Image credit: NASA's Cosmic Times
For over a thousand years, astronomers wondered about the nature and development of nebulae. Nebulae appear to be faint clouds of gas and dust in the distant universe. Until recently, there have not been enough observations to classify nebulae based on their features or qualities.
During his studies of the spiral nebulae, Dr. Hubble proposed a system to classify all nebulae. He sorted them into three basic categories: elliptical, spiral, and irregular. These categories were further subdivided according to shape (spherical to elongated ellipses, for example) and structure (hazy to distinct spiral arms, barred spirals, etc.).
Hubble's system shows a sequence of evolutionary change but was "based primarily on the structural forms of photographic classification which should be entirely independent of theoretical considerations." Future astronomical studies and evidence will be the test for this new classification system.