Neil deGrasse Tyson Explains How Many Stars You Can See In the Night Sky

chuck i got a question for you okay uh how many stars can you see with the naked eye on a perfect night on a mountaintop no clouds just pick a number billionaires and billions uh you know gary larson had a comic where the caption gary larson you know yet the caption was carl sagan as a child and he's looking up and he said there's so many stars there must be hundreds of i know that's funny hundreds hundreds and hundreds of them so um so dude i don't wha i'm i mean seriously if you're in a really like like when uh where i where i went to summer camp and it was on top of a mountain camp sky mount okay and it was in the summer with the i mean when you would look up it seemed like millions of stars and campers talk about laying out on their sleeping bag looking up of course if you're camping you're not in the city or somewhere far away from the city so there's no city lights probably there's no pollution and campers talk about you know just being able to reach out and touch the stars and grab them so on any given night if you have good vision and you're looking up and there's no moon you can see about three thousand stars what i'm just telling you don't shoot the messenger okay come on three thousand lousy stars and if that's just in half the sky the other half there's another three thousand the part that hasn't risen yet or will never rise because you gotta be in the southern hemisphere for it so the total number of stars that are brighter than the detection threshold of your human eye and the retina is about six thousand it's only six thousand that sucks and so i didn't ask you to get emotional about it this is well you know information chuck because it because that is so okay let's that's so less romantic than looking up and and just like the millions of stars in the comments all here just for me that's not like okay so now watch let's keep going so now let's introduce the moon right all right so the moon has light reflected light from the sun right which by the way thousands of years ago people thought the moon had its own light they didn't really think that through um but as before science the methods and tools of science were brought to bear on such questions but so then see now now see now you got me because now i want to know what happened to let someone know that it was actually a reflection of the sun as opposed to being just its own luminosity well so i don't know if there's an exact moment but if you go back to biblical genesis where it says god made two great lights one to light the day and the other to light the night this is very sort of they're making their own light you know that's the that's the only sensible way to interpret that sentence and then you assign the moon to the night and the sun to the daytime as though they are their own agencies all right so um but once you realize that the moon goes around the earth and the earth and the moon go around the sun there's no other way to understand and then you get the phases come out for free out of that right okay so you bring out the moon there's only one phase of the moon that rises at sunset and sets at sunrise okay why is that sunset and and therefore it's up all night and that's the phase of the moon that's exactly opposite the sun okay okay so what phase would that be it's got to be a floor that's a full moon right any other phase is rising at setting at some other time of day and night right so let's take a full moon night because that's up all night the moon makes so much light that the number of stars you see in the night sky drops because it changes it washes out the low threshold of what you're able to detect so when a full moon comes out the 3000 drops to about 300. oh my goodness about 300. that that's that's that's awful it's it's sad it's so this the moon is just stealing all these all the stars glory all the stars glory and it's so if you have a friend who's an astronomer who's ready to go out never say oh i hope your moon is bright and you're it's like yeah that's like screw you buddy have a really lousy like stargazing we are it's a it's a not very hidden secret that astronomers hate the moon we hate the moon when you apply for observing time at telescopes okay and it's competitively awarded there's dark time and there's bright time all right and the bright time is sort of relegated to like if you have a bright scrub scientist if you're like a scrub scientist then you get bright time you know what i mean it is the it is the least coveted time right i have a very uh clever colleague uh i knew him at princeton when i was there and he applied for time at one of the caltech back when he was at caltech at one of the caltech telescopes and he's his favorite objects are very deep sky faraway galaxies all right that you would never see during bright time he applied for time during bright time okay and and of course he got it because there was a very low demand for that time but people why did he do that what what and and they said well you know where where is he right name is jim gunn where is jim gunn tonight oh he's at the telescope and then you wait around and you watch and then that night was a total lunar eclipse so he my boy knew him he knew what he was doing right and so the full moon goes away and he has you know a couple of hours of really good observing time at when otherwise people are avoiding it but so anyhow so there's not that many stars so if you whip out a pair of binoculars even simple ones that number goes up okay and generally the bigger the the the lens okay the the size of the lens is is is capturing light from the sky and so anything that's bigger than the pupil of your eye is going to see more is going to bring more starlight to you than your eyes alone that's why even simple binoculars will up that number from three thousand to ten thousand to a hundred thousand then you get honkering binoculars that you basically you take it to millions and so yeah so it doesn't take much to improve on the human eye this basically improves your eyesight and then you can see much farther away objects okay so the real the real moral of this story is does this have a moral yeah a chocolate i don't want yes i don't want to imbue my stuff here with okay because morrow is the wrong because mauro uh uh connotes that something um a lesson for you a lesson for life exactly a lesson to live by would be okay so uh but the i think the real takeaway here is that human beings have lousy eyes that's that's the thanksgiving should be the name of this segment it's our most cherished uh sense right because it is