From the January 2024 issue

Red Spider Nebula

By | Published: January 1, 2024

The Red Spider Nebula (NGC 6537) is a planetary nebula in Sagittarius located 2.5° north of the bright open cluster M21. At 13th magnitude and roughly 1′ across, it can easily fall below the radar of observers among the more interesting objects in that area of the sky. However, don’t let the name fool you. The red color is photographic in nature, not visual. And the spider shape is beyond all but the largest telescopes. For most, it may look like a disk with irregular edges. NGC 6537 can be seen in modest telescopes under excellent sky conditions, but its small size makes it a challenge. There is a similar object that’s much easier to see: the Bug Nebula (NGC 6302) in Scorpius. It has a similar shape, which is visible in modest instruments. Both are classified as bipolar planetary nebulae.

NGC 6537’s odd shape is thought to be related in large part to a companion of the hot white dwarf that generated and now energizes this nebula. That white dwarf has a surface temperature as high as 50 times hotter than the Sun’s. The density of gas surrounding the center makes the suspected binary impossible to see. Some astronomers think that unusual magnetic fields created the bipolar structure. The pressure from the star’s solar wind created waves not unlike those made by wind blowing across a lake — although these waves are a tad larger!

The distance to NGC 6537 is subject to much uncertainty. The current best guess is between 1,900 and 4,000 light-years.