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.