# The HR Diagram - and Others! Stars come in all shapes and sizes. The previous four articles should've established this - but I'm going to reiterate it anyway. Astronomers use **H-R** diagram, among other things. These tell us about stellar populations - they're typically drawn like the graph below: ![[Pasted image 20230707090327.png]] The graph here denoted a logarithmic brightness relationship, where $L$, the luminosity of the star given in units of the luminosity of the sun, is plotted against the temperature of the body. This essentially denotes stars that have passed above or away from the main sequence are on the main sequence - stars around that blue line will be on the main sequence, while stars that have moved away and are inching towards the top right of the diagram and stars towards the bottom of the spectrum are white dwarfs. # Understanding the Spectrum Remember that a star's energy arises from fusion processes. There are several factors which affect how fast this fusion actually happens, ranging from: - The metallicity of the star, which is the ratio of elements heavier than helium against the total stellar mass. This is as "metals" actually interact differently with the photons created by the fusion process - The core temperature of the star - The mass of the star Now we can generalise this into a proportional regression based on all of these values! $R \propto (n_{A}n_{B} \times v_{AB} \times \sigma)$ Where: $\sigma$ is the cross-sectional area of the intermolecular collision $n_A$ is the particle density of hydrogen $n_B$ is the particle density of helium $R$ is the reaction rate $v_AB$ is the relative velocity of the products # The Yerkes Classification --> yerkes observatory -- classified with roman numerals give scale here: - VI is subdwarf - V is main sequence - IV is subgiant - III is giant - II is bright (luminous) giant - I is supergiant leave as outlier/object of note - 0 is hyperghiant # The Colour-Magnitude Diagram //color-magnitude diagram: ![[Pasted image 20230710094941.png]] // understand what the B-V color INDEX (on the x axis) is --> the higher the color magnitude the redder the star is --> e.g rigel, B8I star, has a B-V of -0.03, but our sun, G2V, has a color magnitude of -0.656. --> **Vega**, a bright star, is the base for the B-V index! --> we can even find the temperature given the B-V magnitude index, formula not introduced in course but.. sure! ![[Pasted image 20230710095327.png]] # A Deeper Dive into Core Particles # What's Next? For the final instalment in our astro-series, go here: [[White Dwarves (Astro)]] To alleviate luminosity pain: [["Brightness" - Luminosity and Magnitude]] For a more complex dive into stellar classification, go here: [[Lifetimes of Stars (Astro)]] To navigate to anything astronomy, here: [[Everything Astronomy]]