30 τ CMa
ILLUSTRATION FROM SPECTRAL TYPE O9II · 33 000 K · 13 100 L☉ · VARIABLE, MAG 4.35–4.41 · PERIOD 1.282 DAYS · PULSE COMPRESSED
OVERVIEW
] STAR30 τ CMa is an in the constellation Canis Major, 2992 from Earth, shining at apparent magnitude 4.37 (). It is catalogued as HD 57061, HIP 35415 and HR 2782.
- WHAT IT IS
- Blue bright giant
- HOW FAR
- 2992
- HOW BRIGHT (V)
The light arriving now left it that long ago.
Bright enough for the naked eye, under a sky dark enough.
THE MODEL
] RADIUS FROM THE LUMINOSITY AND THE TEMPERATURECALCULATED · NOT AN OBSERVATION · RADIUS FROM THE LUMINOSITY AND THE TEMPERATURE · SPECTRAL TYPE O9II · 33 000 K · 13 100 L☉ · VARIABLE, MAG 4.35–4.41 · PERIOD 1.282 DAYS
Drawn from the radius from the luminosity and the temperature and from the same properties the picture at the top of this page uses, at the distance a flight to it would stop. Drag it, or use the arrow keys, to look from another direction. The stars behind are the 498,631 this catalogue holds real distances for, drawn from where the camera actually is — but every one of them is within 170 parsecs of the Sun and this object is much further out, so what is in the frame is that whole neighbourhood seen from outside it, piled into a knot around the Sun. The rest of the sky from here is not something this catalogue knows.
OBSERVING
] FROM WHERE YOU AREWhat it takes to see 30 τ CMa for yourself. Everything in this part of the page is worked out for a real horizon, a real night and the equipment you say you have.
Say where you are, and this line says when 30 τ CMa is worth going out for.
THROUGH AN EYEPIECE
] 150 MM TELESCOPE · 25 MM EYEPIECEBright enough to see with your eyes alone, once you are away from town lights. Its brightest companion sits 0.10″ away, which at 48× is blended into one star.
- 48×
- 1.08
- 1.25
- 13.0
- · Aa,Ab
- 0.10 · blended into one star
- · AD
- 1.4 · cleanly split
- · AE
- 0.90 · blended into one star
- APPARENT MAGNITUDE ()
- 4.37 · within reach
A is where the pair was the year it was measured — the ones above, between 2002 and 2020, and a pair in a real orbit has moved since.
APPARENT PLACE
] EQUINOX J2026.7Where a telescope has to be pointed today, which is not quite where the catalogue records the object: the coordinate grid itself has turned since the year 2000 the catalogue is referred to.
- RIGHT ASCENSION
- DECLINATION
- GRID TURNED SINCE
- 15.4
- PROPER MOTION SINCE
- 0.11
Computed for 2026-08-28 00:00 UT. Two things separate it from the catalogued position above: 30 τ CMa has moved under its own proper motion, and the coordinate grid has turned under it — about fifty arcseconds a year of precession since 2000, and the small nodding of the Earth’s axis on top of that. Aberration and refraction are not included, and neither is parallax; each is under an arcminute, which is also about as well as the catalogued position is known. The same figure for any date.
VARIABILITY
] BETA LYRAE-TYPE ECLIPSING BINARY- RANGE, AS SAMPLED
- 4.35 – 4.41
- PERIOD
- 1.28214 days (30.8 h)
- PHASE
- 0.238 ± 0.039
- NEXT MINIMUM
- 2026-08-28 23:26 UT---------- --:-- your time
- THEN
- 2026-08-30 06:12 UT---------- --:-- your time
- THEN
- 2026-08-31 12:58 UT---------- --:-- your time
Computed for 2026-08-28 00:00 UT. A phase of zero is minimum light, and 30 τ CMa was last there 7.3 h before that. The ± is what the published elements can still promise: 9,967 cycles have gone by since the epoch they were measured at, and the last digit of the period compounds over every one of them — 72 min by now. It is the precision of the published figures rather than an error bar the catalogue supplied, so where that catalogue has marked a period doubtful the real spread is wider — and it does not contain the star. A binary with a third body in the system, or a pulsator whose period is slowly changing, walks away from a straight-line ephemeris by an amount only observation settles. Times are corrected for where the Earth is in its orbit, which is worth up to 8.3 minutes — the light of a star reaches us that much earlier in one season than the other, and an epoch measured a century ago has the same correction in it.
