Many old ideas about ellipticals have been shaken up by discoveries made over the past couple of decades. Among these is the notion that ellipticals contain hardly any interstellar matter. When looked at closely, about a third of them show features due to absorption by dust. The dust seems to be distributed in rings or disks, in some cases aligned with either the major or minor axes and in other cases warped. Ellipticals contain modest amounts of cool and warm gas, though not as much as found in spirals galaxies and not usually enough to support much star formation. A few have extended disks of neutral hydrogen. X-ray observations show that many ellipticals contain between 1 billion and 10 billion solar masses of gas at temperatures of 10 million K, typically in the form of a pressure-supported atmosphere around the galaxy. The surface brightness of ellipticals doesn't always decline smoothly with radius. When a smooth luminosity profile is subtracted from the actual surface brightness, concentric shells or ripples are often seen. These shells are somewhat bluer than the rest of the galaxy and appear to be composed of younger stars.
The orbits of stars in ellipticals are in random directions, and often very elongated, taking them close to the center and then far into the outskirts. It was once thought that ellipticals were oblate spheroids – shaped like a hamburger bun – and that they kept this shape because they were slowly rotating. It is now know that they are often prolate, like a rugby ball, or even triaxial, meaning they have different diameters in all three directions. They hardly rotate at all; instead the shape is maintained because the stars move faster in the direction of the long axis, and so can travel further from the center before gravity turns them back.
Related category GALAXIES
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