Here's why:

* Brightness: Rigel is one of the brightest stars in the night sky, indicating it's very luminous. High-mass stars are much more luminous than low-mass stars.

* Spectral Type: Rigel is classified as a B-type star. B-type stars are generally hotter and more massive than our Sun (a G-type star).

* Lifetime: High-mass stars have much shorter lifespans than low-mass stars. Rigel is expected to live for only a few million years, whereas our Sun has a lifespan of about 10 billion years.

In summary, Rigel's high luminosity, spectral type, and short lifespan all point to it being a high-mass star.