How big is Cave Nebula? | Scale of the Universe

The Cave Nebula is a faint cloud of glowing gas where new stars are forming, roughly 2,400 to 3,000 light-years from Earth in the constellation Cepheus. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. NASA gives the nebula a span of roughly 40 light-years, or 380 trillion kilometers. Its name comes from a dark opening framed by a bright arc of gas, which looks like the mouth of a cave.

Size

The Cave Nebula is an emission nebula, a cloud of gas that gives off its own light. The bright arc at the mouth of the cave is an H II region, where the hydrogen glows because starlight has stripped electrons from its atoms. The nebula is faint and spread out, so its edges are hard to trace.

Its size also depends on its distance, which is uncertain. NASA puts it roughly 3,000 light-years away, while studies of the surrounding star-forming region use about 2,400 to 2,600 light-years. A 1959 catalog lists the nebula as 60 arcminutes, or 1 degree, wide on the sky, where an arcminute is 1/60 of a degree. At 2,400 light-years that is about 42 light-years, and at 3,000 light-years about 52.

Size comparisons

The Cave Nebula is about 1.7 times as wide as the Orion Nebula, which spans about 24 light-years.
It is more than nine times the distance from the Sun to Proxima Centauri, the nearest star.
About 10 copies of the tallest of the Pillars of Creation, each about 4 light-years long, would stretch across it end to end.
On the sky, the nebula is about twice as wide as the full Moon.
If the Cave Nebula were shrunk to the length of a marathon, 42.195 kilometers (26.2 miles), the orbit of Neptune would be about 1 meter (3.3 feet) across. Proxima Centauri would be about 4.5 kilometers (2.8 miles) from the Sun.

Stars made by starlight

The Cave Nebula is part of a larger cloud of cool gas and dust called Cepheus B. The bright arc marks the edge of that cloud, where it faces a group of young, hot stars. One of them, a massive star called HD 217086, lights the arc from outside the cloud.

This star's radiation is also thought to be making new stars. It drives a wave of pressure into the cloud that squeezes gas together into stars, while it boils away the cloud's outer layers.

Observations with NASA's Chandra X-ray Observatory and Spitzer Space Telescope support this idea. Stars inside Cepheus B are about 1 million years old, and 70–80% of them still have disks of gas and dust where planets may form. Farther out, the stars are 2–3 million years old, and farther still 3–5 million years old; these older stars have fewer disks. This pattern is what astronomers expect if a wave of star birth is moving into the cloud.

A hard target

The Cave Nebula shines at about magnitude 8, on a brightness scale where lower numbers are brighter. Its light is spread thin and low in contrast, which makes it one of the hardest objects to find in the Caldwell catalog, where it is number 9. Its first known record came in 1959, when the astronomer Stewart Sharpless identified it on sky-survey photographs and listed it as number 155 in his second catalog of H II regions, giving it the name Sh2-155.

In 1995, the Hubble Space Telescope photographed a small part of the nebula near the mouth of the cave. Astronomers were looking for planet-forming disks around its young stars, like the disks Hubble had found in the Orion Nebula.