Cygnus
What is Cygnus?
Constellations· Summer evenings (N) · Northern Cross on the Milky Way. A swan flying south along the summer Milky Way. Deneb, at its tail, is one of the three stars of the Summer Triangle, and because its stars form a large cross it is also called the Northern Cross.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Deneb | 1.3 |
| Sadr | 2.2 |
| Albireo | 3.1 |
The 3 brightest of the 6 stars used in the figure (5 lines). Lower magnitude means brighter.
Deep-sky objects near this constellation
Constellation basics
| Season | summer–early autumn |
|---|---|
| Brightest star | Deneb (1.3) |
| Shape | a cross flying down the Milky Way |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
61 Cygni and Albireo — what measuring star distances revealed
The first distance ever measured to a star other than the Sun was measured in Cygnus. In 1838 the German astronomer Friedrich Bessel detected a tiny yearly shift in the position of a star called 61 Cygni. The shift, called parallax, happens because Earth views the star from different places as it orbits the Sun. From its size he worked out a distance of about 10 light-years; the modern value is 11.4.
Bessel chose this star for a reason. 61 Cygni moves more than five arcseconds across the sky each year, earning it the nickname the Flying Star, and nearby stars tend to appear to move fastest. It is a pair of orange stars orbiting each other, and a small telescope shows the two separately.
Distance measurements still change how we see stars. Albireo, at the swan's beak, shows a gold star and a blue one side by side and was long taken for a true orbiting pair. When the Gaia satellite measured each star separately, the gold one came out at about 365 light-years and the blue one at about 395, some 30 light-years apart. Whether the two are real partners or just lined up by chance is still not settled.
The constellation in depth
Cygnus is a swan flying south down the middle of the summer Milky Way. The long line from Deneb at the tail to Albireo at the beak is the body, and the stars reaching out on either side are the wings. Together they form a large cross, which is why the figure is also called the Northern Cross. The Milky Way runs thickest here, so the area is especially fine under a dark sky.
At magnitude 1.3, Deneb looks a little fainter than Vega and Altair, its partners in the Summer Triangle. Its distance changes the picture. Vega is 25 light-years away and Altair 17, but Deneb is thought to be about 2,600 light-years away. To look this bright from so far, it must give out roughly 200,000 times as much light as the Sun.
Near the swan's neck lies Cygnus X-1, a strong source of X-rays that is widely regarded as the first object confirmed to be a black hole. A black hole cannot be seen directly, because not even light escapes it. It gives itself away by pulling gas from a large companion star; the gas heats up as it falls in and glows in X-rays.
From 2009 the Kepler space telescope spent four years staring at one patch of sky between Cygnus and Lyra. It was watching for the tiny dip in starlight that happens when a planet passes in front of its star. Using that method, more than 2,000 planets were confirmed in that small field alone.
Frequently asked
- When can I see Cygnus?
- Cygnus is best seen in summer–early autumn.
- What is the brightest star in Cygnus?
- The brightest star in Cygnus is Deneb, at magnitude 1.3.
- In one line, what is Cygnus?
- Constellations — A swan flying south along the summer Milky Way.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.