GALAXIES · BEGINNER · 4 MIN READ
A galaxy is a whole world of stars
The Milky Way is our home galaxy. Other galaxies let us compare systems of stars, gas, dust and dark matter across a remarkable range of shapes.
A system held together by gravity
A galaxy contains stars and their remnants, often gas and dust, and an extended dark-matter component inferred through gravity. A solar system is much smaller: a star and its orbiting companions can be just one small part of a galaxy.
Our view of the Milky Way is from inside its disk. For nearby external galaxies, we can study an entire system from the outside. Those different viewing positions change what a photograph looks like.
Shapes give us clues
Spiral galaxies have a disk and spiral structure; many contain a central bar. Star formation often highlights parts of their arms. Elliptical galaxies look smoother and generally contain older stellar populations with less ongoing star formation. Irregular galaxies do not fit a regular spiral or elliptical outline.
These labels describe broad morphology. They are not a simple age sequence, and they do not summarize everything about a galaxy. Interactions, mergers, gas supply and internal dynamics can all affect its history.
A change of viewpoint can change the picture
A disk seen nearly edge-on can look like a thin band; the same kind of disk seen face-on can reveal arms. Dust can hide parts of the structure in optical light. A single image alone does not give a full three-dimensional map.
Keep the galaxy and its black hole distinct
M87 is an elliptical galaxy. M87* is its central black hole, a much smaller part of the system. A galaxy photograph and an image of emission around a black-hole shadow probe very different physical scales.
CHANGE SOMETHING. NOTICE WHAT HAPPENS.
Explore the idea
Spiral: a disk with spiral structure. Viewing angle can hide the arms.
Try it in Sky Orbit
Compare spiral galaxies M31 and M51 with elliptical M87. Then select M87* to distinguish the host galaxy from its central black hole.
The app may reveal a target below your horizon to show its direction. A marker does not guarantee naked-eye or telescope visibility.
CHECK YOUR UNDERSTANDING
Two questions to take away
Think of your answer, then open each question to compare.
Is a spiral galaxy necessarily younger than an elliptical galaxy?
No. Shape is not a universal clock. Galaxies have complex histories and contain stars of different ages.
Does an edge-on galaxy have to be physically thin like a line?
No. A three-dimensional disk can look narrow because we are viewing it from the side.
Sources & further reading
Beginner reference lesson · reviewed October 2026. Diagrams are original teaching illustrations; model assumptions are labeled.