# And you think our Sun is big? HAH! **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [June 24, 2006, 4:43am UTC](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954 "2006-06-24T04:43:52Z")\ **Posts on this page:** 11\ **Page:** 4
**Author:** ![iampunha](https://avatars.discourse-cdn.com/v4/letter/i/74df32/32.png) [@iampunha](https://boards.straightdope.com/u/iampunha)\ **Post date:** [June 28, 2006, 12:44am UTC](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954/61 "2006-06-28T00:44:11Z")
> [@Polycarp](#): > > For the record, there are stars are larger even than Antares. For many years the record holder in diameter was µ Cephei, sometimes known as “Herschel’s Garnet Star” from its obvious garnet-red color. It remains the largest star (in terms of distention/radius) visible to the naked eye. However, there are three supergiants with radii of 7 A.U., about 1,500 times the diameter of the Sun: KW Sagitarii, V354 Cephei, and KY Cygni. These are stars so big that the edge of the photosphere is halfway between Jupiter’s and Saturn’s orbits. ([Cite](http://www.universetoday.com/am/exec/view.cgi/1/1971).) Someday, someone is going to find a topic that doesn’t find you writing about it with intelligence and depth seemingly extemporaneously. I kinda hope that day doesn’t come, y’know? ---
**Author:** ![tofergregg](https://avatars.discourse-cdn.com/v4/letter/t/bbe5ce/32.png) [@tofergregg](https://boards.straightdope.com/u/tofergregg)\ **Post date:** [June 28, 2006, 1:33am UTC](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954/62 "2006-06-28T01:33:09Z")
Two things: 1. When parts of the comet Shoemaker-Levy 9 collided with Jupiter in 1994, some of the explosions were _bigger than Earth_. Yikes. 2. Despite all those large and menacing objects in space, space is so sparsely populated that the odds of hitting something (comet, asteroid, planet, etc.) on a voyage to a star and its solar system is so vanishingly small that there wouldn’t be any need to navigate around anything along the way. -Tofer ---
**Author:** ![Beware\_of\_Doug](https://avatars.discourse-cdn.com/v4/letter/b/278dde/32.png) [@Beware\_of\_Doug](https://boards.straightdope.com/u/Beware_of_Doug)\ **Post date:** [June 28, 2006, 2:50am UTC](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954/63 "2006-06-28T02:50:47Z")
\*\* If This GLOBE were the Size of the SUN and Placed Here Then Comparatively; The DAILY NEWS BUILDING would Be 34 Million miles in Height, Approximately the Distance from the EARTH to MARS. \*\* ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [June 28, 2006, 11:52am UTC](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954/64 "2006-06-28T11:52:46Z")
> [@Beware of Doug](#): > > [The New York Daily News Globe and Fun Facts about the Universe](http://www.flickr.com/photos/carljohnson/tags/dailynews/) > > All measurements given in miles. Adds up. “Ah, that’s a 1930s style orrery!” :flees: ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [June 28, 2006, 1:29pm UTC](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954/65 "2006-06-28T13:29:33Z")
> [@RickJay](#): > > But one day the Sun WILL be a red giant, as it dies out, just like Betelgeuse. It’ll eat some of the inner planets, it’ll get so big. This gets a massive “yahbut” that requires a **lot** of clarification. Warning: Long post about stellar evolution, massively oversimplified, follows. If you don’t want to read it, stop here. I’m not prepared to suggest what makes a protostar start condensing from interstellar gas clouds; one of our astronomy professionals can deal with that. But allow me to start with the idea that there’s already a local thickening in an interstellar gas cloud, for whatever cosmological-physics reasons may apply. Being a locally-higher-gravity area, it attracts more adjacent gas to itself. Which infalls in a spherical shape. Net result, an infalling blob of gas that constitutes a protostar. As more gas continues to accumulate and press inward under gravity, the core of the protostar condenses and heats up (conversion of gravitational potential energy to heat). Eventually it gets to the point where it begins fusion. (As noted above, deuterium fusion starts first, and in the smallest protostars is all that happens.) This produces energy (fusion reactions are exothermic below iron) and the newborn star goes through a series of hiccups, which have technical names based on where stars have been observed doing them. Let me offer T Tauri stars as one step in this process, and leave the others for those of even greater wonkitude than myself. Essentially what happens is that the heat of fusion expands the core and radiates/convects outward, causing