When the prospect is deceiving.

M35 and NGC 2158 are two open clusters visible in the constellation Gemini. From this image the two clusters would seem next to each other, but this is only due to the fact that our eyes are not able to recognize the depth of what we are observing in the sky.

M35, on the left, is in fact 2800 light years away from Earth, while NGC 2158 is located four times farther.

The differences between the two clusters don't stop there: as you can see, the two clusters are in fact different in color!

This is due to their different physical characteristics. M35 is in fact made up of about 2500 stars enclosed in a sphere with a diameter of 30 light years. The cluster was formed only 150 million years ago and therefore its stars, still young, shine of an intense blue color.

On the opposite side, NGC 2158 contains many more stars in a smaller volume. Furthermore, its stars are yellow in color as they are on average ten times older than those of M35. All these aspects (color and concentration) make NGC 2158 appear similar to a globular cluster, despite being classified as an open cluster.

Credit: Bart Delsaert.