What is the farthest-away star visible to the naked eye?

Question

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?

Answer 1

Following the questions raised by Rob Jeffries, I have completely rewritten my answer:

What is the farthest-away star visible to the naked eye?

Indeed, this question has many defensible answers. It is not just the concept "visible to the naked eye" that is fuzzy. The stars we see are seldom single objects, but rather binary and multiple star systems. Do we allow doubles and multiples? The wording of your question ("let's restrict to individually distinguishable stars") suggests to exclude multiple star systems. But the candidate farthest visible star put forward (Eta Carinae) is exactly that: a multiple. And like most highly luminous stars Eta Carinae is also a variable star. Should variable stars be allowed? Should we allow variable stars that in the recent past were visible to the human aye, but currently are not? If so, do we also allow cataclysmic variable stars? Do we allow novae and supernovae? Apart from all these ambiguities, as stressed by Rob Jeffries in below comments, there is also the issue of (often considerable) uncertainty in cosmic distances. How do we handle these uncertainties?

Let's first define what we mean by "visible to the naked eye". Which stars are visible to your naked eye depends on the light pollution of 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. The threshold stars you can see in such a night sky have apparent magnitude 6.

So we can eliminate a key ambiguity by changing the question into "which star brighter than 6th magnitude is farthest away?".

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."

This answer puts forward a variable star, but clearly excludes supernovae as that would have resulted in much larger distances (more about that later). Rob questions the apparent five-digit accuracy in this answer. A bit of research reveals that the distance figure is derived from the central value in the measured parallax of 0.22 +/- 0.59 mas (milli-arcseconds). This means that we have no more than a 50% confidence that the distance is indeed 16 kly (kilo lightyear) or more.

We should not blindly accept a 50% confidence level. Rather, we should agree on a confidence level that is deemed sufficiently strict for the intended purpose of selecting the most distant star. Yet another ambiguity to resolve!

I propose to use the one standard deviation upper range of the parallax measurement (in the case of V762 Cas 0.22 + 0.59 = 0.81 mas) to derive distances. This gives us an estimated distance of 4.0 kly with a confidence of about 85% that the actual distance is at least this value. (As Rob points out, a more recent parallax measurement for V762 Cas results in 1.18 +/- 0.45 mas. If we would combine both parallax measurements to derive a chi-square estimate of the actual distance, we arrive at a value compatible with 4 kly.)

This results in the conclusion that the often quoted V762 Cas (see e.g. here and here) is unlikely the most distant naked-eye-visible star. For instance, HIP 107418, put forward by Rob as candidate most distance star, has a lower one standard deviation upper range of parallax of 0.62 mas, corresponding to a 85% confidence distance of 5.3 kly.

I do not have the means to analyze extensive star data bases, but offer this candidate most distant naked-eye-visible star: AH Sco, with a one standard deviation upper range of parallax of 0.48 mas, leading to a 85% confidence that its distance exceeds 6.8 kly.

Finally, what answer do we arrive at if we allow for a broader range of variable stars, including supernovae?

I propose SN 1885A at a distance of 2.6 million light years (!) as the most distant single star that was once (almost 130 years ago) visible to the naked eye.

Answer 2

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!

Answer 3

Rho Cassiopeiae is my favourite candidate. I know it is a variable but is visible at present on very clear nights. One of the reasons for choosing it is that it is listed as being 8,200 light years away (further in some lists) which means that light was on its way from it 6,000 years ago when some say the universe was created. This would require the light to have been created in transit. Cassiopeiae is easy to spot and Rho is slightly up from the top of the W when the Big Dipper is below Polaris.

Answer 4

Here are a bunch of Gaia DR3 (2022) results, and code to facilitate further study.

It seems clear that under the right conditions, we can see stars at least 10,000 light years away! But note that the answers to this question are around the accuracy limit of even Gaia's parallax data, so I present a range of possibilities, not a definitive conclusion.

