6 Gem
ILLUSTRATION FROM MEASURED TEMPERATURE · 3790 K · LUMINOSITY CLASS · VARIABLE, MAG 6.36–6.62 · TIMING NOT TO SCALE
OVERVIEW
] STAR6 Gem is an in the constellation Gemini, shining at apparent magnitude 6.51 (). It is catalogued as HD 42543, HIP 29450 and HR 2197, among 5 designations in total.
- WHAT IT IS
- Red supergiant
- HOW BRIGHT (V)
Binoculars reach it; the naked eye does not.
OBSERVING
] FROM WHERE YOU AREWhat it takes to see 6 Gem 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 6 Gem is worth going out for.
THROUGH AN EYEPIECE
] 150 MM TELESCOPE · 25 MM EYEPIECEToo faint for your eyes alone. Binoculars reach it. It will be a point of light in anything: a star is far enough away that no aperture on Earth gives it a width, and all the equipment changes is how easily you find it.
- 48×
- 1.08
- 1.25
- 13.0
- APPARENT MAGNITUDE ()
- 6.51 · within reach
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
- 22.5
- PROPER MOTION SINCE
- 0.06
Computed for 2026-08-28 00:00 UT. Two things separate it from the catalogued position above: 6 Gem 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
] SLOW IRREGULAR SUPERGIANT (LATE SPECTRAL TYPE)- RANGE, AS SAMPLED
- 6.36 – 6.62
No period is published for this star, so there is no cycle to count. The catalogue records that it varies and by how much, and not how often.
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 is the General Catalogue of Variable Stars’s. These elements as data.
SPECTRUM
] A SPECTRUM OF ITS TYPENo survey in this catalogue has measured 6 Gem’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, M2 I — the nearest class the atlas holds to M1-M2Ia-Iab. 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 6 Gem.
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.
- HEAVY ELEMENTS The catalogued iron figure for 6 Gem is -0.42, which puts it at about a third of the Sun's share of the elements heavier than helium. The figure is a logarithm: nought is the Sun, and every whole step is a factor of ten. It comes from published measurements of the star itself, not from the spectrum above — which belongs to the type.
Pickles (1998) stellar spectral flux library. M2 I — 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 5 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 56 Gem is a red supergiant near the end of its life; some time between the next few centuries and the next million years it will collapse and explode as a supernova, leaving a neutron star.
- COLLAPSING CLOUDtook about two hundred thousand years
- MAIN SEQUENCElasted about forty million years
- SUPERGIANTcenturies to a million years leftNOW
- SUPERNOVAbright for weeks, over in a second
- NEUTRON STARfor 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 two hundred 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 forty 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.
- SUPERGIANTcenturies to a million years leftNOW
- 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.
- NEUTRON STARfor the rest of time
- What is left of the core: a couple of solar masses in a ball the width of a city, so dense that its protons and electrons have been pressed into neutrons.
- HOW FIRM How long a red supergiant has left depends on which fuel its core is burning, and that cannot be seen from outside — it is read from the star's pulsation, by working out which of its periods is the whole star ringing rather than its surface churning. The reading is disputed for every one of these stars, including the most-watched of them: one way round the core is still on helium and there are of the order of a hundred thousand years to go, the other way round it is already on carbon and there are centuries. That is why the figure above is a span. Nothing measured so far closes it.
- ALSO 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.
- ALSO It sits close to the dividing line, near eight times the Sun's mass, between the stars that end quietly as white dwarfs and the ones that explode. Nothing in this catalogue settles which side of it this star is on.
Which stage 6 Gem 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 nine times the Sun's mass, from its temperature, 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 — except the stage this star is in, which that relation cannot reach, and which is given as the range the published readings actually cover. They are orders of magnitude and none of them is a date.
CATALOG DATA
] 16 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
- Surface temperature
- 3789
- Surface gravity (log g)
- Metallicity ([Fe/H])
- Variability
- Slow irregular supergiant (late spectral type)
- Variability type ()
- LC
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)
- −0.91
- Proper motion (Dec)
- −2.18
- Radial velocity
- 22
OTHER NAMES AND NUMBERS
The identifiers other catalogues file this object under, for anyone joining this data to theirs.
- Variable-star designation
- BU Gem
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 (1989ApJS...71..245K, quality C)
- Stellar parameters from
- Spectroscopic determinations compiled from the literature (PASTEL, Soubiran+ 2016)
- Metallicity from
- One spectroscopic determination, compiled from the literature (PASTEL, Soubiran+ 2016)
ALSO KNOWN AS
] 5 DESIGNATIONS- Bayer/Flamsteed 6 Gem
- HD HD 42543
- HIP HIP 29450
- HR HR 2197
- Variable star BU Gem
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 2197
Cross-identifications, measured values and the reference behind each of them.
- ADSobject:"HR 2197"
Papers indexed against this object rather than against the string, newest first.
- VizieR5″ around 93.0796 +22.9085
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. 6 Gem [catalogue entry]. Data release DR1 (2026-08-21). https://spacecatalog.org/object/hd-42543
@misc{spacecatalog_hd-42543,
author = {{SpaceCatalog}},
title = {{6 Gem}},
howpublished = {SpaceCatalog, data release DR1 (2026-08-21)},
year = {2026},
url = {https://spacecatalog.org/object/hd-42543},
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.