Eta Cassiopeiae

Primary of Djuli.

The primary star in the Eta Cassiopeiae system has a stellar classification of G0 V, which makes it a G-type main-sequence star like the Sun. It therefore resembles what our Sun might look like if we were to observe it from Eta Cassiopeiae. The star has 97% of the mass of the Sun and 101% of the Sun's radius. It is of apparent magnitude 3.44, radiating 129% of the luminosity of the Sun from its outer envelope at an effective temperature of 6,087 K. It appears to be rotating at a leisurely rate, with a projected rotational velocity of 3.15 km s–1.

The cooler and dimmer magnitude 7.51 companion is of stellar classification K7 V; a K-type main sequence star. It has only 57% of the mass of the Sun and 66%[8] of the Sun's radius. Smaller stars generate energy more slowly, so this component radiates only 6% of the luminosity of the Sun. Its outer atmosphere has an effective temperature of 4,036 K. Compared to the Sun, this star, and the primary component, show only half the abundance of elements other than hydrogen and helium—what astronomer's term their metallicity.

This is a spectroscopic binary system, indicating that its binary nature was first detected by observing shifts in the spectrum. The pair are orbiting around each other over a period of 480 years.

Based on an estimated semimajor axis of 12″ and a parallax of 0.168″, the two stars are separated by an average distance of 71 AU, where an AU is the average distance between the Sun and the Earth. However, the large orbital eccentricity of 0.497 means that their periapsis, or closest approach, is as small as 36 AU, with an apoapsis of about 106 AUs. For comparison, the semi-major axis of Neptune is 30 AU.