The range is the spread the Hipparcos satellite measured, converted to the visual scale. It is approximate: the satellite sampled the star on a schedule of its own, and where a star spends little of its cycle at an extreme — the floor of an eclipse, most obviously — the sampling can miss it altogether. The classification, the period and the epoch are the General Catalogue of Variable Stars’s. The phase at any instant.
DOUBLE STAR
] 5 PAIRS| COMPONENTS | SEPARATION | POSITION ANGLE | MAGNITUDE | MEASURED |
|---|---|---|---|---|
| Aa,Ab | 0.1″ | 310° | 5.33 | 2020 |
| AE | 0.9″ | 87° | 9.7 | 2018 |
| AB | 8.3″ | 90° | 10.2 | 2015 |
| AC | 14.2″ | 87° | 11.2 | 2002 |
| AD | 83.5″ | 78° | 8.22 | 2020 |
Separation and position angle are as measured in the year given; a position angle is measured from north, through east. Washington Double Star Catalog 07187-2457.
SPECTRUM
] A SPECTRUM OF ITS TYPENo survey in this catalogue has measured 30 τ CMa’s own spectrum: the one that reaches deepest saturates on the brightest stars, and no spectrograph has been pointed at the whole sky. What is drawn below is the average of several real stars of the same class, B2 II — the nearest class the atlas holds to O9II. It shows what a star like this one shows, and every line named on it is a line of the type rather than a measurement of 30 τ CMa.
A star gives out light at every wavelength at once. The dark gaps are atoms in the thin gas above its surface, each one removing the single colour it is able to absorb — which is how a list of ingredients is read off something nobody will ever touch.
Point at a labelled line — or an element above — to see which atom makes it and where it falls.
Pickles (1998) stellar spectral flux library. B2 II — the average of several real stars of this type, not an observation of any one star. Sampled every 5 angstroms; 1,895 points rebinned onto 1,024 log-spaced display points and normalised to the median flux. Brightness is scaled to this spectrum’s own middle value, so the shape is the measurement and the height is not. The 6 lines named above are the ones that stand clear of the scatter and of the features around them; a spectrum at this sampling holds many more that it cannot separate. The samples themselves, as JSON, CSV or VOTable.
LIFE OF THIS STAR
] STAGE 3 OF 530 τ CMa is a blue bright giant near the end of its life; within about two hundred thousand years it will collapse and explode as a supernova, leaving a black hole.
- COLLAPSING CLOUDtook about ten thousand years
- MAIN SEQUENCElasted about two million years
- BRIGHT GIANTabout two hundred thousand years of itNOW
- SUPERNOVAbright for weeks, over in a second
- BLACK HOLEfor the rest of time
NOW The same swelling, on a star heavy enough to keep going: helium, then carbon, then a run of heavier elements each burning faster than the last, until the core is iron and fusion can release nothing more.
NEXT The iron core collapses in about a second and the rest of the star is thrown off at a tenth of the speed of light. For a few weeks it outshines the galaxy it is in.
EVERY STAGE ON THIS TRACK
- COLLAPSING CLOUDtook about ten thousand years
- A cold clump of gas and dust falls in on itself and heats as it falls. It stops being a cloud and starts being a star at the moment its core is hot enough to fuse hydrogen.
- MAIN SEQUENCElasted about two million years
- Hydrogen fuses to helium in the core, and the outward push of that exactly balances the star's own weight. This is the long, steady stretch — nine-tenths of a star's life — and it ends when the core's hydrogen is gone.
- BRIGHT GIANTabout two hundred thousand years of itNOW
- The same swelling, on a star heavy enough to keep going: helium, then carbon, then a run of heavier elements each burning faster than the last, until the core is iron and fusion can release nothing more.
