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Thread: dead stars shining

  1. #1 dead stars shining 
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    is it true? that some of the stars that we see don't exist anymore because by the time their light reaches Earth they cease to exist? ... and if so, how can there be light if the source for that light is no more? ...


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  3. #2  
    Universal Mind John Galt's Avatar
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    That is correct. Light is the fastest thing in the Universe - around 300,000 km / second - but the Universe is huge.
    Stars live anything from a few millions of years to many billions. The main factor controlling this is their mass. Large stars have the shortest lifes, because they 'burn up' their nuclear fuels much faster than the small ones.
    The galaxy we are located in is 100,000 light years across, so several of the stars that can be seen will actually have ceased to exist by the time their light reaches us. When we go beyond the galaxy and look at other galaxies billions of light years away you can imagine that most of them will have stopped shining a long time ago.
    The light is emitted by the stars as photons. The photons are independent of the star once they are emitted. It doesn't matter what happens to the star, those photons just keep on travelling for millions and billions of years until they strike the back of your eye, or a photographic plate, or just about anything else.


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    wow... pretty cool :-D ... are there any ways to know which start from the sky is 'dead' and which one is still burning?
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    Forum Junior Cuete's Avatar
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    There's no straight way to know if a star isn't there any more, because the only information we have available is the one that gets here as light or radio waves (which takes a while to get here, as Ophiolite wrote).

    But we've got some clues or indirect ways to know:
    By measuring the intensity, colour and x-ray emissions of a star we can know if it's a young or an old star what we are observing, based on that calculate how much lilfe time it has left...

    For example if you see a supernova (like the one you have on your avatar) probably that star exploded some time ago... and it's likely to be or about to be gone.
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    thank you for making time to explain that to me :-D
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    Forum Junior Cuete's Avatar
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    Quote Originally Posted by F (i L i) P
    thank you for making time to explain that to me :-D


    You are welcome. :wink:
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  8. #7  
    Universal Mind John Galt's Avatar
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    F(iLi)p, you may also find this a useful summary.

    The ages of stars are determined by knowing their current surface temperatures, luminosities and masses. The masses allow astronomers to compute from the Theory of Stellar Evolution, how rapidly the star can evolve in temperature and luminosity as it burns various fusion 'fuels' in its core. The current luminosity and temperature then pinpoints a particular evolutionary stage in these models. The age of the star is then estimated from the theoretical evolutionary models appropriate to the star's mass. A second important parameter in this calculation is the 'metalicity' of the star which is simply how abundant the elements heavier than helium are in the star relative to hydrogen. Other affects that influence the stellar age calculations are the detailed mechanisms occurring within the star to transport energy from the core to the surface (convection and radiative transport) and the degree of rotation of the star. If this process seems a bit arbitrary to you, you need to understand that the evolution of a star is determined by two processes; gravity and thermonuclear fusion. These are the two, best known and best understood, areas of physics and so stellar evolution is based on very firm experimental foundations drawn from a variety of areas in physics. Debates over the details of a particular star's evolution arise mainly from a lack of detailed knowledge about the element abundances in the core of the star, and the precise mass of the star.

    Source: http://www.astronomycafe.net/qadir/q1591.html

    You might find it useful to google for Hertzsprung-Russell diagram. This is a very useful concept in astrophysics that helped lead to the current understandings of stellar evolution. If you plot luminosity against surface temperature you find that most stars group in certain areas of the diagram. We find that the mass of the star, and to a lesser extent its metallicity and helium content, determine its brightness, its surface temperature, its expected life span, the kinds of nuclear reactions it experiences, the character of its death, etc.
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    Forum Radioactive Isotope mitchellmckain's Avatar
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    I should like to point out that most stars end their life in a cooling white dwarf which still hot like a star and still shines like a star and takes a very very long time to cool down. However because of the greatly decreased diameter the luminosity is very greatly decreased and as a result may be no longer be detectable at its distance. Many stars radically change their luminosity over many different time scales, disappearing and reappearing in this sense all the time.
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  10. #9  
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    thanx
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    Lets look at this with a simpler analogy - I'm standing 100yds away from a man, and shoot him just as he shoots me, say some fractions of a second later. He's dead as soon as my bullet hits him in the head, but his bullet is still on its way to me. I get shot by a guy who was dead when I was hit.

    In the same way, the light has to travel a distance and take a time to do it, but the light is still on its way to us even as the source of the light has died.
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    nice example... thank you
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    Just wondering. There are said to be 10^23 stars in the universe. How accurate is this? Is there a chance they counted some of the dead stars?
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    Forum Sophomore NimaRahnemoon's Avatar
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    :? I don't think that they counted it. I think you are confused with a mol. A mol is a representation of molecules and atoms. In 1 mol there is 6.02x10^23 particles.

    "How accurate is this?"
    I don't know... I have not gotten the chance to count them :P
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    I don't understand. One particle equal one star. Is this correct? Also, how did they get the number?
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    Forum Junior Cuete's Avatar
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    Maybe you'll find this helpful:

    For the Universe, the galaxies are our small representative volumes, and there are something like 10^11 to 10^12 stars in our galaxy, and there are perhaps something like 10^11 or 10^12 galaxies.

    With this simple calculation you get something like 10^22 to 10^24 stars in the Universe. This is only a rough number, as obviously not all galaxies are the same, just like on a beach the depth of sand will not be the same in different places.

