How far is it from here to the edge of the solar system? And how big is that distance relative to the size of the Milky Way? If you are an astronomy writer, these are the kinds of scales you have to put in perspective all the time–for readers like me who have absolutely no idea.
Last week, I posted about size comparisons in science writing—for things both bigger than a breadbox and substantially smaller. But to tackle the mind-bogglingly large, I sought help from astronomy writers—the kind of people who know that a supermassive black hole might have the mass of a billion suns.
“It’s hard to capture the vastness of space in a way that makes it tangible,” notes Nadia Drake, who writes the No Place Like Home blog. “Even the familiar objects we use are almost impossibly large to internalize.” The conventional tools are to cast things in terms of Earths, suns and light-travel distances, she says. For example, Neptune is a planet with 17 Earth-masses, and Eta Carinae lost more than 10 solar masses of material during its great eruption in the 1800s. “I once wrote about a black hole estimated to be “a long light-weekend across–or roughly the distance light would travel over a holiday weekend,” she says. Average orbital distances can also be useful: “Betelgeuse is a red giant star so big that if you parked it where the Sun is, it would fill the orbit of Jupiter,” Drake says.
Robert Frederick (bio) suggests using the amount of time it would take a jet to reach or to circle a given space object. For example, flying at 900 km/hour, it would take 1100 years to go around the surface of a red hypergiant (the largest type of star) just once, he says.
“I try to explain things in terms of other scales that people are familiar with,” says Rebecca Boyle, who writes about space and astronomy for Popular Science and other publications. “You need some other distance to use as a yardstick.” For instance, from here to the moon is about 86 times the distance between Los Angeles and New York City, Boyle notes. And the distance from Earth to the sun is 387 times the Earth-moon trip. Of course, that distance–from here to the sun–is the astronomical unit, an official quantity used for measuring everything else in the solar system, she notes.
Sometimes scaling things down can be helpful, Boyle adds, “If you shrink Earth to the size of a peach, and you put the moon and sun at their scaled distances, the moon would be about 7 feet away, and the sun would be 56 miles away. Jupiter would be more than 280 miles away.”
Drake points to a similar type of example, “In DC recently, Natalie Batalha said that if the Milky Way were collapsed to the size of the United States, and Earth were the Capitol building, the nearest habitable planet would be a stroll down the mall, near the Washington Monument. She based that analogy on an estimate that says the nearest habitable planet is about 15 light-years away — probably orbiting an M-dwarf star. It’s obviously just an estimate, but I think the comparison nicely captures both the nearness and the vastness of space. It’s kind of fun to ask people to guess where they think that planet would be — nobody guesses it’s that close!”
For really long distances, astronomers use a unit called the parsec, which is about 3.25 light years (wow, Han Solo got that wrong). The center of the Milky Way is 8 kiloparsecs away. “I think parsecs are mostly an astronomical term, but most people have at least heard of light years,” Boyle says. “So here is one more that I like: The observable universe (which is expanding) is 98 billion light years across.”
Outer space also specializes in some really dense stuff. “Neutron stars are incredibly dense, with a bit more than a sun’s worth of material crammed into an object about as wide as San Francisco (and a teaspoonful of neutron star would weigh a billion tons),” Drake says. “White dwarfs are slightly less dense, with about a sun’s worth of stuff squeezed into something about the size of Earth.”
So here, in the outer reaches of space, we end our series on scale. I’ll leave you with two final, sweeping overviews. The film “Powers of Ten” zooms in and out over 40 orders of magnitude centered around a late 1970s Chicago picnic. Meanwhile this animation covers 60 orders of magnitude from the size of the observable universe down to the Planck length. Happy zooming.
Image credit: Courtesy of NASA/SDO and the AIA, EVE, and HMI science teams.