Cosmic Distance Ladder: Cepheid Variables
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- Опубликовано: 16 июл 2024
- An introduction to the class of variable stars known as Cepheid variables and their use as standard candles for measuring cosmic distances. Covers Henrietta Leavitt's discovery of the relationship between period and luminosity and an example of how this was used to measure the distance to the Andromeda galaxy which proved the existence of other galaxies.
Here are the links to Henrietta Leavitt's key papers on Cepheid variables:
adsabs.harvard.edu/abs/1908AnH...
adsabs.harvard.edu/abs/1912Har...
And a recent paper on V1 in the Andromeda galaxy including graphs of the light curves
arxiv.org/abs/1111.0262
Just click on the pdf link to see the documents
Let us know what you think of these videos by filling out our short survey at tinyurl.com/astronomy-pulsar. Thank you!
Being a professional science writer, I tip my hat to you! Extremely well done. I feel I learned something that used to be enigmatic and frustratingly elusive.
Thank you SO MUCH for this, it's amazing! I gotta do this presentation on cepheids tomorrow and I had started panicking already but this really saved my life. Thank you.
I'm glad this was able to help. I would recommend having a glance through some of the papers I posted (don't read the entire way through, just check out the introductions, discussions and some of the graphs to get the main important points but not the technical details) and check out the last video in this series (the Other Challenges video) for a few more details relevant to Cepheid Variables that might be helpful for your presentation.
Wow, an excellent easily understood explanation of Cepheid variables and how they can be used to measure distances. I searched numerous sources on the internet for an explanation and this video is the best. Thanks.
The discovery of Cepheid variables, (the name is derived from Delta Cephei, one of the stars of a quadruple system in the constellation Cepeu, and the first standard candle to be discovered) had a profound effect on astronomy. Cepheids were first noted
in 1912 by Henrietta Leavitt, a Radcliff College-educated astronomer who also made several other significant cosmological findings during her lifetime. Because of her gender, the best job she could obtain in her field was as a 30-cents-an-hour research assistant at Harvard, comparing photographic plates. Although to this day, many of her discoveries continue to contribute to our understanding of the universe and easily exceed the subjective and often what-were-they-thinking criteria used to award (or not award) many Nobel Prizes, she received no recognition during her lifetime. She died two years before the Hooker Telescope was completed, but never would have been allowed to operate it. George Hale, who planned and led the telescope’s construction, considered women a distraction, and visiting astronomers weren’t even allowed to bring their wives. Even the chauvinistic Hale, however, later admitted that she deserved a Nobel Prize.
recent papers calculate the distance to Polaris at about 434 light-years (133 parsecs). Some suggest it may be 30% closer which, if correct, is especially notable because Polaris is the closest Cepheid variable to Earth so its physical parameters are of critical importance to the whole astronomical distance scale. Evans, N. R.; Sasselov, D. D.; Short, C. I. (2002). "Polaris: Amplitude, Period Change, and Companions". The Astrophysical Journal 567 (2): 1121.
thanks soooo much, I am doing my physics GCSE and I had no idea how to do this until I found this video
Great explanation, thank you!
As I mention at 6:30 in the video, once Leavitt noticed the period luminosity relationship, Hertzsprung identified some Cepheids in star clusters inside the Milky Way, and measured their distances and brightness exactly as you say. There are only a couple Cepheids close enough to measure their parallax (with the Hipparcos satellite), including the North Star, which is a Cepheid (with a period of ~4 days, which is rather funny since poets take it as a symbol of constancy)
Thanks for watching.
Well done. I taught this on my channel as well. I like your style.
Thank you very much for this excellent lecture❤
Very very good. Thank you for posting this.
this is the best video on the subject, many thanks sir
Now I know what Cepheid variables are! Thanks!
wow, amazing explanation. Thanks a lot
maybe a little late but really appreciate this video, exectly what i needed for my paper
Thanks, that did help! I got another 10 minutes of presenting that way :)
good explanation
thank you verry much
Thank you. Brilliant! :)
Thank you so much
Thank you for making this video. At 5:20 it is stated that she knew all of these objects were in the large and small Magellanic clouds. If it was not yet known how to use the period to determine the luminosity and thus the distance, how did she know that those objects were in the clouds? Didn't she only know which direction they were in?
Great question. You're right that she would have only strictly known that they were in the same direction, but the data itself can show that it is extremely likely that they were at approximately the same distance.
