Standard candles

Standard candles are astronomical objects that have a known intrinsic brightness (luminosity). They are used as reference points to measure distances in the universe. By comparing the known luminosity of a standard candle to its observed brightness as seen from Earth, astronomers can determine how far away the object is. One of the most commonly used types of standard candles is certain types of variable stars, such as Cepheid variables, whose pulsation periods are directly related to their luminosity.

BL Herculis is a type of variable star that belongs to the class of "cataclysmic variables," specifically categorized as an "artificial system" consisting of a binary star system. In these systems, one star is a white dwarf, and the other is typically a red dwarf or main-sequence star. BL Herculis is notable for its periodic outbursts, which occur as a result of mass transfer from the companion star to the white dwarf.

A Cepheid variable is a type of star that exhibits regular and predictable variations in brightness over a specific period of time. These stars are essential for astronomers because their periodic brightness fluctuations are closely related to their intrinsic luminosity, allowing them to serve as important distance indicators in the universe. Cepheid variables are classified into two main types: **Classical Cepheids** and **Type II Cepheids**.

Classical Cepheid variables are a type of pulsating star characterized by periodic changes in brightness due to expansion and contraction of their outer layers. They are typically supergiant stars that belong to the spectral types F or G, and they have well-defined periods of variability that range from a few days to several weeks. The key features of Classical Cepheids include: 1. **Pulsation**: These stars undergo regular pulsations, caused by changes in temperature and pressure in their outer layers.

The Planetary Nebula Luminosity Function (PNLF) is a statistical distribution that describes the luminosities of planetary nebulae (PNs) within a given population, typically in a certain galaxy or specific region of space. It is an important tool in astrophysics for understanding the properties and evolution of stars, particularly those that have evolved off the main sequence and shed their outer layers.

RR Lyrae variables are a type of short-period variable stars that are characterized by their pulsating nature. They are named after the prototype star, RR Lyrae, which was discovered in the constellation Lyrae. Here are some key features of RR Lyrae variables: 1. **Pulsation**: RR Lyrae stars exhibit periodic brightness changes due to pulsations in their outer layers. These changes in brightness typically occur over a period ranging from about 0.2 to 1 day.

Surface Brightness Fluctuation (SBF) is an astronomical technique used to measure the distances to galaxies, particularly in the context of resolving extragalactic structures. This method is based on the idea that the brightness of the light from a galaxy is not uniform, but rather exhibits small variations or "fluctuations" due to the uneven distribution of stars within the galaxy.

The "tip of the red-giant branch" (TRGB) refers to a specific stage in the evolutionary life cycle of stars, particularly for stars with masses similar to or slightly greater than that of our Sun. A star reaches this phase after it has evolved off the main sequence, having exhausted the hydrogen in its core and transitioned to hydrogen shell burning outside of an inert helium core.