This artist’s illustration depicts the system of Delta Orionis A. In the center, two closely separated stars orbit around each other every 5.7 days and this pair is known as Delta Orionis Aa. The more massive, or primary, star in Delta Orionis Aa weighs about 25 times the mass of the Sun, whereas the less massive, or secondary star, weighs about ten times the mass of the Sun. The chance alignment of this pair of stars allows one star to pass in front of the other during every orbit from the vantage point of Earth. This special class of star system is known as an “eclipsing binary,” and it gives astronomers a direct way to measure the mass and size of the stars. Meanwhile a third star, dubbed Delta Orionis Ab, orbits the eclipsing binary with a period of over 400 years.
Image credit: NASA/CXC/M.Weiss
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