First Light Guide #16: Tau Tauri

The object: Tau Tauri

A detailed guide to finding / observing the double star in Taurus

First Light Guide 16 graphicFirst Light Guides #16 is a detailed written and video reference to finding and observing the double star Tau Tauri in the constellation of Taurus. This guide is ideal for beginners, but many advanced amateur astronomers may find it useful. It is designed to help you locate this double star with almost any telescope. There is a video to using a magnified finderscope, red dot finder, or setting circles along written tips on observing Tau Tauri.

Visitors who are new to this site may wish to get started here, while more advanced observers can search the First Light Guides by constellation, or by right ascension

Find Tau Tauri in the sky

Tau Tauri is a double star within our own Milky Way galaxy. It consists of a main sequence B-class dwarf star, and an A-class white star. This is actually a three star system, but the third component is very close 0.1 arc seconds away from the brighter primary star. The slight difference in temperature and visual brightness sometimes shows an actual color difference to some observers and with a wide separation, anyone can find and observe them. The separation of the two easily seen components has remained the same since they were first seen about 200 years ago. The brighter primary star's visually-invisible companion revolves around it every 58 years, though this is only seen via spectroscopy and/or lunar occulation.

Incidentally, do not confuse Tau Tauri (sometimes shown as Tau Tau) with T Tauri, a small protostar, or TT Tauri, a red carbon star. 

If you have a magnified finderscope on your telescope, start here to find Tau Tauri:

If you have a red dot finder on your telescope, start here to find and observe Tau Tauri:

Observing tips:

Tau Tauri is an easily split double star, with a wide separation of 63 arc seconds between the brighter components. As a zodiac constellation, Taurus is visible from most anywhere on Earth, and is ideally placed for best viewing when it is at or near the meridian. In this way, it will be at it's highest point (culmination), providing the least amount of atmosphere to look through in order to find and observer the star. Use a long focal length eyepiece.

The two stars are close in spectral type, with the 4th magnitude brighter primary a B-class blue-white star, and the 7th magnitude secondary an A-class white star. It has been described as "bluish-white and lilac" though observer's perception of colors varies depending on many factors such as their own eyesight, telescope aperture, and atmospheric clarity. More color is more easily seen through smaller aperture telescopes. What color(s) do you see?

Useful filter(s): None needed.

What should I see?

Below is an approximate view of Tau Tauri and the surrounding stars as seen with a 70mm telescope at 38x magnification, and a 1.40 degree telescopic field of view.  

Tau Tauri at 38x in 1.4 degree FOV

Details of Tau Tauri

Type: Double star

True binary: Uncertain

Orbital period: Unknown, but based on 7,800 AU distance, perhaps 200,000 years

Distance: 398 ± 43 light years

Apparent separation: 63 arc seconds

Apparent magnitudes: 4.3, 7.1

Right ascension: 04h 42m 14.7s

Declination: 22° 57′ 25

Because telescopes and observers are all different, here are some alternate sketched/drawn views of Tau Tauri:

None available (want to make one and send it to me?)

The constellation: Taurus

General information can be found here about Taurus the Bull, where Tau Tari is located.  This will help you know where to find the constellation in the sky and be able to locate and identify its brightest stars.


General information about Taurus

Name of constellation: Taurus

Abbreviation: Tau

Genetive form: Tauri

Common names: Bull

Associated asterisms: No major ones

Original 48 of Ptolemy: Yes

Area by size: 797 square degrees

Relative size: 17 out of 88 (Cygnus is next larger, Camelopardalis is next smaller)

First Light Guides objects: Tau Tauri

Brightest stars in Taurus, by magnitude 

First lists the Bayer designation, then the "traditional" star name (often Arabic, but not always - see each star's notes for details). Graphic shows constellation at culmination on the meridian, facing south.

  • α (Alpha) Tauri (Aldebaran) magnitude 0.9 it is the 14th brightest star in the sky, and resides at 65 light years distance. The traditional name comes from the Arabic phrase al-dabarān that means "the follower" perhaps due to the fact that it follows the Pleiades across the sky.
  • β (Beta) Tauri (Alnath or El Nath) magnitude 1.3, and is 131 light years distant. Traditional name means the Arabic النطح an-naţħ meaning "the butting" (horns). This star is often shared with the shape of Auriga the Charioteer. 
  • Alcyone A is a 2.9 magnitude star in the Pleiades star cluster, the brightest of this open group. It is named after the mythological figure of Alcyone
  • ζ (Zeta) Tauri magnitude 3.0 is the other "horn" star of Taurus.