# What is behind the pulsation of the light of a Cepheid?

1. Jan 28, 2006

### Leonardo Sidis

What is behind the pulsation of the light of a Cepheid?

2. Jan 28, 2006

### Labguy

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3. Jan 28, 2006

### Leonardo Sidis

Thanks for the help!

I was also wondering how the pulsation of a Cepheid star would help us determine the distance from Earth of it or another star. Wouldn't we first have to find how far the Cepheid is away from us before being able to determine the distances of other objects from us based on this distance? If parallax was used to find the distance to a Cepheid, why can't we use the same method to determine the distance of another star from Earth?

Last edited by a moderator: May 2, 2017
4. Jan 29, 2006

### Labguy

The second paragraph in the first link I gave explains this in a roundabout way. Parallax can be used to find the distance to only nearby stars, Cepheid or otherwise. The first accurate distance measurements (a long time ago) were by the parallax method, but at long distances the parallax angle shift is too small to measure. So, some of the first parallax measurements happened to be for a few Cepheids and it was then noticed that the Cepheids pulsate in brightness (Magnitude) over regular periods and reach very near the same magnitude highs and lows.

It is the magnitude that is used to determine distances. Since a particular Cepheid type reaches the same brightness peaks and valleys they were then used to measure how far others were. It is referred to as the "standard candle" method where if you know how far one Cepheid is by parallax, then a more distant Cepheid would show the same magnitudes so one can measure the light from a more distant one. The distance is farther since the light reaching the instruments is dimmer. Apply the inverse square law of EM radiation and you could calculate the distance (with several adjustments needing to be made for other factors).

At even greater distances, astronomers now use another "standard candle" and that is a Type Ia supernova. We know how bright they are at their peak, so a dimmer one must be farther away; that's the simple version. Type Ia supernove are being used now to measure the distance to the most distant galaxies, etc. that can be seen by any instruments.

BUT, as far as Cepheids go, old E. Hubble and friends used the Cepheid method to measure distance to the Andromeda galaxy and came up with a distance of appx. 1.1 million light years. That was used for many years and so were their distance measures to other galaxies.

What they didn't realize at the time is that there are two types of Cepheids, Population I and Population II. Each type has different magnitude and time variations in their light curves. Once this dawned on the astronomers they made corrections for this and then found Andromeda to be 2.2 million LY away. The Hipparcos satellite made more accurate measurements and found Andromeda to actually be 2.9 million LY. This has been adjusted again though, and now most use 2.5 million LY as the distance to Andromeda. The same kind of "revisions" are also being made to large distances found by using Type Ia supernova, but the measurements seem to be getting more refined lately.

Next year, we will have more revisions I'm sure, but it is fun to follow the changes and the reasons for changes....

5. Jan 29, 2006

### Leonardo Sidis

Thanks, that cleared things up for me. It seems kind of weird though, that there are two exact types of a star that hold true in every circumstance. Shouldn't the magnitude, size, density, etc. of a star vary indeterminably?

6. Jan 29, 2006

### SpaceTiger

Staff Emeritus
They're not all exactly the same. In fact, astronomers expend a great deal effort trying to quantify the differences between one Cepheid (for example) and the next. Labguy split Cepheids into two groups, Pop I and Pop II, and this is an excellent approximation for some purposes, but one might want to take this further and establish a continuum relationship for age, heavy element content, etc. And even once this is done, stars will never be exactly as we predict them to be -- there will always be some natural variation. We do our best to figure out how much variation there is so that we can estimate the typical error in our distance measurement.

7. Jan 29, 2006

### Leonardo Sidis

This is off the subject but how do you quote multiple seperate sentences out of a larger post, and then reply seperately to each sentence?

8. Jan 30, 2006

### Nereid

Staff Emeritus
There's a tag [ quote ] (without the spaces) and its partner [/ quote ].

How you choose to efficiently cut and paste is a rather personal choice - hit the "QUOTE" button then insert quote/quote tags where you want? Copy the whole lot to a word processor, do your stuff, and copy/paste back? Make use of the quote/quote tool (it's one of the buttons you get in your Message box)?

As to the distance question - one powerful technique is to measure the radius (diameter, actually) of a Cepheid at various stages of its cycle. Direct imaging is out, of course (no optical telescope has the resolution), but there are plenty of other techniques (finding an appropriate eclipsing Cepheid is my favourite)! They all have distance limitations ... except, perhaps, the EB (eclipsing binary) approach.

9. Jan 30, 2006

### Labguy

Space Tiger is correct in:
which is what I was referring to in my earlier post (#4) when I said:
But, I didn't know if you wanted the detail at that point.. There is a lot more to add, but it is called a "book". I type way too slowly to try to post it here..

I could provide a lot more links, but it would be just as easy for you to Google a search and find more than you'd ever need.