it brings in information from the farthest distances relative to your other senses so it understandably has great value it also has great value in the history of evolution because sight has developed in multiple ways at multiple times separate from each other and from one another so if it's so important then why is our eyesight so doggone loudly no it's good to know if the tiger's chasing you that's what you need wait get out of here what is that out here you hear it and then you look i'm running the other way right but i'm saying if you couldn't see you'd just be standing there going what is that i hear oh if you couldn't see it right right right so you wouldn't be able to know it at a distance and then and then run you you'll be much closer perhaps the sense of smell goes to great distances but it doesn't leave the air right right you can see through a vacuum and so so our sense of sight understandably is highly valued um but i'm just saying that you don't have to know that there are millions of stars in the night sky for your survival so there was no evolutionary pressure to build our eyes to be even better than what they are that's all i'm saying gotcha yeah and that makes perfect that makes perfect right so you can't totally blame god for that right so uh but the fact is though it's still far less romantic to think that there are 3 000 lousy stars up there and then on a full moon there's only 300 it's like nobody even came to the concert so now if you're really geeky okay and i've done this many times if you're really geeky your fist at arm's length held up to the sky spans 10 degrees okay okay so one way to test this is you stick your fist out at arm's length and put your bottom of your fist on the horizon then put another fist on top and on top like you're climbing a rope and you keep doing this and then when you get straight up it should be nine fists so 90 degrees from horizon to the top okay now you might say well suppose somebody has like really big hands right ask it what what if somebody has really big hands they generally have long arms ah that would make the difference let's make the difference so right this is one of these ratios that is relatively constant for humans your fist at arm's length but it does not work for orangutan yeah if you have a high ape index that's a problem exactly if your knuckles have to track you are dragging on the ground why are my measurements always off i can never get the right message so you just got to go to the other page we have the ape rules rather than the human rule so here's what you do so take your fist your fist you know my fist is 10 degrees top to bottom and maybe 8 degrees left and right but that doesn't matter for this exercise call it 10 by 10. okay okay so your fist at arm's length in the sky blocks 10 by 10 degrees which is 100 square degrees right okay 100 square degrees here's what you do find any random spot on the sky put your fist up and then move your fist aside and count how many stars you're blocking okay just count it okay all right i know how many square degrees there are on the sky there's 42 253 square degrees in the entire sky wow okay if i last i did my math correctly it's it's like trig math to get that right okay so because it's spherical it's a spherical thing and you it's squares and but anyhow for about 42 000 square degrees so how many hundred square degrees are in 42 000 square degrees there is uh 420 square degrees up 420 patches of 100 square degrees right okay so get your number multiplied by 400. and you should come up with about 6 000. plus or minus of course and i've done this many times it bears out wow i'm telling you that i'm not gonna do that okay i'm i'm gonna sit there and look at the stars i'm not gonna do that i'm not doing that and i want to say i'm disappointed chuck but it's just i i i that is too geeky even for me man [Laughter] i'm down i'm down for the telescope i'm down for the stargazing i'm down for the laser pointer i'm down for seeing what constellations all right one last thing one last thing but once once you start getting you know to making you calculate 40 000 degrees okay all right all right so um so let me leave you with a positive note okay okay all right all right here you go um the farthest object visible to the unaided eye is a galaxy beyond the stars of the constellation andromeda so you find a drama in the sky and there's stars and then there's like a fuzzy patch right okay the stars you see are just sitting on our nose in our own galaxy the milky way the fuzzy patch is well beyond this two million light years away wow okay you can see it because there's so many stars there and the light is but you can't resolve it so it's just a puddle of light in fact it wasn't until 1920. no 1926 that edwin hubble the man not the telescope put his telescope on this fuzzy patch and resolved it into stars wow okay so before then people just called it a nebula it was like the andromeda nebula they thought it was just a fuzzy cloud so that this is why what things look like to you aren't always what they are all right so he does this and you realize it's an entire other galaxy that galaxy is visible to the naked eye so that galaxy has 200 400 billion stars in it right so if you want to if you want to think differently about what the human eye can see you take your 3 000 that are in our galaxy up there and then add to it the hundreds of billions in the andromeda galaxy which you can also see wow look at that and in that case when you look up if you include the andromeda galaxy in your field of view you get to say you're seeing billions of stars again billions and billions billions of problems solved problem solved so there you go chuck that's fascinating that's great i don't know if you ever thought about how many stars you see at the night sky but that's kind of more than you ever cared to know about it yeah well you know i don't here's the thing i don't have to think about it anymore so there it is another explainer video from the universe and from star talk all right we're done here uh this is star talk chuck always good to have you always good to be here you

Channel: StarTalk

Views: 203,745

Rating: 4.9516706 out of 5

Keywords: startalk, star talk, startalk radio, neil degrasse tyson, neil tyson, science, space, astrophysics, astronomy, podcast, space podcast, science podcast, astronomy podcast, niel degrasse tyson, physics, Chuck Nice, stars, night sky, Carl Sagan, Moon, full Moon, visible stars, dark time, bright time, telescopes, eyesight, Andromeda, nebula, galaxy

Id: t82Fdsv-bBg

Length: 15min 16sec (916 seconds)

Published: Tue Sep 01 2020