Question 1

What is the Northern Cross?

Accepted Answer

The bright stars of Cygnus themselves, seen as a cross rather than as a swan. It is an asterism, it is far larger than the Southern Cross, and it points nowhere useful — the Southern Cross earns its keep by indicating the south celestial pole, and this one does not.

Question 2

Where in the sky is Cygnus?

Accepted Answer

Its boundary is centred near right ascension 20h 30m, declination +45.3°. It stands at its highest around midnight in late July, which puts it in the evening sky for the two months after that and behind the Sun half a year later.

Question 3

Can I see it from where I live?

Accepted Answer

Every part of Cygnus clears the horizon from latitudes between 22° S and the north pole. Outside that band some or all of the region never rises at all, whatever the time of year — a constellation is fixed on the sky, and it is your latitude that decides how much of the sky you get.

Question 4

What is the brightest thing in it?

Accepted Answer

The brightest catalogued object in Cygnus is Deneb, at apparent magnitude 1.25 (V). That is apparent brightness — how it looks from Earth — so it mixes how much light a thing gives off with how far away it is, and the brightest object in a region is rarely the most interesting one.

Question 5

How are the stars in it named?

Accepted Answer

The brightest carry Greek letters, assigned by Johann Bayer in 1603 roughly in order of brightness and followed by the Latin genitive of the constellation's name. Fainter stars take a number from John Flamsteed's catalogue, and fainter still a designation from whichever survey recorded them.

Question 6

What exactly are the boundaries?

Accepted Answer

Lines of constant right ascension and declination, drawn by Eugène Delporte for the IAU in 1930 and referred to the sky as it stood in 1875. Precession has tilted the sky against them since, which is why the boundaries on a modern chart are very slightly askew.

Question 7

Are the stars in Cygnus actually near each other?

Accepted Answer

Almost never. The stars in a constellation lie at wildly different distances and are not related to one another in any way — the pattern is a line of sight from one particular planet. Two stars that look adjacent can be a hundred times further apart than either is from the Sun.

Question 8

What is the difference between a constellation and an asterism?

Accepted Answer

A constellation is one of the 88 regions the IAU recognises, with a fixed boundary and an official name. An asterism is any memorable pattern that is not one — the Plough inside Ursa Major, the Summer Triangle drawn across three constellations, the Great Square shared between two.

Question 9

Why are the names in Latin?

Accepted Answer

Because the list was assembled in Latin. Forty-eight of them come down from Ptolemy's catalogue of the second century, and the other forty were coined between the sixteenth and eighteenth centuries to fill the southern sky and the gaps. The genitive form is what star names use: Alpha Orionis means the alpha star of Orion.

Cygnus

THE FAMOUS ONES

BRIGHTEST OBJECTS

COMMON QUESTIONS