In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. Because stars, which comprise most of the visible matter in the universe, are composed mostly of hydrogen and helium, astronomers use for convenience the blanket term “metal” to describe all other elements collectively. Thus, a nebula rich in carbon, nitrogen, oxygen, and neon would be “metal-rich” in astrophysical terms even though those elements are non-metals in chemistry. This term should not be confused with the usual definition of “metal”; metallic bonds are impossible within stars, and the very strongest chemical bonds are only possible in the outer layers of cool K and M stars. Earth-like chemistry therefore has little or no relevance in stellar interiors.
The metallicity of an astronomical object may provide an indication of its age. When the universe first formed, according to the Big Bang theory, it consisted almost entirely of hydrogen which, through primordial nucleosynthesis, created a sizeable proportion of helium and only trace amounts of lithium and beryllium and no heavier elements. Therefore, older stars have lower metallicities than younger stars such as our Sun.
Image credit:NASA, ESA, and H. Richer (University of British Columbia)
arcadians: Dione. photographed by Cassini, 28th March 2012.
Evander Crater and central peak complex, 57°S 145°W. At 350km wide, Evander is the biggest crater on Dione. At bottom left is half of Erulus Crater and its central peak.
In legend and poem, Evander was a wise king of Arcadia, and an ally of Aeneas in the Trojan war. As far as I know the crater’s central peak has no name; “Mt. Pallantium” might serve, after the city that Evander founded, later to become part of Ancient Rome.
Erulus is a curious and obscure character, possibly mythological but only recorded in Vergil’s Aeneid; he was born of the fertility-harvest goddess Feronia, and had three souls and six arms. Accordingly, Evander was compelled to kill him three times before he was slain.
moth: Comet and solar corona, photographed by SOHO, April 2004.
Comet Bradfield (C/2004 F4) at and just after perihelion. This animation uses 23 images taken 17th-19th April, about one every two hours. At its closest, the comet was about 25 million km from the sun (16% of the Earth-Sun distance).
C/2004 F4 was discovered by William Bradfield, amateur astronomer and prolific comet hunter: The most prolific comet hunter of all was Jean-Louis Pons (1761-1831) with 37; the second William Robert Brooks (1844-1921) with 26; Bradfield has discovered 18.
The most prolific comet detecting instrument, incidentally, is SOHO, with more than 2,500 discoveries.
It is suspected that in this case, Hubble had locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in this remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288.