# The contraction and the expansion of the star while moving

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#1
Hi!

I was doing some A-level Physics Unit-5 past papers. Now I'm stuck at this question :

Here is the mark scheme:
( https://www.thestudentroom.co.uk/att...hmentid=966938 )

The first point of the answer says ''When star contracts (front of) star is moving away from observer'' I did not understand this.

I have read the Textbook but could not find anything about the ''contraction and the expansion of the star as the
mechanism for the movement of the source''

I also searched the internet but could not find any useful information related to it.

Any help with a logical explanation would be highly appreciated :-)
Last edited by tahmidbro; 1 month ago
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1 month ago
#2
Cepheids are pulsating variables - they change in size.

The centre of mass of the star is (for most stars*) always moving away from us... but the light we can see from a star is the light emitted at the surface and in a pulsating star the surface is changing it's distance from the centre of mass of the star as the star expands and contracts in size... so you'd expect to see a cyclic change in the Doppler shift of the light coming from a pulsating star that corresponded to the period of it's magnitude cycle. It'd be additional to the constant** Doppler shift due to the radial velocity of the stars centre of mass relative to earth.

*some are coming towards us e.g. stars in the Andromeda galaxy

** certainly near enough over the short periods at which cepheids cycle.

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Last edited by Joinedup; 1 month ago
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#3
(Original post by Joinedup)
Cepheids are pulsating variables - they change in size.

The centre of mass of the star is (for most stars*) always moving away from us... but the light we can see from a star is the light emitted at the surface and in a pulsating star the surface is changing it's distance from the centre of mass of the star as the star expands and contracts in size... so you'd expect to see a cyclic change in the Doppler shift of the light coming from a pulsating star that corresponded to the period of it's magnitude cycle. It'd be additional to the constant** Doppler shift due to the radial velocity of the stars centre of mass relative to earth.

*some are coming towards us e.g. stars in the Andromeda galaxy

** certainly near enough over the short periods at which cepheids cycle.

---
why will it particularly 'contract' if the star moves away from us? ( according to the mark scheme)
Last edited by tahmidbro; 1 month ago
0
1 month ago
#4
You sound like you've got cause and effect swapped
The star isn't expanding and contracting because the star is getting nearer and further away

It's that the surface of the star is moving towards us when the star expands and the surface of the star is moving away from us when the star contracts

The surface of the star is the only part of the star that is sending photons out

Due to the Doppler effect those photons will be blue shifted when the star is expanding and red shifted when the star is contracting
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#5
(Original post by Joinedup)
You sound like you've got cause and effect swapped
The star isn't expanding and contracting because the star is getting nearer and further away

It's that the surface of the star is moving towards us when the star expands and the surface of the star is moving away from us when the star contracts

The surface of the star is the only part of the star that is sending photons out

Due to the Doppler effect those photons will be blue shifted when the star is expanding and red shifted when the star is contracting
I've got it. Thank you very much !
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