For this week's APOD, I chose the picture called Flame Nebula Close-Up, uploaded on November 26, 2010. It is a picture of the big Flame Nebula, also known as NGC-2024. NGC-2024 is an emission nebula in the constellation of Orion. The nebula appears to be on fire, but it isn't actually on fire.
The nebula appears to have a reddish color, which is because of the glow of hydrogen atoms. The nebula is in the process of forming a star cluster. The hydrogen atoms are on the edge of Giant Orion, which is a molecular cloud about 1,500 light-years away from our solar system. The hydrogen atoms have been stripped of their electrons, or in other words, they have been ionized. By losing electrons, the atoms become positively charged ions. The glow is caused by the the atoms and electrons recombining with each other. In this picture of the nebula, there is a central dark lane of absorbing interstellar dust, which is prominent as a dark silhouette when compared to the hydrogen atoms glowing next to them. This can actually hide the true source of the Flame Nebula's energy from optical telescopes. At infrared wavelengths, young stars can be seen behind the dark areas. The most probable source of ultraviolet radiation and ionization of hydrogen gas in this nebula is a young, massive star in the cluster.
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