Stellar Evolution – Life of a Star Stellar evolution is the process in which the forces of pressure (gravity) alter the star. Stellar evolution is inevitable; stars deplete their own fuel source. Stellar evolution is very important. It is responsible for the production of most of the elements (all natural elements after H and He). As well, it aids in the formation of galaxies, new stars and planetary systems.

Stellar Evolution – Life of a Star The rate of stellar evolution is primarily due to mass. More mass requires greater fuel consumption and faster evolution. Recall from the Hertzsprung- Russel Diagram, the measurements of star luminosity and star temperature highlight a link to mass.

Stellar Evolution – Life of a Star New stars are found in the Main Sequence of the Hertzsprung-Russel Diagram. Based on size and surface temperatures, the hot-bright-blue stars occur in the upper left corner while the cool-dim- reddish stars occur in the lower right.

Stellar Evolution – Life of a Star Stars often begin as a nebulae. A nebulae is a cloud of gas and dust in space. Some nebulaes are regions where new stars are being formed, while others are the remains of dead or dying stars. The word nebulae comes from the Latin word for cloud. In the nebulae, gravity pulls the materials together. The concentration increases internal temperatures. When the temperature reaches OC, the nuclear reaction FUSION begins and the star “turns on”.

Stellar Evolution – Life of a Star The fundamental property shared by all Main Sequence stars is THERMAL EQUILIBRIUM. The liberation of energy from the interior of the star is balanced by the energy released at the surface of the star. The energy is produced by hydrogen burning in the core of star (conversion by fusion of H to He). A second property is HYDROSTATIC EQUILIBRIUM. There is sufficient pressure produced by burning H in the core to support the pressure (i.e., weight of the outer plasma layers).

Stellar Evolution – Life of a Star This is the state of our Sun…a MAIN SEQUENCE STAR. It has been burning for about 4.5 billion years…and will continue to burn as such for another 4.5 billion years. Most stars are Main Sequence Stars (recall the Hertizsprung-Russell Diagram)

Stellar Evolution – Life of a Star Low mass stars are Red Dwarfs. A Red Dwarf consumes its hydrogen slowly, losing its mass very slowly. In the end, all that remains is the very hot core of the star: a White Dwarf. A typical White Dwarf is half as massive as the Sun, yet only slightly bigger than the Earth. This makes White Dwarfs one of the densest forms of matter, surpassed only by Neutron Stars.

Stellar Evolution – Life of a Star As the supply of H in the core is depleted, thermal imbalance occurs and the pressure in the core lessens. With the change, the star beings to collapse inward because the fewer particles inside cannot maintain the pressure needed to support the outer layers. Under the outer weight, the core begins to collapses. This event increases the internal pressure, raises the core temperature and increases surface luminosity. The escaping energy increases the surface temperature so that the outer layers of H begin fusion. This is called SHELL HYDROGEN BURNING.

Stellar Evolution – Life of a Star At this point, there is no H fuel and the burning in the core ceases. The core collapses again. Without H, the star converts its gravitational energy into thermal energy to maintain thermal equilibrium.

Stellar Evolution – Life of a Star He is now the fuel source, but the temperature and energy needed to ignite He fusion is greater than H. Thus, the energy released by He fusion in the core is greater than needed to support the weight of the outer layer. The excess energy expands the outer layers beyond its previous radius and star’s volume and mass increases. This expansion is a Red Giant. The added mass and pressure increases the star’s core temperature to start helium fusion. The burning reestablishes thermal equilibrium, stops gravitational contraction and restarts outer layer expansion.

Stellar Evolution – Life of a Star A Red Giant can be a Red Giant or a Red Supergiant. The only difference is radius. Red Giant is up to 100X larger than our Sun while the Red Supergiant is over 1000X larger. Due to its enormous size, the Red Giant’s outer layers are very cool relative to the core. This creates the colour and brightness of the star. What? Red is cool? Why?

Stellar Evolution – Life of a Star After a long period, SHELL HELIUM BURNING begins, but the surface is very thin and the star becomes unstable. Temperature and energy increase, and the outer layer ignites into fusion by thermonuclear reactions. This stage is called THERMAL PULSE. Luminosity increases by a factor of ten. The outer layer expands, and it is ejected emitting ultraviolet radiation. The radiation ionizes any ejected gases creating a glow called planetary nebula. The death is a LOW-MASS STAR SUPERNOVA.

