What kind of star is 51 Pegasi?
51 Pegasi, catalogued as 51 Pegasi (Helvetios; HD 217014), appears in the official constellation Pegasus. Its apparent visual magnitude is about 5.49, while its estimated distance is about 50.6 light-years. Astronomers classify the object as G2 IV–V Sun-like star. These labels describe temperature, luminosity class or system components; they should not be read as a claim that every naked-eye point is a single isolated star.
51 Pegasi is a slightly evolved G-type star broadly comparable with the Sun. Its planet, 51 Pegasi b or Dimidium, has roughly half Jupiter's mass and circles at only about 0.05 astronomical unit, completing a year in 4.23 days. The planet does not transit from Earth's viewpoint; it was found through periodic Doppler shifts in the stellar spectrum. Such a tight gas giant contradicted expectations based solely on our Solar System and drove theories of planetary migration. Later observations detected the planet's reflected-light spectrum, but no telescope shows it as a conventional resolved globe beside the star. Page copy should call it the first planet confirmed around a Sun-like star, not the first exoplanet of any kind.
The coordinate pair RA 22h 57m 27.98s, Dec +20° 46′ 07.8″ fixes the object's direction on the celestial sphere for the J2000 reference epoch. Right ascension works like celestial longitude measured in hours, while declination works like latitude north or south of the celestial equator. Proper motion and improved measurements can refine catalogue positions, but these coordinates are sufficiently precise for a finder chart or planetarium search.
51 Pegasi data at a glance
| Proper name | 51 Pegasi | Catalogue designation | 51 Pegasi (Helvetios; HD 217014) |
|---|---|---|---|
| Constellation | Pegasus | Apparent magnitude | about 5.49 |
| Spectral or system type | G2 IV–V Sun-like star | Estimated distance | about 50.6 light-years |
| Right ascension (J2000) | 22h 57m 27.98s | Declination (J2000) | +20° 46′ 07.8″ |
Values are rounded for an observer-friendly overview. Consult the linked SIMBAD record for component-level identifiers, bibliography, measurement provenance and newer catalogue values.
The name 51 Pegasi
“51 Pegasi” is its Flamsteed catalogue designation. The IAU-approved proper name Helvetios, honouring Switzerland, was selected through the 2015 NameExoWorlds campaign; its planet 51 Pegasi b is named Dimidium.
Michel Mayor and Didier Queloz announced 51 Pegasi b in October 1995 after observations with the ELODIE spectrograph at Haute-Provence Observatory. Pulsar planets had been found earlier, but this was the breakthrough around a normal Sun-like star. The discovery helped earn Mayor and Queloz half of the 2019 Nobel Prize in Physics and launched intensive radial-velocity surveys around the world.
Modern professional catalogues retain identifiers such as the Bayer designation alongside a proper name. The International Astronomical Union standardizes which name belongs to which component—an important distinction for systems whose combined light looks like one star from Earth.
How to find 51 Pegasi
Use the Great Square of Pegasus and a detailed chart to locate this faint naked-eye star west of the square. Binoculars make the identification easier. The planet cannot be seen separately, but precision spectroscopy detects its four-day gravitational pull on the host.
Visible to unaided eyes under dark skies from most inhabited latitudes, best during northern autumn and southern spring. Visibility on a particular night still depends on latitude, season, time, horizon obstructions, weather and local light pollution. Use the interactive atlas below to check the surrounding field before observing.
Observation checklist
- Best evening months are September through November.
- Dark skies are needed for reliable naked-eye identification.
- Use the Great Square of Pegasus with a detailed finder chart.
- The planet is detected spectroscopically and cannot be visually separated.
Interesting facts about 51 Pegasi
- 51 Pegasi b was the first confirmed planet around a Sun-like star.
- Its year lasts only about 4.23 Earth days.
- The planet established the hot-Jupiter class as an observational reality.
- The discovery contributed to the 2019 Nobel Prize in Physics.
- Helvetios and Dimidium are IAU-approved proper names for star and planet.
Continue from 51 Pegasi into the night sky
Use the Pegasus constellation guide to place this star within its larger pattern. When you are ready to observe, compare dark-sky places, find a nearby observatory, or follow our practical guide to finding a star in the night sky. Symbolically registered names are kept separately in the searchable Star-Register catalogue.
Sources and further reading
Measurements are rounded for readability. Multiple systems, variable stars and distant supergiants can have component-specific or model-dependent values. Survey panels show photographic sky fields, not the physical surface of the star.