Gamma Cassiopeiae (Tiansi): Central Star of Cassiopeia’s W

Gamma Cassiopeiae (γ Cas) is a multiple star system located at a distance of 550 light years in the constellation Cassiopeia. Officially named Tiansi, it is normally the third brightest star in the constellation, after Schedar and Caph. It is one of the five stars that form Cassiopeia’s recognizable W asterism.

Gamma Cassiopeiae has an apparent magnitude of 2.39. The variable star has been observed to brighten to magnitude 1.6 and fade to magnitude 3.0. At its brightest, it has outshone its W neighbours to become Cassiopeia’s brightest star.

Informally known as Navi, the massive bright star is a supernova candidate. It is the primary component in a multiple star system that may contain as many as eight components.

Tiansi (Gamma Cassiopeiae), image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

What type of star is Gamma Cassiopeiae?

Gamma Cassiopeiae has the stellar classification B0.5 IVe, indicating a subgiant star appearing blue or blue-white in colour. However, despite presenting as a subgiant, a star running out of its supply of hydrogen, astronomers believe that Gamma Cas is a main sequence star, still fusing hydrogen into helium in its core.

The “e” suffix in the star’s spectral class indicates hydrogen emission lines in the stellar spectrum. These lines are caused by a circumstellar disk of material lost from the star because of its high spin rate.

Gamma Cassiopeiae has a mass around 15 times that of the Sun and a radius 7.9 – 10.9 times that of the Sun. With a surface temperature that varies between 17,300 and 26,500 K, the star is around 19,000 times more luminous than the Sun. Most of its energy output is in the invisible ultraviolet part of the spectrum.

Tiansi rotates very rapidly and has an oblate shape. As a result of its fast rotation, both its radius and effective temperature vary by latitude. The star has a radius of 7.9 solar radii at the poles and 10.9 solar radii at the equator, and a temperature of 26,500 K at the poles and 17,300 K at the equator.

Gamma Cassiopeiae has an estimated age of only 8 million years. Even though it is still a very young star, it will not be long before it runs out of hydrogen to burn in its core and evolves into a supergiant.

Evolutionary phase

Gamma Cassiopeiae is still on the main sequence, the longest stage of any star’s lifetime. This stage is characterized by core hydrogen burning, which provides the main source of energy. The thermal energy generated from the nuclear fusion of hydrogen provides the pressure that keeps the star in a hydrostatic equilibrium.

Massive stars like Tiansi evolve into supergiants after leaving the main sequence. Their lifespans are measured in only millions of years, while low-mass stars like the Sun can keep burning hydrogen for billions of years.

With a mass of 15 solar masses, Gamma Cassiopeiae is a supernova candidate. When it evolves away from the main sequence, it will keep fusing progressively heavier elements until it develops an iron core. At that point, it will suffer a core collapse and end its life as a luminous supernova.

Related: Life Cycle of High-Mass Stars

Fast spinning Be star

Gamma Cassiopeiae is an exceptionally fast spinner. It has a projected rotational velocity of 389 km/s. When corrected for the star’s axial inclination, the true value for the spin rate climbs to 450 km/s, very close to the star’s breakup velocity.

As a result, Tiansi has an equatorial bulge. Its equatorial radius is larger than its polar radius, giving the star a flattened shape.

As another effect of the star’s fast rotation, a lot of material from the surface is ejected and forms a hot circumstellar disk of gas. The decretion disk causes both brightness variations and hydrogen emission lines in the star’s spectrum.

Gamma Cassiopeiae is classified as a Be star, a non-supergiant class B star showing Balmer (hydrogen) emission lines in its spectrum. Be stars are fast spinning stars with gaseous disks of expelled material that cause one or more of these emission lines.

Tiansi was the first star to be identified as a Be star, by the Italian astronomer Angelo Secchi in 1866. The brightest star belonging to this group is Achernar in the constellation Eridanus, the ninth brightest star in the sky. Achernar was only recognized as a Be star in 1976, long after Tiansi.

