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How far away is Deneb?

Here is the beautiful Summer Triangle asterism, now about to come back into convenient evening view for another season. An asterism isn’t a constellation. It’s just a recognizable group of stars. This one is made of three bright stars in three different constellations. Now notice the star Deneb, one of the three Summer Triangle stars. When you gaze at Deneb, you’re gazing across a great distance of space. The exact distance to Deneb is not known for certain, but the currently accepted distance of around 2,600 light-years. That makes Deneb one of the most distant stars we can see with the eye alone.

Why don’t astronomers know the distance to Deneb exactly? In fact, there are varying estimates for this star’s distance. The answer is a glimpse into the process of science, and the way that different astronomers or teams of astronomers – using advancing technologies – try to improve on what was learned earlier, sometimes years before.

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Astronomers use the parallax method to find distances to nearby stars. But Deneb is too far away for accurate parallax measurements from Earth’s surface.

Estimates for Deneb’s distance have been obtained by a variety of methods, some of which involved theoretical models related to the way stars evolve and some of which assumed Deneb’s membership in the Cygnus OB7 association of stars. The most important modern distant measurement for Deneb came in the 1990s, with ESA’s Earth-orbiting Hipparcos Space Astrometry Mission. Hipparcos gathered astrometric data on Deneb. Early analyses of the data indicated a distance of somewhere around 2,600 light-years.

Since then, various groups of astronomers have re-analyzed Hipparcos data. Consider that computer power gets stronger with each passing year, which helps to improve techniques for analysis. So, for example, a 2009 study published in the peer-reviewed journal Astronomy and Astrophysics used a newer method of analysis, which resulted in a distance for Deneb that’s barely half the widely accepted value.

Why does Deneb’s distance matter? It matters to astronomers because – if they don’t know exactly how far away the star is – they can’t get accurate numbers of its true size, mass and energy output.

ESA now has a second astrometric satellite – called Gaia – in orbit around Earth. Its goal is to measure the positions and distances of stars with more precision than ever before, and it’s in the process constructing what ESA says will be:

… the largest and most precise 3D-space catalog ever made.

A new estimate for Deneb’s distance wasn’t included in Gaia’s first data release, in September 2016. Maybe a new distance estimate for Deneb will be included in Gaia’s second data release, planned for April 2018.

And so science marches on!

We can see Deneb from such a great distance because it is a large and very hot supergiant star. Here’s its place on the Hertzsprung Russell diagram. Image via Wikimedia Commons/ ESO.

Bottom line: The star Deneb – part of the famous Summer Triangle – is one of the most distant stars you can see with your eye alone. Why don’t we know its distance precisely?

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