# Determining the radius of a star

#### VordLoldemort

How can I find the radius of star if I know only the following information:
-It's pulsation period is 9.83 days
-It's contiuum spectra peaks at 490 nm
-The aboslute magnitude of the Sun is 4.83
-The temperature of the Sun is 5780 K

Thank you!

#### Geomartian

You can’t. The properties of a star changes according to the neighborhood (mass density of space, which equates to the rate of time). That is why Black Holes are not found out here in the galactic arms. They blow up since their atomic/electric forces of repulsion increase with the time rate.

The repulsive forces within a star changes with the space you are in. A nice well-behaved star near the galactic center might turn into a red giant out in the galactic arms.

You also need the space-time parameters for the position of the star in order to lessen the variables.

Have you tried to measure the star by counting the number of steps?

#### Helio

How can I find the radius of star if I know only the following information:
-It's pulsation period is 9.83 days
-It's contiuum spectra peaks at 490 nm
-The aboslute magnitude of the Sun is 4.83
-The temperature of the Sun is 5780 K

Thank you!
Is this a homework question? Sounds rather specific.

The 490nm peak will reveal its temperature, or very close to it. [see Wien's Displacement Law.] This alone might give you the size if it's a main sequence star. But you mention the pulsation rate. That is a clue it might be a Cepheid class star and the pulsation rate alone should lead you to the radius, IIRC.

#### VordLoldemort

Is this a homework question? Sounds rather specific.

The 490nm peak will reveal its temperature, or very close to it. [see Wien's Displacement Law.] This alone might give you the size if it's a main sequence star. But you mention the pulsation rate. That is a clue it might be a Cepheid class star and the pulsation rate alone should lead you to the radius, IIRC.

Thank you for your reply.
I believe it is a Cepheid variable, yes. How does the pulsation rate lead to the radius?

Thanks

#### VordLoldemort

You can’t. The properties of a star changes according to the neighborhood (mass density of space, which equates to the rate of time). That is why Black Holes are not found out here in the galactic arms. They blow up since their atomic/electric forces of repulsion increase with the time rate.

The repulsive forces within a star changes with the space you are in. A nice well-behaved star near the galactic center might turn into a red giant out in the galactic arms.

You also need the space-time parameters for the position of the star in order to lessen the variables.

Have you tried to measure the star by counting the number of steps?

Thank you for your reply.

What do you mean by number of steps?

Thanks

#### rod

"-It's pulsation period is 9.83 days"

There are specific Cepheids reported now this period. https://ui.adsabs.harvard.edu/abs/2018MNRAS.478.3825L/abstract, "As part of a wider investigation of evolved massive stars in Galactic open clusters, we have spectroscopically identified three candidate classical Cepheids in the little-studied clusters Berkeley 51, Berkeley 55, and NGC 6603. Using new multi-epoch photometry, we confirm that Be 51 #162 and Be 55 #107 are bona fide Cepheids, with pulsation periods of 9.83 ± 0.01 d and 5.850 ± 0.005 d respectively, while NGC 6603 star W2249 does not show significant photometric variability."

wikipedia has an article on Cepheid pulsation model algebra used.

Here is a report too, http://www.astronomy.ohio-state.edu/~ryden/ast162_4/notes16.html, PULSATING STARS

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