flares and other interesting phenomena, until all inherent instabilities have been worked through. Net result, however, is that the star, after a bunch of childhood and adolescent growing pains, settles down as a Main Sequence star. And the interesting thing about this is that size, brightness, and lifespan are closely interconnected: the bigger the star, the hotter it burns, and therefore the faster it uses up the hydrogen in its core. An extremely hot bright high-mass Type O star will have a lifespan of only a few million years; a cool, dim low-mass Type M will have a lifespan well in excess of the age of the universe. The Sun, a relatively small Type G star, a bit hotter and brighter, and faster-burning, than the modal Type M red dwarf, has a lifespan of ten billion years on the Main Sequence, which it’s approximately halfway through. When the star has effectively used up the hydrogen at its core (there’s still some left, of course, but it’s a trace compared to the helium produced by hydrogen fusion), there is no longer any reaction going on in the core to continue the heat/energy pressure outwards. Some fusion, of course, continues at the interface between the core and the hydrogen-bearing next layer out. But the core itself collapses in on itself under the force of its own gravity, creating a yet hotter and denser core area. When this heat and pressure reach a given point, the helium can then fuse to carbon, turning the outward pressure of radiation from the core back on. But that intense heat and pressure then acts on the outer layers of the star to distend (and hence cool) them, turning the star into a red giant. In larger stars, this process continues, with yet more interior heat and pressure being generated, to produce oxygen and other elements, eventually producing silicon which then fuses to produce iron. The result is a supergiant, a vastly distended star held at that immense diameter by the extreme heat and pressure of the core caused by and resulting in the reactions that produce silicon, and, for a few hours, iron. Iron is a dead end – except for an equilibrium reaction transforming iron into nickel and back, **any** thermonuclear reaction involving iron is energy-_consuming_ rather than energy-producing. The net result is catastrophic collapse, into white dwarf, neutron star, or black hole, with concomitant shock waves that cause the layers outside the core to blow up and off, producing a supernova explosion (one of two types of supernova, and we won’t get into the details of what causes the other). However, getting to this point requires the mass to cause the collapse of the core to proceed to the point where silicon and iron production are possible, which takes a fairly large star to begin with. Smaller stars cannot get beyond the carbon-producing phase (and quite possibly the smaller dimmer M stars will never even get that far). When the size of the star has caused the core to collapse and heat up as far as that particular star will go, it ends up collapsing to white dwarf size, probably belching off planetary nebulae in the process. The Sun will likely never get anywhere near supergiant stage. It will almost certainly be able to burn helium, and possibly carbon, but it’s not heavy enough to collapse the core to the heat and pressure needed to get beyond that. So as a red giant it will be a fairly small one, probably somewhere on the order of Pollux at maximum distention. The result, though, will still be catastrophic for the planets: the vastly increased radiating surface, even though somewhat cooler, will end up providing much more insolation to all the planets, heating them up. On Earth, the oceans will heat up and eventually evaporate, producing a greenhouse effect that will turn it into another Venus. Mercury will be for all practical purposes melted down, even if the Sun doesn’t expand quite to Mercury’s orbit (different popular analyses say it will or won’t, and I have inadequate knowledge of stellar physics to judge who’s right). I haven’t seen any analyses of what is likely to happen to other planets, though the results for Mars and for Jupiter and its large moons may be intriguing. We are, however, talking a period five **billion** years from now for these results, and geologic time – probably several million years – for the Sun to transform from main sequence through the instability stage into a red giant. Red giants and supergiants, by the way, are very short-lived on an astronomical time-scale – a few thousand years at most, I believe. The final stages of high-level fusion and the catastrophic collapse are short by comparison – days or even hours (I believe I recall reading that a star does iron fusion for only about six hours from start of Si\>Fe fusion to collapse.) The Sun won’t turn into a Betelgeuse-size supergiant; it isn’t massive enough. It will, however, distend substantially from its present Main Sequence size when it goes through its red giant phase, five billion years from now. ---
