How do we know if an object is redshifted?












4












$begingroup$


I have some questions about redshift. I think that the first two hang on the last two, but feel free to point out anything you think I should know.



How do we know if an object is actually redshifted, and that we aren't just seeing the regular spectrum of the star?



How can we tell the extent to which a star is redshifted?



Do astronomers have a full baseline spectrum that they compare the measured spectrum against? Or do they just use the hydrogen spectrum since all stars are mostly hydrogen?






the-sun distances redshift







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    4












    $begingroup$


    I have some questions about redshift. I think that the first two hang on the last two, but feel free to point out anything you think I should know.



    How do we know if an object is actually redshifted, and that we aren't just seeing the regular spectrum of the star?



    How can we tell the extent to which a star is redshifted?



    Do astronomers have a full baseline spectrum that they compare the measured spectrum against? Or do they just use the hydrogen spectrum since all stars are mostly hydrogen?






    the-sun distances redshift







    share|improve this question







    New contributor




    Gimcrack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
    Check out our Code of Conduct.







    $endgroup$















      4












      4








      4





      $begingroup$


      I have some questions about redshift. I think that the first two hang on the last two, but feel free to point out anything you think I should know.



      How do we know if an object is actually redshifted, and that we aren't just seeing the regular spectrum of the star?



      How can we tell the extent to which a star is redshifted?



      Do astronomers have a full baseline spectrum that they compare the measured spectrum against? Or do they just use the hydrogen spectrum since all stars are mostly hydrogen?






      the-sun distances redshift







      share|improve this question







      New contributor




      Gimcrack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.







      $endgroup$




      I have some questions about redshift. I think that the first two hang on the last two, but feel free to point out anything you think I should know.



      How do we know if an object is actually redshifted, and that we aren't just seeing the regular spectrum of the star?



      How can we tell the extent to which a star is redshifted?



      Do astronomers have a full baseline spectrum that they compare the measured spectrum against? Or do they just use the hydrogen spectrum since all stars are mostly hydrogen?






      the-sun distances redshift




      the-sun distances redshift






      share|improve this question







      New contributor




      Gimcrack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.











      share|improve this question







      New contributor




      Gimcrack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.









      share|improve this question




      share|improve this question






      New contributor




      Gimcrack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.









      asked 9 hours ago









      GimcrackGimcrack

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      211




      New contributor




      Gimcrack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.





      New contributor





      Gimcrack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






      Gimcrack is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






















          1 Answer
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          $begingroup$

          There are two methods, one more reliable than the other (though both are pretty good.)



          Key point: The brighter a star is, the more detail we can see in its spectrum -- you can think of it as being able to magnify the spectrum more so as to be able to see finer details. This also allows us to see fainter lines (not all spectral lines are equally intense.)



          We have very extensive reference spectra of all the elements under a wide variety of conditions, so when a star is bright enough we see many spectral lines and can match them up to reference spectra. No two elements have similar spectra, so if you can see many lines, you can't easily confuse one for another.



          When you can account for nearly all of the lines in a star's spectrum by a plausible set of elements in plausible abundances and all with the same red shift, you have a very, very reliable match and accurately know the object's red shift.



          But distant stars (and galaxies) are very faint and so the spectra we get are very low resolution and show only a few of the strongest spectral lines. They can frequently be matched because hydrogen is so common that the strongest lines seen will be hydrogen lines and even if you can only spot two or three of them, if they bear the same relative positions as bright hydrogen lines, you can safely identify them and read off the red shift. This is less reliable, but still reliable enough that it rarely is a problem.



          In the very dimmest cases you may only see a single line. You have a very rough idea of the redshift from the object's brightness and all you can do is assume it's the strongest hydrogen line and see if it's in roughly the right position given the distance deduced from the object's brightness and type. This works too, but is still less reliable. (Much of Hubble's original work relied on just a single line for the more distance objects, but proved to be correct when observed later with dramatically better equipment.)






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            1 Answer
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            1 Answer
            1






            active

            oldest

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            active

            oldest

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            active

            oldest

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            5












            $begingroup$

            There are two methods, one more reliable than the other (though both are pretty good.)



            Key point: The brighter a star is, the more detail we can see in its spectrum -- you can think of it as being able to magnify the spectrum more so as to be able to see finer details. This also allows us to see fainter lines (not all spectral lines are equally intense.)



            We have very extensive reference spectra of all the elements under a wide variety of conditions, so when a star is bright enough we see many spectral lines and can match them up to reference spectra. No two elements have similar spectra, so if you can see many lines, you can't easily confuse one for another.



            When you can account for nearly all of the lines in a star's spectrum by a plausible set of elements in plausible abundances and all with the same red shift, you have a very, very reliable match and accurately know the object's red shift.



            But distant stars (and galaxies) are very faint and so the spectra we get are very low resolution and show only a few of the strongest spectral lines. They can frequently be matched because hydrogen is so common that the strongest lines seen will be hydrogen lines and even if you can only spot two or three of them, if they bear the same relative positions as bright hydrogen lines, you can safely identify them and read off the red shift. This is less reliable, but still reliable enough that it rarely is a problem.



