Eta Carinae.
Caption: The Carina nebula is one of the brightest optical emission nebulae in the Milky Way. It stretches over some 2-1/2 degrees of sky which, at a distance of 2.6 kpc (8000 light years), is 300 light years across-about twenty times the size of the Orion nebula. Two clusters of young stars are primarily responsible for heating the nebula. Each cluster contains one of the brightest known stars in our galaxy. One is HD93129 A, which has a mass estimated to be about 120 times that of the Sun and releases most of its five million solar luminosities as ultraviolet radiation. The second bright star is Eta Carinae.
Eta Carinae, which also is one of the most luminous stars in our galaxy with a bolometric or total luminosity exceeding 10 40 ergs sec -1, emits most of its energy in the infrared. Eta Carinae is today the brightest infrared source outside our solar system observed at wavelengths of 10 and 20 microns. At about 7th magnitude, Eta Carinae is not visible to the naked eye today, but the star's brightness has changed by large amounts over the centuries. Halley estimated it to be 4 times the magnitude observed in 1977. About one hundred years later it was observed to be two magnitudes brighter. During the mid-1840s, it had brightened to magnitude -1 and was the second brightest star in the sky! Eta Carinae remained brighter than first magnitude until 1857, then faded to magnitude 8 by 1900. However, by the late 1970s, it had brightened to magnitude 6.2. It continues to show slow changes in its brightness.
Copyright: (c) Smithsonian Astrophysical Observatory
Credit: Courtesy of National Optical Astronomy Observatories