The Milky Way From Outside, Built Only From Measured Stars

This is Vishala Space. Every picture you have ever seen of our galaxy from the outside is a painting. Nobody has been out there. No camera has. This one is different. Every dot in it is a star, or a cloud, whose distance somebody measured. About seven thousand of them.

That gold point is the Sun. Tonight we pull back from it until the whole disc is in view, and we draw a line between what was measured and what was drawn. Here is the problem. We live inside the thing we want to photograph. Where we sit, the disc of the Milky Way is about a thousand light-years thick, and about a hundred thousand across. To see it from outside, a camera would have to climb out of that disc. Nothing we have built has come close.

Voyager One, the farthest machine we have ever sent, is about one light-day from the Sun. The disc is a thousand light-years thick. That is more than three hundred thousand times farther. Put another way: if the disc were this thick on your screen, the farthest thing we have ever sent would still be inside the pixel that marks the Sun. So we are like a passenger on a Ferris wheel, trying to draw the whole wheel from one seat. The nearby seats are clear. The far side is behind the hub.

Astronomers did it anyway. Not with a photo. With distances. Measure how far each star is, and in which direction, and put it in its place. Do that seven thousand times, and a shape appears. Let us build it from the inside out. First, the neighbourhood.

The Yale Bright Star Catalogue lists the nine thousand one hundred and ten stars you can see with your own eyes on a dark night. Three thousand one hundred of them carry a distance in that catalogue. Here they are, each in its place. Almost all of them are within a few hundred light-years. That is our whole naked-eye sky, and on the scale of the galaxy it is a speck. If the disc were the size of a country, these stars would fit inside one town. Second, the Local Arm.

The gold points are twenty-eight radio sources, glowing clouds around newborn stars, whose distances were measured by radio telescopes spread across the Earth, by a team led by Mark Reid. They trace the arm we live in. Whether it counts as a major arm is still argued. We will not settle that tonight. Third, the near disc. One thousand four hundred and thirteen clouds of glowing hydrogen, seen by the WISE telescope.

And two thousand two hundred and fourteen pulsing stars called Cepheids, from the OGLE survey. The pale blue haze underneath them is a model. It is there to help your eye. The gold is the data. Now the whole disc. Watch the pill in the corner. It says DATA when you are looking at measured points, and MODEL when the haze is doing the work.

From here, a ruler. It starts at the Sun and runs to the centre of the galaxy. That distance was measured in twenty twenty-two by the GRAVITY instrument, from the orbits of stars circling the central black hole. Eight thousand two hundred and seventy-seven parsecs. About twenty-seven thousand light-years. About one hundred and sixty quadrillion miles. The ruler has a tick every five thousand light-years.

Five of them, and a bit. If you have read twenty-six thousand, that was an older value. The new one is more precise, and every modern measurement rounds to the same spoken number. And the far side, beyond the centre, goes dim now. It is drawn, not measured. The tag says so. Here is the shape the data give.

In the middle, a bar. A straight band of old stars, about sixteen thousand light-years from the centre to each tip, tilted about thirty degrees from our line of sight. That comes from star counts through the bulge. The bar is old. The arms are where new stars are born, which is why the radio sources trace them. Now the arms. The one hundred and ninety-nine radio sources light up, arm by arm.

Where they line up, a line is drawn through them. Those lines are a fit. They are the model. Here is Reid's fit from twenty nineteen. Four long arms, plus our local one. Two main arms in the inner galaxy, and irregular pieces outside.

Both teams used measured points. They disagree about the lines. That is what a model is. Now the dark wedge. Within about twelve degrees of the centre, there are almost no measured distances. Not because nothing is there: the surveys cannot pick out faint, distant sources among the many bright nearby ones. Reid's team says so in their own paper.

And on the far side, there is exactly one measured point. A water maser, measured in twenty seventeen, about sixty-six thousand five hundred light-years from the Sun. Plus nine thousand, minus seven thousand. That error bar is the glow you see. One point, for half a galaxy. So the top view is honest now. Gold where it was measured.

Blue where it was drawn. Dark where nobody knows. But a top view hides the thing that surprised me most. The galaxy is not a flat plate. Seen from the side, it is a thin, bent sheet. And the Sun is not quite in the middle of it. Let us tip the whole thing over.

