. All of these resemble a mixture of smoke rising from a chimney and in watercolour broad and fine dark brushstrokes, at many wavelengths.
