Galaxies

Galaxies

Middle School

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Learning Objectives

• Analyze the role of gravity in the formation of stars and galaxies.

• Define a galaxy and identify our own galaxy, the Milky Way.

• Classify the three main types of galaxies: spiral, elliptical, and irregular.

• Describe the key characteristics that differentiate galaxy types.

Key Vocabulary

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Galaxy

A galaxy is a huge collection of gas, dust, and billions of stars, all held together by gravity.

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Gravity

Gravity is the natural force that attracts any two objects with mass, pulling them toward each other.

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Star

A massive, luminous sphere of plasma held together by its own gravity, producing energy through nuclear fusion.

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Nebula

An enormous cloud of dust and gas between stars that acts as a nursery for new stars.

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How Gravity Builds Stars

• Gravity is the essential force that attracts objects with mass, building stars.

• Gravity pulls together huge gas and dust clouds called nebulae, or "star nurseries."

• As the nebula collapses, its spinning center becomes extremely hot and very dense.

• The hot core ignites nuclear reactions, marking the birth of a new star.

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Multiple Choice

What is the main role of gravity in the process of star formation?

• It pushes gas and dust apart.

• It pulls vast clouds of gas and dust together.

• It causes the star to start spinning.

• It creates the light of the new star.

Galaxies and Their Types

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Spiral Galaxy

• It has a flat, rotating disk with a central bulge.

• Features bright arms that spiral outwards from the center.

• Contains many young, hot stars, along with gas and dust.

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Elliptical Galaxy

• They have a smooth, oval shape with no spiral arms.

• Contains mostly older, cooler stars and has very little gas.

• Few new stars are formed due to less gas and dust.

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Irregular Galaxy

• These galaxies have a chaotic and undefined shape.

• They do not fit into spiral or elliptical classifications.

• Often contains a mix of both old and new stars.

Multiple Choice

What do the terms spiral, elliptical, and irregular refer to?

• The main types of galaxies

• The different sizes of stars

• The stages in a star's life

• The various colors of planets

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Common Misconceptions

Misconception	Correction
The Milky Way galaxy is the entire universe.	The Milky Way is just one of billions of galaxies.
All galaxies look the same and have new stars.	Galaxies have different shapes; elliptical galaxies have older stars.
Stars are fixed and do not move in the sky.	Stars constantly move and orbit the center of their galaxy.

Multiple Choice

An astronomer observes a galaxy with a smooth, oval shape and mostly older, cooler stars. How would this galaxy most likely be classified?

• Elliptical

• Spiral

• Irregular

• Nebula

Multiple Choice

What is a key difference between a spiral galaxy and an elliptical galaxy?

• Spiral galaxies are held together by gravity, but elliptical galaxies are not.

• Spiral galaxies have arms and contain gas and dust, while elliptical galaxies are smooth and have less.

• Elliptical galaxies have many young, hot stars, while spiral galaxies have older, cooler stars.

• Elliptical galaxies have a defined, chaotic shape, while spiral galaxies are oval-shaped.

Multiple Choice

If a large amount of gas and dust were suddenly introduced into an old elliptical galaxy, what might begin to happen?

• The galaxy would immediately become a spiral galaxy.

• New stars might begin to form in pockets.

• The older stars would become hotter and brighter.

• The galaxy would break apart due to gravity.

Multiple Choice

A new galaxy is discovered that has no defined shape and contains many young, hot stars. What does the presence of these young stars indicate about this galaxy?

• It is likely an old, elliptical galaxy.

• It has very little gas and dust remaining.

• Star formation is actively occurring.

• It is moving away from Earth very slowly.

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Summary

• Galaxies are vast systems of stars, gas, and dust held together by gravity.

• Gravity pulls matter together to form stars, the building blocks of galaxies.

• Galaxies are classified by shape as spiral, elliptical, or irregular.

• Our home galaxy, the Milky Way, is a spiral galaxy.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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• 3

• 4