Cepheid variables are unique stars studied in space. They have been utilized in measuring cosmic distances. While stellar parallax is limited to stars within hundreds of parsecs, Cepheid variable stars and supernovae can measure larger distances, including those between galaxies and galaxy clusters.

The history of Cepheid findings can be traced back to the late 18th century when John Goodricke discovered Delta Cepheid. In addition, he correctly suggested the mechanism behind the star’s variability.

However, the breakthrough discovery happened much later in the early 20th century. Henrietta Swan Leavitt discovered the direct relationship between a Cepheid’s luminosity and its pulsation period. This finding established a basis for measuring cosmic distances. This article presents an overview of the discovery of Cepheid variables and their function in determining the cosmic distance.

What Is a Cepheid Variable Star

Cepheid star is a special type of variable star. What makes it different from the other kinds of variable stars is that it is pulsating. Pulsating is a term used to describe the star’s behavior, where it brightens and dims periodically.

The regular changes in the star’s brightness are not random but follow a precise and predictable pattern. The luminosity variations of a Cepheid are due to periodic expansions and contractions in the star’s outer layers. It was influenced by the internal mechanisms of the star, related to helium and hydrogen fusion.

Cepheid variables are identified by their regular and distinct pulsation patterns. Astronomers use light curves, graphs that show the brightness of a star over time, to detect these patterns. The presence of a regular pulsation cycle, typically ranging from a few days to months, helps in classifying a star as a Cepheid.

What Is Special About Cepheid Variable Stars

The significance of Cepheid variables depends on their role as “standard candles” in astronomy. Because these stars have a specific relationship between their luminosity (actual brightness) and pulsation period (the duration of a full cycle of brightening and dimming). This relationship is known as the Leavitt Law or the Period-Luminosity Relationship. It allows astronomers to determine their absolute brightness based on their pulsation period.

The Leavitt Law or the Period-Luminosity Relationship

The Period-Luminosity Relationship discovered by Henrietta Swan Leavitt describes the correlation between the pulsation period of a Cepheid variable star and its intrinsic luminosity (brightness). Leavitt had noticed a striking pattern among Cepheid variables – those with longer pulsation periods appeared to be intrinsically brighter than those with shorter periods.

By observing the pulsation period of a Cepheid variable star, astronomers could now determine its absolute luminosity. Since the apparent brightness of a star diminishes with distance (due to the inverse square law of light), comparing the star’s absolute brightness with its observed brightness allows astronomers to calculate its distance from Earth.

** All the detections and measurements of brightness, luminosity, and pulsating period are done via telescopes, photometers, and other instruments. In this article, we only present the general knowledge and overview of the method. To understand more about this topic, readers can refer to scientific journals or articles.

Why Do Cepheid Stars Pulsate

The pulsation of Cepheid stars is caused by the “Eddington valve,” or kappa mechanism. This mechanism involves the opacity changes in the ionization zones of helium and hydrogen within the star. The term “kappa” refers to the Greek letter κ, which symbolizes opacity in astrophysics.

As these layers become more opaque, they trap heat, causing the star to expand. When the opacity decreases, the outer layers cool and contract. This cyclical change in opacity and temperature drives the regular pulsation.

How the Kappa Mechanism Works

1. Opacity Variation: The kappa mechanism relies on the variation in the opacity of the star’s outer layers. Opacity is a measure of how transparent or opaque a material is to radiation. In Cepheid variable stars, the primary element responsible for the opacity changes is helium.
2. Ionization Zones: The outer layers of the star contain ionization zones where helium transitions between different ionization states. Helium can exist in three states: neutral helium (He), singly ionized helium (He+), and doubly ionized helium (He++). In Cepheids, the critical zones are where helium transitions from He+ to He++.
3. Increase in Opacity: As the star contracts under its gravity, the temperature in the ionization zones increases. When helium becomes doubly ionized (He++), the opacity of the star’s outer layers increases. This increase in opacity means that radiation from the core is trapped more effectively, causing the trapped energy to build up.
4. Expansion Phase: The trapped energy creates an outward pressure, causing the outer layers of the star to expand. As the star expands, the temperature in the ionization zones decreases. When helium recombines from He++ back to He+, the opacity decreases.
5. Decrease in Opacity: With lower opacity, radiation can escape more easily, reducing the outward pressure. This leads to a decrease in the expansion rate, and eventually, the outer layers of the star begin to contract again due to gravity.
6. Contraction Phase: As the star contracts, the cycle repeats: the temperature increases, helium ionizes to He++, opacity increases, and the cycle of expansion and contraction continues.

What Is the Closest Cepheid Variable Star to Earth

The nearest known Cepheid variable to Earth is Polaris situated in Ursa Minor constellation. Its approximate location is 448 light-years away. Polaris is an interesting star because it is both a Cepheid variable and a current North Star. The North Star is one of the pivotal points in the night sky for navigation.

Here is a listing of 10 famous Cepheid variable stars. These Cepheid variable stars are crucial for astronomers in measuring distances and studying the structure and scale of the universe due to their predictable brightness variations.

• Delta Cephei: This is the prototype of Cepheid variables and one of the most studied. Located in the constellation Cepheus, it’s approximately 887 light-years away.
• Zeta Geminorum: Also known as Mekbuda, this Cepheid is in the constellation Gemini and is about 1,200 light-years distant.
• Beta Doradus: Found in the southern constellation Dorado, it is approximately 1,050 light-years away.
• Eta Aquilae: Located in the constellation Aquila, this star is about 1,200 light-years from Earth.
• RS Puppis: One of the brightest Cepheids, located in the constellation Puppis, about 6,500 light-years away.
• T Monocerotis: This Cepheid is situated in the constellation Monoceros and is approximately 3,260 light-years distant.
• X Cygni: Found in the constellation Cygnus, this star is about 1,800 light-years away.
• W Sagittarii: Located in Sagittarius, this Cepheid is approximately 1,500 light-years from Earth.
• SU Cassiopeiae: In the constellation Cassiopeia, this star is about 2,900 light-years away.
• S Vulpeculae: Found in Vulpecula, this star is approximately 8,700 light-years away.

Summary

Cepheid variable stars are more than just celestial objects. They are fundamental tools to measure and understand the vast scales of our universe. By enabling precise distance measurements across galaxies, Cepheids have proven to be very invaluable in space study. Moreover, it will continue to help us to further the discovery of the cosmos.

Disclaimer:

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