Galaxy Nebula Evolution

Each Galaxy Nebula object in a Galaxy represents a collection of stars, gas, and dust, orbiting through the galaxy together. In a real galaxy, stars form from clouds of gas, and then each star evolves with time. In Universe Sandbox, the stars represented by a Galaxy Nebula are not simulated using the Stellar Evolution model for individual Star objects. Instead, the appearance of the nebula reflects the amount of stars, gas, and dust within the nebula, as well as the temperature of the stars, and these properties are automatically set by Universe Sandbox to represent different galaxy types.

## Related Properties & Settings

### Properties

* Galaxy nebula evolution can be toggled on and off using an individual Galaxy Nebula's Enable Nebula Evolution property in the Nebula tab of the properties panel. Galaxy nebula evolution can also be toggled on and off for all of the nebulae at once in the Galaxy's Evolve Nebula Evolution property in the Galaxy tab.
* The number of stars represented by a single Galaxy Nebula is represented by the nebula's Star Count property. The total number of stars in the Galaxy is given by the galaxy's Star Count property.
* The relative amounts of gas, stars, and dust in the nebula are represented by the Gas Fraction, Star Fraction, and Dust Fraction properties. All three of these locked properties are percentages, which always add up to 100%.
* The Average Star Temperature property of a nebula controls the color of the nebula.
* The Density of a nebula affects its brightness: denser nebulae, which represent more stars and gas within the same volume, will appear brighter.

## Models

### Appearance

The properties of a Galaxy Nebula object listed above affect the nebula's appearance.

#### Stars, Gas, and Dust

Stars in a Galaxy Nebula object are represented by small points of colored light. Gas and dust are more diffuse, or spread out, in a nebula. The gas content in a Galaxy Nebula represents the interstellar gas that forms stars, while the dust content represents the interstellar dust. The simulated gas emits light while the simulated dust blocks light.

#### Star Temperature

The color of a Galaxy Nebula is based on the Average Star Temperature property, just as the color of Star objects is based on their Effective Temperature. Higher Average Star Temperatures indicate younger, hotter, more massive stars, and make the nebula more blue. Lower Average Star Temperatures indicate older, cooler, less massive stars, and make the nebula more red or yellow.

### Irregular Galaxies

The Average Star Temperature of a Galaxy Nebula in an irregular galaxy is randomly chosen from the range of 3500-36500 K. Nebulae in irregular galaxies are given a Dust Fraction of 0%, a Gas Fraction of 60%, and a Star Fraction of 40%.

### Elliptical Galaxies

The Average Star Temperature of a Galaxy Nebula in an elliptical galaxy is set to 3500 K. Nebulae in elliptical galaxies are given a Dust Fraction of 0%. The Gas Fraction of a nebula depends on its distance from the black hole at the center of the galaxy, and will change as the nebula orbits closer to and farther from the center. Nebulae closer to the center will have higher Gas Fractions than nebulae at the outer edges of the galaxy. The Star Fraction will represent the remaining mass in the nebulae that is not in the gas, i.e., the Star Fraction is 100% minus the Gas Fraction.

### Spiral Galaxies

The Average Star Temperature of a Galaxy Nebula in a spiral galaxy depends on the nebula's distance from the black hole at the center of the galaxy and will change as the nebula orbits closer to and farther from the center. Nebulae closer to the center will have a lower Average Star Temperature compared with nebulae at the outer edges of the galaxy.

In the bulge of the galaxy, the Gas Fraction of the nebulae also depends on the distance of the nebulae from the center, with nebulae closer to the center having larger Gas Fractions. The nebulae in the bulge are given Dust Fractions equal to 0%, and Star Fractions such that the sum of the Gas Fraction and the Star Fraction is 100%.

In the disk of the galaxy, the relative star, gas, and dust content is based on the nebula's True Anomaly. This allows Universe Sandbox to simulate real effects seen in spiral galaxies, like bright spiral arms and dark dust lanes, that are created by physical processes that are not currently simulated by Universe Sandbox, like shock waves and star formation. When the nebula is in a spiral arm, as measured by its True Anomaly, the nebula's Dust Fraction is 0%, its Gas Fraction is 60%, and its Star Fraction is 40%. When the nebula is between spiral arms, its Gas Fraction is set to 0%, its Dust Fraction is calculated based on how close the nebula is to the next spiral arm, and its Star Fraction is set such that the sum of the Dust Fraction and the Star Fraction is 100%.