Theta Tauri — Double Star in Taurus
HIP 20885; Theta1 Tauri; 77 Tauri
About Theta Tau
Description
Theta1 Tauri is a G9 III yellow giant about 154 light-years away, the fourth-brightest member of the Hyades cluster at magnitude 3.84. It forms a naked-eye optical pair with Chamukuy (Theta2 Tau) across 5.6 arcminutes. Despite the close apparent pairing, the two Theta stars are roughly 20 light-years apart — but both are legitimate Hyades members.
Observing Tips
Theta1 and Theta2 Tau form an eye-catching pair just east of the main V, only about 5 arcminutes apart — one of the most accessible wide naked-eye doubles in the sky, visible even from urban skies. Binoculars clearly separate them and show their nearly matched yellow-orange colors. Best observed November through March.
History
The Theta designation was assigned by Bayer in 1603. The two Theta Tau stars are both classical Hyades members and have long been used as test subjects for cluster-membership criteria. No traditional name has been widely applied.
Fun Facts
Theta1 and Theta2 Tauri are a useful demonstration that even within a single open cluster, small spatial variations can create apparent double stars of physically unrelated origin. Their combined gravitational influence on local cluster dynamics has been mapped by Gaia's highly precise parallax measurements.
Observe
1Physical Properties
2Position & Identifiers
3How easy to split?
| Telescope | Bortle 3 | Bortle 4 | Bortle 5 |
|---|---|---|---|
| 80 mm refractor 80mm refr. | Hard | V. hard+ | V. hard |
| 150 mm Newton 150mm Newt. | Medium | Hard+ | Hard |
| Celestron C8 (203 mm SCT) C8 203mm | Medium+ | Medium | Hard+ |
Bortle 3 = rural · 4 = outer suburbs · 5 = suburbs
4Visibility
Set a location in User Settings to see visibility data.
5Multiple Star System Quadruple
Eyepiece View
A: 3.8 · B: 12.0 · Sep: 268.3″ · PA: 75° · N up, E right
Resolved · 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
Discover
15Stellar Notes
16
Light Travel Time Machine
17
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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