# Stellar astrophysics

Stellar astrophysics is a subfield of astrophysics, which is in itself a subfield of physics, that is focused on the study of stars as opposed to the study of galaxy formation and evolution, planetary systems, or cosmology. Stellar astrophysicists aim to understand these through a mixture of observational astronomy and theoretical physics (including nuclear physics, particle physics, fluid dynamics, etc).

## Overview
### Observational Stellar Astrophysics

While not the start of observational stellar astrophysics studies, the Hertzsprung–Russell diagram (H-R diagram for short) provides a great introduction to the study of stellar evolution. The H-R diagram is typically plotted as luminosity as a function of either temperature or color and shows the main phases in a star's life, e.g. main sequence, red giant or red supergiant, horizontal branch, and white dwarfs or neutron stars. This allows for the H-R diagram to be an excellent probe into stellar evolution. While not on the H-R diagram, observational studies of stellar astrophysics also include studies of supernovae. One specific type of supernova, the Type-1a supernova, can act as a standard candle to help measure our distance from other interstellar objects. These supernovae are typically caused by a binary star system where one of the stars is a white dwarf. Once the white dwarf in this system reaches a certain mass, known as the Chandrasekhar limit, the white dwarf will collapse into a neutron star. Thus, a white dwarf is an excellent standard candle as there is a strict upper limit (around 1.44 solar masses), and an upper limit of luminosity. Some neutron stars, known as pulsars, have been used in the past to try and measure a gravitational wave background (see NANOgrav).

### Theoretical Stellar Astrophysics

As of recently, most theoretical stellar astrophysics is done through numerical simulations to probe the inner workings of stars. The equations produced from the study of theoretical stellar astrophysics are typically too complicated to directly solve, even in the most idealized cases, which makes these simulations necessary for scientists to understand the stars. One of the larger examples of stellar simulations is the evolution code MESA. MESA allows stellar astrophysicists and anyone with coding knowledge to produce evolution tracks given an expanding set of parameters, such as the mass of a star. Earlier models of stars can be produced through numerical integrations of the Lane-Emden equation, which equates a star's density as a function of radius given a relation between central pressure and central density. Other such examples of computational stellar astrophysics include 3D models of massive stars.

### Importance

Stellar astrophysics touches many of the other sub-fields in astrophysics. On a galactic scale, we can determine whether a galaxy is currently forming stars from the light that emits from the galaxy (see galaxy). In cosmology, we can study the times when stars form, allowing scientists to determine the composition of our universe. The study of heliophysics is a more direct analysis of our sun, such as observational probes such as studying the emitted neutrinos.