NGC 206

Lore & Background

NGC 206 was discovered by German-born English astronomer William Herschel in 1786, and possibly even two years earlier when he observed 'a streak of milky nebulosity, horizontal, or part of the 31st Nebula.' It was originally identified by Edwin Hubble as a star cluster, but due to its size it is now classified as an OB association. The star cloud contains hundreds of stars of spectral types O and B. NGC 206 has a double structure: one region has an age of around 10 million years and includes several H II regions in its border; the other region has an age of 40 to 50 million years and includes a number of cepheids. The two regions are separated by a band of interstellar dust.

Reader's Guide

NGC 206 holds significance as the richest and most conspicuous star cloud in the Andromeda Galaxy, making it a key feature for studying star formation within a major spiral galaxy. Its classification as an OB association, rather than a star cluster, reflects its large size and the nature of its stellar population. The presence of two distinct age regions—one about 10 million years old with H II regions, and another 40 to 50 million years old containing cepheids—provides insight into sequential star formation processes. The star cloud's location in a spiral arm, in a zone free of neutral hydrogen, adds to its unique characteristics. As one of the largest and brightest star-forming regions in the Local Group, NGC 206 serves as a comparative benchmark for understanding similar structures in other galaxies. Its discovery by William Herschel in the late 18th century, with a possible earlier observation, places it within the early history of extragalactic astronomy.

Did You Know?

• NGC 206 contains more than 300 stars brighter than Mb=−3.6.
• The star cloud has a double structure with two regions of different ages separated by a band of interstellar dust.
• NGC 206 is located in a spiral arm of the Andromeda Galaxy in a zone free of neutral hydrogen.