Orion Nebula Gets New Milepost Marker

Oct. 11, 2007 — One of the most famous and scrutinized heavenly objects is 10 to 20 percent closer than we thought, say two teams of radio astronomers who have made some of the most precise cosmic distance measurements ever, with a telescope nearly as big as Earth.

The Orion Nebula is the closest major stellar nursery to Earth, so it has been heavily studied to learn about the lives of stars. Its distance from Earth, however, has long been a matter of uncertainty, with an estimate made about 25 years ago in need of revision.

"Over the years people stopped citing the uncertainty" of the quarter-century-old measurement, said astronomer Geoff Bower of the University of California at Berkeley. He is part of one team of astronomers who have offered a new measurement based on the same technique the human brain uses to judge distance using two eyes: parallax. Their results appear in the Oct. 10 issue of the Astrophysical Journal.

In the case of the Orion Nebula, the "eyes" are radio telescopes from Hawaii to the Caribbean in what's called the Very Long Baseline Array (VLBA) — a virtual giant radio telescope that can resolve an extremely small patch of the sky.

The VLBA was aimed at one of the few radio-wave emitting stars in Orion, which was viewed twice in a single a year. The almost 200-million-mile width of Earth's orbit around the sun allowed the VLBA to serve as one eye, then again as the other eye six months later.

This arrangement allowed astronomers to discern a slight shift in the Orion Nebula star's position relative to powerful radio quasars — active galactic centers — in the very distant background.

The measurement of that small shift made it possible, via a bit of basic trigonometry, to calculate the distance to the Orion Nebula.

Bower and his colleagues came up with a distance of 1,270 light-years, give or take 76 light-years. That compares with the previous estimate of 1,565 light-years, give or take 266.

"The 'gold standard' for astronomical distance measurements is the trigonometric parallax, which uses the ancient surveying technique of triangulation," reported Mark Reid of the Harvard-Smithsonian Center for Astrophysics.

But Bower and his team may already have to give way to Reid and his group, who have submitted an even finer-tuned VLBA measurement of four stars in the Orion Nebula. Reid and his cohorts' measurement, also slated for publication in the Astrophysical Journal, is a slightly more distant 1,350 light-years away with a margin of error of just 23 light-years.

The continuing refinement of the radio parallax technique is expected to be handy far beyond the Orion Nebula, said Bower. For instance, it can be used to detect very small wobbles of stars caused by small planets orbiting around them, and even direct measurements of far more distant objects like M-33, the Triangulum Galaxy, he said.

"The limitation is that we can only do it on very bright radio sources," said Bower.

Very Long Baseline Array

An Orion Nebula photo album

How Stuff Works: Parallax