Shell events

Gamma Cassiopeiae is an eruptive variable star that sometimes shows unpredictable variations in brightness when material is expelled from the star. The periods when the circumstellar disks are particularly strong and the star’s brightness increases are called shell episodes.

Gamma Cassiopeiae underwent two major shell events in 1935-36 and 1939-40, when it brightened to above magnitude +2.0 and then rapidly dimmed to magnitude +3.4. The star’s brightness later slowly increased to around magnitude 2.2. At its brightest, the star outshone both Schedar (mag. 2.24) and Caph (mag. 2.28).

Shell stars like Gamma Cassiopeiae often show variability in their brightness as the shell features change or disappear. These stars are known as Gamma Cassiopeiae variables.

Variations in brightness

Many Be stars are variable in brightness. If there is a transient disk, they are classified as Gamma Cassiopeiae variables. If they are believed to be pulsating stars, they are classified as Lambda Eridani variables.

Gamma Cassiopeiae and other variables of this type are eruptive stars. Typically, they are fast spinning class B subgiants, giants or main sequence stars, that show brightness variations with larger amplitudes than other Be stars. They have decretion disks around the equatorial region, which sometimes reform or disappear.

Gamma Cassiopeiae variables

Gamma Cassiopeiae serves as the prototype for the class of variable stars known as the Gamma Cassiopeiae variables. These stars exhibit irregular brightness variations over periods of decades. Their brightness varies with an amplitude of about a magnitude.

Gamma Cassiopeiae itself usually shines at magnitude 2.25, but it has been observed to fade to magnitude 3.0 and brighten to magnitude 1.6. The variations in brightness are caused by shell episodes, where shell features appear in the star’s spectrum, significantly increasing or decreasing brightness.

Stars classified as Gamma Cassiopeiae variables include Gomeisa in Canis Minor (mag. 2.84 – 2.92), Pleione in the Pleiades cluster in Taurus (mag. 4.8 – 5.5), Pi Aquarii in Aquarius, Lambda Pavonis in Pavo (mag. 4.00 – 4.26), and Kappa Canis Majoris (mag. 3.4 – 3.97) and 27 Canis Majoris (mag. 4.92 – 5.39) in Canis Major.

Example of gravity brightening

Like its neighbour Caph, Gamma Cassiopeiae is an example of gravity brightening (or gravity darkening). Its poles are closer to the centre of mass and therefore hotter and more luminous than the equator. They are said to be gravity brightened, while the equatorial region is gravity darkened.

Other famous examples of gravity brightening include Achernar in the constellation Eridanus, Alderamin in Cepheus, Altair in Aquila, Rasalhague in Ophiuchus, Regulus in Leo, and Vega in Lyra.  All these are fast-spinning stars whose shapes have been distorted into oblate spheroids.

X-ray source

Gamma Cassiopeiae was identified as an X-ray source in 1976, but it was unclear whether the star itself was the source of the intense X-rays or it had a mass-accreting binary companion. The hot blue star was found to have an X-ray emission around 40 times stronger than that typically detected in similar massive stars. For decades, the source of the emission was a mystery.

The plasma responsible for the intense X-rays fluctuates rapidly, reaching temperatures of over 100 million degrees. In recent decades, researchers ruled out a neutron star and a stripped star as possible sources.

Discovery of the white dwarf companion

In 2026, a team of astronomers led by Yaël Nazé at the University of Liège and Masahiro Tsujimoto at the University of Japan confirmed that the star’s strong, variable X-ray emission comes from an accreting white dwarf companion. The presence of the white dwarf was long suspected, but the discovery was not confirmed during previous observations, including those with the CHARA array.

The researchers analyzed data obtained with the Resolve instrument on the XRISM space telescope (X-Ray Imaging and Spectroscopy Mission) and found that the hot plasma changes velocity following the orbital motion of the small companion and not the primary component.

The results confirmed that the plasma is associated with the magnetic white dwarf, and that the dwarf’s magnetic field directs the incoming material towards its poles. The study was published in Astronomy & Astrophysics in March 2026.

