Figure 26.5: After a supernova, the remaining core may end up as a neutron star. ( 20 )If the core remaining after a supernova is more than about 5 times the mass of the Sun, the
core will collapse so far that it becomes ablack hole. Black holes are so dense that not even
light can escape their gravity. For that reason, black holes cannot be observed directly. But
we can identify a black hole by the effect that it has on objects around it, and by radiation
that leaks out around its edges.
Measuring Star Distances
The Sun is much closer to Earth than any other star. Light from the Sun takes about 8
minutes to reach Earth. Light from the next nearest star, Proxima Centauri, takes more
than 4 years to reach Earth. Traveling to Proxima Centauri in spacecraft similar to those
we have today would take tens of thousands of years.
Light-years
Because astronomical distances are so large, it helps to use units of distance that are large
as well. Alight-yearis defined the distance that light travels in one year. One light-year is
9,500,000,000,000 (9.5 trillion) kilometers, or 5,900,000,000,000 (5.9 trillion) miles. Proxima
Centauri is 4.22 light-years away, which means that its light takes 4.22 years to reach us.
One light-year is approximately equal to 60,000 AU and 4.22 light-years is almost 267,000
AU. Recalling that Neptune, the farthest planet from the Sun, orbits roughly 30 AU from