Stars and constellations
Name: Kristina Matarazzo, Mary, and Burbank School
Why do stars always stay in the same place. For example, I can always find
the constellations. Why do not the stars change into other constellations?
Stars seem to stay in the same place for a long time, long compared to our
lifetime. This happens because of the large distance that separates us from
the stars. We can observe only changes in the angular position of stars.
An object at distance D will appear to move by one degree across the sky if
it moves a distance D/60. For a star at a distance of few light years from
us, this displacement is also of the order of a light year. Given the
average velocity of stars, this distance will be covered in a few thousand
years So these two reasons contribute, large distance to stars and small
relative velocity of stars, in making stars appear stationary.
Jasjeet S Bagla
Constellations are human inventions. Usually the stars that make up a
constellation have no relation to each other; they are generally not part of
the same cluster (an exception is the Big Dipper; many of its stars actually
do belong to a single open cluster.) A very bright star that is very far
away may look similar to a dimmer star that is closer; and if we see two (or
more) similar-looking stars in the same part of the sky, it is not unreason-
able to associate them. It is believed that the constellations were
originally invented thousands of years ago as an aid for learning the stars
so that they could be used for navigation by sailors.
They form in the minds of people looking at the stars! Most of the stars in
constellations are actually very far apart from one another. All we can
tell is how bright the stars look from here and where they are in 2 dimen-
sions - the angular position in the sky. We cannot tell right away how far
they are, and that means that stars that may look like they are forming a
group together are actually in totally different places. The constellations
are a useful way of getting to know the night sky, however - they make the
patterns of stars very recognizable, and if you go look at them once a week
or so you will soon know them very well!
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Update: June 2012