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How to measure star distances

Here is the beautiful Summer Triangle asterism, now about to come back into view for another season. An asterism is a recognizable group of stars that isn’t a constellation. This one is made of three bright stars in three different constellations. You can even see the Summer Triangle on a moonlit night or from light-polluted cites. Now notice the star Deneb, one of the three Summer Triangle stars. When you gaze at Deneb, 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.

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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.

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 see your finger appear to jump from side to side with respect to background objects. If you hold your finger farther from your nose, it’ll appear to jump a smaller distance.

As Earth orbits the sun, astronomers can measure the parallax of the nearer stars against the more distant starry background, first from one side of Earth’s orbit and then – six months later – from the opposite side. Measuring stellar distances directly by parallax (trigonometry) only works for the nearer stars, however. For more, read Wikipedia’s parallax page.

Indirect means – which may be subject to errors – must be employed to estimate the distances to the more distant stars, like Deneb. Deneb’s given distance represents an educational guess, not a certainty. But there is little doubt that Deneb is one of the most distant stars that you can easily see with the unaided eye.

Bottom line: Deneb is one the three brilliant stars in the famous Summer Triangle asterism, which you’ll see over the east-northeast horizon by mid to late evening tonight. At our mid-northern latitudes, Deneb will light up the evening sky from now till the end of the year.

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Delta Cephei, prototype of Cepheid variable stars

How far is a light-year?

Deborah Byrd