PHYS 005 Lecture Notes - Lecture 8: Galactic Center, Spiral Galaxy, Density Wave Theory
5 Jul 2018
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Department
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4/18
Reading- Ch. 23 Sect. 5,6
Being tiny does not = insignificant
●Ex. brain
●Think of ourselves as being PART OF THE universe
*midterm- READ QUESTIONS CAREFULLY
How do spiral arms form?
●66% of spiral galaxies have a bar & spiral arms
○Radio emissions allow us to see how many spirals
○Bar- bulge made of older stars- not spherical
○Spiral arms- coming out of the bar
■2/3rds of galaxies have a bar
●***milky way may have 2 bars
○You do not need a bar to have spiral arms (other 1/3rd of galaxies)
Spiral Nature of Galaxies
Stars- hydrogen gas falls toward center once it collapses
●Fund law of physics- energy is only transferred/ transformed
○ENERGY TURNS INTO ROTATIONAL MOVEMENT
■Explains why the galaxy ROTATES
■BECAUSE IT ROTATES SO FAST, IT BECOMES A FLAT DISK
●Created from the huge collapse that made galaxy
●Conservation of angular motion
The Winding Dilemma
How did spiral arms form?
●As you fall further and further behind, the spiral winds tighter
What keeps the spiral arms in its model?
●Pattern disappears in 300-400 million yrs-> what creates spiral arms
Density-Wave Model
Bertil Lindbad (1925)- new formed stars- blue & brightest stars- overtake brilliance of arms-> arms are
blue
●Where the particles are tightly squeezed-creates new stars eventually (everyone gets to ride the
wave)
○Tightly squeezed- makes higher density
Lin & Shu (1964)- further developed idea- what is the peak/ what is happening that makes things
squeezed
●Stars move in slightly elliptical orbits (not perfect circles)
○Each one further away from center is rotated
a little bit compared to previous one
●They all get more dense @ a certain spot
○Creates a wave in the disk- wave front & wave back
Document Summary
Think of ourselves as being part of the universe. 66% of spiral galaxies have a bar & spiral arms. Radio emissions allow us to see how many spirals. Bar - bulge made of older stars- not spherical. Spiral arms - coming out of the bar. You do not need a bar to have spiral arms (other 1/3rd of galaxies) Stars- hydrogen gas falls toward center once it collapses. Fund law of physics- energy is only transferred/ transformed. Because it rotates so fast, it becomes a flat disk. Created from the huge collapse that made galaxy. As you fall further and further behind, the spiral winds tighter. Pattern disappears in 300-400 million yrs-> what creates spiral arms. Bertil lindbad (1925)- new formed stars- blue & brightest stars- overtake brilliance of arms-> arms are blue. Where the particles are tightly squeezed-creates new stars eventually (everyone gets to ride the wave)
