Phys141 Principles of Physical Science Chapter 18 The Universe Instructor: Li Ma Office: NBC 126 Phone: (713) Webpage: Department of Computer Science & Physics Texas Southern University, Houston Nov. 24, 2004

Topics To Be Discussed The Sun The Life Cycle of Stars Galaxies Skip –§18.2 The Celestial Sphere –§18.3 Classifying Stars –§18.6 Cosmology

The Universe The totality of all matter, energy, and space In this chapter, stars, galaxies, and the celestial sphere will be discussed Milky Way Galaxy is only a tiny part of our universe Our solar system only occupies a very small volume of Milky Way Galaxy

The Sun The Sun is a star – a self-luminous sphere of hot gas The hot gas is held together by gravity and energized by nuclear reactions in its core The diameter of the Sun is almost four times the distance between the Earth and the Moon

The Sun (cont) Table 18.1 on page 460 –diameter, mass, density, distance from the Earth, and so on Figure 18.1 on page 460 –A cross-sectional view of the Sun The bright, visible “surface” of the Sun is called photosphere –The distinct feature of the Sun’s surface is the periodic occurrence of sunspots

The Sun (cont) Sunspots are huge patches of cooler material on the surface of the Sun –Figure 18.2 & 18.3 on page 461 The number of sunspots appearing on the Sun varies over an 11-year sunspot cycle

The Life Cycle of Stars In general, a star’s life cycle is: be born, radiate energy, expand, possibly explode, and then die The exact details depend on a star’s mass and its composition The greater the mass of a star, the faster it moves through its life cycle

The Life Cycle of Stars (cont) The life cycle of low-mass stars is: protostar, main-sequence star, red giant, planetary nebula, and finally white dwarf Stars may increase in brightness by a factor of 100 to millions in hours A star undergoing such a drastic increase in brightness is called a nova, or “new” star

The Life Cycle of Stars (cont) A nova is the result of a nuclear explosion on the surface of a white dwarf Supernova is a gigantic explosion of stars A Type II supernova results from the collapse of the iron core of a massive red supergiant, leaving behind either a neutron star or a black hole

The Life Cycle of Stars (cont) Evolution of a high-mass star is: when they leave the main sequence, they become supergiants and eventually explode as Type II supernova, scattering much of their material into space and leaving behind a neutron star, or in the case of most massive star, a black hole –Figure on page 477

Galaxies A galaxy is an extremely large collection of stars bound together by mutual gravitational attraction Galaxies are the fundamental components of the structure of the universe Our solar system occupies a relatively small volume of space in very large system about 100 to 200 billion stars known as Milky Way Galaxy

Galaxies (cont) Galaxies have been classified into 4 types based on how they look in photographs: –Elliptical galaxy –Normal spiral galaxy –Barred spiral galaxy –Irregular galaxy Milky Way Galaxy is a normal spiral galaxy

Assignment Homework Assignment –Review Questions (page 488 – 489): 3,4,32,40,44,49,50 –Exercises (page 490): 3 (steps of calculation are required since answer is already given in the book) –It’s due Monday, 12/6/04