How big is Barnard's Loop?

Barnard's Loop is a huge, faint arc of glowing hydrogen gas in the constellation Orion. It curves around the eastern side of Orion's Belt and Sword, and its arc spans about 330 light-years from end to end. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. The loop lies roughly 1,300–1,400 light-years from Earth.

Size

Barnard's Loop is an arc, the edge of a thin shell of gas seen from the side, so its size is the distance from one end of the arc to the other. Seen from Earth, the arc stretches about 14 degrees across the sky. How large that is in space depends on how far away the loop is.

A 2023 study took the loop's distance to be 400 parsecs, where a parsec is about 3.26 light-years. At that distance, 14 degrees of sky corresponds to roughly 100 parsecs, or about 330 light-years. A 2011 study used a distance of 440 parsecs and gave a length of 110 parsecs, about 360 light-years. Both figures are approximate.

Size comparisons

On the sky, Barnard's Loop is about 27 times as wide as the full Moon.
If the Sun were shrunk to the size of a basketball, Barnard's Loop would be about 530,000 kilometers (330,000 miles) across, about 1.4 times the distance from Earth to the Moon.
The Great Nebula in Carina, a giant cloud of glowing gas in the Milky Way, spans about 390–650 light-years, about 1.2 to 2 times the length of Barnard's Loop.
The Tarantula Nebula, in the Large Magellanic Cloud, is about three times as long as Barnard's Loop.
The Orion–Eridanus Bubble, a much larger shell of gas around the stars of Orion, is about 380 parsecs (1,240 light-years) long, more than three times the loop's length.

What makes it glow

Hot, massive stars in Orion's Belt and Sword give off strong ultraviolet light. That light strips electrons from the hydrogen atoms in the loop, and the gas glows with the red light of hydrogen as a result. The gas is warm, about 6,000 kelvins (about 5,700 degrees Celsius), but extremely thin. Each cubic centimeter, a cube 1 centimeter on a side, holds only about two free electrons.

Because its glow is so faint, the loop was found only in photographs: by W. H. Pickering in 1889 and by E. E. Barnard, after whom it is named, in 1894. It is a popular target for amateur astronomers' cameras.

How the loop formed

Massive stars push away the gas around them with their light, with winds of particles and, when they die, with supernova explosions. Barnard's Loop was long thought to be the bright eastern part of the Orion–Eridanus Bubble, which Orion's stars have blown over millions of years. Between 10 and 20 supernovae may have exploded in the Orion region over the past 12 million years. Recent studies suggest the loop is a separate, smaller shell nested inside the larger bubble.

A 2023 study used three-dimensional maps of the dust around Orion. It found that the loop matches a large cavity around a young cluster of stars called OBP-B1. Orion's three main clouds of cold gas and dust, called Orion A, Orion B and Lambda Orionis, lie on the shell of this cavity.

The study concluded that the cluster's light, winds and at least one supernova probably shaped the loop. Its exact origin is still debated, more than 130 years after its discovery.