Cosmic Exploration
A field guide to what the sky contains — the classes of objects out there, the stellar spectral sequence from hottest to coolest, and the instruments humanity points at all of it. The object catalogue itself is just getting started.
The catalogue so far
The Local Group
Everything gravitationally bound to us: two big spirals, a third smaller one, and forty-odd dwarfs strung out to the edge of the group's reach. Radius on the map is true distance on a log scale — each ring is ten times further out than it looks — and the angle groups objects by what they orbit, so it is not a direction on the sky.
Where this is soft
The distances and diameters here hold up — every apparent size they imply lands on the catalogued value, and Andromeda comes out at 3.4°, which is what you see. These are the parts that do not.
- The mass column mixes two different quantities. The Milky Way, Andromeda and the Large Magellanic Cloud are quoted at their halo masses — 1.2, 1.5 and 0.14 trillion suns, dark matter included. The Small Magellanic Cloud, M32, M110 and NGC 6822 are quoted at something closer to what shines. Both are legitimate numbers; printing them in one column is not, and it makes the LMC look twenty times the SMC when the ratio people usually mean is nearer two.
- Most objects carry no mass at all, and the empty ones are not marked as unknown so much as simply absent.
- The coordinates are here, and unused. Right ascension, declination and distance are stored for every object, so subgroup membership could be checked in three dimensions — M32 and M110 should sit within a few hundred thousand light-years of Andromeda, and if they did not, something would be wrong. This page does not yet do that arithmetic, so "Andromeda subgroup" is still an assertion rather than a test.
- Diameters for dwarf galaxies are a choice of definition, not a measurement: half-light radius, tidal radius and the visible extent differ by factors of two or three. Crater 2 is here at 1.06° and is often quoted at 2°, and both are honest — they are measuring different edges.
- — objects, of roughly eighty known members. The missing ones are mostly ultra-faint dwarfs found since 2015, and which of those are galaxies rather than star clusters is itself unsettled.
The stellar spectral sequence — hottest to coolest
Field guide: classes of celestial objects
Instruments watching the sky
Observatories
Discoveries, in order
Space missions
Researchers on this page