Cygnus X-1
Where is Cygnus X-1?
Look south, about 73° up at 20:20 in Cygnus. It sinks lower through the evening and sets at 06:21.
- Rises
- 09:38
- Highest
- 20:02
- Sets
- 06:21
- Best after dark
- 20:20 · 73°
Can I see Cygnus X-1?
No. A black hole gives off no light of its own that you could see. What you can do is look toward it.
From London tonight, the direction of Cygnus X-1 is above the horizon after dark. It’s highest soon after dark, around 20:20, in the south at 73°.
When is it highest?
Its part of the sky is highest in the evening, around 10 pm, in August.
It crosses the meridian, its highest point, at 20:02, 74° up, in daylight or twilight. Times for London.
What can you see in this direction?
Eyes
Direction onlyNothing visible; the companion star is too faint.
Binoculars
The companion star, HDE 226868, is around magnitude 9: a faint point near Eta Cygni.
Telescope
A small telescope shows the blue supergiant companion as an ordinary-looking star. The black hole itself can’t be seen.
About
Cygnus X-1 is a black hole about 7,200 light-years away, orbiting a blue supergiant star every 5.6 days. It was discovered as a strong source of X-rays in 1964 and became one of the first objects widely accepted as a black hole.
The black hole, around 21 times the mass of the Sun, pulls gas from its companion. The gas heats up as it spirals inward and glows in X-rays.
In 1974, Stephen Hawking bet physicist Kip Thorne that Cygnus X-1 was not a black hole. Hawking conceded in 1990.
You can’t see the black hole, but its companion star is visible in binoculars.
The black hole spins very fast, close to the maximum possible. Its companion star is about 40 times the mass of the Sun and is gradually losing material to the black hole.
Astronomers identified the visible companion in 1971, when radio observations pinned down the X-ray source’s position.
Visible nearby
Questions
Can I see Cygnus X-1?
How do we know it is a black hole?
Sources: SkyMate; star chart: Wikidata (CC0). Positions computed for your place and time with astronomy-engine. Data & Licences