The stars you can see

On a dark night the sky looks like a dome with the stars painted on it, all the same distance away. They are not. Some are close and some are hundreds of times further off.

Sirius, the brightest, is under 9 light years away. Deneb, another bright one, is about 1,400. The light you see from Deneb tonight set out around the year 600.

That is why the patterns are not real groups. The three stars of Orion’s Belt sit in a neat row, but the middle one is given as nearly three times further away than the other two. Stand somewhere else in the galaxy and the row would fall apart. People have joined the dots for thousands of years, and today the sky is divided into 88 official constellations, but each is a pattern seen from here.

Every star you can see by eye belongs to our own galaxy, the Milky Way, and almost all of them are in our own small part of it. Nine in ten are within 1,000 light years. The galaxy is 100,000 light years across.

In the 3D view you can do what nobody can do from the ground: move away and look at these stars from the side.

What the eye can see is a sample of the galaxy, but a very unfair one. This level looks at how many stars there are to see, how far away they are, and why the sample is so lopsided.

How many. Brightness in the sky is measured in magnitudes, where a smaller number means brighter. Sirius is -1.46, and the faintest stars the eye can pick out from a dark place are about magnitude 6. The star catalogue this site draws holds 4,940 stars of magnitude 6 or brighter, over the whole sky. Go a little fainter, to 6.5, and the count is about 9,000. Half the sky is below the horizon at any moment, and haze near the horizon hides more, so from one dark place you can see perhaps 2,500 at once.

How far. These distances come from the same catalogue. They are worked out here from its positions.

Of the 4,940 stars to magnitude 6	How many, or how far
Within 100 light years	466, fewer than one in ten
Half lie within	About 350 light years
Within 1,000 light years	4,463, nine in ten
The furthest	About 3,000 light years

The further away a star is, the less sure its distance. Figures of a few hundred light years are good. Figures in the thousands can be wrong by a large part.

Some familiar stars.

Star	Distance in light years	Magnitude
Sirius	9	-1.46
Vega	25	0.0
Arcturus	37	-0.1
Canopus	310	-0.6
Polaris, the Pole Star	430	2.0
Betelgeuse	500	0.5
Rigel	860	0.2
Deneb	1,400	1.3

Canopus looks only a little fainter than Sirius from more than thirty times further off. Light spreads out with the square of distance, so Canopus has to be giving out hundreds of times more light to manage that.

Why the sample is unfair. The nearest-stars lesson showed that the typical star is a small red one, too dim to see even from next door. The night sky is the opposite selection. A star that gives out thousands of times the Sun’s light can be seen from a thousand light years away, so powerful stars are collected from a huge volume of space. Dim stars are collected only from very close by. The result is a sky full of rare giants, with the ordinary stars left out. The Sun itself would drop out of sight by eye from only about 55 light years away.

Patterns, not groups. A constellation is an area of sky, and there are 88 official ones. The stars in one usually have nothing to do with each other.

Pattern	Distances of its stars
Orion’s Belt	Mintaka about 690 light years, Alnitak about 740, Alnilam given as nearly 2,000
The Plough	Five stars between 80 and 86 light years, with Dubhe at 124 and Alkaid at 104

The Plough is the exception that shows what a real group looks like: its five middle stars are at nearly the same distance. Orion’s Belt is the usual case.

The edge of what you can see. All of this lies in a ball a few thousand light years across, in a galaxy 100,000 light years wide. The pale band of the Milky Way is the combined glow of far more distant stars, too faint to pick out one by one. Beyond our galaxy, only a handful of things can be seen by eye at all, and the furthest most people can see is the Andromeda Galaxy, about 2.5 million light years away.