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Grade 9 / Middle 3 (age 14-15)

grade 9 Galaxies & the Expanding Universe

Galaxies and the Expanding Universe

A galaxy is a vast collection of hundreds of billions of stars held by gravity, and our Milky Way is a spiral galaxy. Space itself expands so galaxies move apart, and more distant galaxies recede faster. Light from receding galaxies is stretched into longer wavelengths (redshift), and Hubble's law v = H₀d captures that link. Change galaxy type and time and expansion and the Hubble relation appear.

What is a Galaxy?

🌌 Intuition — A Vast Star City
①Galaxy: hundreds of billions of stars bound by gravity
②The Milky Way: a spiral galaxy containing our Solar System
③Types: spiral, elliptical, irregular
④The universe has about 2 trillion galaxies
Spiral Galaxy
📝 Galaxy Types
①Spiral: arm structure (our galaxy)
②Elliptical: ball-shaped, many old stars
③Irregular: no clear shape

The Universe is Expanding

🎈 Dots on a Balloon
①Inflate a balloon: distances between dots grow
②From every dot, others recede
③Same with space between galaxies
④Farther galaxies recede faster
5 / 10

Hubble's Law and Redshift

🔴 Redshift
①Light from receding objects gets longer wavelengths (redshift)
②Hubble: more distant galaxies show greater redshift
③Therefore distant galaxies recede faster
④That is evidence of cosmic expansion!

Summary

Hubble's Law
v = H₀ × d
Recession velocity = Hubble constant × distance (H₀ ≈ 70)
Evidence of Expansion
Redshift → moving away
Longer wavelength = receding galaxy
Big Bang Theory
Universe began ~13.8 Gyr ago at a single point
Reversing cosmic expansion converges to one point
🎯 Exam Points
①Galaxy: huge stellar group (spiral/elliptical/irregular)
②Milky Way: spiral, ~200 billion stars
③Redshift: evidence galaxies are receding
④Hubble: v = H₀d
⑤Big Bang: universe began ~13.8 Gyr ago

Examples and Unit Test

Example 1
When we observe the light from a distant galaxy, its wavelength is longer than the original (a redshift). What does this tell us about how that galaxy is moving?
1
Light from a receding object has a longer wavelength, shifted toward the red. This is called redshift.
2
So a galaxy that shows a redshift is moving away from us.
▸ Redshift = longer wavelength → that galaxy is moving away from us
Redshift is key observational evidence that the universe is expanding.
Example 2
Hubble found a relationship between a galaxy's distance and its speed of recession. The farther away a galaxy is, what happens to its speed of moving away?
1
Hubble's law is recession velocity v = H₀ × d (distance), so the recession speed is proportional to distance.
2
So the farther away a galaxy is, the faster it moves away.
▸ Hubble's law v = H₀d → the farther the galaxy, the faster it recedes
That farther galaxies recede faster shows that the whole universe is expanding.
Unit Test
Which statement about galaxies and the expanding universe is correct?
①Redshift is evidence that a galaxy is moving toward us
②By Hubble's law, the farther a galaxy is, the more slowly it recedes
③The farther away a galaxy is, the faster it moves away
④Our galaxy is an elliptical galaxy
⑤The universe stays a constant size and does not change
▸ ③ The farther away a galaxy is, the faster it moves away
1
Redshift is evidence that a galaxy is receding (① wrong), and by Hubble's law a farther galaxy recedes faster (② wrong).
2
Our galaxy is a spiral galaxy (④ wrong), and the universe is expanding (⑤ wrong); by Hubble's law v = H₀d, a farther galaxy recedes faster (③ correct).
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Stars and the Universe
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