When is the best time to observe Tau Canis Majoris?

Tau Canis Majoris is best observed from December through February, with peak visibility during January when it transits around midnight in the Northern Hemisphere.

Where is Tau Canis Majoris located in the sky?

Tau Canis Majoris lies in the constellation Canis Major at right ascension 07h 19m and declination -24° 57′.

What telescope do I need to split Tau Canis Majoris?

Splitting Tau Canis Majoris requires an aperture of roughly 80 mm or larger — the components are separated by about 8.3″ at magnitudes near 4.4. The companion is about 5.8 magnitudes fainter, so glare from the primary — not separation — sets the minimum aperture. Larger apertures and steady seeing help resolve the pair cleanly.

Tau Canis Majoris — Double Star in Canis Major

Magnitude 4.4^m DoubleStar Canis Major (CMa) Visible

Star Map + List + Plan

Observe

1Physical Properties

Magnitude 4.40

Range 4.38 - 4.44

Period 30.8 hours

Variable Type Eclipsing Binary (Beta Lyrae-type)

Spectral Type O9Ib supergiant

Star Color Blue-white (B-V -0.15)

2Position & Identifiers

RA 07h 18m 42.4s

Dec -24° 57' 15.0"

Constellation Canis Major (CMa)

HR 2782

HIP 35415

HD 57061

SAO 173446

Bayer Tau

Flamsteed 30 CMa

Double Cat 5977

SIMBAD

3How easy to split?

Primary 4.4 mag Companion 10.2 mag Separation 8.3″

Sign in and configure your equipment and default location to see a personalized row.

Telescope	Bortle 3	Bortle 4	Bortle 5
80 mm refractor 80mm refr.	Hard	Hard	Hard
150 mm Newton 150mm Newt.	Medium	Medium	Medium
Celestron C8 (203 mm SCT) C8 203mm	Medium+	Medium+	Medium+

Easy Medium Hard Very hard Impossible

Bortle 3 = rural · 4 = outer suburbs · 5 = suburbs

4Visibility

Set a location in User Settings to see visibility data.

Best season Dec – Feb (peak: Jan)

5Light Curve

6Multiple Star System Quintuple D: optical

Components 5 (quintuple)

Component IDs AP

Separation 8.3″

Companion Mag 10.2

Companion Sp B2V

Position Angle 90°

Star Colors A: Blue-white P: Blue

Discoverer HJ 3948

Notes

B, 10v at 8" fixed. D, 8.22V, -0.19(B-V), -0.93(U-B), B2V at 85".

Separation over time

Measured 1835 → 2015 (180 y)

Separation drift 8.0" → 8.3" (+0.30")

Rate +0.0017" / y

PA drift 86° → 90° (+4°, +0.022°/y)

Essentially fixed on human timescales — the same view your grandchildren will see.

Measured from the WDS observational archive. No orbital solution has been derived — most likely the period is too long to fit an orbit to the available measurement arc.

Eyepiece View

Telescope Eyepiece

Log in to set your own equipment

80x Dawes: 1.9″ TFOV: 0.6°

Seeing

Scale Realistic = true angular size

A: 4.4 · B: 10.2 · Sep: 8.3″ · PA: 90° · N up, E right

Resolved · Rayleigh: 2.3″ · Dawes: 1.9″ · Eff: 2.3″

Explore

8

Size Comparison

Querying VizieR for stellar data…

9

Compare Stars

Compare with another star

10

Spectral Classification

11

Hertzsprung-Russell Diagram

Loading HR diagram…

Read: The Hertzsprung-Russell Diagram — Reading Stars Like a Map

12

Stellar Lifecycle

13

Blackbody Spectrum

14

Stellar Absorption Spectrum

Simulated absorption spectrum based on spectral type. Hover over lines to identify elements.

15

Stellar Fusion

Discover

16Stellar Notes

Variable Star

3.90 - 3.96p. Possibly same period as SB.

Spectroscopic Binary

ADS 5977A, 154.90d, K 49.5k/s, V0 +43k/s, asini 101.

Color

Color excess E(B-V) = +0.15.

Group/Cluster

In cluster NGC 2362 #23; in HII reg. S310. Although a neutral hydrogen cloud about 0.5d in diameter is in the region, it | is at a much greater distance and not associated with the cluster.

Spectral

uv1548 absorption strong, emission weak.