Is it true that the stars we see are already dead?

Question posed by Jo
Artist's impression of a dead star called a 'pulsar'. Credit to NASA/JPL-Caltech

Light is quick stuff
Light is really, really fast*, but it still takes time to get from one place to another. When you 'see' anything, what you are really doing is using your eyes to detect the light that is either given off or reflected by it. If you look at the clock across the room, you're not seeing it as it is now, but as it was a tiny, tiny fraction of a second ago. Of course, light is so fast that over such a short distance it doesn't really make a difference.

So next time you're able to, take a look at the moon, and think about how far the light that it is reflecting has to travel before it hits your eye. It's a lot further away than that clock was, and it still takes only a little bit more than one second to get to you: that means that when you see the moon, you're not seeing what it is like now; you're seeing what it was like a second ago.

Now imagine looking at the Sun (do not actually look at the sun- this could potentially blind you). The light given off by the Sun travels further still- it takes about eight minutes to get to Earth, so when Earth-based telescopes look at the Sun, they're seeing it as it was a whole eight minutes ago. Put another way, something could happen on the surface of the Sun- a sunspot pops up, for example- and we wouldn't know about it for eight minutes.

Let's go further out, to the next closest star to us, Proxima Centauri. That's so far away that it takes light four years to get here (it's four 'light years' away). It's not likely to do so, but if it popped out of existence, exploded or turned into a giant frog right now, we wouldn't know anything about it for another four years.

Now think about all the other stars up there- most of them are much, much further away: hundreds, thousands, millions and billions of light years away. If a star is a million light years away, and it died a million years ago, we'll see it die now. If another star died four million years ago, but is a hundred million light years away, we'll still be able to see it for another 96 million years.

The image below shows the results of the Hubble Ultra Deep Field experiment. The Hubble Space Telescope was pointed at a tiny patch of sky that appeared to be empty to any ground-based telescopes, and the shutter left open for just under one million seconds (about 11 days). This is what it saw:
Each spot of light in this image is an entire galaxy. There are around ten thousand of them here, some of which were around only a few hundred million years after the universe first came into being. Most of these, although we can see them now, are long gone, and possibly even replaced by other younger galaxies that we won't be able to see for many billions of years.

Are they all dead?
No. Some of  the stars that you can see at night may be long gone, with the light from their death yet to reach us. But plenty more are still burning bright right now, although we see them as younger than they actually are. As well as that, new stars are being born all the time- there will be many burning at this moment that we can't see yet because their light hasn't had time to reach us.

So yes, some are dead, but some are very much alive, and some are there but yet to be seen!




* It's the fastest thing in the universe.

Comments

  1. I knew all this, but still find it absolutely mind-blowing.

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  2. Glad I can help to tickle your amazement bone from time to time! Please feel free to ask questions you /don't/ already know the answers to... ;-)

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  3. dude what about the distortion of images that confirmed the presence of dark matter....itz some pretty weird stuff too.....

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  4. I just terribly loved this info. Thank You Very Much for posting. I was unaware of this and I used to gaze at stars at night-time and feel the unspeakable feeling...you know...thank you very much again

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  5. Abhinav: Yes, dark matter is pretty weird stuff! But I don't think it's relevant to this post.

    Navaneet: Thanks for commenting! It's good to have a comment from somebody I don't know for once (that isn't advertising something...) Feel free to ask your own questions either in the comments or using the form (reach it by clicking the "Ask a Question" link at the top of this page).

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  6. Hi, just a totally hypothetical question, so if one was able to achieve the speed of light and travel away from the earth for 1 year, would looking back at the earth appear to be 1 year in the past?

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  7. Hi Ben: Yes, you're absolutely right. If you were one light year away from the Earth you'd see Earth as it was a year ago.

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    1. Hi..i love this stuff. so hence the need to be able to travel at the speed of light to achieve time travel?

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    2. Traveling close to the speed of light would achieve time travel of sorts...

