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Different kinds of Stars. In an Hertzsprung-Russell Diagram (or H-R diagram or color-magnitude diagram) each star is plotted as a point. The horizontal axis of the H-R diagram can be any of several measures of surface temperture:.
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In an Hertzsprung-Russell Diagram (or H-R diagram or color-magnitude diagram) each star is plotted as a point.
The horizontal axis of the H-R diagram can be any of several measures of surface temperture: • The surface temperature itself, often called the effective temperature or Teff • The color • The spectral class
Spectral Class A classification based on the taxonomy of a star’s spectrum Now known to be a sequence of surface temperture: • hot O, B, A, F, G, K, M cool • Often split into subclasses 0—9, e.g., G2 • And further qualified with a roman numeral giving the luminosity or size, e.g., G2V (V = dwarfs, III = giants, I = supergiants) • The Sun is a G2V star • A few other examples; • O & B stars are massive and shortlived • M dwarfs are the common low mass stars
The stars do not lie in random locations in the H-R diagram but lie on distinct sequences.
We find different types of star because: • We find stars in different evolutionary stages • Stars have different initial masses • While one might expect that different chemical composition is important, most stars have a composition much like that of the sun: H 70%, He 28% • The evolutionary phases after core H-burning are short compared to the core-H burning phase. Hence these stars are relatively rare.
At least as for the most important difference between stars is the initial mass. • As M increases self gravity increases • to maintain the balance with gravity as M increases the pressure P must increase. • the increase in P requires an increase in the interior temperature T • the increased T leads to increased nuclear burning rate and increased luminosity L • Indeed L increases rapidly with M .
Core H-burning stars of differing mass define the main-sequence in the H-R diagram.
amount of fuel rate of burning mass luminoisity 2M 25L More massive stars have shorter lifetimes lifetime Consider Sirius: M ~ 2M L ~ 25L M L lifetimeSirius ~ 0.1 ~ 0.1 lifetime ~ 1 Gyr There is no intelligent life at Sirius Indeed, no star with M > 1.2 M is a good SETI candidate
But… Massive stars are still important for SETI because…
For stars more massive than 10M, carbon and eventually several other nuclear species ignite. The final nuclear burning stage of such a star is sort of an onion-like structure with many nuclear shells. The core is usually called the “iron core” although it is actually composed of mostly of 56Ni.
56Ni is the most strongly bound nucleus at the immense densities found in the core. (56Fe is more stable at Earth-like densities---hence the confusing nomenclature.) No more energy can be obtained from nuclear fusion---the core is the ultimate nuclear ash. The “Fe” core is supported by degeneracy pressure from electrons. Basically it is an Fe white dwarf in center of a massive star. However it is a white dwarf which is increasing in mass since the nuclear burning shells are dumping more “Fe” on its outer parts.
The White Dwarf Mass Limit 1930 Chandrasekhar worked out structure of white dwarfs. He was able to derive a relation between the radius and mass For WDs bigger than 1.4 Mthere was no stable WD configuration. Degeneracy pressure cannot balance gravity White dwarf or Chandrasekhar mass limit ~ 1.4 M.
If a WD grows in mass, when M > 1.4 M collapse So when the mass of the Fe core exceeds 1.4 M it will collapse. Ordinarily neutrons are unstable: neutron proton + electron + anti-neutrino n p + e- + However if the pressure is high enough the reaction can be forced to reverse. p + e- n + Don't worry about the neutrini and anti-neutrini.
When the pressure in the WD is big enough the e- start combining with the n in the Fe nuclei • But the pressure holding up the WD is supplied by the e-, • so when the number of e- decreases pressure starts to drop • so gravity is bigger than • contraction • Fe core adjusts to get higher pressure • p + e- n + goes even faster • this cycles back to the beginning and we have a runaway. • In about 1 second the Fe core collapses almost to nothing.
When it has a radius of about 10 km the collapse is stopped by a repulsive component of the nuclear force & degeneracy pressure from neutrons The core bounces; the exterior of the onion which is following the core inward hits the bouncing core and there is a huge explosion. The exterior with all of the “nuclear ashes” is blasted back into the interstellar medium. Most of the common elements from carbon up to iron are made in exploding stars like this. The remnant core is a ball of neutrons called a neutron star. These exploding massive stars are observed as supernovae (SN) (In particular, the scenario described is a Type II SN)
Ejecta of Type II Supernovae are enriched in heavy elemants: C, O, Ne, Mg, Si, S, Ar, Ca …. • Other kinds of SN and red giants produce other elements. • Almost all elements and isotopes heavier than He are made in stars. • Which elements are abundant in the galaxy now is determined by nuclear physics • Element building (nucleosynthesis) was very active in the early galaxy (Age < 1 Gyr)
Conclusions---stars • During the time life has evolved on Earth the Sun's luminosity has increased 20—30%. • Stars bigger than ~1.2 M do not live long enough for intelligent life to evolve. • Since we tend to see high luminosity, hence high M stars, most familiar naked eye stars are not good ETI candidates.
The elements of which we are made were produced in earlier generations of stars. Which elements are common is determined by the laws of nuclear physics. • We are made (mostly) of the most common elements and such elements are common throughout the Galaxy (and Universe) • “First generation” stars are deficient in the elements (C, O. Si, out of which living things are made. (Such stars are fairly rare.) • The “heavy elements” built up very rapidly in the early Galaxy.There have been adequate heavy elements to make planets and critters throughout most of the life of the Galaxy. (Remember:Age of Galaxy 15 Gyr; Age of Solar System 5 Gyr) • The other terms in the Drake Equation also affect the value we adopt for R. We will estimate a new value later.
