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All the members of Orion are within the Milky way, and some of them look pretty close to a neighbor. And could the bow* be an effect similar to the Radcliffe Wave?

*shield

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    $\begingroup$ What is a "Radcliffe Wave"? What does "All the members of Orion" mean? How many stars are you talking about? Thanks! $\endgroup$ – uhoh Feb 16 '20 at 1:49
  • $\begingroup$ Maybe include a link. It's an interesting question and the article suggests that the Radcliffe wave is in the same part of the sky (when viewed from Earth) as Orion's belt and other constellations, but viewed from Earth and in the same part of space don't necessarily go hand in hand. It says star formation happens in the Radcliffe wave, but the stars in Orion's belt are HUGE and two of the systems are fairly close to each other, the 3rd is quite far. My guess would be different formation events, but it's just a guess. space.com/radcliffe-wave-largest-milky-way-structure.html $\endgroup$ – userLTK Feb 16 '20 at 6:35
  • $\begingroup$ Orion's belt, sword and nebula are more-or-less at the same distance $\endgroup$ – ProfRob Feb 16 '20 at 9:17
  • $\begingroup$ @userLTK Betelgeuse is not in Orion's belt. The three belt stars are Alnitak, Alnilam and Mintaka. Despite what you read on wikipedia they are (along with sigma Ori) thought to be at a similar distance. There can be major problems with Hipparcos parallaxes to bright, multiple stars. $\endgroup$ – ProfRob Feb 16 '20 at 16:31
  • $\begingroup$ @RobJeffries I know, but you said belt, sword and nebula - which I may have miss-read. I should have checked. There's still about 800 light years between the more distant star and the other two in the belt (star systems not stars - minor point). Maybe that can happen in a few million years if the one star has high enough velocity. I'll delete the other comment, but it's still about 800 light years. $\endgroup$ – userLTK Feb 16 '20 at 18:53
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The three stars in Orion's belt, along with Sigma Orionis and the Orion Nebula cluster, along with several other young clusters and star forming regions in the central Orion region are all part of the Orion OB1 association at a distance of 350-420 pc.

A review of Orion OB1 can be found in Bally (2008). The three belt stars are probably a similar age and distance and form the brightest stars of the OB1b sub-association. Measured distances to these stars are individually rather uncertain, but better estimates come from the low-mass, co-moving stars around them. It is therefore true to say that the individual distances to each star are not known well enough to be sure that they are close together in distance.

The other bright stars in Orion - Betelgeuse, Rigel, Bellatrix, Saiph are significantly closer than this by 150-300 pc and probably not at a common distance. It has been speculated in the past though that these stars and the Orion OB1 association are part of one, large star forming complex, extending 200-300 pc along the line of sight (Bouy & Alves 2015).

The recent paper, claiming the discovery of a "Radcliffe wave" gas structure in the solar vicinity, does include the young stars in Orion as part of this structure.

So to spell it out: The belt stars are within 70pc of each other in terms of distance. If they are at a similar distance of 400pc, their angular separation on the sky of around 3 degrees, translates to a tangential distance separation of about 20 pc. Other members are more separated than this.

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    $\begingroup$ I think the question involved how close these stars are to each other rather than how close they are to us. $\endgroup$ – antlersoft Feb 16 '20 at 17:13
  • $\begingroup$ @antlersoft How far away they are from us is what mostly determines their relative separation, since at a distance of 400pc, the belt stars have a tangential separation of 20pc at most. The other stars are separated by 100s of pc. $\endgroup$ – ProfRob Feb 16 '20 at 18:16
  • $\begingroup$ Without Gaia-type data you probably can’t answer that, how else would you plug in all those member coordinates, the Orion’s belt scale gives a naive (angular without radial) thumb method of approximating those distances, thanks. $\endgroup$ – Gabe Fernandez Feb 17 '20 at 18:21
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Based on links on this page, the closest visible star in Orion (that is, the region of the sky called Orion) is Pi3 Orionis (π3 Orionis), a.k.a Tabit, a.k.a al-thābit, a.k.a. Zhāng Qí Liù. This is the point where the arm touches the shield/bow. That page says it is 26.2 light-years away; Wikipedia says its 26.32 ± 0.04 light-years or 8.07 ± 0.01 parsecs away. Both call it a main sequence F6 star (F6V). In any case, it's a very nearby star and much closer, and smaller, than the bright stars of Orion that you are probably thinking of, which are all hundreds of light-years away.

