Mapping Stellar Evolution (HR-diagram)
Above and in our EBook Stellar Radiation we introduced the Hertzsprung-Russell diagram which plots stars according to their temperature versus their luminosity. If you haven’t worked with the HR-diagram before, we strongly suggest to go back to the EBook Stellar Radiation, pages 14 – 16, before continuing here.
We saw that stars appear only in specific regions of the graph, which thus tells much about these stars, such as mass and size. The HR-diagram is also a powerful tool to study the evolution of stars, which we will explain here.
As we discussed most stars appear in a more or less diagonal band across the diagram from bottom right to upper left. These are the stars in the Main Sequence which have a fairly strict relationship (correlation) between temperature and luminosity (hence the linear structure). These are the stars that are in the main phase of their existence, in which they fuse hydrogen into helium. But how did they get to the main sequence and what will happen when their hydrogen runs out? That is what we will discuss below. We discussed all of that in this EBook, but now we will see what these stages look like on the HR-diagram.
Entering the Main Sequence
All stars form out of collapsing clouds of gas and dust as we discussed earlier in this EBook. Such “stellar nurseries” will be relatively cool (as compared to stars themselves) but yet quite luminous because these clouds are very large and thus have a large light emitting surface. On the HR-diagram these star forming clouds will therefore be on the right hand side (cool) and about half way up on the luminosity scale.
Once a proto-star collapses it becomes denser, the temperature goes up and the luminosity goes slightly down as the surface area decreases. So a forming star will move in the diagram from where the star forming region is to where it will end up on the main sequence (see image).
At that point the star is formed and the hydrogen fusion has started. As we discussed before, the star will then be relatively stable between self-gravity and radiation pressure from the fusion as long as there is enough hydrogen in the core.
Where the star ends up in the main sequence is entirely dependent on how much matter has collapsed, i.e. how massive the star is. We see the cool red dwarfs at the bottom right and the very hot blue stars in the upper left of the main sequence.
The mass of the star also very much determines how long a star will survive, as large stars fuse their hydrogen much faster and have a much higher temperature than smaller stars. So the location of a star in the main sequence gives a clear indication of both the mass of the star and of its expected lifetime.
Leaving the Main Sequence
Let us first follow the sequence for a Sun-like star. The diagram shows where the star ends up in the main sequence (1). Once the hydrogen in the core runs out and has been mostly replaced by helium, the core collapses which then heats the outer layers of the star, causing hydrogen to start fusing in a shell around the core and the formation of a Red Giant star as we discussed before. This becomes cooler on the outside (moves to the right in the diagram) and the luminosity increases because the surface area increases, so it also moves up in the diagram (2).
When the helium in the core starts fusing (2) (sometimes referred to as "Helium Flash") the star gets hotter but slightly less luminous and it follows the “Horizontal Branch” to the left (3). When after about 100 million years the helium in the core runs out the star swells due to both hydrogen and helium fusion in shells around the core and it follows the “Asymptotic Giant Branch” back to the right and at higher luminosity (4).
When the Helium fusion becomes unstable the core separates from the star’s envelope (4), producing a planetary nebula. At that point the core that has shrunk to the electron degenerate state, becomes exposed. It is very hot so it moves to the far left. Because of the collapse the luminosity goes down, so we see the resulting white dwarfs in the lower left of the diagram, very hot but low luminosity (5). Eventually and very slowly they will cool down and follow a path towards the right and also further down in luminosity, until they disappear at the bottom right off the diagram as a black dwarf, and into oblivion. The latter could take ten trillion years.
If the star is much bigger than our Sun, it will end up on the main sequence further to the left, with higher luminosity and higher surface temperature. It will stay there much shorter as the rate of fusion is much higher. When the hydrogen in the core runs out it will swell to a Red Super Giant (1). It is much more luminous than an ordinary Red Giant, but still relatively cool at the surface.
When Helium starts to fuse to Carbon and Oxygen in the core the star swells again and the surface temperature goes up. It becomes a Blue Super Giant (2). When the helium in the core is exhausted and starts to fuse outside the core, the star swells up and thus becomes more luminous. It becomes a Red Super Giant again (3).
As we discussed before large mass stars can continue fusion all the way up to Iron.
Ultimately the Iron core will collapse and a supernova will occur. Because the remnant of the star is incredibly hot and luminous it will disappear off the diagram far above the top left. The temperature can go up to trillions of degrees K and the luminosity to billions of main sequence stars combined. So the exposed core is literally way off the HR-diagram from the top left. The remnant core forms a neutron star which is incredibly compact and thus has very low luminosity, but extremely high temperature. Hence neutron stars will be off the diagram to the far lower left. If the mass of the remnant core is big enough a black hole will form as we discussed before. A black hole by definition has zero luminosity so it will be on the bottom axis but far to the left. So for big stars that ultimately become neutron stars or black holes, only the main sequence part up to the Red Super Giant stage on the HR-diagram is of any relevance.
A great tool
Astronomers studying stellar evolution make extensive use of the HR-diagram because similar stars occupy the same areas on the diagram during their evolution. Once a particular star is located on the diagram, much of its characteristics and also its evolutionary history are known.
The image shows the final stage of stars with various mass, from 0.5 Msun to 15 Msun. All stars move to the right while their surface temperature decreases. But the more massive stars maintain most of their luminosity and thus move horizontally. The most massive stars have an oscillating surface temperature as clearly shown in the diagram.
ESA’s Gaia spacecraft was launched in December 2013 and is located in the L2 Lagrangian Point on the shadow side of the Earth. Gaia performs three main types of observations: astrometry (stellar position, parallax, and proper motion), photometry (magnitude) and spectroscopy (for radial velocity and astrophysics).
This will allow classification of some billion stars in our entire Milky Way galaxy and will uncover not only stellar evolution and characteristics to unprecedented detail, but also clarify the origin and evolution of the galaxy itself.
More than four million stars within five thousand light-years from the Sun are plotted on this diagram using information about their brightness, colour and distance. The information is based on the second data release from the Gaia satellite. This Hertzsprung-Russell diagram, is the most detailed to date made by mapping stars over the entire sky.