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My girlfriend and I were watching Cosmos, and something Carl Sagan said got us wondering what the farthest-away visible star is. Obviously "visible to the naked eye" is a fuzzy concept that might have many defensible answers, but hopefully not too many.

To make the question a little more interesting, let's restrict to individually distinguishable stars; otherwise the answer is pretty clearly some Local Group galaxy, and there aren't many of them to check.

The closest thing to a reasonable answer we came up with was this wikipedia list of stars that are more luminous than any closer star. The farthest-away star on that list with a plausibly visible apparent magnitude is Eta Carinae (7500 ly away, magnitude 4.55). However, there are several reasons why I'm not willing to consider this definitively answered:

  • It's a wikipedia article, and a poorly sourced one at that. So I don't entirely trust it.
  • It sorts stars by bolometric luminosity rather than visual luminosity, so perhaps there's some farther-away star whose spectrum is better-centered in the visible range.
  • The farthest-away visible star isn't actually guaranteed to be on a list of that sort, even assuming the other two points are cleared up. Perhaps the farthest-away visible star is only barely visible, and there's some star both closer and absolutely brighter than it which makes the list.

Given all these points, is it actually the case that Eta Carinae is the farthest-away visible star, or is there some visible star that's farther from us?

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@zephyr: I meant to suggest that with the word "individual", but I guess I should edit to be a little clearer about it. If you don't impose that restriction, the answer is pretty clearly either the Andromeda or Triangulum Galaxy, depending on how stringently you want to define "visible". –  Micah Dec 3 '12 at 3:52
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A possible interesting side questions would be how distant is the furthest supernova that was distinguishable to the Mk I eyeball? –  dmckee Dec 3 '12 at 4:00

2 Answers 2

up vote 9 down vote accepted

It all depends on the site you are observing from and the atmospheric conditions (and obviously also on you eyesight). A so-called "magnitude 6 sky" is often taken as the standard for a good dark site with no light pollution. This means that the threshold stars you can see have apparent magnitude 6. According to this article :

"The farthest star we can see with our naked eye is V762 Cas in Cassiopeia at 16,308 light-years away. Its brightness is magnitude 5.8 or just above the 6th magnitude limit."

The above five digit figure for the distance in lightyears needs to be taken with a grain of salt, as no doubt it has been obtained by converting a single digit figure in kilo parsecs (5 kpc) into lightyears. Another astronomy blog quotes a distance of "about 15,000 lightyears" for V762 Cas.

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Where does the 5 significant figure distance to V762 Cas come from? The revised Hipparcos parallax reduction by van Leeuwen (2007) gives a parallax-based distance of about 850+/-350 pc. –  Rob Jeffries Sep 11 at 23:00
    
@RobJeffries - the (rather silly) 5-digit figure in ly is no doubt obtained by converting 5 kpc into lightyears. –  Johannes 2 days ago
    
Yes, that would be numerically about right, but please give a source for the 5kpc that isn't someone's blog. –  Rob Jeffries 2 days ago
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@Rob: About right? 5kpc is 16,308.17 light year. Do you really need a source for the 5 kpc? –  Peter Shor yesterday
    
@Peter Shor Yes. It is traditional in science to cite recognised, and preferably the primary, sources for measurements. It turns out the 5kpc appears to be based on an out of date (since 2007) Hipparcos parallax, which in any case had an uncertainty larger than itself. This has then repeatedly been propagated as fact (even to 5 significant figures) around the internet, apparently including Physics SE. However I thought that maybe there could be another source of the 5kpc figure... there are other ways of estimating distance. –  Rob Jeffries yesterday

I have taken the revised Hipparcos parallax catalogue, produced by van Leeuwen (2007, Astronomy & Astrophysics, 474, 653) and taken a subset of stars with Hpmag <6 (i.e. roughly the naked eye limit) and accepted only those objects with a parallax/error in parallax > 2.5. Anything with a larger fractional error in parallax really can't be trusted.

If I then look at this list, sorted by the reciprocal of parallax I see a number of candidates for the most distant naked eye star. I will list the top 4.

Name          Hpmag  |  Plx (mas)  |  e_Plx (mas) | Distance (pc) | Name/Spectral Type

HIP 107418    4.39      0.48           0.14         2080            nu Cep  A2 I  
HIP 22783     4.29      0.52           0.19         1920            alpha Cam  09.5 I  
HIP 54463     4.09      0.52           0.17         1920            chi Car  G0 I   
HIP 107259    3.91      0.55           0.20         1820            mu Cep  M2 I  

Note that V762 Cas (=HIP 5926) has a distance of only 850 pc (parallax 1.18 +/-0.45 mas) according to this catalogue. The oft-quoted Deneb has a parallax of 2.31 +/- 0.32 mas and thus is likely closer than 1000pc.

The top 4 are all blue supergiants or yellow/red hypergiants. The last one on the shortlist is a well known and very well studied object. The size of the error bars is such that it is hard to say which (exactly) is the most distant, and there are another few further down the list that could be more distant within their parallax uncertainties. The Gaia results in early 2017 will resolve this issue.

Eta Carina does not have a parallax in the Hipparcos catalogue, but this maybe because it was too faint at the time or more likely it is not a sufficiently point source (surrounded by nebulosity) for the data analysis to work properly. The SIMBAD CDS catalogue lists is as V=6.21 (from the Ducati [2002] photometry catalogue) and that is also its magnitude in the Yale Bright star catalogue. Of course, it is a variable and has been much brighter in the recent past, so was a naked eye object and it currently is a naked eye object. The system has a binary nature, though the secondary companion is of much lower mass and contributes only a small fraction of the light (Mehner et al. 2010, ApJ, 710, 729).

Allen & Hillier (1993, PASA, 10, 338) give a distance of 2200 +/- 200 pc using a so-called "expansion velocity" method. The star may be part of a larger association that includes the Tr 14 and 16 clusters that have a distance of 2900 +/- 300 pc (Hur et al. 2012, AJ, 143 41). So probably it is more distant than the 4 objects I listed above and is a current naked eye object (late 2014) with V$\simeq 5$ (see http://etacar.fcaglp.unlp.edu.ar/plots/historic.jpg).

An issue in "seeing" Eta Car with the naked eye is that it is mixed up in a lot of bright nebulosity, so it is hard to say to what extent you are seeing the star or the star plus a lot of its surroundings - see below for a Digitized Sky Survey image - messy!

R-band DSS image covering 30x30 arcminutes (size of the full moon) around Eta Car

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Eta Carina is a multiple star system. If we allow these, we might as well allow star clusters and galaxies. –  Johannes 20 hours ago
    
@Johannes True, but have you checked how much light comes from the secondary compared to the primary? Mehner et al. (2010, ApJ, 710, 729) for instance, deduce that the secondary is less luminous than 1e6 Lsun and therefore contributes less than 20% of the system luminosity. i.e. the naked eye object IS the primary star. –  Rob Jeffries 19 hours ago

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