Scorpius & the Galactic Centre

Scorpius is one of the very few constellations that looks like the thing it is named after: a head of three stars, a bright red heart, and a long body curling south and then hooking up into a stinger. It is also one of the few whose brightest stars are genuinely related — most of them belong to the Scorpius–Centaurus association, the nearest region of massive star formation, 400–500 light-years away.

Antares — magnitude 1.06, and the reddest bright star in the sky. Interferometry puts it at about 680 times the Sun's radius: put it where the Sun is and its surface would swallow Mars and reach two-thirds of the way to Jupiter. The name means anti-Ares, the rival of Mars, and the resemblance is real enough that the two are genuinely confusable when Mars passes 7.6° away through the head of the Scorpion.

Follow the body down and round. 17.3° from Antares, the tail hooks upward and ends in two stars less than a degree apart: Shaula (1.62) and Lesath (2.70). They are the stinger, they look like a pair and are not — Shaula is 570 light-years away, Lesath 570 more. This is the part of the figure that sells the whole thing.

10.6° east of the stinger, Sagittarius. Ignore the archer — nobody has ever seen an archer here — and find the Teapot: Alnasl is the spout, Kaus Media and Kaus Australis the base, Nunki and Ascella the handle. Once you have the Teapot you have the direction that matters, because the steam comes out of the spout.

Just off the spout is the centre of the Galaxy. The marked point is Sagittarius A*, the four-million-solar-mass black hole the whole disc turns around, 26,700 light-years away and hidden behind thirty magnitudes of dust — you will never see it, and the Event Horizon Telescope needed a planet-sized radio array to. What you can see is everything in front of it: this is the densest, brightest stretch of the Milky Way, and M8, the Lagoon Nebula, is a naked-eye patch of it at magnitude 5.

1.3° west of Antares sits M4, a globular cluster of a hundred thousand stars at 5,500 light-years — one of the closest globulars to us, and easy in binoculars once you know it is a fingerwidth from a first-magnitude star. It is a useful reminder of scale: Antares is a single star 550 light-years away, and the faint smudge beside it is a hundred thousand stars ten times further off.

This is the lesson's real content, and the readout tells it. Antares sits at declination −26.4°, so how much of the Scorpion you get is decided entirely by your latitude. Watch the altitude: comfortable from Miami, marginal from New York, a horizon-scraping rumour from London — and from Sydney it goes almost overhead. The best Milky Way on Earth is a southern-hemisphere privilege, and no amount of dark sky at 51°N buys it.

Six hours. From a northern site the whole figure slides along the southern horizon rather than climbing over you, which means you are looking through two or three atmospheres of haze the entire time — plan for the hour either side of the altitude peak and do not bother outside it.

The Greeks said a scorpion killed Orion and the gods put them on opposite sides of the sky so it could not happen twice. They were describing a fact: Betelgeuse is 152° from Antares, so the measuring arc dives straight into the ground, and the Hunter's altitude is the negative number on the readout. When the Scorpion is up, Orion is not. The myth is a mnemonic for the calendar.