This artist’s impression visualises the massive star gamma-Cas and its small-but-dense white dwarf companion. New data from XRISM show that the disc of material ejected by gamma-Cas is being consumed by the white dwarf star, generating the highly variable X-ray emission detected from the system. The new observations are the first to detect that the X-rays closely follow the orbital motion of the white dwarf, and not gamma-Cas itself. This closes the case on a mystery that has puzzled astronomers for more than fifty years. Image credit: European Space Agency, Y. Nazé (CC BY-SA 3.0 IGO)

New class of Be and white dwarf binary systems

The XRISM observations established that the Gamma Cassiopeiae system belongs to the long-suspected class of Be + white dwarf binaries, which had previously never been directly observed.

Decades of observations revealed around 20 stars with similar properties. They are called Gamma Cassiopeiae analogues (γ Cas analogues). They include Tianguan (Zeta Tauri) in the constellation Taurus, Pi Aquarii in Aquarius, and BZ Crucis in Crux.

All these stars emit unusually hard X-rays and show a complex variability, caused by a white dwarf companion accreting material from a massive Be star’s disk.

Gamma Cassiopeiae star system

Gamma Cassiopeiae is a multiple star system consisting of up to eight components.

The bright primary component is a spectroscopic binary system with an orbital period of 203.523 days. The component Gamma Cassiopeiae Aa is a massive B-type star and the secondary Gamma Cassiopeiae Ab is a white dwarf with an estimated mass of 0.93 solar masses. They are separated by only 1.632 astronomical units.

Based on the Hipparcos data, astronomers suspect the presence of another component, Gamma Cassiopeiae Ac, with an orbital period of 60 years. The component is separated by 0.233 arcseconds from Gamma Cassiopeiae Aab.

The component Gamma Cassiopeiae B appears at a separation of 2.054 arcseconds and shines at magnitude 11. It takes over a thousand year to complete an orbit around Gamma Cassiopeiae A.

Gamma Cassiopeiae C, a magnitude 13 star almost an arcminute away, was listed as another component of the system but revealed to be only a visual companion. Data obtained in Gaia Data Release 3 places the star at a much greater distance than Gamma Cassiopeiae.

Gamma Cassiopeiae D lies at a separation of 21 arcminutes and is a proper motion companion of Gamma Cassiopeiae. It is a quadruple star system catalogued as HD 5408 (HR 266).

HD 5408

HD 5408 (Gamma Cas D) is a multiple star system located approximately 540 light-years away. It lies about 1 parsec (3.26156 light-years) from Tiansi but is believed to be associated with the Gamma Cassiopeiae system. With a combined apparent magnitude of 5.57, it is faintly visible to the unaided eye on a very clear night.

The system consists of two main visual components that shine at magnitudes 6.12 and 6.57. The component A is a triple star system composed of two hot B-type main sequence stars and an A-type main sequence star. The component B is another B-type star. Another component, a faint red dwarf, is a more distant companion of the system.

HD 5408 A and B orbit each other with a period of 84.10 years and are separated by 0.237 arcseconds. The components Aa and Ab have an orbital period of 4.899 years and a semi-major axis of 0.0309 arcseconds, and Ab1 and Ab2 complete an orbit every 4.241146 days.

HD 5408 lies 21 arcminutes from Gamma Cassiopeiae. The system is believed to part of the Gamma Cassiopeiae association, which includes more than 140 stars within 6 degrees of Tiansi and HD 5408.

Gamma Cassiopeiae and HD 5408, image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Facts

With an average apparent magnitude of 2.39, Tiansi is normally very slightly fainter than Merak in the constellation Ursa Major, Eta Centauri in Centaurus, and Alpha Lupi in Lupus. It is similar in brightness to Enif in Pegasus, and Ankaa in Phoenix, and it just outshines Kappa Scorpii in Scorpius, Sabik in Ophiuchus, and Phecda in Ursa Major.

