ASTR-101 4/4/2018 Stellar Evolution: Part II Lecture 19
WHEN S THE NEXT TEST?!?!?!? If anyone is following the syllabus, you know that it says there is a test today. The test will be on April 11 th (a week from today) Topics will include: The Sun Measuring the Stars Interstellar medium Star Evolution Star Death Obstacle course for extra credit-any injures will result in F
Possible Extra Credit Option Kinematics of Parsec-scale Jets of Gamma-Ray Blazars and Connection between Jet events and Gamma-Ray activity 2:00 pm, Thursday, Room 190, Physics & Astronomy Please let me know if you are going so that I can attend as well. Summarize: What is the goal of the presented research Describe the experimental setup What issues they are encountered What are the results. At least a half a page Points will be awarded based on completeness and at my discretion. Up to 5% of a single test grade
More Massive Stars
Quantum Degeneracy Pressure Quantum mechanics states that particles (electron, protons, and neutrons) exist in discrete states. If all possible states for a given system are filled, no more particles can be added. The result is an emergent pressure against compression of matter into smaller volumes of space. This is the pressure that holds together a White Dwarf: White Dwarfs are in hydrostatic equilibrium White Dwarfs have no fusion With quantum degeneracy pressure, gravity would crush the star
Novae If the Main-Sequence binary companion is close enough, the white dwarf is able to steal the upper layers from its companion. As the hydrogen envelope is accreted onto the white dwarf, fusing begins and the accretion disk is quickly ignited. This process can happen many times, depending on the mass of the companion star.
Type1a Supernovae As the hydrogen envelope is accreted onto the white dwarf, fusing begins and the accretion disk is quickly ignited. This process can happen many times, but eventually comes to a stop once the mass becomes large enough. If the mass gets above the Chandrasekhar limit, 1.4 solar masses, the star cannot support the force of gravity. The star suddenly fusses all the carbon at once.
Type II supernovae-core Collapse Super Novae Hydrostatic equilibrium is lost because Iron does not burn. Before this point core was held up by electron degeneracy pressure and nuclear fusion. Core is compressed, due to gravity, by overlying layers. Electron degeneracy pressure is not strong enough and the core collapses, compressing objects even further. Photodisintegration occurs and many neutrons are created as heavy nuclei are ripped apart. Neutrons get squeezed inside of the core. Collapsing star is halted by neutron degeneracy pressure, resulting in a violent, fast, rebound of the collapsing material.
Death of Massive Starts Core-burning nuclear fusion stages for a 25-solar mass star 25 M star Main Main Process Temperature fuel products Duration (K) hydrogen burning hydrogen helium 7 10 7 10 7 years triple-alpha process carbon burning process neon burning process oxygen burning process silicon burning process helium carbon carbon, oxygen Ne, Na, Mg, Al 2 10 8 10 6 years 8 10 8 10 3 years neon O, Mg 1.6 10 9 3 years oxygen Si, S, Ar, Ca 1.8 10 9 3.5 months silicon iron 2.5 10 9 5 days Stars with a mass greater than 8 solar masses will end their lives in a massive explosion called a core collapse supernova. This happens when the core starts to fuse Silicon in Iron. Unlike lighter elements, fusing Iron into heavier elements does not produce energy. It takes energy away from the system.
Star Clusters Stars that form in the same molecular cloud have very similar initial conditions. Groups of stars gravitationally bound are called star clusters. Often they are moving around a common central point. Stars will differ in mass, but be compositionally the same. Massive stars evolve fast. Low mass stars evolve slow.
16,000 light-years away, Omega Centauri, most massive globular cluster
Globular Clusters nearly symmetrical round systems of, typically, hundreds of thousands of stars. Globular clusters are the oldest part of our Milky Way. If Earth orbited one of the inner stars in a globular cluster, the nearest stars would be light-months, not light-years, away. Found mostly in the Halo of our galaxy.
Open Cluster Open clusters contain up to a few hundred stars and are not much older than our Sun. The youngest open clusters are still associated with the interstellar matter from which they formed. 30 light year diameter The average speed of the member stars may be higher than the cluster s escape velocity, The stars will gradually evaporate from the cluster
Stellar Association Stellar Associated stars are a group of less than 50 hot bright young stars scattered over 100-500 light year diameter. the constellation Orion is an example. Also contains many low mass proto-stars. Always shrouded in dense ISM.
Main Sequence Turnoff Once a star stops burning hydrogen, it will turn off of the main sequence. The more massive the star the quicker it will make this turn. What population is still left on the Main Sequence dictates the age of the cluster. Open clusters turn out to be as young as 1 to a few 100 million years old. Globular clusters have main sequence stars that turn off at a luminosity less than that of the Sun (10 billions years)
What type of cluster is this?
Heavy Metals in Clusters Like our Sun, open clusters contain 1-4% metals (things heavier than hydrogen and helium). Globular clusters contain much less heavy metals (< 1%). Heavy elements are created from star death, which enriches the ISM. The first generation of stars were only hydrogen and helium because not enough time had pasted for stars to die. This means that the first generation of stars that formed in our Galaxy would not have been accompanied by a planet like Earth
Using Parallax to map out the Milky Way Gaia, launched by ESA, has been measuring the position and distances to almost one billion stars with an accuracy of a few tenmillionths of an arcsecond. Giving us a three-dimensional map of a large fraction of our own Milky Way Galaxy. These precision measurements help improve the distances measurement techniques using the H-R diagram.
Roughly ~30,000 parsecs
Variable Stars Recall that the apparent brightness of an object decreases with the square of the distance to that object. A light curve represents how bright a star is as a function of time What would the light curve of our Sun look like?
Pulsating Variable Stars Cepheid and RR Lyrae variables are pulsating variable stars, meaning they change diameter as a function of time. The expansion and contraction can be measured using Doppler shift. This physical phenomenon happens during the brief Red Giant phase of some more massive stars. As the star expands, it cools. Once it becomes cool enough, it contracts (gravity dominates pressures).
The Period-Luminosity Relation Cepheid have a very consistent period of luminosity fluctuation occurring between 3-50 days. Because they are in the red giant phase, they are 1000-10,000 times brighter than the Sun (easy to spot). The amazing part: The brighter the star, the longer the period. Measuring the period of the fluctuations gives the intrinsic brightness of the star. By comparing absolute brightness and relative brightness measured on Earth and using the inverse square law, an distance can be measured!
Henrietta Leavitt Leavitt discovered hundreds of variable stars in the Large Magellanic Cloud and Small Magellanic Cloud. She concluded that all variable stars in these galaxies were at about the same distance. Any difference in their apparent brightnesses must be caused by differences in their intrinsic luminosities. Leavitt found that brighter stars have longer periods and concluded that their period must be related to the absolute brightness of the star.
Type1a Supernovae Type1a supernovae always explode at 1.4 solar masses -> always have the same brightness. Crucially they also have an easily identified light curve. Using Cephied variable stars to measure the distance to supernovae, astronomers can use typ1a supernovae light curves to measure distances.