by Ann » Thu Sep 27, 2012 5:12 am
Oh, this is a wonderful APOD! Really and truly! And Marco Lorenzi is one of the great astrophotographers who regularly grace the Recent Submissions thread in the Astrophotography forum here. Congratulations, Marco!
Note, in the picture, that interstellar dust is typically reddish-brown in color, because it scatters away blue light and preferentially lets through long-wave yellow and red light. But when the dust scatters light in our direction, that light can be very blue. Particularly if it emanates from blue stars, of course!
Note the whitish color of the ancient globular at upper right. (What a skyscape, by the way, with ongoing star formation in the foreground and a circa 10-12 billion-year-old globular in the background. Its whitish color is testimony to the "metal-poor" gas that the stars in the globular are made of. Elements heavier than hydrogen and helium are "cooked up" inside stars, and these elements become part of the "interstellar medium" when the stars that made them die and return much of the gas they were made of to the space around them, either by exploding as supernovae or by ejecting their "outer shells" in planetary nebulae. The older the universe gets, the more generations of stars will have come and gone, the more stars will have returned their metal-enriched innards to space, and the more metal-rich the gas inside galaxies will become. But the globular in this picture was born when the ambient gas was metal-poor. Metal-poor gas gives rise to bluer stars, particularly because a certain kind of old, bright and blue stars, the blue horizontal branch stars, can only exist if the star it evolves from was born metal-poor.
What a snapshot of galactic evolution this picture is, and how beautiful it is!
Ann
Oh, this is a wonderful APOD! Really and truly! And Marco Lorenzi is one of the great astrophotographers who regularly grace the Recent Submissions thread in the Astrophotography forum here. Congratulations, Marco! :D :D :D
Note, in the picture, that interstellar dust is typically reddish-brown in color, because it scatters away blue light and preferentially lets through long-wave yellow and red light. But when the dust scatters light in our direction, that light can be very blue. Particularly if it emanates from blue stars, of course!
Note the whitish color of the ancient globular at upper right. (What a skyscape, by the way, with ongoing star formation in the foreground and a circa 10-12 billion-year-old globular in the background. Its whitish color is testimony to the "metal-poor" gas that the stars in the globular are made of. Elements heavier than hydrogen and helium are "cooked up" inside stars, and these elements become part of the "interstellar medium" when the stars that made them die and return much of the gas they were made of to the space around them, either by exploding as supernovae or by ejecting their "outer shells" in planetary nebulae. The older the universe gets, the more generations of stars will have come and gone, the more stars will have returned their metal-enriched innards to space, and the more metal-rich the gas inside galaxies will become. But the globular in this picture was born when the ambient gas was metal-poor. Metal-poor gas gives rise to bluer stars, particularly because a certain kind of old, bright and blue stars, the blue horizontal branch stars, can only exist if the star it evolves from was born metal-poor.
What a snapshot of galactic evolution this picture is, and how beautiful it is! :D
Ann