Sadr (Gamma Cygni)
Sadr (Gamma Cygni) is a star in the constellation Cygnus, magnitude 2.2.
The heart of the Northern Cross — a yellow-white supergiant set among glowing clouds of gas.
Sadr sits where the beams of the Northern Cross meet, and its name fits: Arabic ṣadr, “chest”, the breast of Cygnus the swan. The same word gave Schedar in Cassiopeia its name. A later catalogue called it Pectus Gallinæ, “the hen’s chest”.
This is an enormous star, about 14.5 times the Sun’s mass and some 180 times its width. If it replaced the Sun, its surface would reach beyond the orbit of Venus. It shines over 33,000 times as brightly as the Sun, yet its surface, near 5,790 kelvin, is almost exactly as hot as the Sun’s. Only about 12 million years old, it is heavy enough to end as a supernova.
It is restless too. Its spectrum shifts slowly over 100 days or more, and it lies close to the zone where stars pulsate like Cepheid variables. It appears wrapped in a faint glowing cloud, IC 1318, the Gamma Cygni nebula.
Sadr (Gamma Cygni) is 1,800 light-years away: the light we see from it set off around AD 230, 100 years after the Pantheon was rebuilt in Rome.
Did you know?
- A yardstick star. Since 1943 Sadr’s spectrum has been one of the fixed anchor points astronomers use to classify other stars.
- The Celestial Ford. In China it is Tiān Jīn yī, the First Star of the Celestial Ford, an asterism that also takes in Deneb.
A yellow-white 2nd-magnitude star where the arms of the Northern Cross meet, between Deneb and Albireo. Sweep around it with binoculars: it lies in a rich stretch of the Milky Way, laced with faint glowing gas.
When and where to see it
It is best placed in the evening sky around August, when it crosses the meridian at about 10 pm local time (around midnight in July). It can be seen from everywhere north of 50°S and never sets north of 50°N.
At its highest it stands 80° up from 50°N, 85° up from 35°N, 50° up from the equator, 15° up from 35°S.
Facts
- Type
- Star
- Constellation
- Cygnus
- Designations
- Gamma Cygni, γ Cyg, 37 Cyg, HR 7796, HD 194093, HIP 100453
- Magnitude
- 2.2
- Position
- RA 20h 22.2m, Dec +40° 15′ (J2000)
- Spectral type
- F8Ib
- Colour
- yellow (B−V 0.67)
- Distance
- 1,800 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Sadr (Gamma Cygni) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.