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#FaveAstroPlot : The HR diagram of brown dwarfs

An awesome thing happened over the weekend. To give you a bit of a backstory, I went a crazy over the last week and started following a bunch of Professors and grad students who pursue astronomy, all around the world. And over the weekend, someone started the hashtag #FaveAstroPlot and tweets started pouring in! There are probably a lot of curated lists of tweets and this is one such is by Emily Lakdawalla.

Now, I thought that it'll be interesting to take up a couple of those plots and write follow up articles detailing what exactly what the plots are about and why they are important/relevant. I also want to get back on the horse and start adding more things to my astro_data_projects repository on Github.

For this week, let's look at the HR diagram of brown dwarfs here. One can find the relevant data here which Trent Dupuy has kindly made available publicly (This is one thing I love about astronomy, most of the data is in the public domain!).

For the uninitiated, let me tell you a bit about the HR diagram and why it's relevant. The HR diagram stands for the Hurtzprung-Russell diagram (wiki), which the scientists Ejnar Hurtzprung and Henry Norris Russell constructed by plotting the absolute magnitudes of stars on the y-axis of the plot and the color (a proxy for temperature) of the star on the x-axis.

Now, the HR diagram is awesome because it gives astronomers a huge amount of information about the stellar evolution or about a star cluster. Once can even squeeze distance to far away galaxies by studying star clusters in said galaxies and constructing their relevant HR diagrams.

The HR diagram are primarily used to explain the evolution of a star during it's life time. As I mentioned, the color (proxy for temperature) of the star is plotted on the x-axis and the temperature in turn is a proxy for the stellar mass (naively speaking, more mass leads to more gravity which leads to more fusion which leads more temperature. naively!). For reasons which I will not get into now, the HR diagram is constructed such that temperature decreases as one goes from left to right on the x-axis and therefore the stellar mass also decreases as one goes from left to right. Stars which fall on the lower right hand corner of the HR diagram are the least massive stars, the least bright and ones with the least temperature, usually brown dwarfs. Stars on the lower left hand corner on the other hand are more massive, much hotter but still just as bright as those on the lower right hand corner, usually white dwarfs.

The other reason this specific plot (a HR diagram of brown dwarfs) is interesting is because the brown dwarfs are stars that stay on the main sequence. Again, to give a bit of introduction, all stars start off on the main sequence and then deviate away from it if they are heavy enough. Given that brown dwarfs are by definition not heavy enough, their stellar evolution is constrained to the main sequence on the HR diagram, which can be clearly seen (the tilted s shaped curve is the main sequence).

To give another example, here's another HR diagram that I made up of 5000 stars from the Hipparchus catalog. You can clearly see the difference between this HR diagram and the first HR diagram, there are stars away from the main sequence, which are heavier stars that have become giants.

I'll stop here and I'll try getting the data and making the plot myself sometime. The data and code for the second plot on the other hand are already available here. I'll take up a different plot the next time and see where it takes us, until then...

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