Is the sun really a dwarf star?

This illustration lays a depiction of the sun's magnetic fields over an image captured by NASA’s Solar Dynamics Observatory on March 12, 2016.

An illustration showing the sun's magnetic fields over an image captured by NASA's Solar Dynamics Observatory on March 12, 2016. (Image credit: NASA/SDO/AIA/LMSAL )

The sun is the biggest object in the solar system; at about 865,000 miles (1.4 million kilometers) across, it's more than 100 times wider than Earth. Despite being enormous, our star is often called a "dwarf." So is the sun really a dwarf star?

Technically, the sun is a G-type main-sequence star — specifically, a G2V star. The "V" indicates that it is a dwarf, Tony Wong, a professor of astronomy at the University of Illinois Urbana-Champaign, told Live Science.

Dwarf stars got their name when Danish astronomer Ejnar Hertzsprung noticed that the reddest stars he observed were either much brighter or much fainter than the sun. He called the brighter ones "giants" and the dimmer ones "dwarfs," according to Michael Richmond, a professor of physics and astronomy at the Rochester Institute of Technology in New York. The sun is currently more similar in size and brightness to smaller, dimmer stars called red dwarfs than to giant stars, so the sun and its brethren also became classified as dwarf stars.

"G" is astronomer code for yellow — that is, stars of a temperature range of around 9,260 to 10,340 degrees Fahrenheit (5,125 to 5,725 degrees Celsius), Lucas Guliano, an astronomer at the Harvard-Smithsonian Center for Astrophysics, told Live Science.

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Wong noted that G2 means it's somewhat hotter than a typical G-type star. "They range from G0 to G9 in order of decreasing temperature," he said. At its surface, the sun is about 9,980 F (5,525 C), Guliano added.

Calling the sun yellow is a bit of a misnomer, however, as the sun's visible output is greatest in the green wavelengths, Guliano explained. But the sun emits all visible colors, so "the actual color of sunlight is white," Wong said.

(On Earth, the sun appears yellow because of the way molecules in the atmosphere can scatter the different colors that make up the sun's white light, according to Stanford University's Solar Center. This is the same reason the sky appears blue.)

G-type stars also range from G0 to G9 in order of decreasing size, Guliano said. Wong explained that class G stars "range in size from somewhere around 90% the mass of the sun up to around 110% the mass of the sun."

The sun is what astronomers call a main-sequence star, a class that includes most stars. Nuclear reactions within these stars fuse hydrogen to become helium, unleashing extraordinary amounts of energy. Among the main-sequence stars, the color is determined by the star's mass.

"The sun is yellow, but less-massive main sequence stars are orange or red, and more massive main sequence stars are blue," Carles Badenes, a professor of physics and astronomy at the University of Pittsburgh, told Live Science.

The sun is slowly changing as it ages. "It has gotten about 10% larger since it started on the main sequence, and it will get much larger," Wong said. Even as it grows, however, the sun will still be considered a dwarf until its last stage of life.

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In about 5 billion years, the sun will run out of hydrogen fuel and begin to swell to become a red giant, leaving its dwarf days behind. "It will engulf the orbit of Venus, and maybe Earth as well," Badenes said, "and its surface temperature will get colder, making it red in color."

Sun quiz: How well do you know our home star?

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Charles Q. Choi is a contributing writer for Live Science and Space.com. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.

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Is the sun really a dwarf star?

Sun quiz: How well do you know our home star?