The Sun and Other Stars How do stars

The Sun and Other Stars • How do stars shine? • How are stars layered? • How does the Sun change over short periods of time? • How do scientists classify stars?

How Stars Shine • A star is a large ball of gas held together by gravity with a core so hot that nuclear fusion occurs. • Nuclear fusion occurs when the nuclei of several atoms combine into one larger nucleus.

How Stars Shine (cont. ) • Nuclear fusion releases a large amount of energy. • A star shines because when energy leaves a star’s core, it travels throughout the star and radiates into space.

Composition and Structure of Stars (cont. ) • There are three interior layers of a typical star. • When first formed, all stars fuse hydrogen into helium in their cores.

Composition and Structure of Stars (cont. ) • The radiative zone is a shell of cooler hydrogen around a star’s core. • In the convection zone, hot gases move toward the surface as cooler gases move down into the interior.

Composition and Structure of Stars (cont. ) Beyond the convection zone are three layers of a star’s atmosphere— the photosphere, the chromosphere, and the corona.

Composition and Structure of Stars (cont. ) The photosphere is the apparent surface of a star, where light energy radiates into space.

Composition and Structure of Stars (cont. ) • The chromosphere is the orange-red layer above the photosphere. • The corona is the wide, outermost layer of a star’s atmosphere.

Changing Features of the Sun: Sunspots • Cooler regions of magnetic activity • Seem to move as the Sun rotates • Number varies on an 11 year cycle Digital Vision/Punch. Stock

Changing Features of the Sun: Coronal Mass Ejections (CMEs) • Huge gas bubbles ejected from the corona • Larger than flares • May reach Earth • Can cause radio blackouts NASA

Changing Features of the Sun: Prominences and Flares • Prominences—clouds and jets of gases forming loops into the corona • Flares—sudden increases in brightness, often near sunspots or prominences SOHO Consortium, ESA, NASA

Changing Features of the Sun: The Solar Wind • Caused by charged particles streaming away from the Sun • Extends to the edge of the solar system • Causes auroras CORBIS

Groups of Stars • Most stars exist in star systems bound by gravity. • Many stars exist in large groupings called clusters. • Stars in a cluster all formed at about the same time and are the same distance from Earth.

Classifying Stars • Scientists classify stars according to their spectra. • Though there are exceptions, color in most stars is related to mass.

Blue-white stars tend to have the most mass, followed by white stars, yellow stars, orange stars, and red stars.

The Hertzsprung-Russell diagram is a graph that plots luminosity against temperature of stars.

The y-axis of the H-R diagram displays increasing luminosity and the x-axis displays decreasing temperature.

Most stars exist along the main sequence.

The mass of a main-sequence star determines both its temperature and its luminosity