30:[{"id":"1ae7c6ab381c3bdd0f7897dceae5c6e8a6639e69e1b20ad90923cde48dd95603af72fe3757a45c9ce2c9936d4185aae03a00c6105a4a74ef0f59a6999e0185bb","href":"https://static.observableusercontent.com/files/1ae7c6ab381c3bdd0f7897dceae5c6e8a6639e69e1b20ad90923cde48dd95603af72fe3757a45c9ce2c9936d4185aae03a00c6105a4a74ef0f59a6999e0185bb","name":"hygSubset.csv","type":"text/csv","lastModified":1732558779351.062,"size":3362416}] 2e:{"title":"30 Day Map Challenge: 25. Heat","cells":[{"id":0,"value":"# 30 Day Map Challenge: 25. Heat","pinned":false,"hidden":false,"mode":"md"},{"id":330,"value":"While I was tempted to create a global climate collapse map for today's challenge, I thought we'd seen plenty of those, so decided to look outwards instead.\n\nWe can estimate the temperature of stars by their colour and by comparing temperature with magnitude (brightness) we can classify them by their age/type.","pinned":false,"hidden":false,"mode":"md"},{"id":333,"value":"## Data preparation\n\nThanks to the [Gaia space observatory](https://en.wikipedia.org/wiki/Gaia_(spacecraft)) we have a catalogue with 1.5 billion records – way too many to visualize directly. Thankfully, the [AT-HYG database](https://github.com/astronexus/ATHYG-Database) has some samples at more usable sizes. I chose the [ATHYG32 subset](https://github.com/astronexus/ATHYG-Database) with around 120,000 stars.\n\nFrom this dataset I selected [right ascension](https://en.wikipedia.org/wiki/Right_ascension) and declination for each star, along with its 'B-V' colour (from which temperature can be derived) and its absolute magnitude value.\n\nRight ascension can be converted into an angle equivalent to longitude by multiplying by 15, and declination is equivalent to latitude (but looking outwards!), enabling stars to be mapped with any map projection.","pinned":false,"hidden":false,"mode":"md"},{"id":5,"value":"hyg = FileAttachment(\"hygSubset.csv\").csv({ typed: true })","pinned":false,"hidden":false,"mode":"ojs"},{"id":336,"value":"## Map Design\n\nIt is common for star maps covering large parts of the sky to use the [Aitoff projection](https://observablehq.com/@d3/aitoff) which I selected for this map.\n\nAlthough the dots representing each star were small I sized and coloured each according to its temperature, which was sufficient to bring out the spatial arrangement of those hottest stars. I used a blue-yellow-red colour scheme to reflect the approximate visible colour associated with each temperature. It's interesting to note that astronomers will be used to thinking of 'red=cold, blue=hot' as the reverse of the convention we might normally expect.","pinned":false,"hidden":false,"mode":"md"},{"id":70,"value":"function starMap() {\n const [w, h] = [900, 510];\n return Plot.plot({\n style: \"background:black; color:white\",\n width: w,\n height: h,\n color: { domain: [-0.3, 2.2], range: starClrs, type: \"linear\" },\n r: { domain: [-0.3, 2.2], range: [1.1, 0.01] },\n projection: GP.geoAitoff()\n .translate([w / 2, h / 2])\n .scale(w * 0.15),\n marks: [\n Plot.sphere({ strokeWidth: 0.6 }),\n Plot.text(\n [\n \"Sample of 120,000 stars from the AT-HYG catalogue showing temperature estimated from spectral class. The hottest stars are blue.\"\n ],\n {\n frameAnchor: \"top-left\",\n fontFamily: \"Orbitron\",\n fontSize: 14,\n lineWidth: 52,\n lineHeight: 1.6,\n dx: w * 0.05\n }\n ),\n Plot.dot(hyg, {\n x: (d) => d.ra * -15,\n y: \"dec\",\n r: \"ci\",\n fill: \"ci\",\n opacity: 0.8\n })\n ]\n });\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":129,"value":"legend = () =>\n Plot.legend({\n style: {\n color: \"white\"\n },\n\n color: {\n label: \"Temperature\",\n domain: [30000, 3000],\n range: [\n \"rgb(97,200,252)\",\n \"rgb(255, 255, 255)\",\n \"rgb(254,255,153)\",\n \"rgb(251,16,0)\"\n ],\n type: \"log\"\n }\n })","pinned":false,"hidden":false,"mode":"ojs"},{"id":338,"value":"Additionally, I wanted to show how temperature and magnitude could be used to classify stars, so showed them on a [Hertzsprung-Russell plot](https://en.wikipedia.org/wiki/Hertzsprung–Russell_diagram).","pinned":false,"hidden":false,"mode":"md"},{"id":22,"value":"function hertzsprungRussell() {\n return Plot.plot({\n style: \"background:black; color:white\",\n width: 800,\n height: 600,\n x: { domain: [-0.3, 2], label: \"Temperature (colder to the right)\" },\n y: { label: \"Magnitude\" },\n color: { domain: [-0.3, 2.2], range: starClrs, type: \"linear\" },\n marks: [\n Plot.dot(hyg, {\n x: \"ci\",\n y: (d) => -d.absmag,\n fill: \"ci\",\n r: 0.5,\n clip: true\n }),\n Plot.text(\n [\n { x: 0, y: -14.5, txt: \"White Dwarfs\" },\n { x: 1, y: -8.8, txt: \"Main sequence\" },\n { x: 1.1, y: -4, txt: \"Subgiants\" },\n { x: 1.5, y: -2, txt: \"Giants\" },\n { x: 0.6, y: 4, txt: \"Bright Giants\" },\n { x: 0.4, y: 8, txt: \"Supergiants\" }\n ],\n {\n x: \"x\",\n y: \"y\",\n text: \"txt\",\n fill: \"white\",\n textAnchor: \"start\",\n fontFamily: \"Orbitron\"\n }\n )\n ]\n });\n}","pinned":false,"hidden":false,"mode":"ojs"},{"id":111,"value":"

Where's the heat?

${starMap()}

${legend()}

${hertzsprungRussell()}

","pinned":false,"hidden":false,"mode":"html"},{"id":72,"value":"---\n\n## Appendices","pinned":false,"hidden":false,"mode":"md"},{"id":346,"value":"_For the Aitoff projection:_","pinned":false,"hidden":false,"mode":"md"},{"id":74,"value":"GP = import(\"https://cdn.jsdelivr.net/npm/d3-geo-projection@4.0/+esm\")","pinned":false,"hidden":false,"mode":"ojs"},{"id":348,"value":"_B-V colour scheme:_","pinned":false,"hidden":false,"mode":"md"},{"id":144,"value":"starClrs = [\n \"rgb(97,200,252)\",\n \"rgb(255, 255, 255)\",\n \"rgb(254,255,153)\",\n \"rgb(251,16,0)\"\n]","pinned":false,"hidden":false,"mode":"ojs"},{"id":249,"value":"","pinned":false,"hidden":false,"mode":"html"}],"files":"$W30","stdlib":"1","titling":"auto"}