From straight above, to dead level. Watch the edge of the disc as we go. About a hundred thousand light-years across, by the usual convention. The measured stars go further, and no edge has been found. About a thousand light-years thick, give or take, and the edges are soft. One to one hundred. If this disc were a dinner plate, it would be thinner than the plate.

Now watch the two thousand Cepheids near the edge. They do not sit on the plane. Out past about thirty thousand light-years from the centre, the disc bends. At forty-nine thousand light-years, it is about two thousand eight hundred light-years off the plane. The north side bends about ten percent more than the south. That is the warp. Nobody drew it.

The Cepheids are the warp. And the Sun? It sits about sixty-eight light-years above the mid-plane. On this disc, that is nothing. Sixty-eight, on a hundred thousand. You could not see it. So the height is drawn fifty times larger, and the label says so.

The real offset is smaller than the dot. The Sun is inside the Local Arm, a little toward the inner edge, moving around the centre at about five hundred and fifty thousand miles an hour. That is the Sun's own speed, measured by Reid's team from those same radio sources. The disc around us turns a little slower, about five hundred and twenty-eight thousand. The difference is the Sun's own drift, about twenty-five thousand miles an hour, in the direction of rotation. Measured, not modelled. At that speed the Sun covers about one hundred and fifty miles every second, and still needs a little over two hundred million years to get around once.

Hold that number. First, one fast move. Hold on. Twenty-seven thousand light-years, in twenty seconds. Through the Local Arm. Across the Sagittarius arm. Into the bar.

Into the bulge. And here, at the centre, something with no light of its own. Sagittarius A star. Four point three million times the mass of the Sun. Measured from the orbits of the stars around it, watched for about thirty years. Nothing here is drawn. Back out.

The Sun goes around this centre. One full lap, if the path were a circle, takes about two hundred and six million years. That is just the distance, divided by the speed. The real path is not a circle. A twenty eighteen study ran the Sun's orbit through a full model of the galaxy's gravity, and got two hundred and twenty-four million years, give or take twenty-two. So say: a little over two hundred million years. Older books say two hundred and twenty-five to two hundred and fifty.

They used a bigger orbit and a slower Sun. The clock in the corner now runs one lap in thirty seconds, and it says so. One lap ago, on Earth, it was the Late Triassic. The first dinosaurs were already walking. Not yet in charge. Half a lap ago, the Sun was on the far side of the galaxy from where it is now. On Earth, that was the Cretaceous.

Tyrannosaurus had not arrived yet. And here is the honest part. The Sun never comes back to the same spot. It drifts in and out over about one hundred and sixty million years, and bobs up and down through the disc about every ninety. A wobble inside a wobble. And the arms do not turn with it. The bar turns faster, roughly once every one hundred and forty million years.

We were on the far side. Which arm? Nobody can say. The Sun is about four and a half billion years old. Divide by one lap. Roughly twenty laps. Not exactly twenty.

Roughly. Now the last trick. The model goes away. This is what is left. One hundred and ninety-nine radio sources. Two thousand two hundred and fourteen Cepheids. One thousand four hundred and thirteen hydrogen clouds.

Three thousand one hundred bright stars. About seven thousand measured distances. That is the Milky Way, as it has actually been measured. Everything else is a drawing on top of it. That is not a complaint. A map that shows its own gaps is worth more than a painting that hides them. And the gaps shrink every year, one measured distance at a time.

How many stars are there in all? You will read one hundred billion. You will read four hundred billion. Nobody counted. The range comes from weighing the galaxy and guessing the average star. That is the largest direct count there is, and it is under two percent of the low estimate.

Even the weight is moving. Older estimates said a trillion. This is not a painting. A painting would have an answer. So here is where we are. Twenty-seven thousand light-years from the centre.

Sixty-eight light-years above the plane. Inside an arm whose status is still argued. On a lap that takes a little over two hundred million years. Measured, not drawn. Seven thousand dots, and the honest dark between them. And beyond the disc there are other galaxies, and beyond those, a limit. What is the biggest single thing we have ever measured?

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