FORMATION OF FORMATION OF STARSSTARS

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OBJECTIVESOBJECTIVES1.1. Name, describe, and give examples Name, describe, and give examples

of several kinds of nebulae and of several kinds of nebulae and explain the relationship between explain the relationship between nebulae and stars.nebulae and stars.

2.2. Describe the formation of red giants Describe the formation of red giants and dwarfs.and dwarfs.

3.3. Describe the formation of novas, Describe the formation of novas, supernovas, neutron stars, and black supernovas, neutron stars, and black holes.holes.

Topic 10 Origin of StarsTopic 10 Origin of StarsBackground:Background: In parts of space between In parts of space between

stars exist huge clouds of very low density stars exist huge clouds of very low density dust (1%) and gas (99% - mostly dust (1%) and gas (99% - mostly hydrogen). The grains of the strange dust hydrogen). The grains of the strange dust are very tiny with diameters of about 1/10 are very tiny with diameters of about 1/10 000 cm [silicon carbide, graphite, 000 cm [silicon carbide, graphite, diamond, and minor amounts of nitrogen diamond, and minor amounts of nitrogen and other elements.]and other elements.]

It is believed that this gas comes from the It is believed that this gas comes from the explosion of stars that have become novas explosion of stars that have become novas or supernovas [Topic 13] that have or supernovas [Topic 13] that have scattered this material over vast areas.scattered this material over vast areas.

TOPIC 10 Origin of a TOPIC 10 Origin of a StarStar

Background:Background: An average cloud is about 25 An average cloud is about 25 light-years in diameter and may contain light-years in diameter and may contain enough material to form many stars. A force enough material to form many stars. A force from outside the cloud [from outside the cloud [shockwave from a shockwave from a supernovasupernova] causes the cloud to begin to ] causes the cloud to begin to condense into stars by triggering the force of condense into stars by triggering the force of gravity that exists between the gas atoms gravity that exists between the gas atoms and dust grains. Huge areas become denser and dust grains. Huge areas become denser throughout the cloud and temperatures throughout the cloud and temperatures increase as the areas contract.increase as the areas contract.

TOPIC 10 Origin of a TOPIC 10 Origin of a StarStar

(a)(a) The great The great nebulaenebulae are clouds of gas are clouds of gas and dust in space of which most are and dust in space of which most are invisible. A invisible. A diffuse nebulaediffuse nebulae is made is made visible from the light of a nearby bright visible from the light of a nearby bright star. [star. [Great Nebula in the constellation Great Nebula in the constellation OrionOrion.].]

(b)(b) Other nebulae, called Other nebulae, called dark nebuladark nebula show up as a dark patch against the show up as a dark patch against the more distant stars. [more distant stars. [The Horsehead The Horsehead Nebula in OrionNebula in Orion]]

TOPIC 10 Origin of a TOPIC 10 Origin of a StarStar

(c) According to theory, stars (c) According to theory, stars continually form wherever dense continually form wherever dense clouds of gas and dust exist.clouds of gas and dust exist.

(d) As the cloud contacts under the (d) As the cloud contacts under the influence of gravity, the temperature influence of gravity, the temperature increases and, if the cloud is large increases and, if the cloud is large enough, parts of it will start to glow. enough, parts of it will start to glow. These large glowing cloud sections These large glowing cloud sections are called are called protostarsprotostars..

TOPIC 10 Origin of a TOPIC 10 Origin of a StarStar

(e) As contraction continues, the protostars (e) As contraction continues, the protostars become hotter and brighter. Eventually become hotter and brighter. Eventually the centre is so hot that a the centre is so hot that a fusion fusion reactionreaction begins and the protostar has begins and the protostar has become a star.become a star.

(f) The star stops contracting when the (f) The star stops contracting when the release of energy from the fusion of release of energy from the fusion of hydrogen in the centre counterbalances hydrogen in the centre counterbalances the force of gravity. At this point the star the force of gravity. At this point the star has reached a has reached a stable state.stable state.

TOPIC 11: Formation of TOPIC 11: Formation of Red GiantsRed Giants

(a)(a) In a stable state a star’s diameter and In a stable state a star’s diameter and radiation remain constant for billions of years. radiation remain constant for billions of years. When so many of the core’s light nuclei are When so many of the core’s light nuclei are used up that the energy of fusion no longer used up that the energy of fusion no longer balances the force of gravity the star loses its balances the force of gravity the star loses its stability.stability.

