An active sun is a somewhat smaller sun | Science News for Students

An active sun is a somewhat smaller sun

The sun shrinks and grows a tiny bit over cycles lasting about 11 years
Jul 9, 2018 — 6:45 am EST
closeup of the sun and solar activity

The radius of the sun gets a smidge smaller the sun is most active, a new study finds.


How big is the sun? That depends on when you look, a new study finds.

Our home star shrinks slightly and expands again as it goes through a solar cycle. That’s a roughly 11-year period. It is characterized by times of high and low magnetic activity, changes in sunspot numbers and more. Two researchers now report finding that when the sun is most active, its radius drops by 1 or 2 kilometers (0.6 to 1.2 miles). That’s not much. The sun’s full radius is about 700,000 kilometers (435 million miles)!

Unlike many planets, the sun has no solid surface. That is one thing that makes computing the star’s size challenging. “It’s a slippery concept,” says Jeff Kuhn. “What does it mean, the radius of the sun?” asks this astronomer who works at the University of Hawaii in Maui. One way scientists have measured the orb’s width is based on how the brightness of the sun falls off from its center. In 2010, Kuhn’s group did that. That turned up no sign that the sun’s radius changed during the solar cycle.

The new study does something different. It measures what’s known as the sun’s seismic radius. Seismic waves travel through the sun’s interior. Any change in the sun’s size would change the frequency of those waves.

This new yardstick has some advantages, says Alexander Kosovichev. He’s an astrophysicist at the New Jersey Institute of Technology in Newark. “By using the seismic radius,” he says, “we can measure more accurately.” And that’s what he and Jean-Pierre Rozelot of Université Côte d’Azur in Nice, France, just did.

To figure out this seismic radius, the two used 21 years’ worth of data on the waves’ frequencies. Two spacecraft had collected those data. How much the sun expanded or shrunk varied by depth, those data show. Some layers within the sun contracted at the same time that others were expanding. Changes in the magnetic fields inside the sun could be behind the sun’s shifting size, the scientists say.

Taken together, the new data point to an overall drop in the seismic radius when the sun is more active.

This new estimate of the sun’s size is not, however, a replacement for measuring the radius in terms of overall brightness. “I think that’s a separate question,” Kuhn says. The two measurements rely on different techniques. They therefore probe different traits of the sun’s behavior.

The seismic radius may help scientists understand how the strength of the sun’s magnetic fields varies at different depths within the star, notes Sabatino Sofia. He’s a retired astrophysicist who used to work at Yale University in New Haven, Conn. There had been hints that the sun’s seismic radius might change over time. However, he says, the new data “really confirms that during the activity cycle, the seismic radius of the sun is changing.”

The paper by Kosovichev and Rozelot has been accepted to appear in an upcoming issue of The Astrophysical Journal.

Power Words

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astronomy     The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

astrophysics     An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

behavior     The way something (often a person or other organism) acts towards others, or conducts itself.

field      (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).

frequency     The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.

gauge     A device to measure the size or volume of something. For instance, tide gauges track the ever-changing height of coastal water levels throughout the day. Or any system or event that can be used to estimate the size or magnitude of something else. (v. to gauge) The act of measuring or estimating the size of something.

magnetic field     An area of influence created by certain materials, called magnets, or by the movement of electric charges.

orb     Something having a spherical shape.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, the object must be big enough to have pulled neighboring objects into the planet itself or to have slung them around the planet and off into outer space.

radius     A straight line from the center to the circumference of a circle or sphere.

seismic wave     A type of wave traveling through the ground (or some other celestial body) produced by an earthquake or some other means.

solar cycle     Sometimes referred to as the sunspot cycle, this is the roughly 11-year period during which the sun undergoes a host of changes as the star swings between periods of minimum and maximum activity. The number of sunspots changes (drops to near zero during the minimum). The magnetic field values at the solar surface also undergoes changes.

star     The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

sun     The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

sunspot     A spot or patch appearing from time to time on the sun’s surface, appearing dark by contrast with its surroundings.

trait     A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.

vertical     A term for the direction of a line or plane that runs up and down, as the vertical post for a streetlight does. It’s the opposite of horizontal, which would run parallel to the ground.

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.


Journal:​ ​​ A. Kosovichev and J.-P. Rozelot. Cyclic changes of the sun's seismic radiusThe Astrophysical Journal, in press, 2018.