<p>When observing starlight and detecting a Doppler shift, it is indeed correct that we cannot directly measure the original wavelength emitted by the star. Instead, what we observe is the shifted wavelength received on Earth. However, by comparing this shifted wavelength to the known properties of specific elements, scientists can infer the presence and magnitude of the Doppler shift, which provides valuable information about the star's motion and speed.</p>
<p>To determine the speed of a star using Doppler shift, scientists rely on the fact that each element has unique spectral lines with well-known wavelengths. These lines act as reference points for comparison. For example, let's consider a star emitting a photon with a wavelength of 500 nm, and upon reaching Earth, it is observed to have a wavelength of 600 nm. From Earth's perspective, it appears as if the star emitted a photon with a wavelength of 600 nm.</p>
<p>To identify the Doppler shift and determine the star's speed, astronomers utilize spectral analysis techniques. They observe the star's light spectrum, which consists of a range of wavelengths, and compare it to the known spectra of various elements. If the spectral lines of a particular element in the star's spectrum are shifted to longer wavelengths (redshift) or shorter wavelengths (blueshift), it indicates that the star is moving away from or towards Earth, respectively.</p>
<p>By measuring the extent of this shift and knowing the rest wavelength of the spectral lines, astronomers can calculate the speed of the star. This is achieved using the formula for Doppler shift:</p>
<p>v = (c * Δλ) / λ</p>
<p>where:</p>
<p>v represents the speed of the star,
c is the speed of light,
Δλ is the observed shift in wavelength (in this case, 100 nm),
λ is the rest wavelength of the spectral line.
Using this formula, astronomers can determine the star's radial velocity, which is the component of its velocity along the line of sight. It is important to note that the Doppler shift alone provides information about the radial velocity but does not convey the direction of motion or the star's transverse velocity.</p>
<p>In summary, while we cannot directly observe the original wavelength emitted by a star, by analyzing the Doppler shift in its spectrum and comparing it to known spectral lines, astronomers can infer the star's speed. This technique has been crucial in understanding the motion and dynamics of celestial objects and has contributed to our knowledge of the universe.</p>
<p>I hope this explanation clarifies the concept of Doppler shift and its application in determining the speed of stars. If you have any further questions, feel free to ask and have a good day!</p>

CC BY-SA 4.0
2023-07-15T10:38:27.737Z
Hello Marx.  Did you see the proposed duplicate? If a question has been asked and answered before it may be best to find another question to answer.  There is quite a lot of this answer that appears to be background information on how to calculate radial velocity from a doppler shift, and isn't directly relevant to the question. While some background is useful, don't feel the need to generate padding for an answer, and you don't need a "I hope I helped you, have a nice day" paragraph.
James K
CC BY-SA 4.0
2023-07-14T12:31:36.923Z
2023-07-14T12:31:36.923Z
Marx
<p>I dont understand how you can look at the light from a star and say that it has been doppler shifted. For example a star emits a photon in the wavelength of 500nm and by the time it reaches earth it has been shifted to 600 nm. How do you know the original wavelength was 500 nm. Would it not from earth just look like the star emitted a photon of 600 nm. so how can you use this to determine the speed.</p>

CC BY-SA 4.0
2023-07-14T11:22:55.333Z
See also https://astronomy.stackexchange.com/questions/12264/how-do-we-know-that-light-is-redshifted-blueshifted-and-not-the-original-light-o https://astronomy.stackexchange.com/questions/11874/how-do-we-know-about-redshift
ProfRob
CC BY-SA 4.0
2023-07-14T13:02:24.703Z
yes this answers the question very well. Should I delete this question now cuz there's one exactly like this
Moiz khokhar
<redshift><doppler-effect>
How does doppler shift work for distant stars
Moiz khokhar
# How does doppler shift work for distant stars

(Asked by: username_0 on 2023-07-14)

I dont understand how you can look at the light from a star and say that it has been doppler shifted. For example a star emits a photon in the wavelength of 500nm and by the time it reaches earth it has been shifted to 600 nm. How do you know the original wavelength was 500 nm. Would it not from earth just look like the star emitted a photon of 600 nm. so how can you use this to determine the speed.

**Tags:** redshift, doppler-effect

> **Comments:**
> **username_1**: See also https://astronomy.stackexchange.com/questions/12264/how-do-we-know-that-light-is-redshifted-blueshifted-and-not-the-original-light-o https://astronomy.stackexchange.com/questions/11874/how-do-we-know-about-redshift  
> **username_0**: yes this answers the question very well. Should I delete this question now cuz there's one exactly like this  

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## 1 answers

### Answer by username_2 on 2023-07-14. Score: 2 | Accepted answer

When observing starlight and detecting a Doppler shift, it is indeed correct that we cannot directly measure the original wavelength emitted by the star. Instead, what we observe is the shifted wavelength received on Earth. However, by comparing this shifted wavelength to the known properties of specific elements, scientists can infer the presence and magnitude of the Doppler shift, which provides valuable information about the star's motion and speed.

To determine the speed of a star using Doppler shift, scientists rely on the fact that each element has unique spectral lines with well-known wavelengths. These lines act as reference points for comparison. For example, let's consider a star emitting a photon with a wavelength of 500 nm, and upon reaching Earth, it is observed to have a wavelength of 600 nm. From Earth's perspective, it appears as if the star emitted a photon with a wavelength of 600 nm.

To identify the Doppler shift and determine the star's speed, astronomers utilize spectral analysis techniques. They observe the star's light spectrum, which consists of a range of wavelengths, and compare it to the known spectra of various elements. If the spectral lines of a particular element in the star's spectrum are shifted to longer wavelengths (redshift) or shorter wavelengths (blueshift), it indicates that the star is moving away from or towards Earth, respectively.

By measuring the extent of this shift and knowing the rest wavelength of the spectral lines, astronomers can calculate the speed of the star. This is achieved using the formula for Doppler shift:

v = (c \* Δλ) / λ

where:

v represents the speed of the star,
c is the speed of light,
Δλ is the observed shift in wavelength (in this case, 100 nm),
λ is the rest wavelength of the spectral line.
Using this formula, astronomers can determine the star's radial velocity, which is the component of its velocity along the line of sight. It is important to note that the Doppler shift alone provides information about the radial velocity but does not convey the direction of motion or the star's transverse velocity.

In summary, while we cannot directly observe the original wavelength emitted by a star, by analyzing the Doppler shift in its spectrum and comparing it to known spectral lines, astronomers can infer the star's speed. This technique has been crucial in understanding the motion and dynamics of celestial objects and has contributed to our knowledge of the universe.

I hope this explanation clarifies the concept of Doppler shift and its application in determining the speed of stars. If you have any further questions, feel free to ask and have a good day!

> **Comments:**
> **username_3**: Hello username_2.  Did you see the proposed duplicate? If a question has been asked and answered before it may be best to find another question to answer.  There is quite a lot of this answer that appears to be background information on how to calculate radial velocity from a doppler shift, and isn't directly relevant to the question. While some background is useful, don't feel the need to generate padding for an answer, and you don't need a "I hope I helped you, have a nice day" paragraph.  

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