(b)(b) When the star loses its stability the centre of When the star loses its stability the centre of the star contracts again. The core gets so hot the star contracts again. The core gets so hot that it causes the star’s outer layers to expand that it causes the star’s outer layers to expand increasing the star’s surface area. The star increasing the star’s surface area. The star again radiates more light and appears again radiates more light and appears brighter. The fusion reaction starts occuring brighter. The fusion reaction starts occuring in the outer layers. The star expands further in the outer layers. The star expands further and becomes a and becomes a red giant or supergiantred giant or supergiant..

TOPIC 12: Formation of TOPIC 12: Formation of White DwarfsWhite Dwarfs

(a) When most of the fuel for the (a) When most of the fuel for the fusion reaction is used up the fusion reaction is used up the temperature and pressure of the temperature and pressure of the core can no longer support the core can no longer support the weight of its outer layers. The giant weight of its outer layers. The giant collapses, the nuclei of the atoms are collapses, the nuclei of the atoms are squeezed tightly together and the squeezed tightly together and the star becomes a star becomes a white dwarfwhite dwarf and is and is probably no larger than Earth.probably no larger than Earth.

TOPIC 12: Formation of TOPIC 12: Formation of White DwarfsWhite Dwarfs

(b) A (b) A novanova [new star] forms from a white dwarf [new star] forms from a white dwarf that has flared up brilliantly, brightening a that has flared up brilliantly, brightening a hundred a hundred to a million times. A nova hundred a hundred to a million times. A nova may be the result of the bombardment by a may be the result of the bombardment by a companion star. Novas fade to their former companion star. Novas fade to their former luminosity in a few years at most.luminosity in a few years at most.

(c) The sun is thought to be 5 billion years old (c) The sun is thought to be 5 billion years old still in its stable stage. It is expected to still in its stable stage. It is expected to remain stable for another 5 billion years remain stable for another 5 billion years before it swells to a red giant and eventually before it swells to a red giant and eventually collapses to a white dwarf.collapses to a white dwarf.

TOPIC 13: SupernovasTOPIC 13: Supernovas(a)(a) Stars with at least 7 times the sun’s mass Stars with at least 7 times the sun’s mass

become red giants in a relatively short few become red giants in a relatively short few million years. million years.

(b)(b) When fusion has stopped, it leaves a central iron When fusion has stopped, it leaves a central iron core. As the star starts to cool, the core core. As the star starts to cool, the core collapses. With the collapse, the pressures and collapses. With the collapse, the pressures and temperatures within the core rise dramatically temperatures within the core rise dramatically and the iron nuclei become fused into heavier and the iron nuclei become fused into heavier elements. In a rush toward further collapse, the elements. In a rush toward further collapse, the star explodes so violently that half its mass is star explodes so violently that half its mass is blown away as a great cloud. The star flares up blown away as a great cloud. The star flares up into an intensely bright object called a into an intensely bright object called a supernovasupernova. [see page 387, figure 21.10]. [see page 387, figure 21.10]

TOPIC 13: SupernovasTOPIC 13: Supernovas(d) The (d) The Crab NebulaCrab Nebula is the is the

constellation of Taurus the Bull is a constellation of Taurus the Bull is a great expanding cloud of gas formed great expanding cloud of gas formed from a supernova observed by from a supernova observed by Chinese astronomers in a.d. 1054. Chinese astronomers in a.d. 1054. this brilliant star faded after a year.this brilliant star faded after a year.

TOPIC 14: Neutron Stars TOPIC 14: Neutron Stars and Black Holesand Black Holes

13(b) A supervova removes only about one-half 13(b) A supervova removes only about one-half of the exploding stars mass. The mass of of the exploding stars mass. The mass of what remains after the explosion is what what remains after the explosion is what astronomers call a astronomers call a neutron starneutron star..

14(a) Astronomers think that in the core of a 14(a) Astronomers think that in the core of a supernova, the forces are so great that every supernova, the forces are so great that every atom’s electrons are crushed into the atom’s electrons are crushed into the nucleus combining with protons to form nucleus combining with protons to form neutrons. All of the core’s nuclei merge into neutrons. All of the core’s nuclei merge into a single, dense mass of neutrons. A typical a single, dense mass of neutrons. A typical neutron star is about 10 km in diameter and neutron star is about 10 km in diameter and trillions of times more dense than the sun.trillions of times more dense than the sun.

TOPIC 14: Neutron Stars TOPIC 14: Neutron Stars and Black Holesand Black Holes

(b) In very massive stars, the nuclear (b) In very massive stars, the nuclear forces between neutrons become forces between neutrons become overwhelmed by the gravitational overwhelmed by the gravitational forces and the star collapses into a forces and the star collapses into a very small volume. These objects very small volume. These objects have gravitational forces so powerful have gravitational forces so powerful that even light cannot escape. These that even light cannot escape. These invisible objects are called invisible objects are called black black holesholes..