Galaxy time: the 230-million-year year

The Sun, with everything orbiting it, travels around the centre of the Milky Way at roughly 230 km per second. Even at that speed one lap takes about 230 million years — a galactic year (estimates run from 225 to 250 million, depending on the speed and radius assumed).

That gives a strange calendar. The Sun and Earth are about 20 galactic years old. The last time the Solar System was at this point in its orbit, the first dinosaurs were appearing in the Triassic. All of recorded human history fits inside the last 1/45,000th of one galactic year.

Five of our galactic neighbours can be seen without a telescope, though not all from the same place. Andromeda and Triangulum are northern-sky objects; the two Magellanic Clouds — small satellite galaxies of the Milky Way — never rise for most of the northern hemisphere and hang high in the southern sky. The chart below draws the real sky over your city and marks each one.

Writing an address from the inside out, the way you would on a letter:

Of the Local Group's galaxies, only three are big spirals. Each has its own page with live sky positions and viewing seasons.

Sitting still is a matter of opinion. Each speed below is measured against a different reference point, and they add up in different directions.

In 1926 Edwin Hubble sorted galaxies by shape, and his diagram — later drawn as a tuning fork — is still the starting point. Ellipticals (E0 round to E7 flattened) are smooth balls of old stars with little gas left to make new ones. Spirals have a disc, arms and ongoing star birth; Sa have big bulges and tight arms, Sc small bulges and loose arms. About two-thirds of spirals, including ours, have a bar of stars across the middle (SBa–SBc). Lenticulars (S0) sit at the fork: a disc but no arms. Irregulars, like the Magellanic Clouds, fit nowhere.

Hubble thought galaxies might evolve along the diagram from left to right. They don't — but the shapes do record history. Big ellipticals are mostly the products of galaxies merging, which is the likely fate of the Milky Way and Andromeda. In modern notation the Milky Way is an SBbc, Andromeda an SA(s)b and Triangulum an SA(s)cd.

In December 1995 the Hubble Space Telescope stared for ten days at a tiny patch of apparently empty sky in Ursa Major — about as big as a tennis ball seen from 100 metres away. The Hubble Deep Field revealed around 3,000 galaxies. The Ultra Deep Field of 2004 found about 10,000 in a similar patch. Scaling up gives an answer in the hundreds of billions for the observable universe.

A 2016 study suggested two trillion, counting faint galaxies too dim to see. But in 2021 NASA's New Horizons probe, far beyond Pluto where the sky is darker than anywhere near Earth, measured the total background glow and found no room for that many hidden galaxies — pointing back to a few hundred billion. JWST is now counting the earliest ones directly.