HD 84406, is a star approximately 241 light-years away in the constellation of Ursa Major. HD 84406 will be the first star to be imaged by the James Webb Space Telescope in order to test the focus of the telescope. The star is a spectral type G star and has a high proper motion.

Why aren't the other more popular stars like Alpha Centauri or Betelgeuse chosen for the first light instead?

Which properties of HD 84406 make it beneficial to the telescope mirror testing? Spectral type? Magnitude? Distance? Does the star have any interesting stuff? Or is it just for the reason that the position of the star is the most convenient for JWST to locate during the test process?

  • 7
    $\begingroup$ because it can never point at HDE 226868? $\endgroup$
    – uhoh
    Commented Jan 26, 2022 at 9:03
  • 2
    $\begingroup$ Nice question.. $\endgroup$ Commented Jan 26, 2022 at 10:32
  • 2
    $\begingroup$ @uhoh - kind of depends where HDE226868's career takes them. A trans-Jupiter observatory might be a fine place to do astronomy from... $\endgroup$
    – Jon Custer
    Commented Jan 26, 2022 at 17:55

2 Answers 2


The most important selection criterion is that the star should be available for observation for a prolonged time.

Because James Webb observes in the infrared, it must hide the Sun (and Earth and the Moon) behind its sunshield, and can hence only observe some 39% of the sky at any given time (source: NASA). Webb needs around three months for its optical alignment, so we need a star that has just entered its field of view$^\dagger$.

In addition to this, we also don't want a star in a field that is too crowded, and HD 84406 is located in a rather isolated region in the NW part of Ursa Major:

HD84406-position Credit: IAU/S&T/Roger Sinnott/Rick Fienberg with my own annotations.

The star should be bright, but probably not too bright (like Betelgeuse), since we don't want to burn MIRI off from the beginning. EDIT: After discussing with several colleagues, damaging MIRI permanently is probably not going to be an issue. Bright sources can however damage the detector temporarily, showing a "ghost image" of the star in subsequent exposures. I've done that myself on a 1.5 m Earth-based telescope, and JWST has a ~20 times larger area, and ~10 times higher resolution, meaning several 1000 times more photons per pixel.

Anyway, as you see there are some important constraints, but many other stars would fit as well; there are quite a lot to choose from.

And the best part: HD 84406 is a magnitude 6.9 star, so while you can't see it with your naked eye, it should be visible in a pair of regular binoculars.

$\dagger$Stars too close to the ecliptic are only available for a more limited time, so a star that is close to the poles is preferred.

  • 9
    $\begingroup$ "but probably not too bright (like Betelgeuse), since we don't want to burn MIRI off" Is damage to the instrument a risk when observing a bright star? $\endgroup$ Commented Jan 26, 2022 at 16:22
  • 4
    $\begingroup$ @CarlKevinson Perhaps not permanently, but temporarily I think yes. I've personally observed a too bright star (with a 2.5 m telescope) where pixels over-saturated to the point that succeeding exposures has suspiciously high values in those pixels. That was a CDD though, maybe JWST's IR detectors works differently. $\endgroup$
    – pela
    Commented Jan 26, 2022 at 21:51
  • 6
    $\begingroup$ But even if you don't damage the detector, imaging of a too bright star is difficult. You'll need extremely low exposure times, probably impossibly small, or else you saturate a large area on the detector, making it very hard to find the centroid of the star (which you want to in order to align the mirror segments). $\endgroup$
    – pela
    Commented Jan 26, 2022 at 21:55
  • 3
    $\begingroup$ @CarlKevinson So, my conclusion after discussing with several colleagues is that we probably shouldn't worry about damaging MIRI permanently, only temporarily. I edited my text accordingly. Thanks for questioning this! $\endgroup$
    – pela
    Commented Jan 27, 2022 at 14:23
  • 2
    $\begingroup$ @pela Thank you for the follow up! I was just curious and I'm happy to learn something! $\endgroup$ Commented Jan 27, 2022 at 15:00

This hot off the press, from Bill Ochs, Webb project manager:

This is just the first step; HD 84406 will be too bright to study with Webb once the telescope starts to come into focus.

  • $\begingroup$ Hi TonyK, thanks, this is an important comment, but not really an answer to the question, so it should probably be converted to a comment. $\endgroup$
    – pela
    Commented Jan 28, 2022 at 14:09
  • $\begingroup$ @pela: My feeling is that it contributes relevant additional information to your nice answer. So I put it where it would be more likely to get noticed. $\endgroup$
    – TonyK
    Commented Jan 28, 2022 at 14:31
  • $\begingroup$ It does, but it's still a comment, not an actual, stand-alone answer to the question, so my guess is that it will be closed/converted. The moderators tend to be quite strict about these rules (for a reason). $\endgroup$
    – pela
    Commented Jan 28, 2022 at 15:20
  • $\begingroup$ Oh for heaven's sake. $\endgroup$
    – TonyK
    Commented Jan 28, 2022 at 16:14

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .