51 Pegasi (abbreviated 51 Peg) is a Sun-like star located 50.9 light-years (15.6 parsecs) from Earth in the constellation Pegasus. It was the first extrasolar Sun-like star found to have a planet orbiting it.
The star is of apparent magnitude 5.49, and so is visible with the naked eye under suitable viewing conditions. The Flamsteed designation for this star, 51 Pegasi, was assigned by John Flamsteed in his star atlas published in 1712.
51 Pegasi has a stellar classification of G5V, indicating that it is a main sequence star that is generating energy through the thermonuclear fusion of hydrogen at its core. The effective temperature of the chromosphere is about 5571 K, giving 51 Pegasi the characteristic yellow hue of a G-type star. It is estimated to be 6.1–8.1 billion years old, somewhat older than the Sun, with a radius 24% larger and 11% more massive. The star has a higher proportion of elements other than hydrogen/helium compared to the Sun; a quantity astronomers term a star's metallicity. Stars with higher metallicity such as this are more likely to host planets. In 1996 astronomers Baliunas, Sokoloff, and Soon measured a rotational period of 37 days for 51 Pegasi.
Although the star was suspected of being variable during a 1981 study, subsequent observation showed there was almost no chromospheric activity between 1977 and 1989. Further examination between 1994 and 2007 showed a similar low or flat level of activity. This, along with its relatively low X-ray emission, suggests that the star may be in a Maunder minimum period during which a star produces a reduced number of star spots.
The star rotates at an inclination of 79+11
−30 degrees relative to Earth.