Gamma Cassiopeiae was first reported to be a binary system in a study published in 2000. The study suggested that the companion had a mass between 0.7 and 1.9 solar masses and was not responsible for the X-ray emission detected from the star.

The binary nature of the star was confirmed in 2011. Since there were no signs of a companion in the spectrum of Gamma Cassiopeiae, the system was adopted as a single-lined spectroscopic binary. Single-lined spectroscopic binary stars are systems in which the spectrum of only one component is seen, and the presence of a companion is inferred from spectral lines periodically shifting towards the blue, then towards the red and then towards the blue again. The shift indicates that the star is moving toward us and away from us, i.e. that it is orbiting around the binary system’s common centre of mass.

The 2011 study suggested a circular orbit with a period between 203.0 and 203.6 days and a companion with a mass of 0.98 solar masses. The companion is believed to be more evolved than the primary and to have transferred some of its mass to the brighter star during an earlier evolution phase.

Cassiopeia’s W

Gamma Cassiopeiae forms Cassiopeia’s W asterism with Segin (Epsilon Cassiopeiae), Ruchbah (Delta Cassiopeiae), Schedar (Alpha Cassiopeiae) and Caph (Beta Cassiopeiae). The star pattern appears as a W during the northern hemisphere spring and summer. In winter, it appears inverted. Seen from locations south of the equator (where the asterism is visible), it always appears like an M.

Gamma Cassiopeiae is the youngest, hottest, most massive and most luminous of the five stars that form the W. Located 550 light-years away, it is also by far the most distant. Segin, the other hot B-type star in the asterism, has a mass of 9.2 solar masses and is the only other supernova candidate among the five stars. It lies approximately 470 light-years away.

The other three stars have already evolved from the main sequence. Schedar is an orange giant with around 5 times the Sun’s mass, located 228 light years away. Caph is an F-type giant located 54.7 light-years away, and Ruchbah is an A-type subgiant 99.4 light-years away.

Cassiopeia’s W, image: Stellarium (annotated for this article)

Deep sky objects near the W

Cassiopeia’s W asterism can be used to find many deep sky objects that appear in this region of the sky. These include the star cluster Messier 52 and the Bubble Nebula (NGC 7635), the famous Heart and Soul nebulae, the Pacman Nebula (NGC 281), the Shrimp Nebula (Sh2-188), the bright Caroline’s Rose Cluster (NGC 7789) and the Owl Cluster (NGC 457) in Cassiopeia and the Double Cluster in the neighbouring constellation Perseus.

Deep sky objects near Cassiopeia’s W, image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Gamma Cassiopeiae Nebula: IC 59 and IC 63

Gamma Cassiopeiae illuminates IC 59 and IC 63, two rim nebulae that appear in the star’s vicinity. Both nebulae have a cometary shape and are 10 arcminutes across. They were discovered by the German astronomer Max Wolf on December 30, 1893.

IC 63 lies closer to the bright star and is the brighter of the pair. It is called the Ghost of Cassiopeia or the Ghost Nebula. Because of its proximity to the luminous star, it has a well-defined ionization front, unlike the more distant IC 59. IC 63 lies 7.5 light-years from Gamma Cassiopeiae and IC 59 is 13.5 light-years from the star.

IC 63 is slowly being eroded away by the star’s radiation. The ultraviolet radiation energizes the electrons within the nebula, and the electrons release the energy as hydrogen-alpha emission, which gives the nebula its red glow. The blue light of the nebula is the light from the star reflected by the dust in the nebula. IC 59 shows less hydrogen-alpha emission and more reflected blue light.

IC 59 and IC 63 (the Ghost of Cassiopeia), image credit: Wikimedia Commons/Savoury Monkey (CC BY-SA 4.0)

Name

The name Tiansi comes from the name of the ancient Chinese asterism, 天駟 (Tiān Sì, Heavenly Quadriga), which is at least 2000 years old. The asterism was part of the traditional Chinese constellation Wangliang (王良) and represented the four horses pulling the chariot. Wang Liang is a legendary charioteer represented by an asterism formed by Caph, Schedar, Achird, Kappa Cassiopeiae and Lambda Cassiopeiae.