**Author:** ![Beware\_of\_Doug](https://avatars.discourse-cdn.com/v4/letter/b/278dde/32.png) [@Beware\_of\_Doug](https://boards.straightdope.com/u/Beware_of_Doug)\ **Post date:** [June 28, 2006, 4:01pm UTC](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954/66 "2006-06-28T16:01:33Z")
> [@Polycarp](#): > > “Ah, that’s a 1930s style orrery!” Don’t you get orrery with ME, youngster… :dubious: ---
**Author:** ![Beware\_of\_Doug](https://avatars.discourse-cdn.com/v4/letter/b/278dde/32.png) [@Beware\_of\_Doug](https://boards.straightdope.com/u/Beware_of_Doug)\ **Post date:** [June 29, 2006, 5:55pm UTC](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954/67 "2006-06-29T17:55:04Z")
Please to read the inscription [here](http://www.flickr.com/photos/carljohnson/94042086/). Unless my math upthread is WAAAAY off (entirely possible), this Struve’s Star is a whole lot bigger than any of the three top supergiants discovered in 2005. Have astronomical methods changed enough in 3/4 century to make it likely that THEIR math was waaaay off compared to ours today? Also, nobody seems to know which star was Struve’s, or indeed, which Struve. There was a whole dynasty of them. One specialized in _double_ stars, which makes me suspect this might be one. ---
**Author:** ![Angua](https://avatars.discourse-cdn.com/v4/letter/a/8baadc/32.png) [@Angua](https://boards.straightdope.com/u/Angua)\ **Post date:** [June 29, 2006, 9:15pm UTC](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954/68 "2006-06-29T21:15:50Z")
> [@Beware of Doug](#): > > Please to read the inscription [here](http://www.flickr.com/photos/carljohnson/94042086/). > > Unless my math upthread is WAAAAY off (entirely possible), this Struve’s Star is a whole lot bigger than any of the three top supergiants discovered in 2005. Have astronomical methods changed enough in 3/4 century to make it likely that THEIR math was waaaay off compared to ours today? Well, telescope resolution has come on in leaps and bounds in the past 75 years. What appeared to be one star even 50 years ago, we can now resolve into multiple stars. Its entirely likely that what Struve saw was a double (or even multiple) star, that he was unable to resolve. In fact, doing a Google search for Struve’s star, brings up newer images of it, showing it to be a double star. ---
**Author:** ![Angua](https://avatars.discourse-cdn.com/v4/letter/a/8baadc/32.png) [@Angua](https://boards.straightdope.com/u/Angua)\ **Post date:** [June 29, 2006, 9:22pm UTC](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954/69 "2006-06-29T21:22:01Z")
Ignore the last sentence of my previous post. Actually, Googling “Struve’s star” brings up a whole load of references to Struve’s (whichever one’s) double star catalogue. Still, I think the sort of maximum radius for a star is about 20-30 times the solar radius, so its probably rather likely that the Struve’s star in the inscription is an unresolved double. ---
**Author:** ![AncientHumanoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ancienthumanoid/32/278_2.png) [@AncientHumanoid](https://boards.straightdope.com/u/AncientHumanoid)\ **Post date:** [June 29, 2006, 10:51pm UTC](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954/70 "2006-06-29T22:51:46Z")
> [@Angua](#): > > Ignore the last sentence of my previous post. > > Actually, Googling “Struve’s star” brings up a whole load of references to Struve’s (whichever one’s) double star catalogue. Still, I think the sort of maximum radius for a star is about 20-30 times the solar radius, so its probably rather likely that the Struve’s star in the inscription is an unresolved double. After looking at pages and pages of sites for different cites about stellar sizes, absorbtion lines, corona temperatures, population densities, dark matter, dark energy, string theory, active galactic cores, and the search for a quantum theory of gravity, I have come to one conclusion: My gawd you’re sexy when you talk shop. ---
**Author:** ![lizardling](https://avatars.discourse-cdn.com/v4/letter/l/f4b2a3/32.png) [@lizardling](https://boards.straightdope.com/u/lizardling)\ **Post date:** [June 30, 2006, 12:05am UTC](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954/71 "2006-06-30T00:05:54Z")
Wow. Those are some amazing pictures right thar. [Previous page](https://boards.straightdope.com/t/and-you-think-our-sun-is-big-hah/361954.md?page=3)