1. FORMATION OF STARS SES4U

2. OBJECTIVES 1. Name, describe, and give examples of several kinds of nebulae and explain the relationship between nebulae and stars. 2. Describe the formation of red giants and dwarfs. 3. Describe the formation of novas, supernovas, neutron stars, and black holes.

3. Topic 10 Origin of Stars Background: In parts of space between stars exist huge clouds of very low density dust (1%) and gas (99% - mostly hydrogen). The grains of the strange dust are very tiny with diameters of about 1/10 000 cm [silicon carbide, graphite, diamond, and minor amounts of nitrogen and other elements.] It is believed that this gas comes from the explosion of stars that have become novas or supernovas [Topic 13] that have scattered this material over vast areas.

4. TOPIC 10 Origin of a Star Background: An average cloud is about 25 light- years in diameter and may contain enough material to form many stars. A force from outside the cloud [shockwave from a supernova] causes the cloud to begin to condense into stars by triggering the force of gravity that exists between the gas atoms and dust grains. Huge areas become denser throughout the cloud and temperatures increase as the areas contract.

5. TOPIC 10 Origin of a Star (a) The great nebulae are clouds of gas and dust in space of which most are invisible. A diffuse nebulae is made visible from the light of a nearby bright star. [Great Nebula in the constellation Orion.] (b) Other nebulae, called dark nebula show up as a dark patch against the more distant stars. [The Horsehead Nebula in Orion]

6. TOPIC 10 Origin of a Star (c) According to theory, stars continually form wherever dense clouds of gas and dust exist. (d) As the cloud contacts under the influence of gravity, the temperature increases and, if the cloud is large enough, parts of it will start to glow. These large glowing cloud sections are called protostars.

7. TOPIC 10 Origin of a Star (e) As contraction continues, the protostars become hotter and brighter. Eventually the centre is so hot that a fusion reaction begins and the protostar has become a star. (f) The star stops contracting when the release of energy from the fusion of hydrogen in the centre counterbalances the force of gravity. At this point the star has reached a stable state.

8. TOPIC 11: Formation of Red Giants (a) In a stable state a star’s diameter and radiation remain constant for billions of years. When so many of the core’s light nuclei are used up that the energy of fusion no longer balances the force of gravity the star loses its stability. (b) When the star loses its stability the centre of the star contracts again. The core gets so hot that it causes the star’s outer layers to expand increasing the star’s surface area. The star again radiates more light and appears brighter. The fusion reaction starts occuring in the outer layers. The star expands further and becomes a red giant or supergiant.

9. TOPIC 12: Formation of White Dwarfs (a) When most of the fuel for the fusion reaction is used up the temperature and pressure of the core can no longer support the weight of its outer layers. The giant collapses, the nuclei of the atoms are squeezed tightly together and the star becomes a white dwarf and is probably no larger than Earth.

10. TOPIC 12: Formation of White Dwarfs (b) A nova [new star] forms from a white dwarf that has flared up brilliantly, brightening a hundred a hundred to a million times. A nova may be the result of the bombardment by a companion star. Novas fade to their former luminosity in a few years at most. (c) The sun is thought to be 5 billion years old still in its stable stage. It is expected to remain stable for another 5 billion years before it swells to a red giant and eventually collapses to a white dwarf.

11. TOPIC 13: Supernovas (a) Stars with at least 7 times the sun’s mass become red giants in a relatively short few million years. (b) When fusion has stopped, it leaves a central iron core. As the star starts to cool, the core collapses. With the collapse, the pressures and temperatures within the core rise dramatically and the iron nuclei become fused into heavier elements. In a rush toward further collapse, the star explodes so violently that half its mass is blown away as a great cloud. The star flares up into an intensely bright object called a supernova. [see page 387, figure 21.10]

12. TOPIC 13: Supernovas (d) The Crab Nebula is the constellation of Taurus the Bull is a great expanding cloud of gas formed from a supernova observed by Chinese astronomers in a.d. 1054. this brilliant star faded after a year.

13. TOPIC 14: Neutron Stars and Black Holes 13(b) A supervova removes only about one-half of the exploding stars mass. The mass of what remains after the explosion is what astronomers call a neutron star. 14(a) Astronomers think that in the core of a supernova, the forces are so great that every atom’s electrons are crushed into the nucleus combining with protons to form neutrons. All of the core’s nuclei merge into a single, dense mass of neutrons. A typical neutron star is about 10 km in diameter and trillions of times more dense than the sun.

14. TOPIC 14: Neutron Stars and Black Holes (b) In very massive stars, the nuclear forces between neutrons become overwhelmed by the gravitational forces and the star collapses into a very small volume. These objects have gravitational forces so powerful that even light cannot escape. These invisible objects are called black holes.