How space works
Galaxies, nebulae, and star clusters
A bright patch in the sky could be a gas cloud, a family of stars, or a galaxy containing billions of stars. Identifying the structure changes how its image should be read.
Updated
A galaxy is a large gravitational system
Galaxies contain stars, gas, dust, stellar remnants, and substantial dark matter. Spiral galaxies have disks and recognizable arms; elliptical galaxies have different distributions of stars; irregular galaxies lack a simple overall shape. These labels describe appearance and structure rather than a single inevitable age sequence.
The Milky Way is our home galaxy. Looking along its disk means looking through overlapping stars and dust from inside the system. A face-on picture of the Milky Way is therefore a reconstruction, while an external galaxy such as Andromeda can be photographed across much of its projected extent. The viewpoint changes what can be measured directly.
Nebulae are clouds with different causes
A nebula is a cloud of interstellar material. Some glow because nearby stars ionize their gas. Others scatter starlight or appear dark because their dust blocks the background. Stellar deaths can also create expanding shells and supernova remnants. A bright color in a processed image may represent light from one specific atomic transition.
The Orion Nebula is associated with active star formation. A planetary nebula, despite its name, is connected to a late stage of stellar evolution rather than the making of planets. Identifying the physical source of the light is more useful than assuming every colorful cloud is a stellar nursery.
Clusters let astronomers compare stars
An open cluster is a group of stars that formed together and remains recognizable for some time. Globular clusters are much more densely populated systems, often containing old stellar populations. Shared distance and formation history make clusters useful for comparing stars of different masses.
Membership still needs evidence. Stars can overlap along the same line of sight without belonging to the same system. Astronomers use position, motion, distance, and spectra to distinguish members from foreground and background objects. A constellation is different again: it is a defined region of our sky, not necessarily a physically connected group.
Images have scale, wavelength, and processing choices
A telescope image combines information about the target with the instrument and the selected wavelengths. Infrared can show regions hidden by visible-light dust; X-rays can reveal hot gas and energetic processes. Assigning visible colors to invisible wavelengths makes the structure accessible, but those colors are a presentation convention.
Compare angular size with distance before judging an object's true scale. A nearby cloud may cover more sky than a much larger distant galaxy. For galaxies far enough away that cosmic expansion matters, distance requires a cosmological model and a clear definition. The Codex's object articles focus on those distinctions rather than treating a vivid image as a simple view from a spacecraft window.