Review of stellar characteristics and the lifetime of typical stars The Hertzsprung-Russell Diagram We will plot stars on a chart which has temperature on the horizontal axis and the luminosity on the vertical axis. Named after two astronomers who
made this type of plot in the 1920s. Use units of solar luminosity and Kelvin Reminder: we need to use the HR diagram as we describe stellar evolution
Here are the main regions on the HR diagram. Stellar masses determine a stars
position along the main sequence, more than other properties do. We believe that most stars form in clusters from a single large cloud that fragments. An example of an Open Cluster is the Pleiades cluster (M45, a.k.a. the seven sisters).
H-R diagram of the Pleiades Open Cluster There are stars all along the main sequence, from blue giants
down to red dwarfs. Cluster Evolution on the HR Diagram Newborn Cluster after 10 million
years Notice that there are already some red giants from massive stars that have already run out of hydrogen fuel.
Young Cluster after 600 million years Notice that the cutoff is at Type A stars and that there are already some white dwarfs.
Globular Cluster Omega Centauri H-R diagram of Globular Cluster Omega Centauri contains no main sequence stars with mass
greater than 0.8 solar mass over 10 billion years old. Interstellar matter Gas and dust Dust grains about 10-7 m in diameter,
about the size of smoke particles Dust causes reddening of the light that passes through it, but NOT redshift. This is due to absorption of the blue components of light (more-so in UV). Light Reddening due to absorption by dust.
Interstellar gas The interstellar gas is very dilute, about ONE atom per cubic centimeter. In some places it is much denser. The distribution of gas is very uneven. It is mostly Hydrogen (90%), Helium (9%), and everything else (1%).
Molecular cloud with EGGs: Evaporating Gaseous Globules these can be
seen as pillars and egg-like objects Some properties of these nebulae: note that these are AVERAGE quantities; the nebulae are actual quite uneven in their density and
temperature. Note the huge mass (hundreds to thousands of solar masses) and sizes. Radio Emission reveals the dark dust cloud. Hydrogen 21-cm Emission (shortwave radio) In visible light, we can
see the Horsehead Nebula, a dark dust cloud. A reflection nebula is seen to the lower left of the Horsehead. This is due to scattered light from a nearby
bright star. The pink nebula is an emission nebula, due to UV illumination by a hot star. Star formation a 7 stage process
1 an interstellar cloud 2 shrinking cloud fragments 3 a fragment is the size of our solar system 4 protostar center reaches 1,000,000 K 5 protostar at ~10 solar radius, 4000K surf. 6 ignition of fusion in core, now a star 7 reaches main sequence
Orion Nebula, A closer look reveals globules or EGGs, some of which may contain protostars. Several disks
that may be protoplanetary disks are found after blowing up the Hubble photo. a.k.a. proplyds A protostar can be
plotted on the HR diagram after reaching stage 4. It is heated solely due to contraction and is fairly cool, but might be 1000 times as luminous
as our Sun, mostly in the infrared part of the spectrum. Plotting a newborn star on the HR Diagram Stage 5 T Tauri
stage has violent surface activity and may form jets Stage 6 core at 10 million K and finally get fusion, so now it is a star Stage 7 reaches
the main sequence Pre-stellar Evolutionary Tracks for stars of other masses The minimum mass needed to get
nuclear fusion and produce a real star is about 0.08 solar mass, or about 80 times the mass of Jupiter. With less mass all we get are brown dwarfs
which are failed stars. Stars with Masses between 0.08 and 0.4 times the mass of the Sun have low core temperatures,
live a long time, convect helium from the core, so it mixes uniformly, and will end up composed entirely of helium. A G-Type Star is similar
to our Sun (1 solar mass) The evolution is shown during an imaginary trek through space. At the end of the red giant stage, the core is small, the envelope huge,
and the outcome depends on the total mass of the star. Evolution of stars with more than 0.4 solar masses Solar Composition Change During stage 7
hydrogen burning causes a build-up of helium in the stars core. We will follow the evolution of a star like the Sun, with one solar mass.
Hydrogen Shell Burning occurs around an ash core, which is mostly helium, and the temperature is T = 10 million K The hydrogen shell burning causes higher pressure on the envelope, which causes the star to expand into a Red Giant.
The star follows the yellow curve on the HR diagram. Stage 8 is the subgiant branch and the radius is about 3 times the solar radius.
An example is the star Arcturus, M = 1.5 Msolar and R = 23 Rsolar, the luminosity is about 100 times solar. Stage 10 follows the Helium Flash,
which is like a huge nuclear explosion of helium flashing or burning quickly into carbon at 108 K Fusion of 3 He-4 nuclei produces a
C-12 nucleus plus other products Then there is the Horizontal Branch Helium Shell Burning on the Horizontal Branch
Reascending the Giant Branch occurs in a way similar to the original move up to a giant. Burning in the H and He shells is even faster than before,
so the star expands even more on this asymptotic branch Planetary Nebulae form when the core cant reach 600 million K, the minimum needed for carbon burning. All but the core of the star blows off to form a
Planetary Nebula . A white dwarf remains. A Planetary Nebula with the shape of a ring, 0.5 pc across, called the Ring Nebula. White Dwarf formation on the HR Diagram
Some heavier elements are formed in the last years of the burning in the shells surrounding the carbon core. H, He, C, O, and some Ne and Mg are expelled from
the star as a planetary nebula High-Mass stellar evolutionary tracks are quite different from the low-mass stellar evolution
tracks. Notice that the core can heat up so fast that the envelope of the star tends to lag behind. Carbon fusion can start before the red giant phase.
Heavy Element Fusion - shells like an onion A Type II Supernova is a core collapse and occurs when the core is finally pure iron, which cannot be fused to other elements. The core collapses to a big ball of neutrons, which causes a
shock wave to bounce back outward, which blows off the entire envelope of the red giant, to form a supernova remnant. M1 the Crab Nebula is from a supernova
seen in year A.D. 1054 The remnant is 1800 pc away and the diameter is currently 2 pc.
Supernova Light Curves fall into two types Sirius Binary System: Sirius B is a white dwarf Sirius B has a high mass for a white dwarf, and probably came from a mass 4 Msolar star. White dwarfs are about the size of the Earth.
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