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We often hear our solar system has properties for life. For example, it is far away from the center of Galaxy. It has Jupiter far away to protect us from asteroids. It has optimum size of territorial planets, plate activity on Earth, and so on.

But besides that, compared with similar stars, does our sun itself also have properties that make life possible? For example, does it have extra stable brightness? Or does it emit less high energy radiation?

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    $\begingroup$ Unique means singular. The Sun is not unique in any sense. In fact nor are any of the properties you list in your question likely to be unique. The combination may be unlikely. But as we can only have emerged on a planet suitable for us, it is hardly surprising that we have done so. A better question would be what variables could you change that would make life (as we know it) on Earth impossible. $\endgroup$ – Rob Jeffries Feb 3 '16 at 7:32
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Other than being kind of in the "Goldilocks" range, meaning, not too big, not too small, (not to be confused with Goldilocks-zone, that's something else), there's nothing special about our sun. There might be something a little unusual about Jupiter's movement over the last 4 billion years and perhaps, something unusual in the just right formation of our moon that stabilizes the Earth's wobble some, but there's nothing special about the sun beyond being a good size.

Now, as far as the "center" vs "outside" of the galaxy, I've heard that too but I'm not sure how well established that theory is. The "OK" range might be fairly broad. Certainly you wouldn't want to be too close to a large star that goes nova, and you "might" not want to be on the outer edge of the galaxy assuming the galaxy has a kind of protective shield like the sun's heliosphere, though I'm not sure how important that is and I'm not sure anyone knows. It's possible there's an ideal ring within the milkyway, but I'm not sure anyone knows that for sure or knows how large it is.

A star can't be too large because there's probably a minimum lifetime for an Earth like planet to form. With a ratio of roughly the power of 2.5 a star twice the mass of our sun would only have a lifespan of 1.5 billion years before it goes red-giant. If you figure it takes 0.5-1.0 billion years for a planet to cool down enough to be a good place for life and for the early bombardment periods to complete and time for the planet to cool and form a magnetic field, 1.5 billion years is pretty tight to allow time for oxygen formation and evolution, so about 2 solar masses is close to the maximum ceiling for earth like planet candidate and that ignores the sun going through increasing luminosity as it approaches it's red giant stage. Ideally you might not want to go too much over 1.5 solar masses (about 4 billion years of main sequence).

Now, you could probably go a fair bit smaller than our sun, cause that would extend the lifespan significantly, but if you get too small, "Flare stars, coronal mass ejections and tidal locking could become issues. Still, there's a pretty good range in the Orange-Yellow dwarf/main sequence range and perhaps upper red dwarf where Earth like planets might be possible.

The bigger question (I think) is stable orbits and planet formation, I suspect. If a star has a gas giant that moves inward and becomes a hot jupiter or a gas giant with a very high eccentricity, I don't think a solar-system like that could maintain and earth like planet. Binary stars might have a hard time providing a billion or more years of stability. Similarly, if there are no large gas giants, There might not be enough early bombardment and water carrying comets, or, there might be too much bombardment later on, which isn't good either.

Other factors like a rotating core, a magnetic field, plate tectonics, and the right mix of gases, for example, too large a planet probably retains hydrogen and you can't get an oxygen atmosphere if there's too much hydrogen, so there's likely a maximum mass beyond which earth like planets couldn't be. There's other issues like snowball earth. Runaway Greenhouse. Planets really need a nearly perfect situation to be earth-like. I suspect there's not much room for error. Fortunately with a billion or so stars of the right size in the milky way, there's likely many potentially Earth like planets.

So, that's my "guess" is that earth like planets has much more to do with solar-system formation than anything else. The star simply has to be within a range of mass, maybe 0.1 or 0.2 to 1.5 or so solar masses. There's still so much we don't know about other solar systems that we really can't say how common earth like planets are. That's one reason I'm looking forward to James Webb Telescope. That should give us much better information on exo-planets and other solar-systems.

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One thing that is, if not unique, unusual about the Sun is the size of it. It belongs to the top 5% brightest stars, meaning that 19 out of 20 stars are smaller than the Sun.

Having a Jupiter size planet is certainly not unique, and plate tectonics we know nothing about for exoplanets.