Most of the previous answers were before even Gaia DR1 came out in 2016. The best previous data from the 2011 Extended Hipparcos compilation was limited to an accuracy of about 1 mas, giving them perhaps 10% error at just 350 light years. Gaia DR3 parallaxes are accurate to about 0.03 mas for stars brighter than 15th magnitude (though as I understand it, stars brighter than G ≈ 5 mag start to saturate Gaia's CCDs, making precise centroiding difficult). This still means that distance estimates for stars beyond perhaps 10,000 ly tend to have significant errors.

I should also note that data other than parallax measurements can also be helpful. And I'll add that real-world observations, especially for those stars nearer the limits of human seeing, should be incorporated!

But for now I'll just provide the results of an analysis of the Gaia data. This comes from my open-source code farthest-naked-eye-star to facilitate further improvements.

The code produces two results, one based on nominal parallaxes, and one produced by adding Gaia's parallax_error data for each star to the parallax to create a lower bound distance, as others have suggested.

Gaia records brightness using the "G" passband, between about 330 nm and 1050, vs the V passband intended to model human perception, which I think is based on just the wavelengths between 500 and 600 nm. The code gets V magnitudes from SIMBAD.

I did some initial queries which suggest that it is common for the G magnitude to be significantly less than V, but rare to find stars with G larger than 6.2, yet V less than 6.0. So the results shown here use a hard cutoff of G < 6.2.

This script implements the following strategy for selecting stars:

• Query Gaia DR3 for stars with Gmag < 6.2
• Compute nominal and conservative lower-bound distances (in ly).
• Form two candidate tables: one sorted by nominal distance and one by lower-bound distance.
• Initially select the top 50 candidates from each.
• Enrich each candidate with SIMBAD data:
  • SIMBAD main identifier, common name, Vmag, spectral type, luminosity class, variable star type.
• Then filter each table to keep only those with Vmag < 6.0. That leaves 19 nominal candidates and 16 lower-bound candidates.

Here is the result sorted by lower-bound. If you scroll to the right you'll get the Gaia DR3 id for each star. Also note that the latest versions of these three tables can be found as csv files in that same github repository for the source code.

main_id	common	Vmag	Gmag	plx	plx_err	ly_nom	ly_lb	Con	RA	Dec	spec	lum	var	Gaia DR3
* rho Cas	TIC 65366071	4.590	4.050	-0.05693	0.09455	-1	86705	Cas	358.59593	57.49937	G2_0			1998148532777850880
V* V810 Cen	TIC 322418709	5.030	4.792	0.15369	0.09270	21222	13238	Cen	175.87991	-62.48939	G0_0-IaFe1	Ia		5333358794709001216
* phi Cas	TIC 332680754	4.980	4.736	0.21417	0.08385	15229	10944	Cas	20.02047	58.23161	F0Ia	Ia		413828761929696256
V* V509 Cas	TIC 313555973	5.130	4.999	0.25073	0.06327	13008	10387	Cas	345.02123	56.94537	G4_0			2009856132592845568
* x Car	TIC 306916925	3.830	3.557	0.22276	0.10044	14642	10092	Car	167.14740	-58.97503	G5_0-Ia	Ia		5337891256535666176
* mu. Cep	TIC 260614141	4.080	2.451	0.11902	0.26369	27403	8522	Cep	325.87694	58.78003	M2-Ia	Ia		2202630001603369856
* omi01 Cen	TIC 451860101	5.097	4.724	0.32537	0.07341	10024	8179	Cen	172.94191	-59.44205	G3_0-Ia	Ia		5336490478704008960
HD 102878	TIC 303884894	5.702	5.591	0.36989	0.03657	8818	8024	Cen	177.61359	-62.64938	A2Iab	Ia		5334608080460780672
HD 70761	IDS 08186-2602 AB	5.883	5.789	0.35960	0.05370	9070	7892	Pup	125.70805	-26.34820	F2Iab	Ia		5695101727992548224
HD 12953	TIC 374968886	5.680	5.477	0.38407	0.04746	8492	7558	Per	32.16907	58.42360	A1Ia	Ia		506778691804372736
HD 199478	TIC 468549377	5.640	5.532	0.39151	0.04014	8331	7556	Cyg	313.95750	47.41764	B7Ia	Ia		2166846047768767488
* 3 Gem	GEN# +1.00042087	5.750	5.736	0.38778	0.06161	8411	7258	Gem	92.43327	23.11345	B2.5Ib	Ib		3425306173127304448
* z02 Car	LLNS 4103	5.190	5.038	0.38506	0.07002	8470	7167	Car	167.14160	-61.94717	B9.5Ia	Ia		5337105105696383360
HD 111904	LLNS 9341	5.770	5.635	0.44359	0.03096	7353	6873	Cru	193.34119	-60.32849	B9Iab	Ia	V*	6056408567538569088
* y Car	TIC 467274664	4.620	4.431	0.39320	0.08828	8295	6774	Car	168.15000	-60.31762	A5Iae:	Ia		5337672556798367360
HD 111613	TIC 435684037	5.720	5.583	0.45345	0.02962	7193	6752	Cru	192.82486	-60.32979	B9.5Iab	Ia		6056416676437740416

