Main Page

Explain xkcd: It's 'cause you're dumb.
Revision as of 21:16, 24 March 2013 by Waldir (talk | contribs) (New here?)
Jump to: navigation, search

Welcome to the explain xkcd wiki!

We have collaboratively explained 6 xkcd comics, and only 2003 (100%) remain. Add yours while there's a chance!

Latest comic

Go to this comic explanation

Hertzsprung-Russell Diagram
The Hertzsprung-Russell diagram is located in its own lower right corner, unless you're viewing it on an unusually big screen.
Title text: The Hertzsprung-Russell diagram is located in its own lower right corner, unless you're viewing it on an unusually big screen.


Ambox notice.png This explanation may be incomplete or incorrect: Fill out the table... And somebody please get the table to fit so it doesn't make the page to zoom out. Do NOT delete this tag too soon.

The Hertzsprung–Russell diagram is a scatterplot showing absolute luminosities of stars against its effective temperature or color. It's generally used to understand a star's age.

The axes are labeled in Kelvin (degrees Celsius above absolute zero) for effective temperature and, unlike many Hertzsprung–Russell diagrams, Watts for luminosity. While most Hertzsprung–Russell diagrams are labelled in units of solar luminosity or absolute magnitude, all three are perfectly valid measures of luminosity, which refers to the total power emitted by the star (or other body). Effective temperature refers to temperature of a blackbody with the same surface area and luminosity. This is meant to provide an estimate of the surface temperature of the object.

Regular Hertzsprung–Russell diagrams cover ranges of about 1,000K to 30,000K, and what is labeled on this diagram as 1021 to 1033 watts—i.e. the upper-left corner. Extended diagrams increase the luminosity range only to include the "Brown Dwarfs". This diagram has been extended to much lower magnitudes on both axes. The joke comes from the absurdity of a diagram meant for stars including much smaller objects, such as planets ... and astronomers.

Though not included in the diagram, the title text notes that the screen displaying the diagram would probably be plotted somewhere in the lower right corner due to its (relatively) low brightness and heat output. Bigger screens have a higher total output (in terms of luminosity) and are thus positioned further towards the diagram's top. An "unusually big screen" would have to be something like a JumboTron or a projector for its luminosity or temperature to put it outside of the lower right corner.


Item Effective Temperature Luminosity Explanation
Betelgeuse 3200 K 1.6 * 1031 W
Vega 10,000 K 1.8 * 1028 W
Sun 5800 K 3.6 * 1026 W
Proxima Centauri 2700 K 2.0 * 1023 W
HD 189733 b 2100 K 4.8 * 1021 W
Interior of a hydrogen bomb during detonation ~108 K ~1020 W
Jupiter 285 K 1.2 * 1018 W
Venus 330 K 5.0 * 1017 W It appears that this might have been misplaced on the temperature axis, being far too closely placed to France and to Earth. In fact Venus is at 735K where Earth has a mean of 287K.
Earth 300 K 3.0 * 1017 W
Mars 255 K 2.0 * 1016 W
Moon 300 K 1.2 * 1016 W
Nuclear Fireball 8000 K 2.0 * 1014 W
France 300 K 2.0 * 1014 W This is part of Earth (and more precisely a part of Europe), the same temperature as Earth, but less luminous in proportion to its surface area. Including this may be a joke referencing the two possible meanings of ‘Europa’ (see the next entry). France emit less light at night than neighbouring countries, perhaps due to lower population density
Europa 90 K 3.5 * 1014 W While this term could refer to Europe (a part of Earth, of which France (the previous entry) is a further part), the temperature and luminosity are both too small for that, so it must refer to the moon of Jupiter instead.
Lightning Bolt 30,000 K 30 GW
Ivanpah Solar Plant Salt Tank 1200 K 1.2 GW The Ivanpah Solar Power Facility is a large solar power generator in the Californian Mojave desert. It concentrates sunlight from 173,500 reflectors onto three boiler towers.
Medium-sized Lava Lake 800 K 32 MW
Cruise Ship 325 K 30 MW
Campfire 870 K 7.0 kW
Blue whale 280 K 78 kW Must be average surface temperature as whales are warm-blooded @ ~100F/37C internally, interestingly this and the cruise ship may be the only entries where a significant amount of power produced is conducted away rather than radiated. Also the power seems high compared to what I can find. These images suggest a surface temperature around 295K - 300K for a Humpback whale when surfacing[email protected]/Visible-and-infrared-spectrum-images-of-various-humpback-whale-surfacing-features.png
Arc lamp 65,000 K 150 W
Lightbulb 4800 K 75 W The temperature value here refers to colour temperature, which for an incandescent bulb is the same as the filament temperature. However tungsten filament lights, commonly referred to as "bulbs", have a colour temperature of between 2400 and 3600 K.
LED Bulb 5800 K 8 W The temperature value here refers to colour temperature, not physical temperature. Color temperature is a better match to effective temperature than physical temperature.
Astronomer 310 K 100 W The body temperature of a human (astronomer or otherwise) is about 310K (37°C). Skin Surface Temperature (which would fit the meaning of effective temperature better) is typically 31°C - 35°C. An astronomer standing outside in a thick coat on a cold night would have a much lower surface temperature.

A human being generating 100W for 24 hours needs 2065 kcal or 8,64 MJ. According to the UN FAO this is e.g. the typical daily energy output of women with weight 55kg between 18 and 59 years having a light activity lifestyle of 1.55xBMR (basic metabolic rate).


Ambox notice.png This transcript is incomplete. Please help editing it! Thanks.
Expanded Hertzsprung-Russell Diagram
[A scatter plot is shown, with the x-axis labeled Effective Temperature (in kelvins), and the y-axis Luminosity (watts).]

Is this out of date? Clicking here will fix that.

New here?

Last 7 days (Top 10)

Lots of people contribute to make this wiki a success. Many of the recent contributors, listed above, have just joined. You can do it too! Create your account here.

You can read a brief introduction about this wiki at explain xkcd. Feel free to sign up for an account and contribute to the wiki! We need explanations for comics, characters, themes, memes and everything in between. If it is referenced in an xkcd web comic, it should be here.

  • List of all comics contains a complete table of all xkcd comics so far and the corresponding explanations. The red links (like this) are missing explanations. Feel free to help out by creating them! Here's how.


Don't be a jerk. There are a lot of comics that don't have set in stone explanations; feel free to put multiple interpretations in the wiki page for each comic.

If you want to talk about a specific comic, use its discussion page.

Please only submit material directly related to —and helping everyone better understand— xkcd... and of course only submit material that can legally be posted (and freely edited.) Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users who repeatedly post such content will be blocked.

If you need assistance from an admin, feel free to leave a message on their personal discussion page. The list of admins is here.