Readers ask about the size of the Milky Way, ancient rainforests and COVID-19

April 25, 2020 cover

Milky Way matters

Computer simulations and observations of nearby galaxies helped astrophysicists determine that the Milky Way stretches nearly 2 million light-years across, Ken Croswell reported in “Astronomers find Milky Way’s edge” (SN: 4/25/20, p. 8).

Reader John Montany thought that the Milky Way must be more massive than previously thought. “I have read other articles that have reported discoveries of previously unknown mass in the universe,” Montany wrote. “Taken collectively, are these discoveries changing theories of dark matter as an explanation for gravitational observations?”  

No, Croswell says, the new work does not suggest the Milky Way has more mass and doesn’t change estimates for how much dark matter the universe has. “Most astronomers believe that dark matter explains such observations as the high velocities of stars in dwarf galaxies and the high velocities of galaxies in galaxy clusters,” he says. “Without the gravitational force exerted by this dark matter, stars in typical dwarf galaxies would escape their galactic homes, and galaxies in galaxy clusters would escape their abodes as well.”

Rainforest reservations

A sediment core analysis revealed that a rainforest flourished near the South Pole around 92 million years ago, Carolyn Gramling reported in “A rainforest once grew in Antarctica” (SN: 4/25/20, p. 14).

Reader Dennis McHenry wondered if the researchers accounted for continental drift. “Ninety million years ago, the Antarctic wasn’t quite where it is now,” he wrote. McHenry also wondered if Earth’s axial tilt 90 million years ago would have kept Antarctica in darkness for months at a time. “It isn’t only the cold that prevents forests, but also the lack of light for many months of the year,” he wrote.

The latitude where this forest once existed, 82° S, is a paleolatitude, Gramling says. R­esearchers determined this ancient location based on calculations of where and how quickly tectonic plates have moved over the last 90 million years. “The drill site is a bit closer to the South Pole, but the ancient site was within about 1,000 kilometers of the pole, even when accounting for the movements of the plates,” she says.

As for axial tilt, the angle of the tilt changes on a 41,000-year cycle, between a minimum of 22.1 degrees and a maximum of 24.5 degrees. “Although that cycle can impact climate, a site at 82° S would still have been plunged into darkness for months each winter,” Gramling says. “That makes it even more remarkable that temperatures remained warm enough for the forest to thrive, and it’s why the researchers determined that such high carbon dioxide levels as well as a basically ice-free continent were both necessary conditions.”


Science News reporters Tina Hesman Saey, Aimee Cunningham, Jonathan Lambert and Erin Garcia de Jesus are following the latest research to keep you up to date on the coronavirus pandemic. The team is answering reader questions about COVID-19.

“Is there something different about coronaviruses whereby we do not have long-lasting immunity once infected?” reader Bob Schalhoub asked.

That’s a million-dollar question. When a virus infects a person, the body typically mounts an immune response to fight off the infection. Once the virus is gone, certain cells help the immune system recognize the virus if it attacks again. But that memory tends to fade over time for some viruses, like coronaviruses, and not for others, like measles virus. Scientists don’t fully understand why there’s a difference.

Because of this short memory, people can be repeatedly infected by cold-causing coronaviruses, studies have shown. It’s unclear whether that’s the case for coronaviruses that have caused outbreaks, including the new corona­virus. The SARS outbreak was contained before that question could be explored. And MERS doesn’t offer helpful insights because it infects so few people a year.

From the Nature Index

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