Introduction to Stellar Evolution 13.7 billion years ago, the Universe as we know it came to being as a result of what we call the “Big Bang”. At that time the Universe contained only protons and electrons. Eventually those protons and electrons combined to form Hydrogen and Helium. Where did everything else come from?

How Do Stars Shine? Stars shine by nuclear fusion: the process of extracting energy from the fusion of lighter elements into heavier elements. Helium is made from 4 Hydrogen atoms, but one Helium atom is slightly less massive than the 4 Hydrogen atoms combined.

Nuclear Fusion/Hydrostatic Equilibrium The “extra” mass is converted into energy according to Einstein’s famous formula: E = mc 2 Fusion generates heat, which keeps stars from collapsing under their own weight.

Next we are going to talk about the changes that stars undergo with time. But before that let’s look at the intrinsic differences between stars.

The 30 closest stars to the Sun: Based on their brightness, color, etc., we can group stars into various classes.

Stellar Brightness/Distance Ancient astronomers assumed that all stars were the same distance away. We now know that the stars are not all the same distance away. A star’s distance from the Earth affects how bright it appears to be. Farther stars appear fainter; closer stars appear brighter

The Inverse Square Law Move 2x as far from a light and it gets 4x dimmer. The light is simply spread out over a larger area.

Therefore, two stars that appear equally bright might be a closer, dimmer star and a farther, brighter one: Luminosity vs. Brightness

Group Question A B C Which star is brightest? a)A b)B c)A and B are equal d)Can’t tell Which star is the most luminous? a)A b)B c)C d)Can’t tell

The color of a star is indicative of its temperature. Red stars are relatively cool, while blue ones are hotter. Stellar Temperature/Color

Stellar Colors/Temperature The color of an object is inversely proportional to temperature. If we know the color of a star, we know the temperature.

Stars: Color vs. Temperature Red starYellow starBlue star

The Hertzsprung-Russell (H-R) Diagram Any plot of (intrinsic) brightness versus color or temperature is called an HR diagram.

Classes of Stars Stars aren’t scattered randomly on the HR diagram, rather they fall in certain clumps. In our neighborhood, about 90% of stars lie on the “main sequence”; 9% are “red giants” and 1% are “white dwarfs”.

Main Sequence Stars Also called dwarfs Normal, run-of-the-mill stars About 90% of nearby stars are MS In H-R Diagram: band from upper left (bright and hot) to lower right (faint and cool) Sun is a MS star (type G2) Cool MS stars are more common than hot MS stars.

Giants MUCH bigger than the Sun (10 to 100x) Red Giants – Coolest (~4000 K) giant stars; appear very red Supergiants – Both bigger and brighter than the average giant

White Dwarfs MUCH smaller than the Sun Very hot, but not very bright Actually remnants of dead or dying normal stars

OK, now that we know that there are hot, blue massive stars and cooler, red less massive stars, let’s talk about what happens to them over time.

Star Formation and Lifetimes Tutorial

Hydrostatic Equilibrium Fusion keeps stars from collapsing under their own weight. Pressure from the outflowing hot gas balances the pressure of gravity. This process is called hydrostatic equilibrium

At first, all stars process hydrogen into helium. In low-mass stars this can take billions of years. Stellar Evolution

Eventually, as hydrogen in the core is consumed, the star begins to die. But stars don’t go down easy! Its evolution from then on depends very much on the mass of the star: Low-mass stars go quietly High-mass stars go out with a bang! The Death of Stars

As the fuel in the core is used up, the core contracts; when it is used up the core begins to collapse. That collapse releases gravitational energy and the layer just outside the core heats up. Hydrogen begins to fuse outside the core: Evolution of a Low-Mass Star

Initial giant stage: the red giant branch -- shell hydrogen fusion Evolution of a Low-Mass Star As the core continues to shrink, the outer layers of the star expand and cool. The star, now a red giant, is as big as the orbit of Mercury. Despite its cooler temperature, its luminosity increases enormously due to its large size.

