# What's the correct distance to Betelgeuse? [duplicate]

Reading [1], [2] and this smartphone app[3], I'm getting different answers to the question.

The smartphone app is stating 427ly. [1] states approx. 700 ly, and [2] states approx. 724ly.

Even if I take [1] as the correct value, why does [2] state 724ly and the app stating 427ly?

Thanks

• – user24157
Jul 2 '20 at 10:08
• Tip: Relying on wikipedia and smartphone apps is not the way to get reliable information. Jul 2 '20 at 12:17
• coincidentally and possibly misleadingly, 427 and 724 are anagrams of each other. Jul 3 '20 at 1:45
• I think this question should be re-opened, as while it is definitely related to the other question, to me this doesn't feel like a duplicate. The other question is about the method of distance determination, while this one is about the actual value itself.
– user24157
Aug 6 '20 at 17:42

Distance measurements for Betelgeuse are a bit of a mess. Solutions based on parallax would be ideal, but Betelgeuse has a rather large angular diameter at most wavelengths thanks to its extended envelope; optical and infrared observations usually fall in the 40-60 mas range (see Dolan et al. 2016 for a recent review), while radio observations show a disk of emission roughly twice that size (O'Gorman et al. 2017). The parallax is expected to be much smaller that the angular diameter, on the order of ~5 mas, and so it depends strongly on the choice of the center of emission.

One of the first decent parallax results was obtained by the Hipparcos satellite in 1997, whose astrometric measurements allowed comparatively precise measurements of location, parallax and proper motion for over 100,000 stars. Hipparcos measured a parallax for Betelgeuse of $$\pi=7.63\pm1.64$$ mas, corresponding to a distance of $$131\pm30$$ pc$$^{\dagger}$$. This is that 427 light-year number the app cited. The Hipparcos-only result was subsequently improved upon significantly by van Leeuwen 2007, who found $$\pi=6.56\pm0.83$$ mas, cutting the old uncertainty in half; this would correspond to a distance of 152 parsecs. If you're going to quote a Hipparcos result, this is the one to pick.

More recent results indicate that this value is likely too low. Combining Hipparocs data with multiple multifrequency radio measurements using the Very Large Array, ALMA and e-MERLIN (Harper et al. 2008, Harper et al. 2017) gives derived values of $$197\pm45$$ pc and $$222^{+48}_{-34}$$ pc, with the former barely consistent with the purely optical results (the latter value converts to 724 light-years). These groups note that the Hipparcos stochastic astrometric solution required the addition of so-called "cosmic error" or "cosmic noise" terms to individual position measurements.

The papers above note that the photocenter at both and optical radio wavelengths does not coincide with the barycenter, and may change on timescales of months to years. Therefore, extended, long-term observations would be necessary to reduce any photospheric "jitter" or other variations which could lead to variations in emission and thus astrometric fitting. Harper et al. 2017 proposed joint ALMA and Expanded VLA/Jansky VLA observations and mm and sub-mm bands over a period several years, but also suggested that this would require "a Herculean effort" to achieve, for logistical reasons (how many telescope committees would be inclined to commit that much time up front?). Perhaps interest in Betelgeuse's recent luminosity dip could motivate this sort of observation.

$$^{\dagger}$$Rob Jeffries makes the point that given the pretty terrible signal to noise ratios of many of these parallaxes (Hipparcos was in particular was bad, but none of the observations are amazing), it's not clear that you can really get good, meaningful uncertainties on the distance measurement from them via $$d=1/p$$. I agree; it's safe to say that the jury is still out on those, and any source that claims a distance and an error should give make that quite clear.

• There's a recent arXiv preprint that proposes a new seismic distance, it's worth a read but has not yet been published: arxiv.org/abs/2006.09837
– user24157
Jul 2 '20 at 15:32
• @antispinwards Looks like I have some reading to do - thanks. Interesting to see MESA involved. Jul 2 '20 at 15:35
• FYI That parallax measurement should not give the distance as shown. The reciprocal of a parallax with a SNR of only ~5 is subject to a significant bias and the uncertainties cannot be calculated in a simple way. Jul 2 '20 at 16:32
• Further, the revised Hipparcos catalogue gives a revised and more precise parallax. Jul 2 '20 at 20:02
• Thanks everyone! Appreciate the information and clarification. This question has been puzzling me to no ends lately. I've based my calculations on the app's answer; but apparently I'm underestimating value. Jul 3 '20 at 1:49

Current measurements give a distance "somewhere between 567 light years and 835 light years", with a "best guess" of 724 light years.

So wikipedia is being honest and saying "about 700". We really don't know any better.

Earthsky.org is giving the "best guess" value.

The app is probably using the distance measured by the Hippcaros space telescope. This value is generally considered to be an underestimate.

Another answer explains why we find it so hard to measure this distance: What will it finally take to accurately measure the distance to Betelgeuse?

• Needs to give details and sources of information/quotes and how the distance was arrived at. Jul 2 '20 at 12:17
• @antispinwards it doesn't give the distance as 131 pc though does it? You can't just invert a poorly determined parallax. Jul 2 '20 at 16:34
• @RobJeffries - I'm aware that's an extremely flawed method of estimating distance from parallax. Nevertheless, that appears to be how the 427 light year value was determined.
– user24157
Jul 2 '20 at 17:32
• @antispinwards who determined it? Jul 2 '20 at 19:59
• @RobJeffries - I'm sorry, I really don't understand your point here. I'm referring to the 427 light years distance mentioned in the original question from the smartphone app, which appears to be a naive 1/parallax determination from HIP1 rather than a typo as asserted by James K. I'm not suggesting that is a good estimate of the distance.
– user24157
Jul 2 '20 at 21:06

Look at this article: https://arxiv.org/abs/1706.06020. Depends on studies distance varies and this is probably the reason of this discrepancy. For instance Hipparcos catalogue gives $$131^{+35}_{−23}$$~pc, assuming pc=3.26ly it gives $$\sim$$427~ly. More details in attached article ;).

We are waiting for results from Gaia mission, because at first it made parallax of fainter stars and very bright, as Betelguse will come next, but I don't know when, maybe it was measured already, but I can't find any credible information. They wrote in https://www.aanda.org/articles/aa/pdf/2016/11/aa29272-16.pdf

"The 230 brightest stars in the sky (G < 3 mag, loosely referred to as very bright stars) receive a special treatment to ensure complete sky coverage at the bright end"

• The abstract of the paper you cite does not give a Hipparcos distance of 131 pc and the means by which your disance is obtained (I doubt it is in the Hpparcos catalogue) is highly dubious. You cannot just invert a noisy parallax to get the probable distance..I doubt that Gaia will make great strides forward on the distance to Betelgese because it is not the precision of the parallax that is the problem, it is its accuracy because of the inhomogeneous atmosphere of Betelgeuse.. Jul 2 '20 at 16:38
• Yes in abstact is different, newer value, but inside the paper they mention: "The release of the original Hipparcos catalog (Perryman et al. 1997; ESA 1997) suggested that Betelgeuse had a distance of $131^{+35}_{−23}$~pc" and this one I cited. What do you mean by "inhomogeneous atmosphere"? That in different wavelength the thickness of atmosphere differs, or because of star pulsation ? Jul 2 '20 at 23:29
• I have found sth which probably is your point: "Instead of the star's atmosphere expanding uniformly due to gas heated to high temperatures near its surface, it now appears that several giant convection cells propel gas from the star's surface into its atmosphere". Therefore, because of inhomogeneity and fluctuation of its, we cannot really properly estimate distance with e.g. parallax method. Ok,, I got it :) Jul 2 '20 at 23:34