Note that Rho Cas, which tops the list, has a small negative nominal parallax of -0.05693 mas. Along with the huge parallax error estimate, this indicates that the data is spurious, though negative values can also reflect some systematic errors. See an interesting discussion of negative parallaxes. Recent papers on Rho Cas adopt a value of 2500 pc, or 8200 ly.

Here is the nominal data.

main_id	common	Vmag	Gmag	plx	plx_err	ly_nom	ly_lb	Con	RA	Dec	spec	lum	var	Gaia DR3
* mu. Cep	TIC 260614141	4.080	2.451	0.11902	0.26369	27403	8522	Cep	325.87694	58.78003	M2-Ia	Ia		2202630001603369856
V* V810 Cen	TIC 322418709	5.030	4.792	0.15369	0.09270	21222	13238	Cen	175.87991	-62.48939	G0_0-IaFe1	Ia		5333358794709001216
HD 152234	TIC 339565401	5.450	5.424	0.15988	0.34058	20399	6517	Sco	253.50765	-41.80640	B0.5Ia	Ia		5966510057279631488
* phi Cas	TIC 332680754	4.980	4.736	0.21417	0.08385	15229	10944	Cas	20.02047	58.23161	F0Ia	Ia		413828761929696256
* x Car	TIC 306916925	3.830	3.557	0.22276	0.10044	14642	10092	Car	167.14740	-58.97503	G5_0-Ia	Ia		5337891256535666176
V* V509 Cas	TIC 313555973	5.130	4.999	0.25073	0.06327	13008	10387	Cas	345.02123	56.94537	G4_0			2009856132592845568
HD 135345	TIC 307739457	5.171	5.064	0.29684	0.44906	10988	4373	Lup	229.01671	-41.49118	A/F			6004302402223120512
* omi01 Cen	TIC 451860101	5.097	4.724	0.32537	0.07341	10024	8179	Cen	172.94191	-59.44205	G3_0-Ia	Ia		5336490478704008960
HD 70761	IDS 08186-2602 AB	5.883	5.789	0.35960	0.05370	9070	7892	Pup	125.70805	-26.34820	F2Iab	Ia		5695101727992548224
HD 102878	TIC 303884894	5.702	5.591	0.36989	0.03657	8818	8024	Cen	177.61359	-62.64938	A2Iab	Ia		5334608080460780672
HD 12953	TIC 374968886	5.680	5.477	0.38407	0.04746	8492	7558	Per	32.16907	58.42360	A1Ia	Ia		506778691804372736
* z02 Car	LLNS 4103	5.190	5.038	0.38506	0.07002	8470	7167	Car	167.14160	-61.94717	B9.5Ia	Ia		5337105105696383360
* 3 Gem	GEN# +1.00042087	5.750	5.736	0.38778	0.06161	8411	7258	Gem	92.43327	23.11345	B2.5Ib	Ib		3425306173127304448
HD 199478	TIC 468549377	5.640	5.532	0.39151	0.04014	8331	7556	Cyg	313.95750	47.41764	B7Ia	Ia		2166846047768767488
* y Car	TIC 467274664	4.620	4.431	0.39320	0.08828	8295	6774	Car	168.15000	-60.31762	A5Iae:	Ia		5337672556798367360
* 6 Cas A	ADS 17022 A	5.590	5.293	0.42644	0.06598	7648	6624	Cas	357.20901	62.21451	A3Ia	Ia		2012942564813958656
* b Vel	TIC 141901042	3.810	3.554	0.43116	0.10079	7565	6131	Vel	130.15651	-46.64872	F0Ia	Ia	V*	5521599549054493696
* psi01 Aur	TIC 258622975	4.750	4.122	0.44257	0.11030	7370	5899	Aur	96.22459	49.28788	K5-M1Iab-Ib	Ia		970675154497903616
HD 111904	LLNS 9341	5.770	5.635	0.44359	0.03096	7353	6873	Cru	193.34119	-60.32849	B9Iab	Ia	V*	6056408567538569088

