Key Ideas

〉How are stars formed?

〉How can we learn about stars if they are so far away?

〉What natural cycles do stars go through?

What Are Stars?• How are stars formed?• Stars are formed from clouds of dust and

gas, or nebulas, and go through different stages as they age.

• star: a large celestial body that is composed of gas and emits light

• light-year: the distance that light travels in one year; about 9.46 trillion kilometers

Stars

• Stars are powered by nuclear fusion reactions.– The core of a star is extremely hot,

extremely dense, and under extreme pressure.

– Nuclear fusion takes place in the core of a star.

– Fusion combines the nuclei of hydrogen atoms into helium.

– When two particles fuse, energy is released.

Visual Concept: Nuclear Fusion

Stars• Nuclear fusion takes place in the core of a

star.• Fusion combines the nuclei of hydrogen

atoms into helium.• When two particles fuse, energy is

released.

-Two hydrogen atoms come together to form 1 helium atom.

This energy given off by the nuclear power, makes the sun, and all stars, shine brightly.

Stars• Energy moves slowly through the layers of a

star.– Energy moves through the layers of a star by

convection and radiation.– During convection, hot gas moves upward, away

from the star’s center, and cooler gas sinks toward the center.

– During radiation, atoms absorb energy and transfer it to other atoms in random directions. Atoms near the star’s surface radiate energy into space.

Structure of the Sun

                      

Studying Stars• How can we learn about stars if they are so far

away?

• The telescope allowed astronomers to study stars in more detail for the first time.

• Some stars appear brighter than others.– The brightness of a star depends on the star’s

temperature, size, and distance from Earth.– The brightest star in the night sky, Sirius,

appears so bright because it is relatively close to Earth.

Studying Stars• We learn about stars by studying energy.

– Stars produce a full range of electromagnetic radiation, from high-energy X-rays to low-energy radio waves.

– Scientists use optical telescopes to study visible light and radio telescopes to study radio waves emitted from astronomical objects.

– Earth’s atmosphere blocks some wavelengths, so telescopes in space can study a wider range of the spectrum.

Studying Stars

• Spectral lines reveal the composition of stars.

– The spectra of most stars have dark lines caused by gases in the outer layers that absorb light at that wavelength.

– Each element produces a unique pattern of spectral lines.

– Astronomers can match the dark lines in starlight to the known lines of elements found on Earth.

Studying Stars• A star’s color is related to its temperature.

– Hotter objects glow with light that has shorter wavelengths (closer to the blue end of the spectrum).

– Cooler objects glow with light that has longer wavelengths (closer to the red end of the spectrum).

– Hot stars emit more energy at every wavelength than cooler stars do.

Visual Concept: Types of Stars

Studying Stars• If you look up in the sky

at night, you see stars!!!• Astronomers, scientists

who study the universe, classify stars according to their physical characteristics.

• These include:1. Color2. Temperature3. Size4. Brightness

Color and Temperature• Stars are very bright and very hot, but…• Like on Earth, the different colors of

flames/light, reveals different temperatures.• Stars range from colors that are red (the

coolest) to Yellow (our Sun) to white and blue colors, which are the hottest stars.

Size• Stars come in different sizes.1. White dwarf- about size of earth, usually

a star that is dying off (running out of fuel)

2. Medium size- our Sun is a medium sized star.

3. Large stars- they are usually the hottest (blue) stars.

4. Super giants- so big that it would fill our solar system from the sun to Jupiter, but they are usually a much cooler star (red color)

Brightness depends on BOTH size and temperature.

H-R Diagram

The Life Cycle of Stars• What natural cycles do stars go through?• In a way that is similar to other natural cycles,

stars are born, go through various stages of development, and eventually die.

• The sun formed from a cloud of gas and dust.– The sun formed about 5 billion years ago.– Stars do not last forever.– The sun was “born” when the process of fusion

began in the core.

The Life Cycle of Stars

• The sun has a balance of inward and outward forces.– The fusion reactions in the core of the sun

produce an outward force that balances the inward force due to gravity.

– Over time, the percentage of the sun’s core that is helium becomes larger.

– Scientists estimate that the sun can continue nuclear fusion for another 5 billion years.

Life Cycle of StarsThe first stage of a star’s life is called:

Protostar1. Stars begin as a Nebula: a large cloud of gas

and dust in space.2. Gravity pulls the gas and dust together to form

what we call a protostar.3. The sun was “born” when the process of fusion

began in the core.

Protostar

Nebula

High Mass Star

Star

4. When a star is “born” it lives (like our sun is right now) for about 10 billion years. Then is starts to run out of fuel.

5. Eventually they will become super giants (which means that the sun will get so big that it will engulf the first 4 planets, including earth)!!!

6. Then it either explodes and forms a supernova, or a white dwarf.

7. (A) If a supernova forms then the star will eventually form either a black hole or a neutron star.

Black Hole- where the gravity is so strong that everything, even light, gets sucked into the hole. No one really knows where it goes!!!

Neutron Star- Similar to a white dwarf but much smaller and more dense.

8. (B) If a white dwarf forms that means that whatever is left after the fuel is gone collapses to form a small glowing sphere in outer space!

High Mass Star

Star

The Life Cycle of Stars• Some supernovas form neutron stars and

black holes.– If the core that remains after a supernova has a

mass of 1.4 to 3 solar masses, the remnant can become a neutron star.

– If the leftover core has a mass that is greater than three solar masses, it will collapse to form a black hole.

• black hole: an object so massive and dense that not even light can escape its gravity

The Life Cycle of Stars• The sun will become a red giant before it dies.

– As fusion slows, the outer layers of the sun will expand.

– The sun will become a red giant.

• red giant: a large, reddish star late in its life cycle– When the sun runs out of helium, the outer layers will

expand and eventually leave the sun’s orbit. – The sun will become a white dwarf.

• white dwarf: a small, hot dim star that is the leftover center of an old star

The Life Cycle of Stars• Supergiant stars explode in supernovas.

– Massive stars evolve faster, develop hotter cores, and create heavier elements through fusion.

– The formation of an iron core signals the beginning of a supergiant’s death.

– Eventually the core collapses and then explodes in a supernova.

• supernova: a gigantic explosion in which a massive star collapses and throws its outer layers into space, plural supernovae

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