Once the core temp has risen to 100,000,000 K, the helium in the core starts to fuse into carbon Helium begins to fuse extremely rapidly; within hours the enormous energy output is over, and the star once again reaches equilibrium Evolution of a Low-Mass Star Next giant stage: the horizontal branch - - core helium burning

As the helium in the core fuses to carbon, the core becomes hotter and hotter, and the helium burns faster and faster. Soon all the helium in the core has been converted to carbon. Again the core contracts and collapses, releasing gravitational energy. Evolution of a Sun-like Star, Cont

Evolution of a Low-mass Star Final giant stage: asymptotic giant branch (supergiant stage) -- shell helium fusion Helium is fused in the first shell, hydrogen in the next.

Interior of an Old Low Mass Star

Death of a Low-Mass Star Low mass stars are not massive enough to turn carbon and oxygen cores into heavier elements (not hot enough) These stars eject most of their outer layers Only the core is left, which “lights-up” the gas that the star has been ejecting – causing a planetary nebula

The core continues to contract, but never gets hot enough to burn Carbon. Meanwhile, the outer layers of the star expand to form a planetary nebula. Death of a Low-Mass Star

The Helix Nebula

NGC 6826 and 7027

Even after the nebula has dispersed into space, the core remains. It is extremely dense and extremely hot, but quite small. This stage is called a white dwarf. White dwarfs are very hot, but not very luminous. Death of a Low-Mass Star

Group Question The force of gravity acts to make a star a)Larger b)Smaller c)Cooler d)None of these

1) hotter 2) smaller 3) larger 4) cooler 5) identical in size Group Question More massive white dwarfs are ______ compared with less massive white dwarfs.

1) hotter 2) smaller 3) larger 4) cooler 5) identical in size Group Question More massive white dwarfs are ______ compared with less massive white dwarfs. Chandrasekhar showed that more mass will squeeze a white dwarf into a smaller volume, due to electron degeneracy pressure.

White Dwarfs White Dwarf – the burned out carbon- oxygen core of a “dead” star Supported by electron degeneracy pressure Less than 1.4x M Sun (Chandrasekhar Limit) Form from MS stars between 0.8x and 8x M Sun

The small star Sirius B is a white- dwarf companion of the much larger and brighter Sirius A: A White Dwarf As a white dwarf cools, its size does not change much; it simply gets dimmer and dimmer

The Hubble Space Telescope has detected white dwarf stars (circled) in globular clusters:

Low-Mass Star Summary Giants – after core hydrogen burning First: shell hydrogen burning (normal giants) Second: core helium burning (horizontal branch giants) Third: shell helium burning (supergiants) Most go through an unstable variable stage White Dwarfs – collapsed remnants of low- mass stars.

Stellar Evolution Sequence (Low-Mass Stars)

Universe Video Ch 2. 3:50 – 7:30. 11:44- eoc

High-mass stars, like all stars, leave the Main Sequence when there is no more hydrogen fuel in their cores. The first few events are similar to those in lower- mass stars – first a hydrogen shell, then a core burning helium to carbon, surrounded by helium- and hydrogen-burning shells. But instead of stopping at Carbon, a star of more than 8 solar masses can fuse elements far beyond carbon in its core, leading to a very different fate. Evolution of High-Mass Stars

Stars more massive than the Sun follow very different paths when leaving the Main Sequence: Evolution of High-Mass Stars

High Mass Star Cores Star has layers of lighter and lighter elements, like an onion.

High Mass Stars Continued Eventually the core is composed entirely of Iron (Fe) Iron is heaviest element that can be created by fusion Core now supported only by electron degeneracy pressure Once core becomes more than 1.4x M Sun, the core collapses

Supernovae Core collapses until the density reaches the neutron degeneracy limit Core collapse comes to a sudden halt and bounces back (core bounce) Outward moving core slams into infalling outer layers of star Star is blown to smithereens (supernova) Elements heavier than iron are formed during the explosion.

A supernova is a one-time event – once it happens, there is little or nothing left of the progenitor star. There are two different types of supernovae, both equally common: Type I -- results from excess mass being dumped onto a white dwarf from a binary companion Type II -- the “normal” death of a high-mass star. Supernova Explosions

Type I vs. Type II Supernovae

Supernova Remnants The supernova explosion lights-up the gas and dust that the star has already ejected, creating a kind of nebula – a supernova remnant.

The Gum Nebula

Supernova 1987A

Star formation is cyclical: stars form, evolve, and die. In dying, they send heavy elements into the interstellar medium. These elements then become parts of new stars. The Cycle of Stellar Evolution

You are made of star dust.

Group Question Red Giants are bright because they are extremely hot. a)True b)False