Objectives
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Determine how distances between stars are measured. Distinguish between brightness and luminosity. Identify the properties used to classify stars. Section 29.2. Objectives. Measuring the Stars.

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Objectives

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Objectives

Determine how distances between stars are measured.

Distinguish between brightness and luminosity.

Identify the properties used to classify stars.

Section 29.2

Objectives

Measuring the Stars


Groups of stars

Long ago, many civilizations looked at the brightest stars and named groups of them after animals, mythological characters, or everyday objects. These groups of stars are called constellations.

Today, astronomers group stars by the 88constellations named by ancient peoples.

Section 29.2

Groups of Stars

Measuring the Stars


Groups of stars1

Some constellations are visible throughout the year, depending on the observer’s location. Constellations that appear to rotate around one of the poles are called circumpolar constellations. Ursa Major, also known as the Big Dipper, is a circumpolar constellation for the northern hemisphere.

Section 29.2

Groups of Stars

Measuring the Stars


Objectives

Unlike circumpolar constellations, the other constellations can be seen only at certain times of the year because of Earth’s changing position in its orbit around the Sun.

Section 29.2

Measuring the Stars


Objectives

The most familiar constellations are the ones that are part of the zodiac. These twelve constellations can be seen in both the northern and southern hemispheres.

Section 29.2

Measuring the Stars


Objectives

Section 29.2

Measuring the Stars

Star clusters

By measuring distances to variable stars and observing how their gravities interact with each other, scientists can determine which stars are gravitationally bound to each other. A group of stars that are gravitationally bound to each other is called a cluster.


Objectives

Section 29.2

Measuring the Stars

Star clusters

An open cluster is a group of stars that are not densely packed.

A globular cluster is a group of stars that are densely packed into a spherical shape.


Objectives

Section 29.2

Measuring the Stars

Binaries

When only two stars are gravitationally bound together and orbit a common center of mass, they are called binary stars. More than half of the stars in the sky are either binary stars or members of multiple-star systems.


Objectives

Section 29.2

Measuring the Stars

Doppler shifts

The most common way to tell that a star is one of a binary pair is to find subtle wavelength shifts, called Doppler shifts. Scientists use Doppler shifts to determine the speed and direction of a star’s motion.


Objectives

Section 29.2

Measuring the Stars

Doppler shifts

 Movie Clip 

When a star moves toward the observer, the light emitted by the star shifts toward the blue end of the electromagnetic spectrum. When a star moves away from the observer, its light shifts toward the red.


Stellar positions and distances

Section 29.2

Stellar Positions and Distances

Measuring the Stars

Astronomers use two units of measure for long distances. One is the light-year (ly). A light-year is the distance that light travels in one year, equal to 9.461 × 1012 km. Astronomers often use a unit larger than a light-year—a parsec (pc), which is equal to 3.26ly, or 3.086 × 1013 km.


Objectives

Section 29.2

Measuring the Stars

Parallax

When estimating the distance of stars from Earth, astronomers must account for the fact that nearby stars shift in position as observed from Earth. This apparent shift in position caused by the motion of the observer is called parallax.


Basic properties of stars

Section 29.2

Basic Properties of Stars

Measuring the Stars

The basic properties of a star are mass, diameter, temperature, and luminosity; which are all related to each other.


Magnitude

Section 29.2

Magnitude

Measuring the Stars

Temperature controls the nuclear reaction rate and governs the luminosity, or apparent magnitude, which is how bright the stars and planets appear in the sky from Earth.


Objectives

Section 29.2

Measuring the Stars

Magnitude

Absolute magnitude is how bright a star would appear if it were placed at a distance of 10 pc. The absolute magnitude compared to the apparent magnitude is used to find the distance to a star.


Objectives

Section 29.2

Measuring the Stars

Temperature

Stars are assigned spectraltypes in the following order: O, B, A, F, G, K, and M. Each class is subdivided into more specific divisions with numbers from 0 to 9.


Objectives

Section 29.2

Measuring the Stars

Temperature

 Movie Clip 


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