Ardra, and a star that is dying while we watch

Ārdrā (आर्द्रा), the sixth of the 27 nakṣatra (नक्षत्र) the moon crosses at about one a night, is the one station in the list whose star you can catch in the act of changing. The star is Betelgeuse, and it is up before dawn just now. Face east and wait for three bright stars in a short row to come up nearly on end. Above them and off to the left stand two more, and the lower of that pair is orange from any city street. Note how bright it looks beside Rigel, the blue-white star at the far corner of the same figure. Then look again next year. The answer will have moved, and your own eyes are the whole instrument.

This file is its life story: where it came from, what is happening to it now, and how it ends. The first two are reasonably well established. The third is certain in kind and unknown in date.

Timing first. By the middle of August, from Delhi, the orange star clears the horizon a little after two in the morning and stands 25 degrees up by four, with the sky properly dark until half past. It rises four minutes earlier each night, two hours a month: before midnight by the end of September, and by the middle of December it comes up as the sky goes dark. Half a fist above it, in what looks like empty sky, sits the faint triangle of the station before this one. That one wanted a dark field and 20 patient minutes. This one you can do from a balcony with a streetlight burning beside you.

Betelgeuse is a red supergiant, which is a description and not a compliment: a heavy star that has used up the hydrogen in its core, swollen enormously, and cooled at the surface. That surface sits near 3,800 degrees where the sun's is near 5,800, and that is the whole reason for the colour. Cool burns red. Put the star where the sun is and it would take in Mercury, Venus, the earth and Mars and reach into the belt of asteroids, on the larger estimates most of the way to Jupiter. Some 15 to 20 times the sun's mass is spread through that volume, so thinly that the outer layers are nearer a vacuum than air.

How far away is it? Nobody can say for certain, but here are the best estimates. Distance normally comes from parallax: the small yearly shift in a star's apparent position as the earth swings from one side of its orbit to the other. Betelgeuse's shift is about 5 thousandths of a second of arc. Its visible disc is 44 thousandths across, nine times wider than the wobble we are trying to measure. That disc is not smooth. Convection cells rise and sink on it and drag the apparent centre about. It is like measuring the sway of a lamp by watching a bonfire. Published distances run from around 500 light years to 724, and a 2020 study using the star's own pulsations gave 548. The careful estimates overlap near 600, and that is the figure I carry.

Born in a hurry

The sun is 4,500 million years old and not yet halfway through what it has to burn. Betelgeuse is younger than 10 million years and nearly finished. That is what mass does to a star: the more of it you have, the harder you crush your own core, and the faster you spend everything you own.

It formed with the rest of Orion and then left. It crosses the space between the stars at 30 kilometres a second, fast enough to pile a shock wave in front of it more than four light years wide. None of that is a metaphor. It is a large object doing what objects of that mass do.

John Herschel recorded in 1836 that its brightness varied, and over the next four years he twice saw it outshine Rigel, in October 1837 and again in November 1839. It is now classed as a semiregular variable, which is an honest label: there is a rhythm and it does not keep time. The main beat runs about 400 days and comes from the whole star breathing, expanding and contracting. A slower beat of some 2,100 days has never been explained. The brightness runs between about 0.0 and 1.6 on the magnitude scale, which counts backwards, so a bigger number means a fainter star. Most of the time it sits near 0.5.

Then, in late 2019, it went outside the record. From about November it faded, and kept fading, and in the second week of February 2020 it reached magnitude 1.614, roughly a third of its usual light and the faintest measurement anyone has made of it. A great many people decided it was about to explode, and some attached that to the year we were all about to have, which is the ordinary business of turning an event into an omen.

What had happened was soot. The star threw off a big bubble of gas. A patch of its surface cooled. In that cool patch the gas turned into solid dust. The dust hung between us and the star's southern half and made that half about 10 times darker in visible light. By April 2020 it was back. The picture at the top of this file is the evidence: the same face four times over 15 months, the shadow crossing it. Astronomers got a gift out of the fright: the first clear view of stardust forming close to the surface that makes it.

The end, and when

One day the core will run out of fuel it can burn, collapse in on itself, and the star will explode as a supernova. That is not in doubt for an object of this mass. The usual figure for when is within the next hundred thousand years, with some work arguing sooner and some later; the spread between those estimates is longer than recorded history.

A hundred thousand years is twenty times longer than people have been writing anything down. So "Betelgeuse will explode" is a true sentence and "Betelgeuse is about to explode" is not one anybody can support. It is not a countdown and it is not about your year. When it does go, the estimates have it shining about as brightly as a half moon for some months, visible in daylight, and at 600 light years doing the earth no harm.

And the part that never settles. Light takes 600 years to cross from there to here. If the star blew up in 1426, the news is still on its way. Everything in this file is a report from a place whose present tense we cannot see.

Now the name. Ardra is the feminine of ārdra (आर्द्र): wet, moist, damp. The word runs on into fresh, green, living, and then into soft, tender, full of feeling. As a plain noun it is fresh ginger. The symbol the handbooks give the station is a teardrop.

Its presiding figure is Rudra (रुद्र), whose name is usually traced to a root meaning to cry out or to howl, so, the roarer. Three hymns of the Rigveda are addressed to him, and dozens more name him. He fathers the Maruts, the storm gods, and in the same hymns he is asked for medicines. Wet, a storm's noise and a teardrop, in one station.

The season fits, and you can check it. On the sidereal reckoning the sun stands in Ardra for about a fortnight from around the 22nd of June, which is when the rains normally reach the north and finish covering the country. The station named for wetness is the one the sun crosses as the water arrives. Whether that is why it was named so I cannot show you. That it lines up is in every panchang.

One honest complication. Each of the 27 stations is a slice of the moon's road, 13 degrees and 20 minutes of it, and Betelgeuse does not stand on that road. It lies about 16 degrees south of the ecliptic, the line the sun keeps to, and the moon strays little more than 5 degrees from that line. The moon has never once passed over this star and never will. Ardra is the arc of sky; Betelgeuse is the landmark whose name ended up on the signpost, standing well back from the verge. Not everyone accepts the identification, and some would rather hand the station a star in the Twins. Inherited lists disagree about several of these junction stars, which is the ordinary condition of this subject and not a scandal.

What the handbooks make of Ardra is mostly storms and upheaval, and I am not passing it on in that form. Take the word itself. Wet is the condition in which a thing can still be changed. A seed opens when it is wet and not before. Clay takes a shape while it is wet. What is dry has finished, and being finished is a loss as much as a comfort. If there is a question in this station it is that one, and it has a person in it: what in my life is still soft enough to be altered, and what have I quietly let set.

There is one piece of work in this file, and it costs a minute of a morning you were spending anyway. Find the orange one above the row of three, look across the figure at Rigel, and decide which of the two is brighter. Do not look the answer up first. Herschel did exactly that and got an answer that surprised him. Write yours down with the date beside it, somewhere you will find it again. One of these mornings, a long way past all of us, somebody will go out and look east and it will not be there. Until then the star is watched by whoever bothers, one dated line at a time. Next August, add another.