• The star Deneb - visible by mid-evening every May - is one of the most distant of the bright stars. When you gaze at this star, you are gazing across a great distance of space. The exact distance to Deneb is not known for certain, with estimates ranging from about 1,425 light-years to perhaps as much as 7,000 light-years. EarthSky veteran sky blogger Larry Sessions said in his post about Deneb:
• The best estimates likely are those obtained by the Hipparcos Space Astrometry Mission in the 1990s. A simple calculation from initial Hipparcos data

gives the figure of 3,230 light-years, whereas the refined data yield just over 1,400 light-years. At any of these estimates distances, Deneb is one of the farthest stars the unaided human eye can see. It is so far, that the light that reaches the Earth today started on its journey well more than 1,000 years ago.
• Why don't astronomers know the distance to Deneb exactly, and why are there different estimates for the star's distance? The answer is that science is not a body of facts. It's a process. Different astronomers or teams of astronomers try to improve on published distance estimates to the stars, and their various estimates are then published and passed along.
• Graphic on right courtesy of Wikimedia Commons
• Deneb is too far away for its distance to be measured by the only direct method - stellar parallax. The distance to stars within a few hundred light-years of the solar system can be determined fairly accurately by parallax. The basic principle of parallax you can demonstrate to yourself by holding a finger in front of your nose and gazing at it with one eye closed, then the other eye closed. When you do this, you

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