M101

CALCULATED · NOT AN OBSERVATION · ANGULAR SIZE AT THE CATALOGUED DISTANCE · HUBBLE TYPE SABc · SURFACE BRIGHTNESS 24.0 MAG/ARCSEC² · MAJOR AXIS 24′ · AXIS RATIO 0.96 · POSITION ANGLE 28° · NO SURFACE — DRAWN AS A VOLUME · AXIS RATIO READ AS AN INCLINATION

An exponential disc with a bulge at the middle and arms wound at the pitch angle its Hubble type implies, with the dust drawn where the dust is: on the inner edge of each arm, ahead of the crest, because inside co-rotation the gas overtakes the spiral pattern and piles up as it goes in. The shape of it is the catalogued morphology; the tilt is the catalogued axis ratio read as an inclination, which is what a round disc's axis ratio is. Nothing measured the depth — a disc that looks this elliptical from the Sun is this far over, and the model is that disc seen from wherever you turn it to. The two colours are two populations: old stars in the bulge and the disc, young ones in the arms, both through the same blackbody table as the star pages. 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.

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  M101 hires STScI-PRC2006-10a

  Image: European Space Agency & NASA
  CC BY 4.0
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  NASA-SpiralGalaxyM101-20140505

  X-ray: NASA/CXC/SAO; Optical: Detlef Hartmann; Infrared: NASA/JPL-Caltech
  Public domain

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

14h 04m 10.3s

DECLINATION

+54° 13′ 08.9″

GRID TURNED SINCE J2000

11.5′

PROPER MOTION SINCE J2000

none published

Computed for 2026-10-11 00:00 UT. No proper motion is published for M101, so nothing here moves the object; what separates this from the catalogued position above is the coordinate grid turning 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.