Earth and space: stars and the universe (grade 12) · lesson

Fusion powers stars and makes elements

💡 Stars fuse light nuclei into heavier ones, releasing energy and making the elements we are built from.

In the Sun's core, hydrogen nuclei fuse into helium and release energy. This is nuclear fusion, not burning. Burning needs oxygen and would have run out long ago.

Older stars fuse helium into carbon, and massive stars make even heavier elements. When stars shed their outer layers or explode, these elements spread into space and end up in new stars and planets.

A star with more mass has a hotter, denser core. It uses its fuel much faster, so it lives a shorter life.

🧠 Let's work it out

Fusion adds protons.
Carbon has 6 protons and helium has 2, so 3 helium nuclei make 1 carbon nucleus.
The Big Bang made almost only hydrogen and helium, so early stars had few heavy elements; later stars formed from gas enriched by earlier stars.

What do you think comes next?

nuclear fusion: joining light nuclei into a heavier nucleus, releasing energy

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Life cycles of stars

💡 A star's mass decides how it lives and how it ends.

Every star begins as a cloud of gas and dust (a nebula) that gravity pulls together until fusion starts. It then spends most of its life on the main sequence, fusing hydrogen.

A Sun-like star then swells into a red giant and ends as a white dwarf. A very massive star becomes a red supergiant and ends in a supernova explosion.

🧠 Let's work it out

Order the stages by asking what must happen first.
A star can't fuse hydrogen until the cloud collapses.
It can't run out of hydrogen until it has used it on the main sequence.

What do you think comes next?

nebula: a cloud of gas and dust in space
supernova: the explosion of a massive star

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Evidence for the Big Bang

💡 Galaxies moving apart and the leftover microwave glow show the universe began hot and dense and is still expanding.

Light from distant galaxies is redshifted, meaning its lines are shifted toward red. That shows the galaxies are moving away. Farther galaxies are moving faster, which fits an expanding universe.

Hubble's law says speed = H × distance. Faint microwave radiation coming evenly from every direction, the cosmic microwave background, is leftover glow from the hot early universe.

🧠 Let's work it out

Use Hubble's law both ways.
With H = 70 km/s per megaparsec, a galaxy 200 megaparsecs away moves at 70 × 200 = 14,000 km/s.
A galaxy moving at 7,000 km/s is 7,000 ÷ 70 = 100 megaparsecs away.

What do you think comes next?

redshift: a shift of light toward red, caused by the source moving away
megaparsec: a huge unit of distance used for galaxies

What starlight tells us

💡 Starlight is old news and a fingerprint: it shows how far away a star is and what it is made of.

A light-year is the distance light travels in one year. Light from a star 8.6 light-years away left it about 8.6 years ago. We see stars as they were, not as they are.

Each element makes its own pattern of lines in a spectrum. If a star's lines match hydrogen's pattern, it has hydrogen. If every line is shifted toward blue, the star is moving toward us; toward red means away.

🧠 Let's work it out

Separate the two clues.
The pattern of the lines tells what the star is made of.
The shift of the lines tells how it is moving.

What do you think comes next?

light-year: the distance light travels in one year
spectrum: light spread out by wavelength

🔗 NASA Space Place: What is a light-year? · NASA Space Place ↗