Plot surface temperature vs. luminosity
The H-R diagram is one of the most important tools in astronomy. Stars on the main sequence are fusing hydrogen in their cores. Main sequence stars are ‘alive’. ALL main
sequence stars are called “dwarf stars” Luminosity class tells us a star’s evolutionary state. Recall: stars are powered by converting mass into energy.
You might think that a more massive star would have more ‘fuel’, so it would live longer. However, more
massive stars crush their cores more and burn hotter, so they actually use up their fuel faster. Most stars form
in clusters. We can use the H-R diagram of a cluster to figure out how old the cluster is.
Blue main-sequence stars have short lives. Red main-sequence stars have long lives.
Red nebulae hydrogen gas Also, dust and blue stars Lots of blue stars lots of young stars.
Note that, where there is a lot of dust, there are a lot of red nebulae and blue stars.
In such nebulae (like orion), we tend to find lots of dark patches, which are places where the gas and dust are
concentrated enough that they block out background light. Often these dark, dusty structures are studded
with very young stars. They are the places where stars form. Usually these nebulae are rich in molecules,
often quite complex ones. They are called molecular clouds. At right, you see a very famous and important
molecular cloud, called Barnard 68 or just B68. B68 is special because, although in visible light it doesn’t
look very interesting.... in infrared light, we can see right through it. We can measure how much light it blocks
and figure out its structure. B68 is in hydrostatic balance. If it got a little cooler, the pressure inside would fall
and gravity would win... As it contracted, conservation of angular momentum would cause to to spin faster. It
would also flatten. You can think of angular momentum as ‘tendency to spin’.
𝐋 = 𝐦 × 𝐯 × 𝐫
Angular momentum is conserved when there are no outside forces* acting on a body. (torques)
As it contracted, conservation of energy would also cause it to heat up. From a molecular cloud fragment to...
...a spinning disk of dust and gas orbiting a protostar to... ...a protostar surrounded by a disk and a pair of
bipolar jets. We tend to see these dusty disks in silhouette against the brighter background gas. Protostellar
jets are one of the few phenomena in the universe that change fast enough that we can watch them doing it.
In the end, we wind up with many red dwarf stars and very few blue giants. Eventually, radiation from
the young stars blows away most of the gas, and you are left with just stars!
How a star will die depends strongly on its initial mass. low-mass star-> white dwarf + planetary nebula.
Gravity wins -> Sun starts to shrink.
Release gravitational potential energy -> ignite hydrogen shell burning.
Hydrogen shell burning is so intense hat it floods the outer layers of the Sun with more energy than they used
They expand dramatically. The subgiant branch is almost the reverse of the Sun’s approach to the main
sequence when it was forming. The Sun’s He core will keep contracting. Eventually, the He
core will ignite! Helium fusion turns on very suddenly. This is called the helium flash. For a brief time, the Sun
“lives again”, fusing helium to make carbon. It moves to the part of the H-R diagram called the horizontal branch and stays there for a while.
Once the Sun has a carbon core, it is dead.
Carbon core starts to shrink -> electrons in the c