Are there stars that do not emit any light in the visible part of the EM spectrum?
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$\begingroup$ do you count black holes as stars? $\endgroup$– John DvorakAug 28, 2014 at 16:42
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$\begingroup$ @JanDvorak - Black holes don't emit any EM radiation. $\endgroup$– HDE 226868 ♦Aug 28, 2014 at 17:33
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1$\begingroup$ @HDE226868 See Hawking Radiation $\endgroup$– YashbhattAug 28, 2014 at 18:16
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5$\begingroup$ I think you could start by outlining your definition of a star, to help the answers. $\endgroup$– harogastonAug 28, 2014 at 18:25
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1$\begingroup$ @HDE226868 Why is it different? $\endgroup$– YashbhattAug 28, 2014 at 18:26
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2 Answers
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There are two possible reasons why a star would not be detectable in the visible part of the spectrum (even with the most powerful telescopes not yet invented), apart from the trivial (too far away, hidden behind screens of dust).
It has too low a temperature. To not emit much in the visible part of the spectrum, a star has to be very cold indeed, at most a few 100K, when radiation is mostly in the infrared (which is still detectable). By definition, a star is an object that undergoes (or has undergone) hydrogen burning (H$\to$He fusion). This sets an lower mass limit of 0.08M$_\odot$. All objects of that mass (or more) ever formed in the universe are still much hotter than 100K. However, in the very distant future, some 'dead' stars (without remaining fusion energy source) will cool down to such temperatures, this includes white dwarves.
It has too high a gravitational redshift for any light to appear in the visible. In fact, the so-called stellar-mass black holes (remnants of supernova explosions of massive stars) may in fact be such objects: strange stars denser than a nucleus (consisting of a quark-gluon plasma as dense as a neutron) but with such a small size and high mass that any radiation emitted from their surface is redshifted by a factor 1000 or larger.
So if you consider those (still hypothetical) strange stars, then these are possible candidates.
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$\begingroup$ I've seen in the past some sources suggesting Jupiter may be or become one of your first category stars. It emits a lot of radiation, but no visible light. $\endgroup$– jwentingSep 3, 2014 at 8:57
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$\begingroup$ @jwenting Jupiter is not a star, but a planet. Stars are defined to undergo (or have undergone) hydrogen burning (H$\to$He fusion). This requires a minimum mass of about 0.08M$_\odot$. Objects smaller than that, but massive enough to have some fusion (Li) early on are brown dwarves. The mass limit here is about 10M$_{\mathrm{Jupiter}}$. $\endgroup$– WalterSep 15, 2014 at 7:55
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Maybe a very old Neutron Star?
A black dwarf would not emit any visible light, but the universe is not old enough for that. Even the oldest and coolest white dwarfs still have a temperature between 2500-4000K (sorry for not remembering the reference for this).
Brown dwarfs (or planemos/sub-brown dwarfs) like WISE 0855–0714 could be as cool as ice. but they don't count as stars anyway.
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2$\begingroup$ This could be right, if the OP ever gets back to the definition of a "star". $\endgroup$– HDE 226868 ♦Sep 16, 2014 at 0:19
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1$\begingroup$ This article arxiv.org/abs/1501.07083 deals with the hypothesis that Fomalhaut b is an old neutron star located in the background of Fomalhaut, but still very close (11Pc). If confirmed, one could imagine how intrinsically faint an old NS can be, given the apparent faintness of Fomalhaut b. $\endgroup$ Feb 23, 2015 at 14:16