# Is there an example of a star that is already dead, but can still be seen on Earth?

While looking up at the stars I wondered, what are the odds that some of them are already dead. I did some research and found a very interesting article here. It states that the odds are rather small, so it is not like about 50% of the stars we see are already dead.

But I still wonder if there is an example of a star we know to be already gone, while it is still visible on earth.

As far as I know, the only way to find out is to watch the stars light. So if the light "goes out", we know the star has been gone for quite some time. Assuming this to be fact, I would have to conclude that such an example does not (and cannot) exist.

Is this true? Or is there in fact a star that is already dead that we know of, which can be seen in the sky?

PS: By "seeing the star" I am talking about a star that looks just like any other star in the night sky (not, for instance, a visible supernova).

Edit: If there is such a star, how did we find out it is already gone?

• Simultaneity is going to come after you :-) . But seriously, I would like to modify this question - is there any detectable star that is dead in our view? My first guess would be a binary star system of which one is live and one dead. (and that would be tough, since any kind of nova event would do bad things to the other star) – Carl Witthoft Aug 29 '17 at 12:09
• I think a duplicate of astronomy.stackexchange.com/questions/10316/… – ProfRob Aug 29 '17 at 15:03
• SN1987a, the light from supernova arrived much later than the neutrino of the same source. – user6760 Aug 30 '17 at 6:57
• – Wayfaring Stranger Aug 30 '17 at 16:18

There is an accepted answer already, but there is a couple known cases of a star we know has gone supernova, and yet we can still see it. This source describes one such unique circumstance. The star that exploded happened to be in a galaxy that was behind another massive one from our point of view. The alignment was just right such that the light formed an Einstein Cross. The light from each point on the cross takes a different path to get here, and each of those paths are different lengths. Thus, the different points on the cross show the star at 4 different times in its final years. The Space.com article I linked was written in 2015. Scientists first noticed the supernova in 2014, and each image of the supernova arrived within a year of when it was first noticed. If I'm understanding that article correctly, as of now (2017) all of the images of that supernova show the supernova aftermath, and not the star beforehand. However, there was a period of time where we could see the star both pre and post-supernova at the same time.

You imply in your question that you are focusing on stars within our own galaxy. I don't know of any such situations of Einstein Cross events closer to home that let us do this.

• Wow - that's all I can say. – MartinV Aug 30 '17 at 8:18

Most high-redshift galaxies — i.e. the very distant ones, billions of light years away — are detected due to the UV light that comes primarily from very hot stars (in particular, copious amounts of Lyman $\alpha$ photons at 121.6 nm are emitted from the gas surrounding the hot stars which is ionized and then recombines). Because these stars "burn their fuel" so fast, they typically live only for a few to a few tens of millions of years. That means that those stars for sure are long gone.

You are, I think, asking two very similar but linked questions:

1. Are there stars - in our galaxy, as stars in other galaxies are not individually visible - that are 'dead' but still alight to us. The Slate article you linked is rather good in this respect: i) the lifespan of a given star is much longer than the time taken for light to reach us so it is unlikely that this is the case and ii) most stars have an extended extinction compared to the time taken for light to reach us, with few going out in a supernova flash. Hence it is pretty unlikely that this is the case at the moment - but it will happen on occasion
2. If there is a specific star which is 'dead' but still alight to us - could we know? Not really, as no useful information will be travelling to us faster than the speed of light. The best we might do is to observe a supernova and then say "this event happened x years ago"

Maybe Betelgeuse in the Orion constellation. It seems it is nearly at the end of its life and it is at around 600 light years from us. (But well it can also be in something like greater than 10000yrs of lifetime so it is difficult to say (still very low at astronomical scales)...)