Re: APOD: Thor s Helmet (2010 Jun 05)
Posted: Sun Jun 06, 2010 5:35 am
Case wrote:
Fantastic! Lamda Aquilae is a young, hot, bright, blue star. Do you realize how rare such stars really are? When we look at the sky we may get the impression that stars like Lam Aql are common, simply because they are bright and can be seen over vast distances. In fact, 99% of all stars we can see in the sky with the naked eye are brighter than the Sun, and most of them are hotter and bluer than the Sun, too, just like Lambda Aquilae. But when astronomers search the sky with their telescopes and make an inventory of the stars, they find that 95% of the stars in the Sun's vicinity are fainter than the Sun! Isn't that amazing? If we approximate the stars we can detect with our telescopes and assume that their spectral class "demographics" is typical of stars everywhere, at least in our own galaxy and in our own time, then, according to Ken Croswell's book Planet Quest, about 5% of all stars are tiny white dwarfs, 9% are K dwarfs, and no less than 80% of all stars are faint little red M dwarfs! Only 4% of all stars are G dwarfs like the Sun, and only 2% of all stars are really brighter than the Sun! And most of those are going to be F dwarfs, just a few times brighter than the Sun! Surely stars like Lambda Aquilae can make up no more than 0.1% of all stars in our galaxy! Imagine that Pioneer 11 is going to sail past one of those rare blue diamonds in the sky! Amazing!
But what do you think about this star?
But speaking of AC+79 3888, however: According to Wikipedia this star is fast approaching us, and 40,000 years from now, when Voyager 1 sails past it, it will only be 3.45 light years from the Sun! That is closer than any star is to us right now (apart from the Sun of course), closer than Alpha Centauri! But Alpha Centauri, which is intrinsically 1.5 times brighter than the Sun, looks big and bright to us in the sky, brighter than all other stars we can see except Sirius and Canopus. When AC+79 3888 sails past us closer than Alpha Centauri 40,000 years from now, it will still be invisible to the naked eye. That's how faint it is.
So Pioneer 11 will sail past one of the rare bright blue jewels in our galaxy, Voyager 1 will sail past one of the true run-of-the mill little stellar midgets of the sky, and Voyager 2 will sail past the brightest-looking star of them all (from our current vantage point), Sirius! Wow! I must say that these three spacecraft are going to run into fascinating denizens of the stellar population of our galaxy!
Ann
Wow, Pioneer 11 will sail past Lambda Aquilae! I'm impressed. Lambda Aquilae is a B9V type star, about twice as hot as the Sun, 52 times as luminous as the Sun in visible light and 84 times as bright as the Sun in bolometric light, that is, it is 84 times as bright as the Sun if you take into account all the energy that the star radiates in all wavelengths.Pioneer 11 is now estimated to be following an escape trajectory from the solar system. Pioneer 11 will pass near the star Lambda (λ) Aquilae in about 4 million years time.
Fantastic! Lamda Aquilae is a young, hot, bright, blue star. Do you realize how rare such stars really are? When we look at the sky we may get the impression that stars like Lam Aql are common, simply because they are bright and can be seen over vast distances. In fact, 99% of all stars we can see in the sky with the naked eye are brighter than the Sun, and most of them are hotter and bluer than the Sun, too, just like Lambda Aquilae. But when astronomers search the sky with their telescopes and make an inventory of the stars, they find that 95% of the stars in the Sun's vicinity are fainter than the Sun! Isn't that amazing? If we approximate the stars we can detect with our telescopes and assume that their spectral class "demographics" is typical of stars everywhere, at least in our own galaxy and in our own time, then, according to Ken Croswell's book Planet Quest, about 5% of all stars are tiny white dwarfs, 9% are K dwarfs, and no less than 80% of all stars are faint little red M dwarfs! Only 4% of all stars are G dwarfs like the Sun, and only 2% of all stars are really brighter than the Sun! And most of those are going to be F dwarfs, just a few times brighter than the Sun! Surely stars like Lambda Aquilae can make up no more than 0.1% of all stars in our galaxy! Imagine that Pioneer 11 is going to sail past one of those rare blue diamonds in the sky! Amazing!
But what do you think about this star?
Lambda Aquilae is so far away that it will take Pioneer 11 four million years to make the journey to its vicinity. But Voyager 1 will sail past a star in only 40,000 years. If we assume that Pioneer 11 and Voyager 1 travel at the same speeds (which is admittedly unlikely) then that star AC+79 3888 would be a hundred times closer to us than Lambda Aquila! Well, that is obviously not the case, since Lam Aql is only 125 light years away, and a star a hundred times as close to us as that would be only 1.25 light years away! To our knowledge there just aren't any stars at all within 1.25 light years of the Sun. Clearly AC+79 3888 is farther away than that. Indeed, according to Wikipedia AC+79 3888 is 17.6 light years away. So it is seven times closer to us than Lambda Aquilae, but it is nevertheless seven magnitudes fainter to the eye! And seven magnitudes fainter means, I think, that it shines about 250 times fainter in the sky than Lambda Aquilae even though it's seven times closer to us. That is why this comparatively nearby star doesn't have a name, just a designation that hardly anyone knows how to pronounce!Voyager 1, in 40,000 years, will float by within 1.6 light years of star AC+79 3888 in the constellation Camelopardalis.
But speaking of AC+79 3888, however: According to Wikipedia this star is fast approaching us, and 40,000 years from now, when Voyager 1 sails past it, it will only be 3.45 light years from the Sun! That is closer than any star is to us right now (apart from the Sun of course), closer than Alpha Centauri! But Alpha Centauri, which is intrinsically 1.5 times brighter than the Sun, looks big and bright to us in the sky, brighter than all other stars we can see except Sirius and Canopus. When AC+79 3888 sails past us closer than Alpha Centauri 40,000 years from now, it will still be invisible to the naked eye. That's how faint it is.
So Pioneer 11 will sail past one of the rare bright blue jewels in our galaxy, Voyager 1 will sail past one of the true run-of-the mill little stellar midgets of the sky, and Voyager 2 will sail past the brightest-looking star of them all (from our current vantage point), Sirius! Wow! I must say that these three spacecraft are going to run into fascinating denizens of the stellar population of our galaxy!
Ann
