Hi Guys, would anyone be able to enlighten me?
I have for years being fascinated about the spectrum of objects in the sky.
Unfortunately i do not take chemistry in sec sch and to makes things worse i think i can`t really find someone to teach me about them. Wonder if any kind soul would help??
pls teach me to understand the following graph:
http://cas.sdss.org/astro/en/tools/expl ... 7266371612
Many thanx in advance
--yongxing
Pls enlighten me
Hi iori86 the "graph" you see is a relatively low resolution spectroscopic analysis of a diffused light source which in this case is a spiral galaxy.The horizontal axis is the measured wavelength in angstrom (10^-10 m) and the vertical axis represents the flux density(intensity) of the signal from the particular wavelength. Located at the bottom of the diagram, there is also a labelled z value of 0.0583. this represents the redshift, and in this case is a considerable low value, implying that the observed galaxy is probably at close proximity to us.
The first obvious feature of the diagram is the H-alpha line. H-alpha is the first transition of the balmer series (n=3->n=2) corresponding to a wavelength of 6562.81 . It is normally the peak line due to the large presence of ionised hydrogen and also due to the fact that n>3 transitions would more frequently result in complete ionisation, and thereafter returning to the ground state. However if u look at the graph closely you will realise the H-alpha line is not at 6562.81nm but rather at a slightly higher (approx. 6900nm). This is due to the redshift of the galaxy as previously mentioned. plug in the z value into this formula z+1=(wavelength observed) / (wavelength emitted) and u will find that they coincide.
Another Feature that are present are the metallic lines. In particular O(I),O(II), Si(II) and Ca(II). O(II) ionization occurs at a much higher temperature/energy in the cores of stars, normally only possible in v v hot O stars. whereas single O(I) ionisation occurs in B stars.However due to poor resolution, can't really tell which is from which. But whatever it is there is evidence of a considerable presence of ionised oxygen, indicating a presence of large MS giants. The Mg(II) Ca(II) and Si(II) belong to cooler stars from F-G as they do not reach core temperature required to consume these elements during MS phase.(as in lithium with brown dwarfs.)
There are also many other intrestring features that can be seen if the scale and resolution of the spectra improves. Most notably the lyman alpha forest around the 1000 angstrom region which can be used to determine intergalatic medium. Hope this helps as an introduction.
The first obvious feature of the diagram is the H-alpha line. H-alpha is the first transition of the balmer series (n=3->n=2) corresponding to a wavelength of 6562.81 . It is normally the peak line due to the large presence of ionised hydrogen and also due to the fact that n>3 transitions would more frequently result in complete ionisation, and thereafter returning to the ground state. However if u look at the graph closely you will realise the H-alpha line is not at 6562.81nm but rather at a slightly higher (approx. 6900nm). This is due to the redshift of the galaxy as previously mentioned. plug in the z value into this formula z+1=(wavelength observed) / (wavelength emitted) and u will find that they coincide.
Another Feature that are present are the metallic lines. In particular O(I),O(II), Si(II) and Ca(II). O(II) ionization occurs at a much higher temperature/energy in the cores of stars, normally only possible in v v hot O stars. whereas single O(I) ionisation occurs in B stars.However due to poor resolution, can't really tell which is from which. But whatever it is there is evidence of a considerable presence of ionised oxygen, indicating a presence of large MS giants. The Mg(II) Ca(II) and Si(II) belong to cooler stars from F-G as they do not reach core temperature required to consume these elements during MS phase.(as in lithium with brown dwarfs.)
There are also many other intrestring features that can be seen if the scale and resolution of the spectra improves. Most notably the lyman alpha forest around the 1000 angstrom region which can be used to determine intergalatic medium. Hope this helps as an introduction.
Hi, new here.
If you find that spectrums are so fascinating, you should begin with stellar spectrums rather than galactic spectrums actually. A galactic spectrum is much more confusing than a stellar spectrum.
One main purpose of spectral lines in astrophysics is to find out the chemical/ nuclear composition and the relative abundance of a celestial object. This gives us information on the surface temperature of the star, which is one piece of information required to understand the life cycle of the star.
In contrast, the galactic spectral is relatively ill-understood yet. Apart from the relative composition of the stars in a galaxy, and the recessional velocity of the galaxy, the other informations are still not well understood.
If you find that spectrums are so fascinating, you should begin with stellar spectrums rather than galactic spectrums actually. A galactic spectrum is much more confusing than a stellar spectrum.
One main purpose of spectral lines in astrophysics is to find out the chemical/ nuclear composition and the relative abundance of a celestial object. This gives us information on the surface temperature of the star, which is one piece of information required to understand the life cycle of the star.
In contrast, the galactic spectral is relatively ill-understood yet. Apart from the relative composition of the stars in a galaxy, and the recessional velocity of the galaxy, the other informations are still not well understood.
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Clifford60
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- Joined: Mon Sep 04, 2006 8:41 pm
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What do you need the individual flux densities for? The SRSS does not seem to publish the actual flux densities, but when you click on specline at the navigating bar at the left, it should provide the necessary information that you need.
The 10th column at the table gives the gaussian height of the absorption or emission line. If the value is positive, it gives an emission line. If the value is negative, it is an absorption line. Do you have particular use for the flux densities?
The 10th column at the table gives the gaussian height of the absorption or emission line. If the value is positive, it gives an emission line. If the value is negative, it is an absorption line. Do you have particular use for the flux densities?