Population II in the context of "Walter Baade"

The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A direct distance measurement of an astronomical object is possible only for those objects that are "close enough" (within about a thousand parsecs or 3×10 km) to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.

The ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy. Instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung.

In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-dark) matter in the universe is either hydrogen or helium, and astronomers use the word metals as convenient shorthand for all elements except hydrogen and helium. This word-use is distinct from the conventional chemical or physical definition of a metal as an electrically conducting element. Stars and nebulae with relatively high abundances of heavier elements are called metal-rich when discussing metallicity, even though many of those elements are called nonmetals in chemistry.

The Fornax Dwarf Spheroidal (formerly known as the Fornax System) is a dwarf elliptical galaxy in the constellation Fornax that was discovered in 1938 by Harlow Shapley. He discovered it while he was in South Africa on photographic plates taken by the 24 inch (61 cm) Bruce refractor at Boyden Observatory, shortly after he discovered the Sculptor Dwarf Galaxy.

The galaxy is a satellite of the Milky Way and contains six globular clusters, an unusually high number for its size; the largest, NGC 1049, was discovered before the galaxy itself. The galaxy is also receding from the Milky Way at 53 km/s. It mostly contains population II stars, but also has populations of young and intermediate age.

Planetary nebula luminosity function (PNLF) is a secondary distance indicator used in astronomy. It makes use of the [O III] λ5007 forbidden line found in all planetary nebula (PNe) which are members of the old stellar populations (Population II).It can be used to determine distances to both spiral and elliptical galaxies despite their completely different stellar populations and is part of the Extragalactic Distance Scale.