# Eddington Luminosity – Definition & Detailed Explanation – Astrophysics Glossary

## I. What is Eddington Luminosity?

Eddington Luminosity is a concept in astrophysics that refers to the maximum luminosity that a star can achieve when the outward radiation pressure balances the inward gravitational force. It is named after the British astrophysicist Sir Arthur Eddington, who first proposed this concept in the early 20th century. Eddington Luminosity is a critical parameter in understanding the behavior of stars, especially massive ones, and plays a significant role in stellar evolution.

## II. How is Eddington Luminosity calculated?

The Eddington Luminosity can be calculated using the Eddington Limit, which is the maximum amount of luminosity that can be produced by a star before the radiation pressure becomes so intense that it overcomes the gravitational force holding the star together. The Eddington Limit is determined by the balance between the radiation pressure generated by the star’s energy output and the gravitational force pulling matter inward.

Mathematically, the Eddington Luminosity (L_Edd) can be expressed as:

L_Edd = (4πGMm_p c) / σ_T

Where G is the gravitational constant, M is the mass of the star, m_p is the proton mass, c is the speed of light, and σ_T is the Thomson scattering cross-section. By calculating this value, astronomers can determine the maximum luminosity that a star can achieve before reaching the Eddington Limit.

## III. What is the significance of Eddington Luminosity in astrophysics?

Eddington Luminosity plays a crucial role in astrophysics as it provides insights into the behavior of stars, particularly massive ones. When a star reaches its Eddington Luminosity, it can have profound effects on its evolution and eventual fate. For example, when a star exceeds its Eddington Luminosity, it can lead to the ejection of its outer layers in a stellar wind or even a supernova explosion.

Additionally, Eddington Luminosity is essential for understanding the dynamics of accretion disks around black holes and other compact objects. By studying the luminosity of these systems, astronomers can gain valuable information about the mass and energy transfer processes occurring within them.

## IV. How does Eddington Luminosity relate to stellar evolution?

Eddington Luminosity is closely tied to stellar evolution, as it dictates the maximum luminosity that a star can achieve during its lifetime. As a star evolves, its luminosity and temperature change, affecting its size, composition, and eventual fate. When a star reaches its Eddington Luminosity, it can undergo significant changes, such as the onset of stellar winds, mass loss, or even a supernova explosion.

For massive stars, exceeding the Eddington Luminosity can lead to the formation of a Wolf-Rayet star, a hot, luminous star with strong stellar winds. These stars are often precursors to supernova explosions and play a crucial role in enriching the interstellar medium with heavy elements.

## V. What are some examples of objects with Eddington Luminosity?

Several astronomical objects exhibit Eddington Luminosity, including massive stars, black holes, and active galactic nuclei. Massive stars, such as Wolf-Rayet stars, can reach their Eddington Luminosity during the later stages of their evolution, leading to the ejection of their outer layers in stellar winds.

Black holes, especially supermassive ones at the centers of galaxies, can also reach their Eddington Luminosity when they accrete matter from their surroundings. This can result in the formation of powerful jets of radiation and particles that can be observed across vast distances.

Active galactic nuclei, which are powered by supermassive black holes at the centers of galaxies, can also exhibit Eddington Luminosity when they accrete large amounts of matter. This can lead to the emission of intense radiation across the electromagnetic spectrum, making them some of the brightest objects in the universe.

## VI. How do astronomers study Eddington Luminosity in the universe?

Astronomers study Eddington Luminosity in the universe using a variety of observational techniques and theoretical models. By measuring the luminosity, temperature, and mass of stars, black holes, and other objects, astronomers can determine whether they are approaching or exceeding their Eddington Luminosity.

One common method is to observe the spectral lines emitted by these objects, which can provide valuable information about their composition, temperature, and luminosity. By analyzing these spectra, astronomers can determine whether an object is undergoing significant changes due to reaching its Eddington Luminosity.

Additionally, astronomers use theoretical models and simulations to predict the behavior of objects at or near their Eddington Luminosity. By combining observational data with theoretical models, astronomers can gain a better understanding of the processes driving the evolution of stars, black holes, and other celestial objects in the universe.