    No one would try to count stars individually, instead we measure integrated quantitites like the number and luminosity of galaxies.
    ESA's infrared space observatory Herschel, to be launched in 2007, will make an important contribution by 'counting' galaxies in the infrared, and measuring their luminosity in this range - something never before attempted.

    Knowing how fast stars form can bring more certainty to calculations. Herschel will also chart the 'formation rate' of stars throughout cosmic history. If you can estimate the rate at which stars have formed, you will be able to estimate how many stars there are in the Universe today.


    Reference: http://www.esa.int/esaSC/SEM75BS1VED_index_0.html
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    Forum Sophomore NimaRahnemoon's Avatar
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    Uhh, I think you misunderstood, I'm not saying that 1 particle is equal to one star, but I was saying that it was a coincidence that there is a mol of stars, and that the source may have been just a theory that wanted to say that there is a mol of stars as a representation.

    The number is avogadro's number. He made it because you can't really hold iron and say that there is exactly 10^23 atoms of iron. He made the unit "mole" (abreviated mol) which corresponds to the atomic mass. So for iron 1 mol of iron is equal to 55.847 grams. Avogadro set his number to the number of particles in 12 grams of Carbon-12.

    That's besides the point though... "There are said to be 10^23 stars in the universe.", who said that? I figure that they were using it as a representation and comparing it to one mol of a substance.
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    http://www.cnn.com/2003/TECH/space/07/22/stars.survey/

    How did they get this number? Is it possible they included dead stars? Can they be off by 50% of the total.
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    Quote Originally Posted by starlight
    http://www.cnn.com/2003/TECH/space/07/22/stars.survey/

    How did they get this number? Is it possible they included dead stars? Can they be off by 50% of the total.
    This si why the Herschel misison willb e itneresting... as we will get a clue on how fast are created new stars; we know how fast they die, but the creation pace is still largely undetermined, so we can count "dead" stars whose shine we still see but we already can't estimate "new born" stars whose bright we don't see already...

    This is why we estimate in galaxy-packs as the gravity of a galaxy (and thus its mass) it's easier to determine than the actual number of stars.
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  20. #19  
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    Sorry, I missed your post Cuete. Thanks everyone!
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  21. #20  
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    Well, what is a dead star? I'm not sure why you're trying to make a distinction between a "dead" star and one that is shining. Stars are merely gravity-created collections of matter, which due to the constraints of the laws of physics when the pressure and temperature get high enough, happen to initiate a fusion reaction and shine. If they're not shining now, so that they're "ex-stars", they can still be the centres for solar systems. So the number of "stars" based on mass is still a pretty valid figure for a number of stars - as a useful unit of astronomical measurement - as opposed to the number of "shining things", which may not be as useful, astronomically.

    From that star survey story, we find scientific inexactitudes that would make you laugh if they didn't make you cry.
    He said there were likely many million more stars in the universe but the 70 sextillion figure was the number visible within range of modern telescopes.

    The actual number of stars could be infinite he said.
    "Many" million more stars than 70 sextillion is .... well, a tiny tiny tiny fraction! To whit, 1/0.00000000000000001 as much. What kind of idiot still uses "million" to indicate "a lot" when you're dealing with 10<sup>22</sup> stars! I trust that it was the idiot writer, and not the supposed credible astronomer that said "many million more stars".
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    Forum Masters Degree invert_nexus's Avatar
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    What always gives me pause is that we can have no certainty that Sol, our own sun, is still shining at this very moment.

    It takes approximately 8 and 1/2 minutes for light from the sun to reach the earth. Therefore, if Sol should happen to explode, it would be at least 8 1/2 minutes before the radiation from said explosion would reach us. The shock wave would take even longer.

    Anyone have any kind of figures on how long a shock wave might take from various sun-ending events?
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    What happens to a star when it stops shinning? I always thought it breaks in pieces.
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    Forum Masters Degree invert_nexus's Avatar
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    Depends on the particular event that causes the star to 'stop shining'.

    Actually, barring any sort of unnatural event, it pretty much depends on the mass of the star.

    Small stars dwindle down to a black dwarf.

    Stars the size of our sun go Nova and shed their outer layers (after some time as a red giant) to form planetary nebulas. The core remains as a white dwarf (which shines faintly for a time) and eventually fades to a black dwarf.

    Bigger stars go supernova and either leave behind a neutron star or a black hole. Neither shed light (I don't think a neutron star does...) but both can shoot out lots of radiation.

    I'm sure other members can go into more detail. I only dabble in this subject.
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    it is correct. if we are, say, 20 billion light-years away from a star, the question is in the answer. we will see light from a 20 billion-year dead star. we are seeing 20 billion-year old light. The question is: “if the universe is infinite, why don’t we see a totally bright sky?” the answer in earlier times would have been dust blocks the light. That has been overruled in earlier times because eventually the dust would heat up and shine as bright as the star itself. So the flaw comes in the more recent theory. People say that the star light hasn’t reached us yet, because the stars are billions and trillions of light years away. This leaves people perplexed, because all matter came from the big bang, and we would just see an earlier version of the star. But, the reason this theory may be true is because the big bang merely put out mass and not developed stars and planets. So a cloud of matter comes, and once billions of light-years away, light will only reach us later, so we don’t have totally bright skies YET. You may think the star would burn out before the light got here, and it probably would, but the light would still be on its way, and therefore we would see stars light long after it burned out.
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  26. #25  
    Universal Mind John Galt's Avatar
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    The Big Bang resolves Olber's Paradox because it involves an expanding Universe.
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