BD Cas

CALCULATED · NOT AN OBSERVATION · RADIUS FROM THE LUMINOSITY AND THE TEMPERATURE · 6.47 L☉ · VARIABLE, MAG 10.93–11.30 · PERIOD 3.652 DAYS · SPOTS OVER 0.20% OF IT · WITH A CORONA

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. It is cool enough to convect at the surface, so it has a magnetic field wound by that convection, and the field is what everything else here comes from: the dark patches are places where it is strong enough to stop the heat arriving, so they are cooler rather than merely darker, and they are drawn at the fraction of the surface a star this active is measured to cover. The bright web between them is the same field swept to the edges of the larger convection cells, and the brighter patches beside the spots are that web bundled up — both of them brighter towards the limb, because what makes them bright is seeing down the wall of a magnetic tube rather than into its mouth. The fringe standing off the edge is the chromosphere, a forest of jets a few hundred kilometres wide, and the flames leaning out of it are prominences: chromospheric gas held up by the field, drawn at the height one reaches and the width one has. A flare is a second, far hotter thing radiating on top of the photosphere rather than the photosphere brightening, which is why the same flare is barely visible on a hot star and doubles a cool one. The streamers reaching out past the limb are the corona, which is a millionth of the disc's brightness and drawn far brighter than that, since at its own contrast nothing short of hiding the disc reveals it. 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 from here that whole neighbourhood is a knot in one direction and the rest of the sky is empty. The view you land on looks the other way; turn back towards the Sun and the stars come with it. What the readout says is which of the two the frame is pointed at. The emptiness is not a missing backdrop — it is the edge of what this catalogue has measured.

Where 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

00h 11m 16.4s

DECLINATION

+61° 39′ 50.8″

GRID TURNED SINCE J2000

13.5′

PROPER MOTION SINCE J2000

0.07″

Computed for 2026-10-26 00:00 UT. Two things separate it from the catalogued position above: BD Cas 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. The catalogue it came from publishes no error for that position, so nothing here says how far this arithmetic can be trusted — the digits above are as precise as the place they were computed from, and no more. Aberration, parallax and refraction are left out. Aberration swings every position by up to twenty arcseconds over a year; parallax stays under an arcsecond for all but the nearest stars; refraction is nothing overhead and about half a degree at the horizon, where it answers to the air, not the sky. The same figure for any date.

RANGE, AS SAMPLED

10.93 – 11.30

PERIOD

3.6516 days

PHASE

0.238 ± 0.048

NEXT MAXIMUM

2026-10-28 18:44 UT---------- --:-- your time

THEN

2026-11-01 10:23 UT---------- --:-- your time

Computed for 2026-10-26 00:00 UT. A phase of zero is maximum light, and BD Cas was last there 20.9 h before that. The ± is what the published elements can still promise: 3,515 cycles have gone by since the epoch they were measured at, and the last digit of the period compounds over every one of them — 4.2 h 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.