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### Course: Cosmology and astronomy>Unit 2

Lesson 4: Cepheid variables

# Cepheid variables 1

Cepheid Variables 1. Created by Sal Khan.

## Want to join the conversation?

• At Sal says that Cepheid variable start help show that there are stars beyond our galaxy and even galaxy's beyond our galaxy. However in the next video in this series (https://www.khanacademy.org/science/cosmology-and-astronomy/stellar-life-topic/cepheid-variables/v/why-cepheids-pulsate) it is explained that the pulsating is due to a change in luminosity and not as is implied at that it has to changing distance relative to other stars. If this pulsating has nothing with movement, if it does not show orbit in a plane other than that of our galaxy, how does it help prove that there are other galaxy's?
• The pulsating is due to change in luminosity. We know that the frequency of the pulsation corresponds to the peak luminosity, so we measure the frequency and then we know the absolute luminosity. When you know the absolute luminosity and you know how bright a star appears from earth, then you know how far away the star is.

When we determine the distance to cepheids, we see that many of them are much too far away to be in our galaxy.
• Why are those stars called Cepheid Variable Stars?
(Especially the first part 'Cepheid' )
• The first Cepheid Variable discovered was Delta Cepheid, in the constellation Cepheus.
• How have we gotten pictures of the Milky Way galaxy if the farthest man-made satellite is just at the edge of the solar system?
• That is not a real picture of the Milky Way Galaxy. There are two options that that could be. It could either be an artist's depiction of the galaxy or it could be computer generated image based on measurements to certain objects.
Hope this helps!
• At 5.20, since the period is the independent variable and the relative luminosity is the dependent variable, why shouldn't period be placed on the x-axis of the graph?
• The period is not the independent variable; independent is sometimes a matter of perspective. but there's no law that dictates 100% of the time how you make a graph,
Focus on the concept of the cepheid variable, not the formatting of the graph.
• I notice that in the picture showing the different galaxies at the start of the video, there's several named galaxies along with "NGC 6822". Is there any particular reason that its assigned that instead of a mythical name like Andromeda or other name?
• There are a few galaxies with names like the Sombrero Galaxy, the Pinwheel Galaxy, Triangulum Galaxy, etc... However, since galaxies are fairly faint, requiring telescopes to see the majority of them, most of them have only been recently discovered (within the past 100 years) and by then, astronomers were finding so many new objects that they were more concerned with cataloging them than naming them.
• Are Cephid Variables common in the milky
• There are likely about 20,000 Cepheids in the Milky Way. You can think of Cepheid Variability as a "phase" of a star's existence, and only a small fraction of all stars are even capable of existing in this phase. Furthermore, the Cepheid Variability phase lasts a very short fraction of a star's lifetime.
• I know this was asked before, however, I haven't seen an answer which explains this clearly so I thought I'd ask for some insight: I understand the concept that the distance of Cepheid Variable stars from earth can be measured by their luminosity. However, this would have to mean that all Cepheid stars with 1 day periods are equal (at least emit the same luminosity) to all other Cepheid stars with 1 day periods, and the ones with 2 day periods have the same luminosity as all other 2 day period Cepheid stars, and all 3 day are the same as all 3 day and so on... Is this something that has been proven? that all Cepheid Variable Stars with the same periods emit the same luminosity? Sorry if it's redundant.. I'm just really curious. Thanks!
• At you state that if you know the absolute luminosity of a cepheid variable star, then you know the absolute luminosity of any other cepheid variable star. How can that be? Isn't that an assumption? Don't they vary in their absolute luminosity from one star to another?