--- title: "Star Clusters" author: Jed Rembold date: February 11, 2025 slideNumber: true theme: tokyo-night-light highlightjs-theme: tokyo-night-light width: 1920 height: 1080 transition: slide --- ## Announcements - Homework 2 is due on Friday - I'm still working on HW1 feedback unfortunately, but will have feedback out to you by the end of today - This feedback takes me time, about 15 min per essay, which makes for about 10 straight hours of grading. - Check-in form if you forgot! - Set yourself reminders, and I'll also set my reminder to ensure it is up promptly on Saturday - Quiz 1 last 30 minutes of class a week from Thursday! I'm getting a study guide posted by Thursday. ## Recap :::{style="font-size: .9em"} - Doppler movement of absorption peaks can be _very_ tiny - Need highly accurate ways to find the peak: fitting a Gaussian peak - Peaks often on a slope, so subtracting out the background continuum helps fit and accuracy - The _luminosity_ of a star is the amount of energy per second it emits _at its surface_ - Can work backwards if distance and apparent brightness known - Absolute magnitude is another way to indicate this - A parsec is a distance that corresponds to one arcsecond of parallax angle - Stars are classified according to their spectra, ordered by temperature (OBAFGKM) - Star size increases from lower left to upper right on an HR diagram ::: ## Today's Plan - Star Formation - HR Diagram implications - Star Clusters ## HR Diagram {data-background-color="var(--gray2)"} ![](../images/standalone/HR_diagram.svg) # Energetics ## Solar Power - The power source for stars is _fusion_: converting hydrogen to helium in the core - Fusion requires getting atoms so close to one another that the extremely strong but extremely short ranged strong nuclear force can bind them together - The nuclei of atoms are all positively charged though, so they **aggressively** repel one another - For fusion to have a chance of happening then, you need extraordinary conditions: - Very high temperatures: implying lots of heat and the atoms really flying around - Very high densities: implying lots of targets for atoms to run into ## Balance #1 ::::::cols ::::{.col style='font-size:1em'} - Gravity (determined by mass) pulls outer gases inwards - As the gases fall inwards, they gain energy, in the form of heat - The heated gas pressure pushes back on the force of gravity, until things are in equilibrium - The weight the pressure needs to support increases as you go deeper into the star - Thus temperature and density also increase to keep things in balance :::: ::::col ![](../images/ch11_human_stack.jpg){width=60%} :::: :::::: ## Balance #2 - Without an energy source (or with too weak an energy source), a star would slowly cool - It's existing temperature would radiate energy out into the universe - With too strong an energy source, a star would swell up and explode! - No "battery" to store amounts of energy - Stars must also thus be in _energy balance_ - Energy produced in core = Energy emitted from surface - Whatever mechanism transfers the energy (and there are several), it has to be able to keep up ## Repercussions - As a result of both balances, stars are remarkably self-regulating - Is the star slightly too cool? - Pressure won't be able to support, so gravity will pull things in tighter - Things getting tighter means denser, which means more temperature, which pushes back and stops gravity - Higher temperatures result in more fusion reactions in the core, which correspondingly results in more energy released from the surface - Takeaway: - As long as a star is in balance, fusing hydrogen, it will remain in the same place on the HR diagram ## Different Scalings ::::::cols ::::col ![](../images/HR_lifetimes.svg) :::: ::::col - A small difference in mass can result in a huge difference in the core temperature of the star, and hence its luminosity - O Star - $60 M_\odot \,\rightarrow\, 100000 L_\odot$ - M Star - $0.2 M_\odot \,\rightarrow\, 0.01 L_\odot$ - A star's mass of hydrogen **is its fuel source though**! :::: :::::: # Stellar Evolution and Clusters ## Out of Sequence :::incremental - What about the stars not on the main sequence? - Fusing hydrogen into helium does not account for these stars - They must then be stars that exhausted their supply of hydrogen - Two main types: - Giants - In crisis, trying to keep from collapsing - "Fuse all the things!" - Dwarfs - Dejected former giants, having lost all their fuel - How did we work this out? - By looking at _star clusters_ ::: ## Example Clusters ::::::cols ::::col ![An open star cluster](../images/ch12_starcluster.jpeg) :::: ::::col ![A globular star cluster](../images/ch12_globular_cluster.jpg) :::: :::::: ## {data-background-iframe="https://www.youtube.com/embed/3z9ZKAkbMhY"} ## Star Cluster Basics - All stars form from collapsing gaseous clouds of hydrogen - Generally then, many are formed around the same time and location - The initial group can slowly "evaporate" over time as stars move away from one another - If there is still enough mass, stars may remain gravitationally bound nearby - Why are they interesting to us? - **All stars in a cluster around about the same distance away** - **All stars in a cluster are around the same age** ## Determining Stellar Evolution ::::::{.cols style='align-items: flex-start'} ::::col - All stars in a cluster formed around the same time, but some will have shorter lifetimes! - By looking at a cluster, you get a snapshot of stars at various stages of their life - By looking at many clusters, we can start to build up an intuition of how stars evolve throughout their lifetimes :::: ::::col :::r-stack ![](../images/python/gaia_hyades_plot.png){.fragment .only-fragment} ![](../images/python/gaia-M67_plot.png){.fragment .only-fragment} ::: :::: :::::: ## Solar Evolution {data-background-color=var(--gray2)} ![](../images/standalone/HR_diagram_paths.svg) ## Dating Clusters - Once we understand the solar lifecycle, we can apply it back when looking at unknown clusters - The key is to look for the main sequence turn-off point - The further toward the upper-left that this corner or "elbow" appears, the younger the cluster - Can sometimes see stars appearing down in the white dwarf region for really old clusters # Types of Clusters ## Open Clusters :::::{.cols style='align-items: flex-start'} ::::col - Found in the galactic disk - Generally up to several thousand stars - Stars tend to be younger, more newly formed - Most famous the Pleiades structure - Visible with your naked eye in even moderately dark skies - A quick scan of the sky will show it easily in your peripheral vision :::: ::::col :::r-stack ![](../images/ch12_pleides.jpg){.fragment .only-fragment} ![](../images/ch12_subaru.jpg){.fragment .only-fragment} ::: :::: ::::: ## Globular Clusters :::::cols ::::col ![](../images/ch12_globular_cluster.jpg) :::: ::::col - Generally found in the galactic halo - Above or below the disk of the galaxy - Much older stars - A very concentrated density of stars - My favorite is M13, in the constellation Hercules :::: :::::