Question 1

What is being measured here?

Accepted Answer

The combined light of the whole group, which is why clusters can rank high without any single member being remarkable. The Pleiades are bright because a few hundred stars are adding up, close by and unobscured.

Question 2

Open or globular?

Accepted Answer

Both are on this list and they are opposite things. An open cluster is a few hundred young stars loosely spread through the disc; a globular is a hundred thousand or more ancient ones packed into a sphere in the halo, and it holds together for billions of years where the open cluster does not.

Question 3

Does brightest mean the most luminous?

Accepted Answer

No. This ranks apparent magnitude — how bright the object looks from here — which mixes how much light it gives off with how far away it is. A modest star nearby outshines a monster across the galaxy, and the ranking says nothing about which is the greater object.

Question 4

Why do smaller numbers come first?

Accepted Answer

Because the magnitude scale runs backwards. It was built from the naked-eye classes of antiquity, where first magnitude was the brightest, and it kept that direction when it was made numerical — so the top of this list holds the lowest numbers, and the very brightest are negative.

Question 5

Why are some star clusters missing?

Accepted Answer

Because no magnitude is recorded for them here. Brightness is also not a single number for everything: a magnitude measured through one filter is not comparable with one measured through another, and every figure on this site says which band it belongs to.

Question 6

Which clusters can be seen with the naked eye?

Accepted Answer

The Pleiades and the Hyades in Taurus, the Beehive in Cancer, and from the southern hemisphere Omega Centauri and 47 Tucanae — the last two globulars bright enough that both were catalogued as stars before anyone resolved them.

Question 7

Are the stars in a cluster really close together?

Accepted Answer

Closer than average, and still enormously far apart. Even in the core of a globular the typical separation is around a tenth of a light year — thousands of times the size of the Solar System — so collisions are rare, though far commoner there than anywhere else.

Question 8

Why do clusters matter so much to astronomy?

Accepted Answer

Because every star in one shares an age, a distance and a composition. That removes three unknowns at once, which is why clusters are where stellar evolution is tested and where several rungs of the distance ladder are calibrated.

The brightest star clusters

COMMON QUESTIONS