Notable H II Regions
Notable Galactic H II regions include the Orion Nebula, the Eta Carinae Nebula, and the Berkeley 59 / Cepheus OB4 Complex. The Orion Nebula, which lies at a distance of about 500 pc (1,500 light-years) is part of a GMC (called OMC-1) which, if it were visible, would fill most of the constellation of Orion. The Horsehead Nebula and Barnard's Loop are two other illuminated parts of this cloud of gas. The Orion Nebula is actually a thin layer of ionised gas on the outer border of the OMC-1 cloud. The stars in the Trapezium cluster and especially θ1 Orionis are responsible for this ionisation.
The Large Magellanic Cloud, a satellite galaxy of the Milky Way at about 50 kpc (160 thousand light years) from the Sun, contains a giant H II region called the Tarantula Nebula. Measuring at about 200 pc (650 light years) this nebula is the most massive and second largest H II region in the Local group. It is much bigger than the Orion Nebula, and is forming thousands of stars, some with masses of over 100 times that of the sun—OB and Wolf-Rayet stars. If the Tarantula Nebula was as close to Earth as the Orion Nebula, it would shine about as brightly as the full moon in the night sky. The supernova SN 1987A occurred in the outskirts of the Tarantula Nebula.
Another giant H II region—NGC 604 is located in M33 spiral galaxy, which is about 817 kpc (2.66 million light years) away from the Sun. Measuring at approximately 240 × 250 pc (800 × 830 light years) across NGC 604 is the second most massive H II region in the Local group after the Tarantula Nebula, however, it is slightly larger than the latter. It contains around 200 hot OB and Wolf-Rayet stars, which heat the gas inside it to millions of degrees producing the bright X-ray emissions. The total mass of the hot gas in NGC 604 is about 6,000 Solar masses.
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