Grade 8 Integrated Science Chapter 12 Lesson 2 on the Sun and other stars. This lesson discusses nuclear fusion and the composition of a star. It introduces the three interior layers and the three exterior layers. It also introduces the changing features of the Sun's surface. There is also a short introduction on the H-R diagram.

1. The Sun and Other Stars
Chapter 12.2 (p422-428)

2. 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

3. 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.

4. • 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.

5. 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

6. • 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)

7. 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.

9. 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.

11. 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.

12. 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

14. 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

16. 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

18. 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

20. Hertzsprung-Russell Diagram

21. 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.

22. 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

23. The Main Sequence
– High-mass stars have a shorter life span than lowmass 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