Chapter 19-Our Galaxy
x Our Milky Way Galaxy holds over 100 billion stars
x Is among tens of billions of galaxies in the observable universe.
x Is a vast spiral galaxy, with spiral arms.
x The galactic is about 100,000 light-years in diameter, 1,000 light-years thick.
x Sun is located at about 28,000 light-years from the galactic center.
x Stars in the disk orbit in roughly circular paths that all go in the same direction in
nearly the same plane.
x Stars in the bulge and halo soar high above and below the disk on randomly oriented
x Like horses on the merry-go-round, individual stars bob up and down through the disk
as they orbit.
x The orbit of a star around the galaxy arises from its gravitational attraction toward the
x The bobbing arises from the localized pull of gravity within the disk itself.
x A star that is too far above the disk is pulled back into the disk by gravity, but because
the density of interstellar gas is too low to slow the star, it will fly too far below the
disk on the other side
x The ongoing process produces the bobbing of stars.
x Up-and-down motions of the disk give the disk its thickness of about 1,000 light-
x Each up-and-down bob takes a few tens of millions of years.
x The orbital velocities of stars near the edge and those near the center are about the
x Stars closer to the center therefore complete each orbit in less time than stars farther
Orbits of Halo and Bulge Stars
x Orbits of stars in the halo and bulge are much less organized than the orbits of stars in
x These stars travel around the galactic center on more or less elliptical paths, but in
random orientation of paths.
x Neighbouring halo stars can circle the galactic center in opposite directions.
x Halo and bulge stars soar to heights above the disk far greater than the up-and-down
bobbing of the disk stars.
Stellar Orbits and the Mass of the Galaxy
x The sun and its neighbours orbit the center of the Milky Way at a speed of about 220
x It takes the sun about 230 million years to complete one orbit around the galactic
x We can determine the mass of a large object when we know the period and average
distance of a much smaller object in orbit around it.
x Every part of the galaxy exerts gravitational forces on the sun as it orbits, but the net
opposite sides of the galaxy virtually cancel one another.
toward the galactic center.
x :LWKWKHVXQ¶VOLJKW-year distance and 220 km/s orbital velocity, we find that
DIAGRAM 19.3 ± star-gas-star cycle.
Gas from dying stars
x All stars return much of their original mass to interstellar space in two basic ways: 1)
low-mass stars) or supernovae (for high-mass stars).
x Low-mass stars have weak stellar winds while they are on the main sequence. Their
winds grow stronger and carry more material into space when they become red giants.
When it ends its life with the ejection of a planetary nebulae, it has returned almost
half its original mass to the interstellar medium.
x High-mass stars lose mass much more explosively, returning almost all their original
mass to interstellar space. The powerful winds from supergiants and massive O and B
stars recycle large amounts of matter, exploding as supernovae at the end of their
lives. The high-speed gas ejected into space by supernovae or powerful stellar winds
from hot stars sweeps up surrounding interstellar material, excavate a bubble of hot,
x The bubbles created by supernovae can have dramatic effects.
x Supernovae generate shock waves ± waves of pressure moving faster than the speed of
moving gas on its leading edge.
x Thus, a supernova remnant is the aftermath of its shock wave, which compresses,
heats, and ionizes all the interstellar gas it encounters.
x Shock waves from supernovae can act as subatomic particle accelerators.
x As fast as the speed of light, these fast electrons emit radio waves as they spiral around
magnetic field lines threading the supernova remnant.
x Supernovae may also generate the cosmic rays that permeate the interstellar medium
and bombard earth.
x Cosmic rays can cause genetic mutations in living organisms.