Like cosmic storm clouds kicked into high gear by some tenacious interstellar weather front, hundreds of gas clouds stream along peculiarly fast orbits around the Milky Way -- above and beyond the main disk.

These high-velocity clouds move with rapidity in the galactic halo, a sparse sphere of objects that defines the outer reaches of our galaxy. Since discovering the objects in 1963, astronomers have been puzzled by the fact that they move faster than the rest of the galaxy.

Sometime, somewhere, something gave the impetus of speed to these clouds.

If you could leave our galaxy and then turn around after a few hundred thousand light-years to view the Milky Way from the edge-on, you would see something like a flying saucer, with a central bulge surrounded by a fairly flat disk researchers call the galactic plane.

Scattered above, below and all around this plane you would notice a few lone stars, as well as dense groups of stars called globular clusters -- all moving through the halo in a manner consistent with the overall rotation of the galaxy.

You'd also see other streams of stars in the halo, representing the remnants of smaller galaxies in various stages of being swallowed, moving at their own pace. Clouds of gas -- sans any stars -- would be scattered throughout the sphere, too, many moving faster than the general galactic rotation.

Though hard to find and even harder to measure, hundreds of these high-velocity clouds -- or HVCs as they are called -- are thought to exist.

For scientists, the trick to learning the origin of HVCs is in studying their composition. If a cloud is made mostly of hydrogen atoms, researchers figure it is probably left over from the early days of galaxy formation. If, however, a cloud was formed from gas ejected by an exploding star (a supernova) some of the hydrogen would have been converted to heavier elements during the star's life and those elements should still be present.

Researchers already know that fountains of hot gas flow away from the center of the Milky Way, pushed into the galactic halo by the ancient supernova explosions from whence they emanated or by the charged particles of stellar winds.

The question, says Philipp Richter of the University of Bonn, was whether these fountains could cool down and eventually become high-velocity clouds of gas. The answer, he tells us, is yes.

In one Nature paper, Richter and his colleagues report on an HVC that contains molecular hydrogen. In space, for hydrogen atoms to coalesce into molecules (thus becoming molecular hydrogen) they need dust grains on which to stick. And interstellar dust, an astronomer will tell you, is composed of heavy elements. So Richter's molecular hydrogen is a signature of supernova ancestry.

The researchers also found a lot of iron in the HVC -- about half of the amount that exists in our sun, bolstering their case. Richter told space.com that the molecular hydrogen in the HVC also means it's possible, in principle, that the cloud could eventually be the birthplace of new stars.

In a method of cloud generation quite different from Richter's galactic fountains, the Milky Way appears to incorporate clouds of gas from outside its system. Details of this process -- one with several permutations -- is described in a second Nature paper.

By measuring the absorption of distant starlight by sulfur in a relatively nearby HVC, Bart Wakker of the University of Wisconsin, along with colleagues, found heavy-element content to be very low -- roughly 9 percent of that found in our sun. This suggests the cloud has not had its heavy-metal content enriched significantly by stellar processes (so it probably did not come from supernovas).

Instead, Wakker said the cloud, and others like it, might be the result of an ongoing, slow evolution of our galaxy, as hot primordial gas is sucked inward. The cloud Wakker studied is only slightly beyond the galactic orbit of our sun, but it sits far above the galactic plane.

"When such a cloud approaches the Milky Way, it gets compressed, cools and becomes denser," Wakker explained in an e-mail interview. "Further, it gets contaminated with heavy elements from the gas in the galactic halo." Wakker says the cloud's orbital speed would be slowed by friction (Yes, friction on a galactic scale!) as it began rubbing against the edges of the Milky Way.

Wakker said previous evidence of lower, slower clouds lends support to this model, which he gives a 25 percent chance of being the right one to explain his findings.

In a more dramatic possibility, Wakker revealed what he thinks is the most likely scenario for the origin of the cloud he studied: A few billion years ago, the Milky Way distorted and swallowed a passing dwarf galaxy. The stars from the dwarf might have separated from the remaining gas cloud, or they might be stretched out in a nearby, unrecognizable stream.

He said other dwarf galaxies, observed on the outskirts of the Milky Way, have heavy-metal quantities that are in line with the HVC he studied, indicating there may well be a link between galaxy gobbling and HVC creation.

The concept of galaxy gobbling is solidly established. Strong evidence for it was discussed in two Nature papers November 4. Together, the recent host of studies bolsters the argument that the Milky Way, especially its outer reaches, formed by the amalgamation of many smaller building blocks -- a process that appears to continue today.

But Wakker and Richter have only measured two clouds out of hundreds, a fact that makes others stop short of calling the findings conclusive. In a separate commentary in Nature, W. Butler Burton and Robert Braun, astronomers in The Netherlands, wrote, "more of these difficult measurements will be needed to finally decide between the competing theories."

While at first glance the two papers might seem incompatible, Wakker and Richter -- the two lead researchers -- say the two types of HVCs are similar objects resulting from wildly different processes, adding that both papers are valid. "The HVCs are an inhomogenous mixture of different phenomena," Wakker said.

And from Richter, of the University of Bonn: "It is clear: The galactic halo is full of gas, originating from different regions. Certainly, both results will change our picture of galaxy evolution."

Richter said he will move his studies from Germany to the University of Wisconsin next year, where he'll work side-by-side with Wakker and others to "further explore the complex nature of the HVCs."