The International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) approved the name Tiansi for Gamma Cassiopeiae on November 13, 2025. The name formally applies only to the primary component in the system.

In Chinese astronomy, Gamma Cassiopeiae was also known as Ce or Tsih (sometimes spelled Cih). The name comes from the star’s Chinese name, 策 (Cè), meaning “whip.” Gamma Cassiopeiae is the sole star in the Chinese Whip asterism. The asterism is part of the Legs mansion, which represents the tail of the White Tiger of the West.

However, the name Ce was originally used for Kappa Cassiopeiae, and Gamma Cassiopeiae represented one of the horses pulling the chariot of Wang Liang. The IAU approved the name Cexing (“whip star”) for Kappa Cas in 2025.

Before 2025, Gamma Cassiopeiae was one of the brightest stars without an official proper name, along with the brighter Gamma Velorum (informally known as Regor). It is one of the brightest stars without a traditional Latin or Arabic name.

In more recent times, the star was sometimes called Navi. The names Navi and Regor (Ivan and Roger spelled backwards) were given to the bright stars by the American astronaut Virgil Ivan “Gus” Grissom, the commander of the ill-fated Apollo 1 mission. Grissom used these names as a joke on his planning charts and succeeding Apollo astronauts kept using them as a memorial. Grissom also named Iota Ursae Majoris Dnoces (“Second” backwards). The names Regor and Dnoces referred to Grissom’s fellow astronauts Roger Chaffee and Ed White. Iota UMa now has the formal name Talitha.

IC 63 — nicknamed the Ghost Nebula — is about 550 light-years from Earth. The nebula is classified as both a reflection nebula — as it is reflecting the light of a nearby star — and as an emission nebula — as it releases hydrogen-alpha radiation. Both effects are caused by the gigantic star Gamma Cassiopeiae. The radiation of this star is also slowly causing the nebula to dissipate. Credit: ESA/Hubble, NASA (CC BY 4.0)

How to find Gamma Cassiopeiae

Gamma Cassiopeiae is very easy to find. It is the central star of Cassiopeia’s W, a prominent star pattern in the northern sky. With a declination of +61°, the star is visible from all locations north of the equator, but cannot be seen south of the latitude 29° S. From the southern hemisphere, it is only visible low above the horizon at certain times of the year.

Location of Gamma Cassiopeiae (Tiansi), image: Stellarium (annotated for this article)

Deep sky objects near Gamma Cassiopeiae

Gamma Cassiopeiae can be used to find several interesting deep sky objects. The emission-reflection nebulae IC 59 and IC 63 lie only several light years from the star. IC 59 appears to the north while IC 63 is more to the northeast. Both nebulae are faint and have a low surface brightness, which makes them challenging targets for visual observation next to the bright star. However, they are popular astrophotography targets.

The large open cluster NGC 225 (the Sailboat Cluster) stretches across 12 arcminutes northwest of the star. It has an apparent magnitude of 7.0 and lies 2,200 light-years away. It can be found roughly halfway between Gamma and Kappa Cassiopeiae.

Gamma Cassiopeiae, NGC 225, IC 59, IC 63 and NGC 189, image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

The open clusters NGC 189 and NGC 129 appear in the area between Gamma Cassiopeiae and its bright neighbour Caph, the rightmost star of the W asterism. NGC 189 has an apparent magnitude of 8.8 and occupies 3.7’ of the sky, and NGC 129 shines at magnitude 6.5 and has an apparent size of 21’.

Closer to Caph, the irregular galaxy IC 10 has an apparent magnitude of 10.4 and lies 2.5 million light years from the Sun. It is the only known starburst galaxy in the Local Group.

