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Earth and Space Science Unit 6: Stars and the universe

Stars form from gas clouds, shine by fusion, and die in ways set by their mass. The universe began about 13.8 billion years ago and is still expanding.

Covers: Life cycle of stars, the Sun, H-R diagram, galaxies, light-years, the Big Bang

Study guide

Stars form in nebulae, clouds of gas and dust that collapse under gravity until the core is hot enough for nuclear fusion. Fusion of hydrogen into helium releases enormous energy, and a star spends most of its life on the main sequence balancing fusion's outward pressure against gravity's inward pull. A star's mass determines its life cycle. Sun-like stars swell into red giants, shed their outer layers as planetary nebulae, and leave white dwarfs. Massive stars become supergiants, explode as supernovae, and leave neutron stars or black holes. Massive stars burn hotter and die much faster. Elements heavier than hydrogen and helium were made in stars and supernovae.

The Sun is a medium-sized, middle-aged main-sequence star about 4.6 billion years old. Its layers include the core (where fusion happens), the radiative and convective zones, and the visible surface called the photosphere. Sunspots, solar flares and the solar wind affect Earth, causing auroras.

The Hertzsprung-Russell (H-R) diagram plots stars by temperature (or color) against luminosity. Most stars fall on the diagonal main sequence; giants and supergiants are bright and cool (upper right), and white dwarfs are hot and dim (lower left). Color shows temperature: blue stars are hottest, then white, yellow, orange, and red stars are coolest.

Galaxies are huge systems of stars, gas, dust and dark matter, shaped as spirals, ellipticals or irregulars. Our Milky Way is a spiral galaxy. Distances are measured in light-years, the distance light travels in a year, about 9.5 trillion kilometers.

The Big Bang theory says the universe began about 13.8 billion years ago from an extremely hot, dense state and has been expanding ever since. Evidence includes the redshift of distant galaxies (they're moving away), the cosmic microwave background radiation, and the abundance of light elements.

How bright a star looks from Earth depends on both its luminosity (the energy it actually gives off) and its distance. Brightness falls off with the square of distance, so a less luminous nearby star can look brighter than a very luminous distant one.

Key ideas

• Stars shine by nuclear fusion of hydrogen into helium.
• A star's mass determines its lifetime and fate: small stars become white dwarfs; massive ones explode as supernovas.
• The H-R diagram plots brightness vs temperature; most stars sit on the main sequence.
• A light-year is the distance light travels in one year (about 9.46 trillion km).
• Redshift of distant galaxies shows the universe is expanding (Big Bang evidence).

Formulas and rules

Common mistakes

• Thinking a light-year is a unit of time. It's distance.
• Assuming bigger stars live longer. They burn out faster.

Words to know

Nuclear fusion

Hydrogen fusing into helium; it powers stars.

Light-year

The distance light travels in one year.

Life cycle of a Sun-like star

Main sequence → red giant → white dwarf.

Redshift

Light from galaxies stretched to longer wavelengths; evidence the universe is expanding.

Blue stars

The hottest stars.

Nebula

A cloud of gas and dust where stars form.

Main sequence

The long, stable stage when a star fuses hydrogen.

Red giant

A swollen, cooler late-stage star.

White dwarf

The small, hot remnant of a Sun-like star.

Supernova

The explosion of a massive star.

Neutron star / black hole

Remnants of the most massive stars.

H-R diagram

Plots stars by temperature and luminosity.

Photosphere

The Sun's visible surface.

Galaxy

A huge system of stars, gas, dust and dark matter.

Big Bang theory

The universe began ~13.8 billion years ago and is expanding.

Cosmic microwave background

Leftover radiation from the early universe.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.