How would I find data on the Luminosity of Cepheid variables?

[phyzguy]

Seems I had a error in the distance calculations. For that I used the distance modulus to calculate the distances.

Host Galaxy / Galaxy Cluster​

Name of Cepheid Variable Star​

Period (days)

Apparent Magnitude​

Absolute Magnitude​

Red shift (z)[dimensionless]​

Recessional velocity [Kms-1]​

Distance (d) [Mpc]​

NGC 4603​

71​

28.00​

27.35​

-8.154​

0.008647

2594.1​

126.134​

I subtracted the apparent magnitude from the absolute magnitude: 27.35 -(-8.154) then I added 5 and divided everything by 5 to get 8.1008. Then I did 10^8.1008 to get the distance in Parces then divided that number by 10^6 to get the distance in Mpc which gave the incorrect value 126.134.

The redshift values are also a bit off as I assumed the redshift values of the cepheids to be the same as their host galaxy. This was becuase I was unable to access the redshfit values for the cepheids. This somewhat harmed the recessional velocity I believed.

Previously as a guide I was using this website, https://www.atnf.csiro.au/outreach/education/senior/astrophysics/variable_cepheids.html However the website suddenly stopped working and couldn't be used anymore.

However I believe I found the source of error, which happened to be the value of the b constant in the Leavitts law which I used as 4.16 however after using 1.35 I received a similar value.

Using the Leavitt's law formula in this Wikipedia article, I get an absolute magnitude of -5.14.

 

[gmax137]

I have no clue what the y-intercept value refers to as I was mainly focused on the hubble constant value from the gradient.

Shouldn't the recession be zero at zero distance? A non-zero y intercept seems fishy to me.

 

[Vanadium 50]

We sure seem to be giving the OP a lot of help.

• Finding the data
• Pointing him to the correct period-luminosity relationship
• Pointing him to the correct formulation of Hubble's Law ("i.e. what is the intercept")
• Correcting the graph (no units, no labels, implausible values on the axes)

The extended essay is an independent, self-directed piece of research, finishing with a 4,000-word paper.

Can we really say that this is independent and this is the OP's own work? Or are we doing the work and he is acting as a scribe?

 

[Da_Rex]

Using the Leavitt's law formula in this Wikipedia article, I get an absolute magnitude of -5.14.

It seems I have used a different value for the constants a and b and thus got a different absolute magnitude value.

Shouldn't the recession be zero at zero distance? A non-zero y intercept seems fishy to me.

I believe the recessional velocity values imbalance caused this y-intercept because I assumed the redshift values of the cepheids to be the same as the galaxies.

 

[Vanadium 50]

I believe the recessional velocity values imbalance caused this y-intercept because I assumed the redshift values of the cepheids to be the same as the galaxie

That's nonsense. More to the point, you don't have a shred of evidence for this. Making up stories without evidence is poor science and poor scholarship, and if you turn this in, you should be graded accordingly.

Further, the equation you fit is not Huuble's Law. Calling a different equation Hubble's Law is also poor scholarship. If you turn this in, you should be graded accordingly.

It seems I have used a different value for the constants a and b and thus got a different absolute magnitude value.

Are you telling us that there are two period-luminosity relationships, that differ by a factor of (in this case) 15? Congratulations! You just disproved your premise!

Making things up is not the key to a good grade here.

To my earlier point - are you planning on citing PF in the paper and outlining all the help you received? Or are you instead going to claim its all your own work?