The Story of Stars History of the HR diagram  Astronomers love to classify things  Most basic classification for stars is based on temperature  We can see general stellar temperature with just our eyes  Betelgeuse looks red – it is coolmost of its photons are longer wavelength  Sirius looks bluish – it is hot  most of its photons are blue  We can determine stellar temperature by measuring a star’s brightness at different wavelengths.  More detailed classification can be determined by looking at the behavior of hydrogen in stellar atmospheres  Stellar classification scheme: OBAFGKM  O stars – hot, massive stars  M stars – cool, low mass stars  Originally based on strength of hydrogen line in spectrum (A was first). When we understood the behavior of hydrogen, classification got rearranged by temperature This shows the brightness of stars of different temperatures as a function of wavelength (white line in each plot). The position of our “visible” band is also given. Blue stars emit more short wavelength photons, while red stars emit more longer wavelength photons. What is a HR diagram?  1911 – 2 astronomers independently plotted luminosities (brightness) of stars versus their color indices (usually the brightness in visible subtracted from the brightness in blue (B-V))  Imagine the surprise when the resulting diagram was not a scatter plot, but showed definite structure!  A Hertzsprung Russell diagram is a graph showing the relationship between stellar brightness and temperature  Theoretical graph plots luminosities vs. spectral type or color or temperature  Observational version plots color indices.  Hottest, brightest stars up left  Cool dim stars low right  Dim, hot stars low left  Bright cool stars up right Lets look at the Different Regions  Most commonly - stars are on the main sequence (80- 90%)  This is where stars are fusing Hydrogen to Helium  Position on main sequence depends on mass – low mass in lower right, massive stars upper left  Most of a star’s “life” is spent on the MS  Giants – Horizontal branch  Evolved stars  Fusing helium to C/O  Notice gap between MS and HB  Cool, extended stars (surface area)  Supergiants – most massive stars  Complicated history  White dwarfs – stellar graveyard  Small, dim, but very hot Color – Color diagram

Don’t worry too much If you don’t know the “types” of your stars, about this one you can still compare their colors in different wavelength.

Main Sequence

The U, B, and V filters are used to measure the brightness Comparing a star’s brightness in different of stars as a function of wavelength. filters can help you identify the star. The curves show the response as a function of wavelength. HR Diagrams of Star Clusters  Star clusters are very important to understanding stellar evolution.  Assume all stars are at the same distance from us  This means that their apparent magnitudes (how bright they look to us) is nearly equivalent to their absolute magnitudes (the magnitudes the stars would have if they were placed at the distance of 10 pc).  A better way to think of it – for stars in a cluster, you can compare their apparent magnitudes and get a good comparison of the actual energy output of those stars.  Assume all the stars formed at the same time  They are all the same age!  The rate at which stars evolve depends on their masses. Massive stars evolve much quicker than low mass stars.  Evolutionary differences are due solely to difference is mass!!!  Looking at the stars in a cluster that are still on the main sequence can give you an age for the cluster  Lets say that you are observing a cluster in which the most massive star still on the main sequency is 1.5 solar masses. Theoretically, you can calculate how long it takes for a 1.5 solar mass star to run out of hydrogen. This gives you an age for the cluster.  With these assumptions – astronomers can create Color –Magnitude diagrams for clusters, with apparent magnitude on the y axis, and Color Index on the x axis. An Example Color-Magnitude Diagram

Apparent Magnitude

• TO – main sequence turn off, marks most massive main sequence stars • HB = Horizontal branch • RGB – Red Giant Branch • MS – Main Sequency • AGB – Asymptotic Giant Branch

All of the different “branches” mark different stages of stellar evolution. Color index = Blue Brightness – V brightness What can we learn? •Age •Distance •Metallicity •Overall properties of stars •Stars above MS, brighter but with cool temperatures have larger radii •Stars below MS, dimmer but w/ same temperatures have small radii Age

 Star cluster evolution

 Main sequence turn off indicates age  Remember low mass stars live longer  Fit the turn off point – that will give the age of the cluster  Fitting isochrones – theoretical shape of a cluster HR diagram at a particular age. Isochrones Distances 1) All stars at same distance so dimmed by the same amount 2) Plot apparent magnitude 3) Compare to zero age main sequence (which uses M) 4) Difference between apparent and absolute magnitude can give distance 5) Log D=(m-M)/5) +1 6) Do have to worry about reddening