189733

HD 189733 is a K-type main-sequence star in the constellation Vulpecula, about 63 light-years away. It has an apparent visual magnitude of approximately 7.7, making it one of the brighter stars known to host a transiting exoplanet. Spectral analysis indicates a temperature around 5000 K and a metallicity near solar, with the star noted for relative activity and a younger age compared to the Sun.

Its planetary companion, HD 189733 b, is a hot Jupiter in a tight 2.218-day orbit. The planet was confirmed by radial-velocity measurements in 2005, with transit observations shortly after, enabling precise determinations of its mass (about 1.15 Jupiter masses) and radius (about 1.15 Jupiter radii). The close-in orbit yields dayside temperatures around 1100–1200 K.

HD 189733 b has become a benchmark for exoplanet atmospheric studies. Transmission spectroscopy during transits reveals a hazy, high-altitude atmosphere with Rayleigh scattering that gives the planet a blue hue, and detections of water vapor and other molecules. The brightness of the host star and the system's favorable geometry have supported extensive follow-up using space- and ground-based facilities.

The HD 189733 system is frequently cited in discussions of transiting planets and atmospheric characterization. It illustrates how stellar activity can complicate measurements while also enabling detailed atmospheric studies through transit and secondary-eclipse observations.