Explain xkcd: It's 'cause you're dumb.
Title text: 'The light from those millions of stars you see is probably many thousands of years old' is a rare example of laypeople substantially OVERestimating astronomical numbers.
Cueball makes the common observation that many of the visible stars in the sky are so distant that it takes thousands of years for light from that star to reach Earth. However, the brightest star Sirius is one of the nearest at a mere 8.6 light-years distance. In other words, the light that was arriving from Sirius in March 2014, when the comic was posted, was emitted some time around August 2005. The previous US president, George W. Bush, was in office from 2001 to 2009 and Megan notes that this isn't a terribly impressive observation.
The title text references the fact that most normal people have a hard time imagining the large scale of astronomical numbers. For example, the distance between astronomical bodies or the size of the Sun are hard to imagine; they typically underestimate them by many orders of magnitude and think they are much smaller than they actually are. See the TV Tropes article "Sci-Fi Writers Have No Sense of Scale" for more detail.
In this case, however, people instead overestimate both the number of visible stars and their distance by quite a bit. It's frequently cited that about 5,000 to 10,000 stars are visible in the sky by the naked eye. The Bright Star Catalogue is a star catalogue that lists all stars of apparent magnitude 6.5 or brighter, which is roughly every star visible to the naked eye from Earth. The catalog contains 9,110 objects, of which 9,096 are stars, ten are novae or supernovae, and four objects outside of our Milky Way (two globular clusters and two open clusters). To see most of these you need good eyes and a very dark night, and at any point you will only be able to see fewer than half of these as the rest are blocked by the Earth.
This list shows the 91 brightest stars. Of these 59 are more than 100 light years away and only 6 are more than 1,000 light years away. The farthest on this list, Aludra, is "only" 3,200 light years away. Our entire Milky Way contains up to 400 billion (400x10⁹) stars and has a diameter of 100,000 light years.
There are visible objects much farther away, like the Andromeda Galaxy which is 2.5 million light years away and made up of billions of stars. And a gamma ray burst GRB 080319B would have been briefly visible to the naked eye, despite being 7.5 billion light years distant.
- All of the panels of this comic are white-on-black.
- [Megan and Cueball stand facing each other, looking up at the sky.]
- Cueball: Just think - the light from that start was emitted thousands of years ago. It could be long gone.
- [Cueball looks at Megan, who is still looking up.]
- Megan: That's Sirius. It's eight light-years away.
- [Cueball looks up again.]
- Cueball: Oh.
- [Both look at one another.]
- Cueball: Just think - the light from that star was emitted in the previous presidential administration.
- Megan: Hmm, doesn't pack quite the punch.
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- The star V762 Cas in the Cassiopeia constellation is listed as being 14818 light years away and still having an apparent magnitude of 5.87 - thus being within the visible 6.5 limit. If Cueball had been able to point this star out, he would have been correct. But it is only visible under perfect conditions.
I knew this because Nick Cave's 2013 album *Push The Sky Away* includes the lyrics "Sirius is eight point six light years away / Arcturus is thirty seven / The past is the past and it's here to stay / Wikipedia is heaven". Obviously Randall has been listening to it! ;-) 188.8.131.52 08:32, 14 March 2014 (UTC)
The comic is about people getting the distance to the stars wrong. Wikipedia - List of brightest stars claims a that total of 9110 stars are visible to the naked eye and they provide a list of the 91 brightest stars. Of these only 59 are greater than 100 ly and only 6 are greater than 1,000 ly. The farthest visible star is 3,200 ly away. When people think of the stars they correctly imagine the vast distances they spread out over. But when lay people observe or imagine the visible stars they grossly overestimate the distances. As implied in the title text, in a world of vast astronomical underestimations, this is one of the few overestimates. 99% of the visible stars are only dozens of ly away.ExternalMonolog (talk) 09:35, 14 March 2014 (UTC)ExternalMonolog
- People aren't overestimating the distance, they are underestimating the speed of light :-). The number of visible stars is the true overestimating. And even that ... the 9110 is number of INDIVIDUAL stars we can see. We can also see Andromeda Galaxy, 2.5 million light years away ... but we can't distinguish any of trillion stars it have from others, the galaxy as whole is less bright that any of those 91 brightest stars. -- Hkmaly (talk) 11:04, 14 March 2014 (UTC)
In the comic, the lifespan of stars is also vastly underestimated. A thousand years is nothing when their age is generally counted in millions or billions of years. What is the probability a near-visible star died in the last thousand years and wouldn't that be a major astronomical event? Ralfoide (talk) 14:21, 14 March 2014 (UTC)
- In the comic, the lifespan of stars isn't even mentioned in passing. The history of supernovae is pretty well documented and goes back nearly 2000 years, so the light from those supernovae is probably not more than 2200 years old...184.108.40.206 15:21, 14 March 2014 (UTC)
- Actually, it is mentioned in passing, in the first panel. "The light from that star was emitted thousands of years ago. It could be long gone." (Emphasis mine.) While it is true that technically this is still true as it always "could" be long gone, it is in fact most likely still around. Even if the light were a few thousand years old, it's the blink of an eye to stars that live for billions of years. After all, it's roughly equivalent (given average lifespans) to "We haven't seen Steve in half an hour. For all we know he could be dead by now." Yes, possible. No, not likely. 220.127.116.11 18:36, 26 January 2015 (UTC)
Of course, one could also add the time it takes for the radiation to reach the surface of the star ;) 18.104.22.168 15:41, 14 March 2014 (UTC)
It is true that the energy released at the centre of the star may take millions of years to reach the surface. But it will not be the light we see until it leaves the surface of the star, as light cannot propagate through the plasma of the stars interior. So - no - we could not add this time;-) Kynde (talk) 19:56, 28 March 2014 (UTC)
What will be different about the photons leaving the surface? They're the same, they've just been bouncing about for thousands, not millions with respect to G-type stars, of years. In that sense, it is the light we see and we must add the time. 22.214.171.124 (talk) (please sign your comments with ~~~~)