The Sun and Other Stars

Chapter 12.2 (p422-428)

New Vocabulary• Nuclear fusion (p423) – a process that occurs when the nuclei of

several atoms combine into one larger nucleus• Star (p423) – a large ball of gas held together by gravity with a core

so hot that nuclear fusion occurs• Radiative Zone (424) – a shell of cooler hydrogen above a star’s

core• Convection Zone (424) – where hot gas moves up toward the

surface and cooler gas moves deeper into the interior• Photosphere (424) – the apparent surface of a star• Chromosphere (424) – the orange-red layer above the photosphere• Corona (424) – the wide, outermost layer of a star’s atmosphere• Hertzsprung-Russell diagram (427) – a graph that plots luminosity

v. temperature of a star

How Stars Shine• When gases are hot enough, its atoms will move quickly

and collide more often with more energy.• If the energy is high enough the nuclei of some atoms will

combine. This is called nuclear fusion.– Nuclear fusion is defined as a process that occurs when the

nuclei of several atoms combine into one larger nucleus.– This releases a great amount of energy that powers the star

• A star is a large ball of gas held together by gravity with a core so hot that nuclear fusion occurs.– It can reach millions or hundreds of millions of degrees Celsius.– The energy leaves the star and radiates through space.

• Looking at this picture what color atom represents protons? How do you know?

Yellow because we know the reactants are hydrogen which have only one proton and the product helium has two.

Composition and Structure of Stars

• Scientists study our closest star, the Sun, using probes and by studying its spectrum using the spectroscope.

• Spectra of the Sun and other stars provide information about stellar composition.

• The Sun and most stars are made almost entirely of hydrogen and helium gas.

• A star’s composition changes over time as hydrogen nuclei fuse into more complex nuclei

• The bottom spectra is the spectra of our Sun. The spectra above it are for galaxies progressively further and further away. Absorption lines shift to the right as objects move away from us, this is called the red shift. (not necessary for exam, but remember what a spectra is and why it is important)

Interior of Stars• When first formed, all stars fuse hydrogen into helium

in their cores.• Helium is denser than hydrogen, so it sinks to the inner

part of the core after it forms.• A typical star has three interior layers:– Core– Radiative Zone – a shell of cooler hydrogen above a star’s

core• Hydrogen is dense here. Light energy bounces from atom to atom

as it moves upward– Convection Zone – when hot gas moves up towards the

surface and cooler gas moves deeper into the interior• Light energy moves quickly upward into the convection zone

• *Energy moves from inside out.

Atmosphere of Stars• Beyond the convention zone there are three outer

layers:– The photosphere is the apparent surface of a star.

• In the Sun, it is the dense bright part you can see, where light energy radiates into space

– The chromosphere is the orange-red layer above the photosphere.

– The corona is the wide, outermost layer of a star’s atmosphere• The temperature of the corona is higher than the photosphere or

the chromosphere.• It has an irregular shape and can extend outward for several

million kilometers.

Changing Features of the Sun

The Sun’s interior features have been stable over millions of years, but the

Sun’s atmosphere is constantly changing.

Sunspots

• Regions of strong magnetic activity are called sunspots

• Cooler than the rest of the photosphere, sunspots appear as dark splotches that move as the Sun rotates

• The number of sunspots changes over time• They follow a cycle peaking every 11 years• They average the size of Earth

Prominence and Flares

• Prominences are clouds of gas that make loops and jets extending into the corona

• They can sometimes last for weeks• Flares are sudden increases in brightness

often found near sunspots and prominences• Both flare and prominences begin at or just

above the photosphere

Coronal Mass Ejections (CMEs)

• Huge bubbles of gas ejected from the corona are coronal mass ejections (CMEs)

• They are much larger than flares and occur over the course of several hours

• Material from a CME can reach Earth, occasionally causing radio blackout or a malfunction in an orbiting satellite

The Solar Wind

• Charged particles that stream continually away from the Sun create the solar wind

• The solar wind passes Earth and extends to the edge of the solar system

• Auroras are curtains of light created when particles from the solar wind or a CME interact with Earth’s magnetic field

• Auroras occur in both the northern and southern hemispheres

Hertzsprung-Russell Diagram

Hertzsprung-Russell Diagram• The Hertzsprung-Russell diagram (H-R diagram)

is a graph that plots luminosity v. temperature of stars.

• The y-axis is increasing luminosity• The x-axis is decreasing star surface temperature• It is an important tool for measuring star

distances• If two stars have the same temperature,

luminosity can be determined and distance can be found.

The Main Sequence• Most stars spend the majority of their lives on

the main sequence• On the H-R diagram, main sequence stars form a

curved line• The mass of a star determines both its

temperature and its luminosity• Higher Mass Stars– Higher the mass the hotter and brighter the star,

because high-mass stars have more gravity, hotter cores, and can produce and use more energy through nuclear fusion

The Main Sequence– High-mass stars have a shorter life span than low-

mass stars– High-mass stars burn through their hydrogen much

faster and then move off the main sequence• Non Main Sequence Stars– The star group outside the main sequence no longer

fuse hydrogen into helium in their cores– Some of these stars, like supergiants, are cooler, yet

brighter and larger– Others, like white dwarfs, are dimmer and smaller,

yet much hotter