First a quick argument, most stars are found inside of galaxies. Apart from globular clusters (which often orbit around larger galaxies and mostly contain older stars and relatively few variable stars) if we don't see many stars outside of galaxies. With variable stars being a tiny percentage of all stars this means that we wouldn't expect many of these variable stars to be in front of or behind the galaxy that we're looking at. So that's an argument why we might expect these stars to be in the same galaxy, but as for any argument, the assumptions that go into it could be wrong or something else could be wrong, so let's look at the data.
Consider the graph of the data I showed around the 7:00 mark (this isn't the actual graph from the original paper but will make the point). If some of the variable stars in the sample were actually closer or further away, their apparent brightness would be significantly higher or lower (respectively) then the other stars in the sample with the same period. We actually do see a little bit of spread in the data because the LMC and SMC are nonzero in size so some of the variable stars would be a little bit closer or further away. If there were variable stars at much closer or further distances then the vertical spread above and below that line of best fit would be much greater.
The last point I wanted to make was that once this period-luminosity pattern was discovered, it still had to be calibrated to find the absolute luminosity of these stars. Luckily, there are star clusters in our own Milky Way galaxy that contain some of these variable stars and are also close enough to measure their distances using other methods. The current GAIA mission from the ESA has recently made high accuracy measurements of over a hundred Cepheid Variables and thousands of other types of variable stars, allowing us to further improve the calibration of this distance measurement technique (arxiv.org/pdf/1804.09382.pdf)
a video from 2012, I wish it was 2012 again
this helped me alot
Great thanks
How does one identify a cepheid variable as opposed to a variable non-cepheid? That is, how do you know that a given variable star's L and T are proportional?
thanks for explaining. my attention span in class is not what it used to be
thnkx
Fascinating... however, since space is full of debris and stellar dust, how can we reliably measure brightness of stars that are so distant from the earth?
It's a significant challenge to the calibration process, and originally caused very significant errors, especially for stars within the galaxy. But we can also detect that effect because dust and gas will affect different parts of the spectrum in different ways.
When looking at Cepheid's in a different galaxy, it's helpful to remember that the space between galaxies doesn't have a lot of dust and gas, it's mostly just within the galaxies, which can be (to a limited accuracy) modelled.
It's also important to note that the error bounds on most distance measurements are quite large in astronomy. For example, the best estimate distances to Andromeda, the uncertainties are usually around 5%, and that's one of the most studied objects. A current challenge is figuring out how to get more and more precise distance measurements.
Hi!
You said that very roughly the flux is 1*10^-15 W/m^2
So this means you can use this as a constant no matter what star you are measuring from earth?
That flux measurement (and the period of changes in it's brightness) is for that particular Cepheid in the Andromeda galaxy. Depending on which star you were looking at (how near it is and its intrinsic luminosity) the flux would be different; for each Cepheid you would need to measure its own flux (and period) individually and then go through the same kind of calculation in the video to get the distance.
thank you i am doing gcse astromany
cephied arts
How do you know the luminosities for objects beyond 1800 ly? You use the distances from parallax method to determine luminosity from measured flux but the limit is 1800 ly from previous lecture. How do you know a luminosity beyond parallax limits? Seems that there are two unknowns: both distance and luminosity. Only flux can be measured. What am I missing?
In the spectroscopic parallax video we can measure the distances to CLUSTERS of stars many tens of thousands of light years away. We have found enough Cepheid variable stars in these clusters to calibrate how their luminosity scales with the period of the changes in their brightness. So the distance ladder goes like: Parallax of nearby stars gives us the information to do spectroscopic parallax out to farther stars, which gives us the information about enough Cepheid stars to go out to even farther distances, (and so on in the next videos)
Wheres that other distance measurement come from? The cause of variability www.thunderbolts.info/tpod/2005/arch05/050527variablexray.htm
Can you please tell me how did you know that period = 31.4, although, the time between 17 dec and 26 jan is not 31.4
Unfortunately this image was the only one that I was able to get showing the variability of that particular star. To actually measure the period accurately, many observations of the star's brightness would be observed over multiple cycles in the brightness, so that's where I got the number from. A fairly recent reference is available here (arxiv.org/pdf/1111.0262.pdf)
Thanks🌷
Why don’t you post anymore
How do you get the flux?
The flux is essentially the apparent brightness of the star as seen from Earth (the incident light power per square meter reaching your telescope), so it is fairly easy to measure
PhysicistMichael I know you replied a long time ago but I forgot to thank you
Ok, So the wavelength period is a function of absolute luminosity, and luminosity is a function of stellar mass? Gosh! I'm sorry? I still don't get it!
How many cephedis L vs frequncey did not agree with brightness determination from nera by star with color? Cherry picking!
Very clever and imaginative but probably false. Astronomers will do anything to derive distance even from nothing.