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School
Arcadia High School *
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Course
111
Subject
Astronomy
Date
Nov 24, 2024
Type
Pages
2
Uploaded by KidGazelle146
The Pleiades is an open cluster of stars. Open clusters are located in the disk of the galaxy and are sites of recent star formation. Why do the stars in the Pleiades
appear to be so blue?
The Sun is just one star among
100 - 400 billion stars
(this is the range of accepted estimates and reflects
the precision of our knowledge) that are gravitationally bound to the Milky Way galaxy. Like all other stars,
the Sun orbits the center of the galaxy. It takes about 230 million years (230 Myr) for the Sun to complete
one orbit around the galaxy. This is a number that is so large that it is meaningless to humans. Try putting
230 million years onto
some other scale
that seems meaningful to you. For example, the dinosaurs became
extinct about 75 million years ago (75 Mya), so, the Sun takes about three ``dino-extinction'' units of time to
travel around the galaxy. Or, the Cambrian explosion occurred about 500 Mya; so the Sun has traveled
twice around the galaxy since the Cambrian explosion.
Review...
1. What do you know about the Milky Way galaxy? How many of the structural components can you name?
2. How would you estimate the number of stars in the Milky Way galaxy, given that it is not possible to count
them all?
Other Galaxies
In detail, galaxies are as unique as snowflakes. However, Hubble noticed that galaxies could be broadly
classified by a few -- and only a few -- different large-scale morphologies. His first hypothesis was that he
might be looking at an evolutionary sequence. There are beautiful spiral galaxies like the nearby
Andromeda
Galaxy
, with bright blue arms where massive young stars are forming. There are also distinct giant elliptical
galaxies like
M87
that astronomers call "red and dead" because they no longer harbor regions of active star
formation. However, the evolution of galaxies is not a simple linear transition from spiral to elliptical. We now
know that galaxies grow by mergers, like the dwarf irregular galaxy,
NGC 4214
. Galaxy mergers trigger new
cycles of star formation.
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