Open Star Clusters M35 and NGC 2158

Open clusters of stars can be near or far, young or old, and diffuse or compact.
Open clusters may contain from 100 to 10,000 stars, all of which formed at
nearly the same time. Bright blue stars frequently distinguish younger open
clusters. M35, pictured below on the lower right, is a relatively nearby at 2800
light years distant, relatively young at 150 million years old, and relatively
diffuse, with about 2500 stars spread out over a volume 30 light years across.
An older and more compact open cluster, NGC 2158, is visible above on the upper
left. NGC 2158 is four times more distant that M35, over 10 times older, and much
more compact as it contains many more stars in roughly the same volume of space.
NGC 2158's bright blue stars have self-destructed, leaving cluster light to be
dominated by older and yellower stars. Both clusters are visible toward the constellation
of Gemini -- M35 with binoculars and NGC 2158 with a small telescope.