It is surprising that even late in 1920, people do not know there are galaxies besides the Milky Way.
Does this mean that all the stars visible to the naked eye are near to us?
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2$\begingroup$ That depends. When I look at the Andromeda galaxy, I see nothing but light from stars. But I do not see a single star. $\endgroup$– SE - stop firing the good guysCommented May 8, 2016 at 9:40
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3 Answers
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All stars visible individually to the naked eye belong to the Milky Way. A handful of galaxies are visible to the naked eye, but here you only see the combined light of millions to billions of stars.
The only exception is when a massive star ends its life, exploding as a supernova. This can be visible to the naked eye for a brief perdiod of time. Most notably, the gamma-ray burst GRB 080319B was visible to the naked eye for roughly 30 seconds in 2008, despite its distance from us being $\sim10^5$ times the size of the Milky Way.
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1$\begingroup$ @uhoh: Thanks, and thanks for the link; I hadn't seen that, actually. $\endgroup$– pelaCommented May 8, 2016 at 9:34
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$\begingroup$ SN 1987A (in the neighboring Large Magellanic Cloud) was visible to the naked eye for about a year -- and one of the three more-or-less simultaneous discovery observations of it was actually naked-eye! (One could quibble about whether a supernova or GRB is technically still a "star", but that seems like being needlessly pedantic.) $\endgroup$ Commented May 8, 2016 at 12:10
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$\begingroup$ @PeterErwin: Yes, the GRB was just an example. There was also one in Andromeda in 1885. $\endgroup$– pelaCommented May 8, 2016 at 12:21
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$\begingroup$ @pela: Sure -- my thought was just that as GRB 080319B is "notable" for being (briefly!) visible in spite of its enormous distance, SN 1987A is "notable" for the extended duration of its naked-eye visibility. (Plus, we know lots of people did in fact see it with the naked eye.) (My impression is that SN 1885A was, at best, just barely visible right at maximum brightness, with V ~ 6; I'm not sure there are any verified naked-eye observations of it.) $\endgroup$ Commented May 8, 2016 at 12:38
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For decades the variable star S Doradus, in the Large Magellanic Cloud, was considered to be the most intrinsically luminous star known. But even though the Large Magellanic Cloud is the second or third closest external galaxy, S Doradus is still too far away from Earth to be seen with the naked eye from Earth. O fall the stars that always or sometimes are intrinsically brighter than S Doradus, only the variable Eta Carinae, in our own Milky Way Galaxy, is close enough to Earth that it is sometimes visible with the naked eye.
Omega Centauri, or NGC 5139, was cataloged as a star by Ptolemy in his Almagest about 150 AD and by Bayer in the Uranometria of 1603 - Bayer named it Omega Centauri. It is at a distance of about 15,800 light years from Earth. There is a theory that Omega Centauri is a remnant of a dwarf galaxy that as captured by our galaxy.
Thus Omega Centauri is an object that was listed and named as a star and may be the remnant of a former galaxy that has been captured by our own galaxy. Thus so far it seems to ALMOST fit the definition of an extra-galactic star visible from Earth with the naked (Human) eye.
Of course Edmund Halley noticed that Omega Centauri was not a star as early as 1677. Today it is classified as a globular Star cluster in our own galaxy (and possibly the remnant of the core of a dwarf galaxy). The light that makes Omega Centauri visible to the naked eye on Earth comes from the light emitted by hundreds of thousands or millions of stars, not one single star.
Even though a globular star cluster has the light of tens of thousands to millions of stars, and our Milky Way Galaxy has over a hundred globular clusters, only a few of them are visible from Earth with the naked eye (47 Tucanae mentioned by RichS is another, and it was also mistaken for a single star at first).
So a single star that was as far away as Omega Centauri 15,800 light years away, or 47 Tucanae 17,000 light year away, would have to shine as bright as tens or hundreds of thousands, and maybe even millions, of ordinary stars to appear just barely visible to the naked eye like those two clusters.
The Canis Major Dwarf Galaxy is believed to be 25,000 light years from earth and the nearest external galaxy, if it really is a galaxy. The Saggitarius Dwarf Elliptical Galaxy is about 70,000 light years from Earth and the Large Magellanic Cloud is about 163,000 light years from Earth. A star in the Large Magellanic Cloud, about ten times as far away as Omega Centauri or 47 Tucanae, would have to be as bright as millions to hundreds of millions of ordinary stars to be seen by the naked eye from Earth.
I suppose it is theoretically possible that one of the only about 6,000 stars visible with the naked eye from Earth might actually be in one of the 2 or 3 closest galaxies or floating alone outside of the disc of our galaxy. But it would have to be a supergiant or hypergiant star, and astronomers would have to somehow not notice oddities in its spectrum that would point to it being so rarely luminous.
I would estimate that the odds against that would be "astronomical".
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There was only one time in recorded history when a star was visible to the naked eye and not within our galaxy. That was Supernova 1987A. When the astronomer, Ian Shelton, first saw it through a telescope, he was amazed at how bright it looked. Thinking it might be naked-eye visible, he stepped out of the observatory and saw it within his own eye.
Since then, all the stars you see are nearby stars within our galaxy. All naked-eye visible stars are within a 17 thousand light years. Most are actually within a few hundred light years, but there are some bright stars within the 47 Tucanae globular cluster that are 17 thousand light years away.
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$\begingroup$ I have downvoted because this is incorrect. The answer already posted shows a more recent naked eye event from outside the Milky Way $\endgroup$ Commented May 10, 2016 at 9:48
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$\begingroup$ @RoryAlsop: Note that no-one actually did see GRB 080319B. We only have photometry showing that the peak magnitude was 5.8, meaning that if someone had looked at this particular point in the sky, at that particular time, s/he would have seen it. But only the camera saw it. $\endgroup$– pelaCommented May 10, 2016 at 10:05
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$\begingroup$ You can't assert that nobody saw it. Only that nobody noticed it and thought it was newsworthy. $\endgroup$ Commented May 10, 2016 at 22:16
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