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  8. hi teakay..to the answer u gave ben..i think if someone travels at the speed of light..he/she would see the same earth for one year even if earth changes..'coz light will also travel with him..if a person travels faster than light..again hypothetical...he/she might be able to see how earth looked earlier..eg..if i travel at twice the speed of light for 1 year..i might be able to see then how earth looked 1 year back...correct me if i'm wrong...

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  9. Hi Cipher,

    My answer was that if you were one light year away from Earth you'd see it as it was a year ago, which is what I interpreted Ben's question to mean: I was looking at journey's end.

    If we look at the journey itself, i.e. travelling at light speed away from the Earth, you wouldn't actually be able to see the Earth at all- as you're travelling at the same speed as the light it won't ever be able to reach your eye. If you were travelling a fraction below the speed of light, the development of the Earth would indeed be slowed down to such a degree that it may well look static to you as an observer.

    Your extrapolation makes sense, but think of it another way: if you travel at twice the speed of light you're travelling twice as fast as the light that you need to reach you in order for you to see anything at all. This means that if you travel faster than the speed of light (this is impossible, by the way), what you would see behind you is... nothing at all.

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  10. wOo.. very interesting post! thnx for the info dude!

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  11. No. The stars we see is relatively close... The ones there are furthest from us is only around 1000 light years away. So the possibility that even a single one of Them is dead by now is very low...

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  12. I still don't quite get it. If im looking at something light-years away im literally looking into the past??? and why is it impossible to travel faster than light? (I assumed it's because light is the most potent unit for measuring time/space)

    If we see a falling star hundreds or thousands of light years away, heading for earth does that mean that we already got hit by the star we can clearly see in the sky that far away hundreds or thousands of years ago? or does that mean we should expect it in the future?

    I'm so confused, i feel like a complete idiot. But thanks for the info, I learned plenty but it only opened my mind into new more complicating questions.

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  13. Hi Karriem!

    Information takes time to travel between places. If you look at your mate across the room, you're not seeing them as they are NOW, you're seeing them as they were a tiny, tiny fraction of a second ago. Over short distances, this makes pretty much no difference, but over thousands of light years of space, it all adds up: If you look at a star that's a thousand light years away, you're seeing it as it was a thousand years ago, not as it is right now!

    If you see a falling star heading for Earth, then the best thing to do is to get out of the way. Falling (or shooting) 'stars' are not actually stars, so they're not hundreds or thousands of light years away- they're bits of rock burning up in our atmosphere.

    Even if a star a thousand light years away was to come towards us, there's no way it could move faster than the light it's giving out- it can't reach us before we've seen it!

    As for your question about why it is impossible to travel faster than light, I'll have a think about it and see if I can write a response suitable for posting on the blog- it's a tricky question that requires knowledge of some quite high-level science!

    Thanks for your comments- feel free to respond or ask more questions if you're interested!

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  14. Just found this, really informative, thanks a lot.

    The idea of being able to see dead stars in the sky fascinates me. It's so haunting but beautiful and strangely profound. Totally mind-blowing.

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  15. No problem, Niels, thanks for commenting!

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  16. Wow, loved this post! Stumbled upon it as I was searching for what different colors of stars meant. I live in the city, but my parents recently moved out into the country...and they suggested I check out the sky at dark. Holy cow, what a sight!!! I was mesmerized by the show in the sky, it looks like millions of stars all bright as could be. Your post was fascinating and you definitely have a way of explaining these complicated things to the regular person, so thanks!!! I think I'm going to go back outside and check the view out again. Breathtaking.

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  17. Wow, Sherry, thanks for the comment! It's always good to get some positive feedback, and some kind of assurance that someone, somewhere is actually reading stuff...

    I have posted about the colour of stars, too: http://blogstronomy.blogspot.co.uk/2012/02/why-are-stars-different-colours.html

    Thanks for visiting! I hope we see you again, and if you have any space-related questions in the future, please come back and ask them!