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  • $\begingroup$ That's because you represent it on a linear scale. Logarithmically, which is a more scale-free way to represent stuff, it's actually pretty average. $\endgroup$ – pela Feb 4 '16 at 7:31
  • $\begingroup$ @pela This is actually just sorted from smallest to largest, no scales involved. It pretty much says that if you chose a star at random, it is very likely to be smaller than the Sun. $\endgroup$ – Hohmannfan Feb 4 '16 at 7:35
  • $\begingroup$ Yes, you're right. If you take a kilogram of random star stuff, it's likely to be in a Solar-type star, though. But for this question, that's not really important, so sorry for interrupting :) $\endgroup$ – pela Feb 4 '16 at 7:51
  • $\begingroup$ The title and the body of the question asks about stars similar to the Sun. This does not answer the question. $\endgroup$ – David Hammen Mar 7 '16 at 10:30
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In terms of absolute size, the sun is at the smaller end of the spectrum (the largest stars can be over 100x the mass of the Sun) but due to the skewed mass distribution in stars, the number of these massive stars is very low and the sun actually lies in the top 5% of stars by mass. The size of the sun is definitely not unique, nor are any of its properties really, but it's size is definitely the reason why it is more stable, the more massive stars burn their fuel much faster and thus have very short lifespans in comparison with our sun, something that wouldn't be helpful to conducing life.

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  • $\begingroup$ The Sun is not a small star - either compared to the mean or the median. $\endgroup$ – Rob Jeffries Feb 3 '16 at 20:55
  • $\begingroup$ @RobJeffries Did you even read my post? I never claimed the sun was small in comparison with the mean or median, in fact I said it is in the largest 5% of stars by mass, but I was explaining how stars can grow to sizes 100 times greater than our sun, making it small compared to these. $\endgroup$ – Dean Feb 3 '16 at 22:18
  • $\begingroup$ @Dean Rob is right that it's not immediately clear what you're saying. "The smaller end of the spectrum" would seem to imply that the Sun is in fact one of the smaller stars, not one of the bigger stars. $\endgroup$ – HDE 226868 Feb 4 '16 at 3:23
  • $\begingroup$ @HDE226868 Its hard to explain this answer without contradicting myself, the Sun IS small when you compare it to the largest known stars, for example VY Canis Majoris is only 20 times more massive than the Sun but it is over 2,500 times wider, or another example is R136a1 which is a Wolf-Rayet star over 260 times more massive than the Sun. BUT these are not typical stars, and the mass distribution isn't even, so even tho the Sun is small in comparison to these it is in the top 5% largest stars by mass when you look at a stellar population on the whole. I hope that is clearer to understand. $\endgroup$ – Dean Feb 4 '16 at 11:38
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Well our sun was not unique 11 billion years ago, Because almost every star in the Universe were all created at once so they all went through Stellar evolution all at once also.Our Sun is 4 billion years old and in a childhood stage compared to the rest of the Stars in the Universe.Our Star systems was one of the last formed from the Big Bangin so only a few stars are in a similar stage as ours as of right now. There still is a few that haven't formed yet, but right now our Sun is a rarity because of it's age and stage giving it a uniqueness in the present Universe.

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  • $\begingroup$ This is incorrect and inconsistent. $\endgroup$ – David Hammen Mar 7 '16 at 10:28
  • $\begingroup$ @David Hammen- Your Working over time I see ;) gotta pay the bills some how huh. $\endgroup$ – user5434678 Mar 7 '16 at 10:37
  • $\begingroup$ Well our sun was not unique 11 billion years ago -- This doesn't make sense. Our sun didn't exist 11 billion years ago. Because almost every star in the Universe were all created at once so they all went through Stellar evolution all at once also. Incorrect. About half of the stars in the universe formed at or before 8.8 billion years ago, about half later than that. Our Sun is 4 billion years old (Almost) correct. The Sun is 4.6 billion years old. Our Star systems was one of the last formed from the Big Bangin Incorrect.We see stars forming now. $\endgroup$ – David Hammen Mar 7 '16 at 11:14
  • $\begingroup$ David Hammen- Thanks for evaluating my question. But I said "our star was ONE of the last not thee last star formed" and Yes stars are still forming I never mentioned they are not. But I answered the question correctly using your data not mine. Those numbers came from the authority not me. My information comes from a different authority and it only knows truth, there are no reason for laws or rules, everything is already known. $\endgroup$ – user5434678 Mar 7 '16 at 12:15
  • $\begingroup$ David Hammen- But sense your around , I want to see if you can post a link showing a documented photo of a star from another system that looks like ours. IN THE SLIGHTEST RESEMBLANCE. you wont be able too, that is why our star is now unique, the new ones forming wont even have a chance to reach our suns stage. And the rest that did resemble our sun have died out long ago.If only you knew who I am, I would be your Best Friend. $\endgroup$ – user5434678 Mar 7 '16 at 12:31

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