# Tiansi (Gamma Cassiopeiae)

Tiansi (Gamma Cassiopeiae) is a star in the constellation Cassiopeia, magnitude 2.2.

**The middle of Cassiopeia’s W — a whirling star with a hidden white dwarf and an astronaut’s nickname.**

In 1866 the Italian astronomer Angelo Secchi turned a spectroscope on this star and saw something new: bright emission lines, the first ever seen in a star. Gamma Cassiopeiae became the first known Be star and the model for a whole class of restless, eruptive [variables](https://astropilot.space/glossary/variable-star).

The cause is spin. This blue-white star of about 17 solar masses turns at some 450 km per second, so fast that its equator bulges far wider than its poles. Shining like 19,000 Suns, it flings gas into a hot disc around its waist — the source of the bright lines and of its fits of brightening and fading. In the late 1930s it rose above magnitude 2, outshining Schedar and Caph, then dropped to 3.4.

It is also unusually bright in X-rays. In 2026 the XRISM observatory showed that the hot gas moves with a companion: a white dwarf a little smaller than Earth, circling every 203.5 days and feeding on the big star’s gas.

Tiansi (Gamma Cassiopeiae) is 550 light-years away: the light we see from it set off around 1475, around the time Columbus crossed the Atlantic.

## Did you know?

- **An astronaut’s star.** Apollo crews used it as a navigation star. In 1967 it was nicknamed Navi — “Ivan” backwards — for Apollo 1 astronaut Virgil Ivan “Gus” Grissom. Despite the popular story, Grissom did not name it himself.
- **From horse to whip.** Chinese astronomers saw it as one of four horses pulling the chariot of the charioteer Wangliang; later it became his whip. In 2025 the IAU named it Tiansi, after the Heavenly Quadriga.
- **A crowded family.** A naked-eye neighbour, HR 266, is itself a quadruple star and seems to travel with Gamma Cassiopeiae. Together the system may hold eight stars, one of the highest counts known.

The blue-white star at the centre of Cassiopeia’s W, usually near 2nd magnitude. It is an unpredictable variable that has ranged from 1.6 to 3.0, so compare it now and then with Schedar and Caph, the W’s other bright stars.

## When and where to see it

It is best placed in the evening sky around November, when it crosses the meridian at about 10 pm local time (around midnight in October). It can be seen from everywhere north of 29°S and never sets north of 29°N.

At its highest it stands 79° up from 50°N, 64° up from 35°N, 29° up from the equator, below the horizon from 35°S.

## Facts

- Type: Star
- Constellation: Cassiopeia
- Designations: Gamma Cassiopeiae, Navi, γ Cas, 27 Cas, HR 264, HD 5394, HIP 4427
- Magnitude: 2.2
- Position: RA 0h 56.7m, Dec +60° 43′ (J2000)
- Spectral type: B0.5IVpe
- Colour: white (B−V -0.10)
- Brightness: varies from magnitude 1.6 to 3
- Distance: 550 light-years
- Discovered: Angelo Secchi, 1866, Known since antiquity; Secchi found the first emission lines ever seen in a star

Open AstroPilot to see when Tiansi (Gamma Cassiopeiae) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view. https://astropilot.space/object/tiansi