51 Pegasi (Helvetios)
51 Pegasi (Helvetios) is a star in the constellation Pegasus, magnitude 5.5.
An ordinary-looking star that changed astronomy: the first Sun-like star found with a planet.
On 6 October 1995, Michel Mayor and Didier Queloz announced a planet orbiting 51 Pegasi — the first ever found around a Sun-like star. Working at the Observatoire de Haute-Provence in France, they had caught the star wobbling as an unseen companion tugged it back and forth. In 2019 the discovery won them a share of the Nobel Prize in Physics.
The planet, 51 Pegasi b, was a shock. With at least half the mass of Jupiter, it races round its star in just over four days, so close that it is heated to around 1,200 °C. Theory said giant planets should form far from their stars, so this ‘hot Jupiter’ pushed astronomers to accept that planets can migrate.
The star itself is a near-cousin of the Sun: slightly older at about 4.8 billion years, 9% more massive, 11.5% larger and richer in heavy elements. It seems to be in a quiet spell with few starspots, like the Sun’s own Maunder minimum.
51 Pegasi (Helvetios) is 51 light-years away: the light we see from it set off in 1975, 6 years after Apollo 11 landed on the Moon.
Did you know?
- Bellerophon. Before it had an official name, the planet was nicknamed Bellerophon, after the Greek hero who rode the winged horse Pegasus.
- Names from Switzerland. In 2015 the star became Helvetios, after the Celtic Helvetii of ancient Switzerland, and its planet Dimidium, Latin for ‘half’ — names proposed by an astronomy society in Lucerne.
A 5th-magnitude yellow star, visible to the naked eye under dark skies, about two degrees west of the midpoint between Scheat and Markab on the Great Square’s western side. Binoculars make it easy to pick out.
When and where to see it
It is best placed in the evening sky around October, when it crosses the meridian at about 10 pm local time (around midnight in September). It can be seen from everywhere north of 69°S and never sets north of 69°N.
At its highest it stands 61° up from 50°N, 76° up from 35°N, 69° up from the equator, 34° up from 35°S.
Facts
- Type
- Star
- Constellation
- Pegasus
- Designations
- Helvetios, 51 Peg, HR 8729, HD 217014, HIP 113357
- Magnitude
- 5.5
- Position
- RA 22h 57.5m, Dec +20° 46′ (J2000)
- Spectral type
- G2IV
- Colour
- yellow (B−V 0.70)
- Distance
- 51 light-years
- Discovered
- Michel Mayor and Didier Queloz, 1995, Discoverers of its planet, 51 Pegasi b
Open AstroPilot to see when 51 Pegasi (Helvetios) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.