Deneb: How astronomers know how far away it is

How far away is Deneb?

The beautiful Summer Triangle is about to come back into convenient evening view for another season. This asterism consists of three bright stars in three different constellations. Now notice the star Deneb in the triangle’s furthest-left corner. When you gaze at Deneb, you’re gazing across a great distance of space. We don’t know the exact distance to Deneb for certain. The distance we see most often online is around 2,600 light-years. That makes Deneb one of the most distant stars we can see with the eye alone.

But estimates vary for this star’s distance. And they vary a lot! Why? The answer is a glimpse into the process of science, and the way that astronomers use advancing technologies to try to improve on previous discoveries.

Discovering Deneb’s distance

Scientists have obtained estimates for Deneb’s distance through a variety of methods. Some of these methods involve theoretical models related to the way stars evolve. Some assume Deneb’s membership in Cygnus OB7, a star-forming complex within our Milky Way galaxy.

ESA’s Earth-orbiting Hipparcos Space Astrometry Mission provided the most significant modern measurement of Deneb’s distance in the 1990s. Hipparcos gathered astrometric data on Deneb. Early analyses of the data indicated a distance of somewhere around 2,600 light-years. That’s the figure you still see most often today.

But, since then, various groups of astronomers have re-analyzed Hipparcos data. This is because computer power, which gets stronger with each passing year, helps to improve techniques for analysis. For example, the peer-reviewed journal Astronomy and Astrophysics published a study in 2009, using a newer method of analysis (skip to the last page for Deneb).

This new analysis showed a distance to Deneb that’s barely half the widely accepted value. The study suggests 1,548 light-years as the distance, with a range between 1,336 and 1,841 light-years. That’s a big ballpark figure.

So is Deneb 1,600 light-years away or 2,600 light-years away? The fact is, we don’t know. Either way, it’s still one of the most distant stars we see with the unaided eye.

Why does Deneb’s distance matter?

Distance matters because it can give us other measurements, too. If astronomers don’t know exactly how far away Deneb is, they can’t get accurate numbers of its size, mass and energy output.

ESA has a second astrometric satellite – the magnificent Gaia space observatory – in a distant orbit similar to that of the James Webb Space Telescope. Gaia launched on December 19, 2013. Its five-year nominal mission ended in July 2019. But it’s still operational, and the mission has been extended to December 31, 2025. Gaia’s goal is to measure the positions and distances of stars with more precision than ever before, and it is exceeding expectations. We really can’t say enough about the incredible things we’ve learnt about our Milky Way galaxy via Gaia. Click here for a few of Gaia’s discoveries.

However, a new estimate for Deneb’s distance wasn’t included in Gaia’s 1st data release, in 2016. And it wasn’t included in Gaia’s 2nd data release in 2018. How about the 3rd data release?

Nope, not there either. What happened, did someone forget to add Deneb to the list?

Deneb is too bright for Gaia

Gaia has produced data on 1.5 billion sources in our Milky Way galaxy. But it can’t image Deneb, the 19th brightest star in our sky. That’s because Gaia cannot measure the distance to bright stars. They saturate Gaia’s sensor and make measurements impossible. Gaia’s brightest possible star is magnitude 1.71. Deneb is brighter, at magnitude 1.25.

It’s not that Gaia isn’t working on it. In 2018, the Gaia team posted an employment opportunity specifically asking for someone to find a way to image bright stars with Gaia. But it seems they’ve not had any luck so far.

As Gaia was launched, its team was working on the problem of imaging bright stars. Paper after paper after poster have addressed the problem of Gaia not being able to image bright stars. They are still working on it.

So, how far away is Deneb? If it is part of the Cygnus OB7 group, then it’s as far away as that group: about 2,050 light-years. But the center of that group is 5.2 degrees to the northeast of Deneb, so Deneb might not be a part of it.

Interestingly, in 1838, the first star for which the distance was calculated was 61 Cygni, which lies less than 8 degrees southeast of Deneb. Maybe, due to its distance and brightness, Deneb will be the last star visible to the unaided eye to receive an accurate distance measurement.

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. But we don’t yet know its precise distance.

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