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== Introduction ==

A '''standard candle''' is an astronomical object that has a known luminosity. By comparing its known luminosity to its observed brightness, astronomers can determine its distance from Earth. Standard candles are crucial for measuring astronomical distances and for understanding the scale of the universe. They play a pivotal role in the field of [[cosmology]], particularly in the study of the [[expansion of the universe]] and the determination of the [[Hubble constant]].

== Types of Standard Candles ==

=== Cepheid Variables ===

[[Cepheid variable]] stars are one of the most important types of standard candles. These stars exhibit a well-defined relationship between their luminosity and their pulsation period, known as the [[period-luminosity relation]]. Discovered by [[Henrietta Swan Leavitt]] in the early 20th century, this relationship allows astronomers to determine the intrinsic brightness of a Cepheid variable by measuring its pulsation period. Cepheid variables are particularly useful for measuring distances within our [[Milky Way galaxy]] and to nearby galaxies.

=== Type Ia Supernovae ===

[[Type Ia supernova]]e are another critical type of standard candle. These supernovae occur in binary systems where a white dwarf accretes matter from a companion star until it reaches the [[Chandrasekhar limit]] and undergoes a thermonuclear explosion. The peak luminosity of Type Ia supernovae is remarkably consistent, making them excellent standard candles for measuring distances to galaxies far beyond the reach of Cepheid variables. Their use has been instrumental in the discovery of the [[accelerating expansion of the universe]].

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=== RR Lyrae Stars ===

[[RR Lyrae star]]s are older, low-mass stars that also serve as standard candles. They are pulsating variables with a well-defined period-luminosity relationship, although they are less luminous than Cepheid variables. RR Lyrae stars are useful for measuring distances within the Milky Way and to nearby globular clusters.

=== Tully-Fisher Relation ===

The [[Tully-Fisher relation]] is a correlation between the luminosity of a spiral galaxy and its rotation velocity. By measuring the rotation velocity of a galaxy, astronomers can infer its luminosity and thus its distance. This method is particularly useful for measuring distances to spiral galaxies.

== Historical Development ==

The concept of standard candles has evolved significantly over time. Early attempts to measure astronomical distances relied on [[parallax]], a method limited to relatively nearby stars. The discovery of Cepheid variables by Henrietta Swan Leavitt and the subsequent calibration of the period-luminosity relation by [[Edwin Hubble]] extended the reach of distance measurements to other galaxies. The identification of Type Ia supernovae as standard candles in the latter half of the 20th century further revolutionized the field, enabling measurements of distances across vast cosmic scales.

== Calibration of Standard Candles ==

Accurate calibration of standard candles is essential for reliable distance measurements. This process involves determining the absolute magnitude of the standard candle, which requires precise observations and often relies on multiple methods. For instance, Cepheid variables are calibrated using parallax measurements from missions like [[Hipparcos]] and [[Gaia (spacecraft)|Gaia]]. Type Ia supernovae are calibrated using Cepheid variables in galaxies where both types of objects are observed.

== Applications in Cosmology ==

Standard candles are indispensable tools in cosmology. They provide the means to measure the [[Hubble constant]], which describes the rate of expansion of the universe. Observations of Type Ia supernovae have led to the discovery of [[dark energy]], a mysterious force driving the accelerated expansion of the universe. Standard candles also help in mapping the large-scale structure of the universe and in studying the [[cosmic distance ladder]], a series of methods by which astronomers determine distances to celestial objects.

== Challenges and Limitations ==

Despite their utility, standard candles are not without challenges and limitations. The intrinsic variability of some standard candles, such as Cepheid variables, can introduce uncertainties in distance measurements. The calibration of Type Ia supernovae is complicated by factors such as [[metallicity]] and [[reddening]] due to interstellar dust. Additionally, the assumption that all Type Ia supernovae have the same peak luminosity has been questioned by recent observations suggesting variations in their properties.

== Future Prospects ==

The future of standard candles looks promising with advancements in observational technology and data analysis. Missions like [[James Webb Space Telescope|JWST]] and [[Vera C. Rubin Observatory|LSST]] are expected to provide unprecedented data on standard candles, improving their calibration and extending their reach. The continued study of standard candles will enhance our understanding of the universe's expansion history, the nature of dark energy, and the overall cosmic distance scale.

== See Also ==

* [[Cosmic distance ladder]]
* [[Hubble's law]]
* [[Dark energy]]
* [[Parallax]]
* [[Redshift]]

[[Category:Astronomy]]
[[Category:Cosmology]]
[[Category:Astrophysics]]