Stellar Evolution – Life of a Star Due to its enormous mass., the core begins to, once again, collapse and fuse. The collapse is not temperature dependent, and as such gravitational collapse stops. The contracted stellar core is a WHITE DWARF.

Stellar Evolution – Life of a Star White Dwarfs are approximately the size of Earth, but their mass and density is much greater. A teaspoon of matter from a white dwarf would weigh 5.5 metric tons on the Earth. A White Dwarf glows for billions of year from the energy released from cooling thermal radiation until it reaches the temperature of surrounding space which is a few degrees above absolute zero.

Stellar Evolution – Life of a Star If the core remnant of a low-mass star is too dense to form a white dwarf, the core collapse may form a NEUTRON STAR. Neutron stars are incredibly small AND extremely dense. Everything is crushed such that one teaspoon of material would weigh a billion tons.

Stellar Evolution – Life of a Star White Dwarfs are also burning, but these stars burn heavier elements (e.g. C). Each burning creates and burns a heavier element (e.g, Ne, O, Si, S and Fe). The burning raises the core temperature and each elemental burning period is shorter than the previous. Fusion cannot continue after Fe, internal mass increases, thermal imbalance occurs and the star collapses. The pressure causes the protons and electrons to become neutrons. The additional neutrons are released as neutrinos by electromagnetic forces. The shock wave caused by this force is a HIGH-MASS STAR SUPERNOVA.

Stellar Evolution – Life of a Star If the gravitational force of a high-mass star supernova prevents the escape of matter, a BLACK HOLE forms. A Black Hole is very dense…and the extreme conclusion of gravity and stellar evolution.

Stellar Evolution – Life of a Star A Black Hole is a region where matter collapses to infinite density, and where, as a result, the curvature of space-time is extreme. As well, the intense gravitational field of the Black Hole prevents any light or other electromagnetic radiation from escaping.

Stellar Evolution – Life of a Star To summarize, a Black Hole is a region where matter and energy disappear from the visible universe. A Black Hole grows by pulling in the mass (…and the associated gravitational energy…) around it. Theoretically, a Black Hole can emit particles. A big Black Hole would emit a particle very slowly; whereas, a small Black Hole would have explosions.

Stellar Evolution - Summary This is a Hertzsprung-Russel Diagram…a very powerful too. It shows the relationship between luminosity (Y-axis) and surface temperature (X-axis). It shows all the possible types of stars. The long band is the Main Sequence. Our Sun is in the middle. Why? The star Gliese is also in the Main Sequence. On the “habitability scale,” it is more habitable than Earth

Stellar Evolution - Summary This diagram illustrates all the possible “star lives.” Our Sun is in the fourth line of stellar evolution.

Just for fun Look at the diagram. Note that you can change the x-axis and the y- axis. Set the x-axis to TEMPERATURE and the y-axis to LUMINOSTITY. Make sure the “Luminosity Classes” is turned ON. Our sun should have a temperature of about 5800 o K and a luminosity of 1.0 (This number is used to compare a star’s brightness to our Sun. Thus, we are 1.0) Below this information is the radius (size) of the star. This number is also compared to our Sun. Again, we are 1.0. Note that you can click and drag the red “X” around on the HR diagram. The star type will be shown compared to our Sun. The temperature, luminosity and size will change. Also note the reset at the top right. Go to this URL of a Hertzsrung-Russel diagram

Fun continued Questions For all questions, draw a thumbnail sketch of the HR diagram. Move the “x” around the region of the Red Giant stars. What is the range (lowest to highest) of temperature? …of luminosity? Move the “x” around the region of the Supergiant stars. How and why do the colours oft hese stars change? Discuss the range of colour, luminosity and size. Most of the stars in the Universe are Red Dwarfs. How would you describe their characteristics (i.e.,size, colour, temperature, luminosity)? What are the characteristics of White Dwarf stars? What characteristic do they share (other than colour)? Where are they on the diagram? Move the “x” along the Main Sequence. Moving from bottom right to upper left, describe the ways that the stars differ. Change the x-axis to “Spectral Type.” What is the spectral class (O, B, A) or type of our Sun?. What is Polaris’ class? Betelgeuse (in Orion) has a temperature of 3500 o K and a luminosity 140,000 X greater than our Sun. Where is it on the diagram? What is its radius compared to our Sun? What is its group and spectral type?

Another Bust-a-Gut Production