Deneb: How astronomers know how far away it is

Deneb: Asterism with three stars in a triangle and the words Summer Triangle on top.
Deneb is in the furthest-left corner of the Summer Triangle.

How do astronomers know distances to stars?

Here is the beautiful Summer Triangle, now 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 Summer Triangle. When you gaze at Deneb, you’re gazing across a great distance of space. The exact distance to Deneb isn’t known for certain. The distance we see most often online is around 2,600 light-years. That makes Deneb is one of the most distant stars we can see with the eye alone.

But estimates vary for this star’s distance. They vary a lot! Why? 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 many years before.

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 the Cygnus OB7 association of stars (Cygnus OB7 is a star-forming complex within our Milky Way galaxy).

ESA’s Earth-orbiting Hipparcos Space Astrometry Mission provided the most important modern distant measurement for Deneb 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. Consider that 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 2009 study (skip to the last page for Deneb) using a newer method of analysis.

This new analysis showed a distance for Deneb that’s barely half the widely accepted value. The 2009 study suggests 1,548 light-years as a distance for Deneb, 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. And, either way, it’s still one of the most distant stars we see with the eye alone.

Dot with a cone above and below with sun at one end and distant stars at other.
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. Image via University of Stockholm.

Why does Deneb’s distance matter?

Deneb’s distance 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 – the magnificent Gaia space observtaory – in a distant orbit similar to that of the James Webb Space Telescope. Gaia was 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 findings about our Milky Way galaxy, via Gaia. Click here for a few of Gaia’s discoveries.

So Gaia is pretty great. But a new estimate for Deneb’s distance wasn’t included in Gaia’s 1st data release, in September 2016. And it wasn’t included in Gaia’s 2nd data release. How about the 3rd data release?

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

Chart showing groups of stars of varying colors.
View larger. | This is the famous Hertzsprung-Russel diagram, which shows the luminosities of stars. See Deneb at the top of the diagram? It is one of the most luminous stars known. Image via Chandra/ NASA.

Deneb is too bright for Gaia

Gaia has produced data on 1.5 billion sources in our Milky Way galaxy. But Gaia can’t image 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. ESA has 2,500 people working for Gaia. One or two more might do the trick.

No one said it would be easy!

As Gaia was launched, its team was working on the problem of imaging bright stars. Paper, after paper after slick-looking card 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 is 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. Why don’t we know its distance precisely?

Read more: Delta Cephei helps measure cosmic distances

May 26, 2022

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