1. 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.)

2. Questions coming …

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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 …

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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 …

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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.

6. 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 pc
works only for the closest stars –
in the Solar Neighborhood.

7. Questions coming …

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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 …

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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 …

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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.)