            In the very dimmest cases you may only see a single line. You have a very rough idea of the redshift from the object's brightness and all you can do is assume it's the strongest hydrogen line and see if it's in roughly the right position given the distance deduced from the object's brightness and type. This works too, but is still less reliable. (Much of Hubble's original work relied on just a single line for the more distance objects, but proved to be correct when observed later with dramatically better equipment.)






            share|improve this answer









            $endgroup$


















              5












              $begingroup$

              There are two methods, one more reliable than the other (though both are pretty good.)



              Key point: The brighter a star is, the more detail we can see in its spectrum -- you can think of it as being able to magnify the spectrum more so as to be able to see finer details. This also allows us to see fainter lines (not all spectral lines are equally intense.)



              We have very extensive reference spectra of all the elements under a wide variety of conditions, so when a star is bright enough we see many spectral lines and can match them up to reference spectra. No two elements have similar spectra, so if you can see many lines, you can't easily confuse one for another.



              When you can account for nearly all of the lines in a star's spectrum by a plausible set of elements in plausible abundances and all with the same red shift, you have a very, very reliable match and accurately know the object's red shift.



              But distant stars (and galaxies) are very faint and so the spectra we get are very low resolution and show only a few of the strongest spectral lines. They can frequently be matched because hydrogen is so common that the strongest lines seen will be hydrogen lines and even if you can only spot two or three of them, if they bear the same relative positions as bright hydrogen lines, you can safely identify them and read off the red shift. This is less reliable, but still reliable enough that it rarely is a problem.



              In the very dimmest cases you may only see a single line. You have a very rough idea of the redshift from the object's brightness and all you can do is assume it's the strongest hydrogen line and see if it's in roughly the right position given the distance deduced from the object's brightness and type. This works too, but is still less reliable. (Much of Hubble's original work relied on just a single line for the more distance objects, but proved to be correct when observed later with dramatically better equipment.)






              share|improve this answer









              $endgroup$
















                5












                5








                5





                $begingroup$

                There are two methods, one more reliable than the other (though both are pretty good.)



                Key point: The brighter a star is, the more detail we can see in its spectrum -- you can think of it as being able to magnify the spectrum more so as to be able to see finer details. This also allows us to see fainter lines (not all spectral lines are equally intense.)



                We have very extensive reference spectra of all the elements under a wide variety of conditions, so when a star is bright enough we see many spectral lines and can match them up to reference spectra. No two elements have similar spectra, so if you can see many lines, you can't easily confuse one for another.



                When you can account for nearly all of the lines in a star's spectrum by a plausible set of elements in plausible abundances and all with the same red shift, you have a very, very reliable match and accurately know the object's red shift.



                But distant stars (and galaxies) are very faint and so the spectra we get are very low resolution and show only a few of the strongest spectral lines. They can frequently be matched because hydrogen is so common that the strongest lines seen will be hydrogen lines and even if you can only spot two or three of them, if they bear the same relative positions as bright hydrogen lines, you can safely identify them and read off the red shift. This is less reliable, but still reliable enough that it rarely is a problem.



                In the very dimmest cases you may only see a single line. You have a very rough idea of the redshift from the object's brightness and all you can do is assume it's the strongest hydrogen line and see if it's in roughly the right position given the distance deduced from the object's brightness and type. This works too, but is still less reliable. (Much of Hubble's original work relied on just a single line for the more distance objects, but proved to be correct when observed later with dramatically better equipment.)






                share|improve this answer









                $endgroup$



                There are two methods, one more reliable than the other (though both are pretty good.)



                Key point: The brighter a star is, the more detail we can see in its spectrum -- you can think of it as being able to magnify the spectrum more so as to be able to see finer details. This also allows us to see fainter lines (not all spectral lines are equally intense.)



                We have very extensive reference spectra of all the elements under a wide variety of conditions, so when a star is bright enough we see many spectral lines and can match them up to reference spectra. No two elements have similar spectra, so if you can see many lines, you can't easily confuse one for another.



                When you can account for nearly all of the lines in a star's spectrum by a plausible set of elements in plausible abundances and all with the same red shift, you have a very, very reliable match and accurately know the object's red shift.



                But distant stars (and galaxies) are very faint and so the spectra we get are very low resolution and show only a few of the strongest spectral lines. They can frequently be matched because hydrogen is so common that the strongest lines seen will be hydrogen lines and even if you can only spot two or three of them, if they bear the same relative positions as bright hydrogen lines, you can safely identify them and read off the red shift. This is less reliable, but still reliable enough that it rarely is a problem.



                In the very dimmest cases you may only see a single line. You have a very rough idea of the redshift from the object's brightness and all you can do is assume it's the strongest hydrogen line and see if it's in roughly the right position given the distance deduced from the object's brightness and type. This works too, but is still less reliable. (Much of Hubble's original work relied on just a single line for the more distance objects, but proved to be correct when observed later with dramatically better equipment.)







                share|improve this answer












                share|improve this answer



                share|improve this answer










                answered 9 hours ago









                Mark OlsonMark Olson

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