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  18. Tk, Tks so much for this headmeltingly amazing information! I stumbled upon this article in the most bizzare way. I'm 38 living in Ireland and have spent the last 20 yrs searching for my birth mother. I found her last November and basically things are great. She came to visit last wknd and we sat outside at night sharing a bottle of wine and talking about the stars and how both of us used to wonder if the other was looking at the same stars etc All a bit 'fifel goes west' but hey there was wine:) I said sometimes the stars are already gone.We goggled it and sat reading ur post at midnight in my garden. If some reason fron all the new memories we are creating this one has impacted me the most. I just literally wanted to say tks apologies if u are vomiting from the cheesey sentimentality of this, but hey it was a giod night.x

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    1. Wow, michellesinhell, thanks for sharing that story! It's heartwarming to think I've played even a tiny part in such a wonderful family reunion!

      Please, please, please give my best wishes to your mother, and thank you both for reading!

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  19. I always wondered what the sky would look like with regard to the the positions of the stars, if we could see them as they are right now in relation to the position of the earth. Rather than seeing them in the positions they were in all those years ago...

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  20. I know this may be a dumb question , but i have always wondered . How can we possibly be the only living race in the entire galaxy or other galaxies ?

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    1. It's not a dumb question at all! It's a very big one, though, and I did a whole series of posts on that subject a while ago: http://blogstronomy.blogspot.co.uk/search/label/drake%20equation

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  21. Just found this blog. Well done. In short, questions and doubts here strongly resemble paradoxes we face with super duper ultra high energy particles called oh-my-good particles.

    Still….
    1 “Hi, just a totally hypothetical question, so if one was able to achieve the speed of light and travel away from the earth for 1 year, would looking back at the earth appear to be 1 year in the past?”

    Assuming you accelerate instantly and then travel at the light speed (and not bothering with everything else modern physics says) my answer here is yes. What is interesting, though- you should be seeing much more. I mean, not only yourself at the start of the journey but also…. 1 sec after, 2 sec later, in the middle of the trip, 1 sec ago, 1 millisecond ago etc- a kind of ‘continuum picture of yourself’. That is, the light would follow you from each single moment of your journey virtually freezing continuous picture of yourself. Quantum mechanics somehow suggests this may be not the case in reality but as said- don’t care about the rest of physics here.
    Other point is what would you be seeing ahead of you? This is even more tricky especially bearing in mind all the relativists effects– goggle mentioned oh-my-good particles, if interested.

    2 Next one
    “hi teakay..to the answer u gave ben..i think if someone travels at the speed of light..he/she would see the same earth for one year even if earth changes..'coz light will also travel with him..if a person travels faster than light..again hypothetical...he/she might be able to see how earth looked earlier..eg..if i travel at twice the speed of light for 1 year..i might be able to see then how earth looked 1 year back...correct me if i'm wrong...”

    No sure on this one. It incorporates a rather heavy assumption that nothing changes in principle if you beat the light speed twice. While we lack even basic understanding of hyper speed effects we are pretty sure this assumption is fake. I would rather expect something similar to the visuals one may get while approaching black hole- you should be able to see quite a lot but rather not from the direction you may expect.

    From the article – comment above last photo
    3 The image below shows the results of the Hubble Ultra Deep Field experiment. The Hubble Space Telescope was pointed at a tiny patch of sky that appeared to be empty to any ground-based telescopes, and the shutter left open for just under one million seconds (about 11 days). This is what it saw:

    True. It may be also interesting what we suspect it may not have seen. According to current theories the inflation of expanding universe should prevent us from seeing the farthest (normally the oldest but with exceptions) parts of universe no matter how good telescopes we create eventually. This is not true, SIC!, however, for gravitational effects which seem to be ‘traveling’ faster than light..

    Regards, stassix

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  22. Hey guys, just a quick question: We see the light of a dead planet, we can almost swear we see the planets shape. We actually kinda do. We shouldnt be able to see its shape, when its already dead, but the last light it actually sended to us. But not its shape. Can someone please break that down for me.
    Science always makes me hype and amazed, but i was never a hight minded guy. Iam sorry but my english knowledge doesnt help me go deeper to the matter.

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