Based on other links on that page, the (very dim) known stars in Orion closer than π3 Orionis are:

Brown dwarfs:

  • WISE J052126.29+102528.4, at 12~21 ly

Red dwarfs:

  • LTT 17897, at 17.5 ly
  • G 099-049, at 17.6 +/- 1.0 ly
  • BD-03 1123, at 18.6 ly
  • Ross 47, at 18.9 ly

(Those 1st 2 MIGHT be 2 names for the same star, based on my attempts to look them up, even if those who made the page obviously didn't think so.)

White dwarfs:

  • G 99-44, at 20.9 +/- 0.1 ly
  • G 99-47, at 26.1 ly

Almost as close is the visible binary star Χ1 Orionis (Chi1 Orionis) (the tip of the club/arm), which is 28.3 ly away according to that page and 28.26 ± 0.07 ly or 8.66 ± 0.02 pc away according to Wikipedia.

If we limit ourselves to only tallking about Betelgeuse, Rigel, Bellatrix, Mintaka, Alnilam, Alnitak, Saiph and Meissa, then the answer is probably Bellatrix (the top shoulder on the viewers right, as opposed to Betelgeuse, which is on the other side). Wikipedia cites this paper* for it estimated distance of 250 ± 10 light-years or 77 ± 3 parsecs away.

That being said, distances are one of the hardest things to measure in astronomy, and, for example, I know that Betelgeuse used to have huge uncertainties in its distance before it was recently narrowed down to being about 25% closer than the older most likely distance. The current distance is 458~597 ly or 153~195 pc, the last range coming directly from the October 2020 paper.* This makes it probably the second closest of those bright stars in Orion I mentioned.

As a final note, it's important to remember that constellations are just stars that happen to be in about the same direction from Earth's perspective. They aren't actual collections of stars, even if there may be actual collections of stars in them, like how several (but definitely not all) of the stars in Orion are suspected to be related to each other. I didn't know anthing about the Radcliff wave until I looked it up just now, nor do I know if the bow/shield ASTERISM (i.e. pattern people see in the sky) might PARTIALLY correspond to anything physical, and I'm not an astronomer.

*van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.

*Joyce, Meridith; Leung, Shing-Chi; Molnár, László; Ireland, Michael; Kobayashi, Chiaki; Nomoto, Ken'Ichi (2020). "Standing on the Shoulders of Giants: New Mass and Distance Estimates for Betelgeuse through Combined Evolutionary, Asteroseismic, and Hydrodynamic Simulations with MESA". The Astrophysical Journal. 902 (1): 63. arXiv:2006.09837. Bibcode:2020ApJ...902...63J. doi:10.3847/1538-4357/abb8db. S2CID 221507952.

In this Wikimedia animation of Orion's appearence from Earth evolving over time from the distant past to the distant future, you can see π3 Orionis moving rapidly towards Bellatrix (not actually towards Bellatrix in space, just towards being roughly between it and Earth). Bellatrix and the other bright stars barely move scross the sky because they are much further away, and so would have to move much faster through space to move the same speed across the sky. The other rapidly moving stars are probably other nearby stars. enter image description here

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  • $\begingroup$ Oops. I just realized that by "in local distance", you probably meant to ask how close the stars were to each other, as opposed to how close they are to Earth, in which case my answer was not very good at all. $\endgroup$ – H. H. Mar 8 at 20:04
  • $\begingroup$ but it is redeemed by that super-cool graphic, though I was hoping that it would show Betelgeuse blowing up in the future ;-) $\endgroup$ – uhoh Mar 8 at 21:42
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    $\begingroup$ I think Betelgeuse might actually survive the whole period of time shown in this video. I remember there was a 2020 study determining what stage of death it was in and determining it still had a while to go before dying. (I think they said it was in some kind of Helium-burning stage, but I'm not sure.) $\endgroup$ – H. H. Mar 14 at 14:06

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