The code can also be used with a custom WHERE clause in the Gaia ADQL query. I've used that to pull in data for most of the other candidates I've encountered in answers here and at What is the farthest star visible naked-eye? - Deep Sky Observing - Cloudy Nights (free login needed).

given_name	Gaia DR3	main_id	common	Vmag	Gmag	plx	plx_err	ly_nom	ly_lb	Con	RA	Dec	spec	lum	var
vv cep	2216536246703152256	V* VV Cep	** WRH 36	4.900	3.940	1.00331	0.10731	3251	2937	Cep	329.16308	63.62555	M2Ibep+B2?ep_sh	Ib
VY Canis Majoris	5616197646838713728	NAME VY CMa B	** B 719B	10.340	7.352	0.41918	0.40791	7781	3943	CMa	110.74301	-25.76754
NML Cygni	2065235853217670656	NML Cyg	OH 080.8-01.9		11.148	0.52792	0.34798	6178	3724	Cyg	311.60641	40.11649	M7/8
Eta Carinae	5350358584482202880	* eta Car	SRGA J104503.3-594104	6.480	4.035	NaN	NaN	-1	-1	Car	161.26474	-59.68446	LBV	V
v762 cas	534889699426804224	HD 7389	TIC 352284047	5.860	4.952	1.31483	0.06929	2481	2356	Cas	19.04957	71.74385	K5
HIP 107418	2203889767044993920	* nu. Cep	TIC 408093341	4.290	4.121	0.93209	0.08651	3499	3202	Cep	326.36216	61.12080	A2Iae	Ia
HIP 22783	482818271843093376	* alf Cam	TIC 281696989	4.290	4.213	0.59157	0.13331	5513	4499	Cam	73.51255	66.34271	O9Ia	Ia
HIP 54463	5337891256535666176	* x Car	TIC 306916925	3.830	3.557	0.22276	0.10044	14642	10092	Car	167.14740	-58.97503	G5_0-Ia	Ia
HIP 107259	2202630001603369856	* mu. Cep	TIC 260614141	4.080	2.451	0.11902	0.26369	27403	8522	Cep	325.87694	58.78003	M2-Ia	Ia
HIP 51623	5351420437845776256	HD 91533	TIC 457766442	6.006	5.885	0.41050	0.02530	7945	7484	Car	158.19915	-58.66673	A2Iab/b	Ia
HIP 52405	5350406619413548032	HD 92964	WISEA J104240.58-591256.7	5.510	5.272	0.47152	0.06689	6917	6058	Car	160.66897	-59.21575	B2.5Ia	Ia
HIP 51192	5351701122540337792	V* V399 Car	TIC 464732822	4.700	4.483	0.49759	0.11508	6555	5324	Car	156.85190	-57.63879	A9/F0Ia/ab	Ia
HIP 52004	5350668303173229824	HD 92207	LLNS 2006	5.450	5.226	0.46747	0.06164	6977	6164	Car	159.36272	-58.73332	A0Ia	Ia
HIP 112444	2705401759471719168	HD 215667	AP J22464026+0454287	7.830	7.794	5.63308	0.02832	579	576	Peg	341.66778	4.90799	A7IV	I
HIP 80493	6045440595422971776	HD 147955	AP J16255065-2634062	7.270	8.037	5.88106	0.40924	555	519	Sco	246.46101	-26.56850	B9V	V
Beta Phoenicis	4933453324590295040	* bet Phe A	WDS J01061-4643A	4.000	3.744	NaN	NaN	-1	-1	Phe	16.52096	-46.71839
