June 30, 2001
On the day you were born, the universe was already ancient beyond measure — and yet, in all those billions of years, it had never once...
Hydrogen, Helium, and the Stars of M10
Birthday · June 30, 2001
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APOD published June 30, 2001. Credit: Till Credner, Sven Kohle (Bonn University), Hoher List Observatory. A publication-date match, not a photo taken on your date. Independent of NASA.
Stars like the Sun use hydrogen for fuel, "burning" hydrogen into helium at their cores through nuclear fusion. But what happens when that hydrogen runs out? For a while, hydrogen burns in a shell surrounding the stellar core and the star expands to become a red giant. The bright reddish-orange stars in this beautiful two-color composite picture of the old globular star cluster M10 are examples of this phase of stellar evolution. Yet the bright blue stars apparent in M10 have evolved beyond the simple, hydrogen shell burning stage. These stars have become "horizontal branch" giants with core temperatures hot enough to burn helium into carbon. In this image, only the barely visible, faint, gray-looking stars are likely to still be burning hydrogen at their cores.
Image credit: Till Credner, Sven Kohle (Bonn University), Hoher List Observatory