Stars Come in all shapes and sizes: • Close and far • Bright and faint • Large and small • Red, white and blue = Cool and hot • Singles, doubles, triples, small clusters, large clusters A cold red giant A double star: A hot white dwarf A star cluster • All stars are born in clusters but get “expelled” pretty soon • Half of the stars are doubles A galaxy But: An individual star • All are part of the Milky Way Galaxy (No “stray” individual stars) • All look like dots in telescopes (Too far to resolve their disks.)

Questions coming …

sec 10 9 11 12 15 14 13 8 7 1 2 3 5 4 6 16 26 17 27 28 30 29 24 25 23 19 18 20 21 22 Question 12 Are all stars part of star clusters? A No, star clusters actually consist of rarified hydrogen gas, not stars. B No, only a few stars have anything to do with any star clusters. C Yes, all stars are part of a star cluster. D All stars were once part of a star cluster but most have left the cluster already. Next question coming …

sec 10 9 11 12 15 14 13 8 7 1 2 3 5 4 6 16 26 17 27 28 30 29 24 25 23 19 18 20 21 22 Question 13 How many double star systems are there? A Very few: out of the billions of stars in the Galaxy, only a hundred are doubles. B Only a few: 99% of stars, like the Sun, are singles. C About the half of the stars are doubles. D Almost all stars are doubles. Next question coming …

sec 8 9 10 12 11 7 6 1 2 3 4 13 30 35 40 45 14 25 16 15 18 17 19 20 5 Question 14 Are there any stars between galaxies? A No. Each star belongs to a galaxy. B Yes. All stars form in galaxies, but they normally leave their parent galaxies soon after birth. C Yes and no: there are no individual stars between galaxies but there are clusters there. D Yes. All stars form in galaxies, but only a few percent leave their parent galaxies after birth. E Yes. Most stars do not form in galaxies.

Parallax How do we know the distance to the stars? Measure parallax: (Closeby stars, for a start.) Parallax • Closeby stars seem to move on tiny circles, once a year • Reflection of the motion of Earth around the Sun • Most (far-away) stars move very little Measure parallax: the closer the star, the larger the circle Parallax = 1 as ↔ distance 1 parsec (pc) 1 pc = 3.26 light years Formula: distance[pc] = 1/parallax[as] Practical limit - precision: • Good telescope can measure where the middle of the blurred image of a star is with a precision of 1/1000 as • Can use parallax method up to distances of 2-300 pc works only for the closest stars – in the Solar Neighborhood.

Questions coming …

sec 10 9 11 12 15 14 13 8 7 1 2 3 5 4 6 16 26 17 27 28 30 29 24 25 23 19 18 20 21 22 Question 15 What is parallax? A Far away stars appear dimmer in the sky. B Stars make one circle around the whole sky in one year. C Stars move in tiny circles in the sky, once a year. D The telescope must follow the star’s apparent daily motion. Next question coming …

sec 8 9 10 12 11 7 6 1 2 3 4 13 30 35 40 45 14 25 16 15 18 17 19 20 5 Question 16 How large an effect is parallax? A Very large: stars move all the way around the sky in a year. B Large. You can see it by the naked eye without any difficulty. C Tiny. We need a very precise telescope to detect the parallax of stars. D Extremely tiny. Not even the largest telescopes can see the parallax of stars because stars are so far away. Next question coming …

sec 8 9 10 12 11 7 6 1 2 3 4 13 30 35 40 45 14 25 16 15 18 17 19 20 5 Question 17 Polaris, the North Star, has its parallax measured as 0.01 arc seconds. How far is it? A One parsec. (That would be ~ 3 light years.) B A tenth of a parsec. (That would be ~ 1/3 of a light year.) C A hundred parsecs. (That would be ~ 300 light years.) D Ten parsecs. (That would be ~ 30 light years.) E A thousand parsecs. (That would be ~ 3000 light years.)