- SUPERNOVAbright for weeks, over in a second
- The iron core collapses in about a second and the rest of the star is thrown off at a tenth of the speed of light. For a few weeks it outshines the galaxy it is in.
- BLACK HOLEfor the rest of time
- Nothing the core could be made of is stiff enough to stop the collapse, so it does not stop. What remains is a region of space that light cannot leave.
- HOW FIRM No mass has been measured for this star, and past the main sequence its temperature no longer implies one — a red giant of one solar mass and a red supergiant of fifteen are the same colour. The figure above comes from its total output instead, and is unlikely to be better than a factor of two.
Which stage 30 τ CMa is in was read from its catalogued class, its temperature and its total output at every wavelength — not from its brightness in visible light alone, which understates a cool star by a factor of several. The mass it started with is about thirty times the Sun's mass, from its luminosity, and that mass is what chooses the track. Every span of time here comes from the standard relation between a star’s mass and how long its fuel lasts, rounded to a single figure. They are orders of magnitude and none of them is a date.
CATALOG DATA
] 15 FIELDSThe raw catalogue values, as the source published them, with the error bars where one was published — sorted by what each set of figures is about rather than by the order they happened to be stored in.
WHAT KIND OF THING IT IS
What the object is in itself, rather than how it happens to look from here.
- Spectral type
- Luminosity
- 13 097.8
- Variability
- Beta Lyrae-type eclipsing binary
- Variability type ()
- EB
- Variability period
- 1.28214
- Reference epoch ()
HOW BRIGHT IT IS
What the light adds up to, measured through one filter at a time. A magnitude is a brightness on a backwards scale — the smaller the number, the brighter the object.
- Colour index (B−V)
- Variable magnitude, brightest
- Variable magnitude, faintest
WHERE IT IS AND HOW IT MOVES
Position, motion across the sky and the size it covers on it. These are measurements of the object as seen from here, so every one of them depends on where here is.
- Proper motion (RA)
- −1.82
- Proper motion (Dec)
- 3.74
- Radial velocity
- 40
OTHER NAMES AND NUMBERS
The identifiers other catalogues file this object under, for anyone joining this data to theirs.
- Variable-star designation
- Tau CMa
- Washington Double Star id
- 07187-2457
WHERE THESE FIGURES CAME FROM
Which catalogue, paper or model each of the fields above was read from, where the source named one.
- Spectral type from
- SIMBAD (2014ApJS..211...10S, quality C)
ALSO KNOWN AS
] 4 DESIGNATIONS- Bayer/Flamsteed 30 τ CMa
- HD HD 57061
- HIP HIP 35415
- HR HR 2782
LOOK IT UP ELSEWHERE
] 3 SERVICESEach link carries the identifier printed beside it. Nothing on this page is a measurement of our own, so a number that disagrees with one of these is worth reporting — and a service that has never catalogued this object under this name will say so plainly.
- SIMBADHR 2782
Cross-identifications, measured values and the reference behind each of them.
- ADSobject:"HR 2782"
Papers indexed against this object rather than against the string, newest first.
- VizieR5″ around 109.6770 -24.9544
Every published table with a row at this position, searched by coordinates rather than by name.
CITE THIS PAGE
] DR1This page is live and its numbers move when an upstream refreshes, so the citation names the release the rows belong to rather than the day you read them. The release itself is frozen, republished as a bundle, and is what a data-availability statement should point at.
SpaceCatalog. 30 τ CMa [catalogue entry]. Data release DR1 (2026-08-21). https://spacecatalog.org/object/hd-57061
@misc{spacecatalog_hd-57061,
author = {{SpaceCatalog}},
title = {{30 τ CMa}},
howpublished = {SpaceCatalog, data release DR1 (2026-08-21)},
year = {2026},
url = {https://spacecatalog.org/object/hd-57061},
note = {Catalogue entry, data release DR1 (2026-08-21). Values from HYG Database v4.1.}
}The values above were taken from HYG Database v4.1 — CC-BY-SA-4.0. Citing this page does not replace citing that catalogue, and its licence is share-alike: the terms reach whatever you build from these rows.