Astronomy Picture of the Day
APOD: 1999 November 21 - Elliptical Galaxy NGC 4881 in Coma
Explanation: Elliptical galaxies are unlike spiral galaxies and hence unlike our own Milky Way Galaxy. The giant elliptical galaxy named NGC 4881 on the upper left lies at the edge of the giant Coma Cluster of Galaxies. Elliptical galaxies are ellipsoidal in shape, contain no spiral arms, contain little interstellar gas or dust, and are found mostly in rich clusters of galaxies. Elliptical galaxies appear typically yellow-red, as opposed to spirals which have spiral arms that appear quite blue. Much speculation continues on how each type of galaxy can form, on whether ellipticals can evolve from colliding spirals, or spirals can be created from colliding ellipticals, or both. Besides the spiral galaxy on the right, all other images in this picture are of galaxies that lie well behind the Coma Cluster.
APOD: 2004 June 16 - Elliptical Galaxy M87
Explanation: Elliptical galaxy M87 is a type of galaxy that looks much different than our own Milky Way Galaxy. Even for an elliptical galaxy, though, M87 is peculiar. M87 is much bigger than an average galaxy, appears near the centre of a whole cluster of galaxies known as the Virgo Cluster, and shows an unusually high number of globular clusters. These globular clusters are visible as faint spots surrounding the bright centre of M87. In general, elliptical galaxies contain similar numbers of stars as spiral galaxies, but are ellipsoidal in shape (spirals are mostly flat), have no spiral structure, and little gas and dust. The above image of M87 was taken recently by the Canada-France-Hawaii Telescope on top of the dormant volcano Mauna Kea in Hawaii, USA.
APOD: 1999 November 3 - M32: Blue Stars in an Elliptical Galaxy
Explanation: Elliptical galaxies are known for their old, red stars. But is this old elliptical up to new tricks? In recent years, the centres of elliptical galaxies have been found to emit unexpectedly high amounts of blue and ultraviolet light. Most blue light from spiral galaxies originates from massive young hot stars, in contrast to the red light from the old cool stars thought to compose ellipticals. In the above recently released, false-colour photograph by the Hubble Space Telescope, the centre of nearby dwarf elliptical M32 has actually been resolved and does indeed show thousands of bright blue stars. The answer is probably that these blue stars are also old and glow blue, reaching relatively high temperatures by the advanced process of fusing helium, rather than hydrogen, in their cores. M32 appears in many pictures as the companion galaxy to the massive Andromeda Galaxy (M31).
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