chapter 22.1; evolution from main sequence to red giants & 22.2 star clusters & 22.3 &22.4 further evolution

zero-age main sequence

• left-hand edge of the main sequence band in the H-R diagram

• zero-age: stars stopped contracting, thus becomes main-sequence, and begins to fuse hydrogen in its core

• continuous line in the H-R diagram that shows where stars of different masses but similar chemical composition can be found when they begin to fuse hydrogen

• when a star’s luminosity and temperature change, the point that represents the star on the H-R diagram moves away from the zero-age main sequence

lifetimes on the main sequence

• how many years a star remains in the main sequence band depends on its mass

• lifetime in a particular stage depends on how much nuclear field it has and how quickly it uses up that fuel

  • more massive stars use up their fuel more quickly than stars of low mass (rate of fusion depends strongly on the star’s core temperature, core temperature is determined by the mass of the star)

main-sequence to red giant

• all hydrogen eventually runs out in a star’s core

• core then contains only helium and some heavier elements

• energy can no longer be generated by hydrogen fusion in the stellar core, and the fusion of helium requires much higher temperatures

• no nuclear energy source to supply heat to core

• thus core contracts; stars energy is partially supplied by gravitational energy (as the star’s core shrinks, the energy of the inward-falling material is converted to heat

• the core becomes hot enough for an outer layer of hydrogen to undergo hydrogen fusion once again

• this energy leaves this hydrogen burning shell and begins to heat up layers of the star farther out, causing them to expand

  • meanwhile, helium core contracts, more heat around it

• increase in luminosity occurs

• with new energy pouring outwards, the outer layers of the star begin to expand, and the star eventually grows and grows until it reaches enormous proportions

• expansion of star’s outer layers cases the temperature at the surface to decrease (colour becomes redder)

• massive stars become red super giants

• lower-mass stars like the Sun become red giants

characteristics of star clusters

globular clusters

• nearly symmetrical round systems, of hundreds of thousands of stars

• most mass in Milky Way: Omega Centauri (16,000ly away, several million stars within it)

  • the brightest stars in this cluster, which are red giants that have already completed the main-sequence phase of their evolution, are red-orange in color, surface temperature of 4000K

• within dense central regions, the stars would be roughly a million times closer together than in our own neighborhood

• 150 known in the Milky Way

• most are in a spherical halo surrounding the flat disk formed by the majority of our Galaxy’s stars

• all are very far from Sun, some at distances of 60,000ly or more away from main disk

• diameters of globular star clusters: 50ly to more than 450ly

open clusters

• found in disk of Milky Way

• range of ages

• smaller than globular clusters (diameters less than 30ly)

• stars appear well separated

• thousands within Milky Way

  • can only see a fraction, interstellar dust dims the others that are more further away

• remains as a cluster for only a few million years

stellar associations

• association: group of extremely young stars, typically containing 5-50 hot, bright O and B tars scattered over a region of space some 100-500 ly in diameter

• e.g., most stars in Orion form stellar associations

• found in regions rich in the gas and dust required to form new stars

main-sequence turnoff

• the location in the H-R diagram where the stars have begun to leave the main sequence

triple-alpha process

• once the core of a red giant reaches 100 million K (occurs due to the the core contracting), three helium atoms can begin to fuse to form a single carbon nucleus

helium flash

• when the triple-alpha process begins in low-mass (0.8-2.0 solar masses) stars, calculations show that the entire core is ignited in a quick burst of fusion

• As soon as the temperature at the center of the star becomes high enough to start the triple-alpha process, the extra energy released is transmitted quickly through the entire helium core, producing very rapid heating. The heating speeds up the nuclear reactions, which provide more heating, and which accelerates the nuclear reactions even more. We have runaway generation of energy, which reignites the entire helium core in a flash.

• by the time a star has reached a helium flash, it has already lost 25% of its mass

what happens after a helium flash?

• star readjusts to the release of energy from the triple-alpha process in its core

• its surface temperature increase

• overall luminosity decreases

• star begins to fuse helium in its core for a while

  • However, at a temperature of 100 million K, the inner core is converting its helium fuel to carbon (and a bit of oxygen) at a rapid rate. Thus, the new period of stability cannot last very long: it is far shorter than the main-sequence stage.

  • all the helium will be used up, and the star cannot generate energy via fusion

• star shrinks

• the star now has a multi-layered structure like an onion: a carbon-oxygen core, surrounded by a shell of helium fusion, a layer of helium, a shell of hydrogen fusion, and finally, the extended outer layers of the star

• As energy flows outward from the two fusion shells, once again the outer regions of the star begin to expand. Its brief period of stability is over; the star moves back to the red-giant domain on the H–R diagram for a short time

evolution of a star with sun’s mass

planetary nebulae

• formed after stars shrink and reach surface temperatures of 100,000K

• produce stellar winds and ultraviolet radiation

  • these winds and radiation heat shells, ionise them, and set them aglow

• shells expand at speeds of 20-30km/s

• diameter of about 1 ly

where are heavier elements formed?

• massive stars

• The outer layers of a star with a mass greater than about 8 solar masses have a weight that is enough to compress the carbon-oxygen core until it becomes hot enough to ignite fusion of carbon nuclei. Carbon can fuse into still more oxygen, and at still higher temperatures, oxygen and then neon, magnesium, and finally silicon can build even heavier elements.

• Iron is, however, the endpoint of this process.

  • fusion of iron atoms produces products more massive than the nuclei being fused and therefore the process requires energy as opposed to releasing energy

nucleosynthesis

• the making new atomic nuclei