UNIT NINE Matter and Motion in the Universe

UNIT NINE: Matter and Motion in the Universe § Chapter 26 The Solar System § Chapter 27 Stars § Chapter 28 Exploring the Universe

Chapter Twenty-Eight: Exploring the Universe § 28. 1 Tools of Astronomers § 28. 2 Galaxies § 28. 3 Theories about the Universe

Section 28. 2 Learning Goals § Classify galaxies according to shape. § Examine the features of the Milky Way galaxy. § Tell how astronomers determine the distances between objects in space.

28. 2 Galaxies § A galaxy is a huge group of stars, dust, gas, and other objects bound together by gravitational forces. § The sun, along with an estimated 200 billion other stars, belongs to the Milky Way galaxy.

28. 2 Types of galaxies § The Milky Way is a flattened, rotating system that contains young to middle-aged stars, along with gas and dust. § Astronomers identify it as a spiral galaxy.

28. 2 Types of Galaxies Astronomers classify galaxies according to their shape. 1. Spiral galaxies consist of a central, dense area surrounded by spiraling arms. 2. Barred spiral galaxies have a bar-shaped structure in the center. 3. Elliptical galaxies look like the central portion of a spiral galaxy without the arms. 4. Lenticular galaxies are lens-shaped.

Can you classify these galaxies?

28. 2 The central black hole theory § Recent studies have suggested that a black hole, with a mass of more than a million Suns, exists at the very center of our galaxy. § The evidence for a huge black hole comes from measurements of the orbital velocities of stars and gas at the center. § One of the strangest predictions of Einstein’s theory of relativity is the existence of black holes.

28. 2 The central black hole theory § The minimum speed an unpowered projectile must have to escape the planet’s gravity is called the escape velocity. § A black hole is an object with such strong gravity that its escape velocity equals or exceeds the speed of light.

28. 2 Distances between galaxies § The distances between stars are 10, 000 times greater than the distances between planets. § The distances between galaxies are a million times greater than the distances between stars.

28. 2 Distances between galaxies § Figuring out the distance between galaxies is one of the more difficult tasks in astronomy. § A faint object in the night sky could be a dim object that is relatively nearby or a bright object that is far, far away.

28. 2 Distances between galaxies § The most reliable method for estimating the distance to a galaxy is to find a star whose luminosity is known. § If the luminosity is known, the inverse square law can be used to find the distance from the observed brightness.

28. 2 Distances between galaxies § The inverse square law shows how the brightness of an object decreases as you move away from it.

28. 2 Distances between galaxies § The inverse square law is important to astronomers because if they know the brightness and luminosity of an object, they can determine its distance by rearranging equation variables.

28. 2 Distances between galaxies § Astronomers have found a way to infer values for luminosity (L) using a source of light called a standard candle. § A standard candle is an object, such as a star, whose luminosity is known. § By determining the unknown star’s temperature (using a spectrometer), they can infer its luminosity by choosing a similar main sequence star on the H-R diagram.

28. 2 Distances between galaxies § A second type of standard candle is called a Cepheid star. § Cepheid stars “pulsate” in regular periods ranging from a few days to a few weeks. § By measuring the period of a Cepheid star, astronomers determine its luminosity and then calculate its distance.

Investigation 28 B Astronomy and Light § Key Question: How do we measure the distance to stars and galaxies?