Alrai — Double Star in Cepheus
HIP 116727; Gamma Cephei; 35 Cephei
About Alrai
Description
Errai (also called Alrai) is an orange subgiant of spectral type K1III-IV at magnitude 3.21 in Cepheus. Located about 45 light-years from Earth, it has a luminosity of roughly 11 times solar. Errai hosts a confirmed exoplanet (Gamma Cep Ab) with about 1.6 Jupiter masses, and will become the pole star around AD 3100.
Observing Tips
Errai lies in the wall of the house-shaped Cepheus, near the north celestial pole. It is circumpolar from mid-northern latitudes. Its warm orange color is apparent in binoculars. Best observed August through December when Cepheus is highest.
History
The name Errai comes from the Arabic 'al-ra'i,' meaning 'the shepherd.' Its exoplanet, first suspected in 1988, was one of the earliest claimed exoplanet detections, though not confirmed until 2003. Due to precession, Errai will be the north pole star around AD 3100-4100.
Fun Facts
Errai will be a closer pole star than Polaris currently is — around AD 4000, it will lie within 0.5 degrees of the celestial pole, making it an excellent pole star for future generations.
Observe
1Physical Properties
2Position & Identifiers
3How easy to split?
| Telescope | Bortle 3 | Bortle 4 | Bortle 5 |
|---|---|---|---|
| 80 mm refractor 80mm refr. | V. hard | V. hard | V. hard |
| 150 mm Newton 150mm Newt. | V. hard+ | V. hard+ | V. hard+ |
| Celestron C8 (203 mm SCT) C8 203mm | Hard | Hard | Hard |
Bortle 3 = rural · 4 = outer suburbs · 5 = suburbs
4Visibility
Set a location in User Settings to see visibility data.
5Multiple Star System
Eyepiece View
A: 3.2 · B: 7.3 · Sep: 1.8″ · PA: 215° · N up, E right
Unresolved · Rayleigh: 2.3″ · Dawes: 1.9″ · Eff: 2.3″
Explore
7
Size Comparison
8
Compare Stars
9
Spectral Classification
10
Hertzsprung-Russell Diagram
11
Stellar Lifecycle
12
Blackbody Spectrum
13
Stellar Absorption Spectrum
Simulated absorption spectrum based on spectral type. Hover over lines to identify elements.
14
Stellar Fusion
15
Exoplanets
1 known planet
View in 3D
| Planet | Radius | Mass | Period | Distance |
|---|---|---|---|---|
| gam Cep b | 12.70R⊕ | 6.60M♃ | 2.5yr | 46ly |
Habitable Zone
Size & Mass Comparison
About exoplanets — how we find them and which host stars you can observe
Discover
16Stellar Notes
17
Light Travel Time Machine
18
Relativistic Travel
Nearby in the Sky
Other targets within a few degrees — pan your scope a little and keep exploring.
Visibility scores assume a 150 mm Newton at Bortle 4.
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