AH Sco	5979926504430920192	V* AH Sco	TIC 45344564	8.100	6.031	0.56324	0.07988	5791	5071	Sco	257.82090	-32.32521	M5Ia-Iab	Ia
rw cep	2005992002061917312	V* RW Cep	TIC 422108142	6.650	5.585	0.11398	0.03425	28616	22004	Cep	335.77920	55.96322	K2_0-Ia	Ia
V640 Mon	3131898712120792064	HD 47129	TIC 220197273	6.060	6.016	0.76148	0.07008	4283	3922	Mon	99.35017	6.13539	O8I+O7.5III	I
AO Cas	394976501194687488	V* AO Cas	TIC 202133804	6.140	6.089	0.75456	0.05794	4322	4014	Cas	4.42941	51.43308	O9.2II+O8V((f))	II
HD 041161	969823376583544576	HD 41161	TIC 416954881	6.764	6.721	0.67137	0.06053	4858	4456	Aur	91.46855	48.24928	O8Vn	V
HD 057682	3047763601330845312	HD 57682	TIC 187458882	6.430	6.375	0.89681	0.07394	3637	3360	Mon	110.50860	-8.97932	O9.2IV	I
P Cygni	2061242908036996352	* P Cyg	TIC 12149245	4.820	4.566	0.62508	0.07292	5218	4673	Cyg	304.44665	38.03290	B1-2Ia-0ep	Ia
EZ CMa	2922367976673391232	HD 50896	TIC 78959225	6.910	6.583	0.65045	0.03662	5014	4747	CMa	103.55433	-23.92833	WN4b
HD 30353	204531088580182016	HD 30353	LAMOST J044854.23+431639.2	7.760	7.485	0.10037	0.04142	32496	23004	Per	72.22229	43.27557	A5Iap	Ia
HD 62745	3028867978609417344	HD 62745	TIC 333817448	8.390	7.183	0.17281	0.02510	18874	16480	Pup	116.23747	-15.69722	M3.3Iab	Ia
HD 48716	3352789979664648064	HD 48716	LAMOST J064514.54+132017.6	7.270	7.179	0.14995	0.02512	21751	18630	Gem	101.31075	13.33593	A0
68 Cygni	1971467916750747264	* 68 Cyg	TIC 138317746	5.000	4.992	1.44978	0.11521	2250	2084	Cyg	319.61330	43.94591	O7.5IIIn((f))	III

Answer 5

The Extended Hipparcos compilation (Anderson+, 2012) provides distances for stars in the Hipparcos catalogue, together with the apparent magnitude in Johnson V (which approximates to how bright a star appears to be to the human eye).

The distance calculation is Dist = 1000/Plx * (1+1.2*(e_Plx/Plx)^2).

Applying the search criteria Vmag<6 & Dist>0 (i.e. not null) identifies 4 stars in Carina that are over 2450 parsecs (over 8000 light-years), all of which are "single components", with some degree of variability:

HIP 51623=HR 4144 2497pc Vmag=5.98 (var < 0.06mag; Hpmag = 6.075)

HIP 52405=HR 4198 (V519 Car) 2492pc Vmag=5.36 (0.06mag < Var < 0.6 mag; Hpmag = 5.446)

HIP 51192=HR 4110 (V399 Car) 2484pc Vmag=4.65 (var < 0.06mag; Hpmag = 4.778)

HIP 52004=HR 4169 (V370 Car) 2475pc Vmag=5.47 (0.06mag < Var < 0.6 mag; Hpmag = 5.47)

Depending on how reliable the distances listed are, any of these might be worthy candidates.