One of the things I’ve always been most curious about on the topic of space is: is it possible for something that emits light (like a star) to emit light such as radio or gamma waves but not visible light? If such an object existed, it would be invisible to us, although it could still be detected. But it such an object existed and happened to be traveling on a course dangerously close to our solar system, we might not know to look for it with special instruments because we can’t see it.
Stars are primarily powered by fusion – fusing hydrogen atoms into helium (sometimes other elements are involved, but these are most common). This produces a wide range of light waves, from gamma to radio. Visible light is in between those extremes, so it’s expected to be emitted as well.
The closest thing to a star that doesn’t emit visible light is a black dwarf (pictured above), which has yet to be discovered. Once a white dwarf star runs out of fuel, it theoretically will turn into a black dwarf, emitting no heat and no light. The only way to observe it would be if another object’s emitted light bounced off of it. There are no known black dwarfs; the universe (we believe) has not been around long enough for any white dwarfs to burn through all of their fuel. So, the chances of a massive, invisible object speeding through space towards us (or a similarly invisible approaching alien fleet) are essentially zero.
Cosmic Brownies 
Jack, thank you for this incredibly informative blog post. I wonder if we detected a black dwarf if scientists would possibly mistake it for a black hole as they could detect a gravitational field around it with no light being emitted. Nice to know we will probably not be wiped out by one though.
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Hi Jack, terrific post. It really makes me wonder about the possibility of black dwarfs; I wonder if they were to already exist, do you think we have the technology to observe them? Or do we need to develop such technology further to see such undetectable objects? I feel that it is already hard enough to observe objects that do emit light and heat, and trying to see a celestial object that emits neither would be nearly impossible to find anyways. Hopefully we’ll find out sometime in the future.
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Personally, I do think that the concept of an invisible star is possible. Although I am not a scientist, the existence of both dark matter and dark energy pose concepts of things that exist but cannot be detected or seen by us. The stars that may be invisible could consist of these dark materials and exist in those conditions without our knowledge. If these invisible stars exist, it would be interesting to then re-evaluate how many stars are in the universe and how many invisible stars there hypothetically are in comparison to visible ones.
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I never really thought that it was possible for some stars to simply not have had enough time in order to die out. Do you think that it’s possible for there to be a black dwarf somewhere outside of our observable portion of the universe?
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Interesting question to pose. I never thought about the idea of an object potentially being invisible to the naked eye, despite being a very real object. However, I think that it is highly improbable, considering there are objects that exist on Earth that are visible to the naked eye that emit waves that are invisible to the naked eye. For example, bananas are slightly radioactive because of potassium-40 isotopes (https://www.universityofcalifornia.edu/longform/what-know-you-go-bananas-about-radiation).
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I never thought that is was possible for some stars to not emit enough light for us to see them. I found the point of a an object coming towards us that we can’t see very interesting. Personally, I think it is possible for an invisible star to exist. If this is true, it makes me wonder how many more stars are out there. Is the universe more massive than we think it is? This truly goes to show how much we don’t know about the solar system.
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