In “Milky Way puts on weight” (SN: 1/31/09, p. 8), you claim to show an image of the Milky Way. This image cannot be real. Worse, it creates misconceptions: As a college educator, I find that most students actually believe NASA has launched probes outside of the Milky Way to take pictures of our galaxy. I hope that printing a correction will help dispel that belief.
Don McCarthy, Tucson, Ariz.
McCarthy, of the University of Arizona’s Steward Observatory, was one of a number of readers who pointed out the impossibility of taking a “real” image of the entire Milky Way galaxy, the subject of the art in the January 31 issue. As reader William Westervelt of East Granby, Conn., put it: “How can we get a plain view of an entity that we are all mixed up with, in the middle of?” To clarify, the image shown is an illustration based on an animation called “Virtual Voyage through the Milky Way” from the Chandra X-ray Observatory website. We did not mean to imply that this was an actual photo. —Eva Emerson
Inspired by change
The editor’s statement (“For both universe and life, only constant is change,” SN: 2/14/09, p. 2) that change is the only constant is a factual idea one encounters infrequently. I was teaching my students this in 1956, spurred by Marcus Aurelius’ Thoughts. He made the point in several places in his writings, though he may not have been the earliest to do so. Early on, I raised the ire of biblical fundies in East Texas and beyond, but I continue to believe and express that change is the only real constant. Thanks for your editorial. The emperor would have loved it!
Franklin H. Mason, Tyler, Texas
Speedy planet building
In “Meteors deliver Earth-like crust” (SN: 1/31/09, p. 15), you say the meteorites with Earth-crust–like materials formed “less than 50 million years after the solar system formed.” What does this say about the rate at which terrestrial planets form? Does this constrain any planet-forming theories as we look for exoplanets around young stars?
Wayne Harris-Wyrick, Oklahoma City, Okla.
Planets can form relatively soon after the material in a solar system begins to coalesce, says Timothy McCoy, curator of meteorites at the Smithsonian Institution in Washington, D.C. In just tens of millions of years, planet-sized bodies can accumulate from nothing but dust and gas, he notes. Very young star systems, even those apparently surrounded by nothing but disks of dust, could have planet-sized bodies hidden in that material, he says.
Top-of-the-world route to America
The study in “Migrants settle Americas in tandem” (SN: 1/31/09, p. 5) shows the X2a haplotype in the Great Lakes area, not along the West Coast. If the top of the Earth was covered with ice, permitting migration, could the Great Lakes settlers have come through Baffin Island by way of Greenland, Iceland, Faroe Island and Scotland? Or Svalbard and Norway? Maybe a route similar to the Vikings’?
Bill Frost, Franklin, Tenn.
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