Gamma Cassiopeiae, Caph, NGC 129, NGC 189 and IC 10, image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Constellation

Gamma Cassiopeiae is located in the northern constellation Cassiopeia. Circumpolar in the northern hemisphere, the Queen constellation has been known to observers since ancient times. It is one of the 48 Greek constellations, first listed by the Greek astronomer Claudius Ptolemy in the 2nd century CE. In Greek mythology, it is associated with Cassiopeia, the vain and boastful queen, wife of Cepheus and mother of Andromeda, who was punished by the gods for claiming that her daughter was more beautiful than the Nereids.

Stretching across an area of 598 square degrees of the northern sky, Cassiopeia is the 25th constellation in size. With its five brightest stars forming a prominent W asterism, it is one of the most recognizable northern constellations. It lies near several other constellations associated with the myth of Perseus: Perseus, Andromeda, Cepheus and Pegasus.

Schedar (Alpha Cassiopeiae), the constellation’s brightest star, is an orange giant that shines at magnitude 2.240 from a distance of 228 light years. It marks the bottom right corner of Cassiopeia’s W.

Other bright stars in Cassiopeia include the yellow-white giant Caph (Beta Cassiopeiae), the blue-white subgiant Ruchbah (Delta Cassiopeiae), the massive blue main sequence star Segin (Epsilon Cassiopeiae), the variable binary star Achird (Eta Cassiopeiae), the massive blue subgiant Fulu (Zeta Cassiopeiae), and the A-type main sequence stars Gang (50 Cassiopeiae) and Theta Cassiopeiae.

Cassiopeia is home to many other interesting stars. These include the variable blue supergiant Kappa Cassiopeiae (Cexing), the yellow hypergiants Rho Cassiopeiae and V509 Cassiopeiae, the red hypergiant PZ Cassiopeiae, the massive blue system called Pearce’s Star (AO Cassiopeiae), the variable multiple star systems Iota Cassiopeiae (Huagai) and AR Cassiopeiae, the Gamma Cassiopeiae variable Omicron Cassiopeiae, and the Mira variable S-type star S Cassiopeiae.

Cassiopeia constellation map by IAU and Sky&Telescope magazine (Roger Sinnott & Rick Fienberg) (CC BY 3.0)

Notable deep sky objects in the constellation include the neighbouring Heart and Soul nebulae (IC 1805 and IC 1848), the emission nebulae NGC 281 (the Pacman Nebula), NGC 7635 (the Bubble Nebula), the Little Rosette Nebula (Sh2-170), and the Fish Head Nebula (IC 1795), the planetary Shrimp Nebula (Sh2-188), the open clusters Messier 52, Messier 103, NGC 457 (the E.T. Cluster), NGC 7789 (the White Rose Cluster), NGC 663, and NGC 654, and the irregular starburst galaxy IC 10, one of our neighbours in the Local Group.

Cassiopeia also contains three well-known supernova remnants: Tycho’s Star (3C 10), the remnant of a Type Ia supernova (SN 1572) observed in November 1572, Cassiopeia A, the brightest extrasolar radio source in the sky, and the Medulla Nebula (CTB 1), an ancient supernova remnant associated with the fast-moving Cannonball Pulsar.

The best time of the year to see the stars and deep sky objects of Cassiopeia is during the month of November, when the constellation appears higher above the horizon in the evening sky. The entire constellation is visible from locations between the latitudes 90° N and 20° S.

The 10 brightest stars in Cassiopeia are Schedar (Alpha Cas, mag. 2.240), Caph (Beta Cas, mag. 2.25–2.31), Tiansi (Gamma Cas, mag. 1.6 – 3.0), Ruchbah (Delta Cas, mag. 2.68), Segin (Epsilon Cas, mag. 3.37), Achird (Eta Cas, mag. 3.44), Fulu (Zeta Cas, mag. 3.59–3.68), Gang (50 Cas, mag. 3.95), Cexing (Kappa Cas, mag. 4.12–4.21), and Theta Cassiopeiae (mag. 4.334).

Tiansi – Gamma Cassiopeiae 

Gamma Cassiopeiae Aa

Gamma Cassiopeiae Ab

Gamma Cassiopeiae B