# Intro - A Massive Star's Final Mission *This was my physics IA topic. It may have taken me ~40-50 hours to get my data because I forgot what extinction was...* Polaris. Delta Cephei. Beta Doradus. Zeta Geminorum. All these stars have something in common - and that is that they all pulsate. Having started their lives as [[Protostars (Astro)#Double H - Herbig-Haro Objects|Herbig-Haro Objects]], they've gone from late O-B stars to large, mutilated chunks of hydrogen, slowly radiating their life force away into space. They're now pulsating, as if a beacon for help, aid, anything to prolong their lifetimes - they've become what we call **Cepheid Variables.** ![[Pasted image 20240221230543.png]] *The long-period Cepheid RS Puppis! Note the* [[Nebulae (Astro)#Light Echoes|light echo.]] # Cepheids - Properties A *Cepheid* is a massive star in its [[Red Giants and the Pre-Death of Stars (Astro)#Horizontal Giant Branch|Red Clump]] stage - where it burns helium to keep itself from collapsing. During this stage, it pulsates - which we can see in its **light curve** - a plot of its brightness we can get from any telescope. One of the first measurements of the [[The Hubble Constant and our Cosmological Flukes|Hubble Constant]] was done using Cepheid stars, using the **period-luminosity relationship.** >[!Success]- Cepheid History Time! > >The Period-Luminosity relationship was first discovered by [Henrietta Swan Leavitt](https://pweb.cfa.harvard.edu/news/remembering-astronomer-henrietta-swan-leavitt) in 1912. However, as a computer, she wasn't allowed to continue working on such stars, and much of the credit was instead transferred to her supervisor [Harlow Shapley](https://www.britannica.com/biography/Harlow-Shapley). > >She would die in 1923 having received barely any recognition for the discovery. Terrible... The graph basically shows us that the relationship's linear - the longer the period, the higher the star's [["Brightness" - Luminosity and Magnitude#Luminosity|luminosity]]. This means that we can use Cepheids to extrapolate the distance to the actual star and galaxy once we take brightness measurements. In other words, it's a *standard candle* - or an object with a constant brightness relationship. >[!Example]- Example Problem >A Cepheid has a period of 4 days. Which solar luminosity would you expect this to correlate with? > >**Answer:** >By looking at the graph, we get an absolute magnitude of $-4$ for the # Cepheid Pulsation Mechanisms (EXTENDED) For many, the story with Cepheids ends there - star pulsates, star glows, star eventually dies. To others, a star is a complex web of plasma hydrodynamics and magnetic tragedies, an object terribly difficult to simulate.