spectography and red shift

[Brainee]

When space body travels far it red shifts, how can one then see what it is made of using spectgraphy?

[exchemist]

14 minutes ago, Brainee said:

When space body travels far it red shifts, how can one then see what it is made of using spectgraphy?

Because the lines in the spectrum, which are characteristic of the elements of which it is composed, are all still there, just moved along a bit, to lower frequency. So you get the same pattern of lines, which can easily be recognised by a spectroscopist. Once you have found one element and determined from that how big the red shift is for that object, you then immediately know by how much all the lines for other elements will be shifted, and so you can assign them to the elements responsible.

P.S. One thing you may possibly not be aware of is that each element emits (or absorbs) not just a single line but a whole series, corresponding to electrons making transitions between different atomic orbitals in the atoms of that element. So it's a whole pattern you are looking for, not just a single line. I agree that if it were only a single line per element, you would not be able to do it. 

 

Edited May 9, 2023 by exchemist

[Brainee]

Sorry, I meant when light from distant body red shifts, how can use spectrography to determine what its made of?

[exchemist]

4 minutes ago, Brainee said:

Sorry, I meant when light from distant body red shifts, how can use spectrography to determine what its made of?

That's what I'm talking about. Here is an article with a picture of some of the lines from hydrogen: https://en.wikipedia.org/wiki/Balmer_series 

 

 

The "visible" hydrogen emission spectrum lines in the Balmer series. H-alpha is the red line at the right. Four lines (counting from the right) are formally in the visible range. Lines five and six can be seen with the naked eye, but are considered to be ultraviolet as they have wavelengths less than 400 nm.

Now, if the star emitting these lines is red shifted, the whole set of these lines will be moved a bit from their normal positions towards the red (lower frequency, longer wavelength) end of the spectrum. But they are all still there and so is the spacing between them. So you can still identify that it is hydrogen emission you are looking at.  

[Brainee]

How do you know how much light has redshifted?

[StringJunky]

1 hour ago, Brainee said:

How do you know how much light has redshifted?

I think they use the emission spectra from a type 1A Supernova as a standard candle.

https://en.wikipedia.org/wiki/Type_Ia_supernova

Edited May 9, 2023 by StringJunky

[Mordred]

You need to understand the process producing the light to get a handle on the initial frequencies of light that should be produced. Hydrogen for example has extremely well understood spectral lines. Standard candles as StringJunky mentioned are also used. Any well understood process can serve as a standard candle.

 Secondly we don't rely strictly on redshift alone to determine distance or motion. Other methods include parallax, in several forms as well as luminosity distance. Though the latter is also subjective to redshift. The luminosity of a star depends primarily on its composition and mass.

 

[exchemist]

7 hours ago, Brainee said:

How do you know how much light has redshifted?

The spectrometer tells you the wavelength or frequency of each line. So you just compare the reading for hydrogen in the star with the standard reading for the same spectral line in a lab here on Earth.

[exchemist]

Here is an example I found on line to illustrate how it is done:

"Once you have process the spectrum with software like Demetra, ISIS or Vspec, it must look like the one below (case of a galaxy with a redshift z = 0.06). The noise level of the spectrum depends on the total exposure time on the target.

We obtain a spectrum strongly shifted in the red with easily identifiable lines and very broad Balmer lines : H Alpha line is thus shifted to red at 6970 Å while its value “at rest” is 6563 Å. "

 

So this is saying this Hα line in the hydrogen spectrum is found at a wavelength of 6563 Angstroms if measured in the lab here on Earth. But with this distant object the same line appears at a wavelength of 6970. It has been shifted towards the red by 6970-6563 = 407 Angstroms.   

Edited May 10, 2023 by exchemist

[Brainee]

what is a standard candle?

[pzkpfw]

12 minutes ago, Brainee said:

what is a standard candle?

See the link from StringJunky: https://en.wikipedia.org/wiki/Cosmic_distance_ladder#Standard_candles