Astronomy and the Solar System

Astronomy questions are mostly about order and cause. The order is the planets and the star life cycle; the cause is what produces phases, seasons and eclipses, and each of those has one common wrong explanation.

The solar system

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Inner, terrestrial planets: Mercury, Venus, Earth, Mars. Small, rocky, solid surfaces, few or no moons.

Outer, giant planets: Jupiter, Saturn, Uranus, Neptune. Large, no solid surface, many moons, all with ring systems though only Saturn's are obvious.

The asteroid belt sits between Mars and Jupiter and separates the two groups.

Facts the test reaches for:

• Mercury is closest to the Sun, but Venus is the hottest planet, because its thick carbon dioxide atmosphere traps heat. That pair is a question almost every time.
• Jupiter is the largest planet.
• Mars is the red planet, colored by iron oxide.
• Saturn is known for its rings.
• Pluto was reclassified as a dwarf planet in 2006, which is why the count is eight. Ceres, the largest object in the asteroid belt, is a dwarf planet too.
• Jupiter's Great Red Spot is a permanent storm, larger than Earth, that has raged for centuries. Mars has Olympus Mons, the largest volcano in the solar system.

The moons worth knowing

Ganymede, Io, Europa and Callisto are the four Galilean moons, found by Galileo in 1610. Titan orbits Saturn, so it is never one of them.

Our own Moon's dark patches are maria ("seas"), plains of old hardened lava.

Rotation and revolution

Two words that get swapped.

Rotation is spinning on an axis. Earth rotates once in 24 hours, which gives day and night.

Revolution is orbiting another body. Earth revolves around the Sun once in about 365 days, which gives the year.

The seasons are caused by the tilt of Earth's axis, about 23.5 degrees - not by distance from the Sun. When the northern hemisphere tilts toward the Sun it gets more direct light and longer days, and that is summer. The two hemispheres have opposite seasons at the same moment, which would be impossible if distance were the cause.

Earth is actually closest to the Sun in early January, during northern winter, which settles the argument.

Moon phases

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The Moon does not produce light. It reflects sunlight, and half of it is always lit. The phase is how much of that lit half we can see from where we are, which changes as the Moon moves around its orbit.

The cycle, in order: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent, and back to new. It takes about 29.5 days.

Waxing means growing; waning means shrinking. In the northern hemisphere the lit part is on the right while waxing and on the left while waning.

Phases are not caused by Earth's shadow. That is the standard wrong answer. Earth's shadow falling on the Moon is a lunar eclipse, which happens only occasionally, not monthly - because the Moon's orbit is tilted and usually passes above or below the shadow.

The Moon always shows the same face to Earth, because it rotates once per orbit. There is no permanently dark side; there is a far side that gets just as much sunlight.

Stars

A star is a ball of gas held together by gravity and held up by the energy of nuclear fusion in its core, fusing hydrogen into helium.

The Sun is an average star. It looks enormous because it is close; there are far larger and far smaller stars.

Star color indicates temperature, and the ordering is the opposite of the everyday association: blue stars are the hottest and red stars the coolest, with yellow like the Sun in between. Anyone reasoning from "red hot" gets this backward, which is why it is asked.

The Sun

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Sunspots are cooler, darker patches on the photosphere. Solar flares are sudden explosive releases of energy from the surface, and a coronal mass ejection throws a huge cloud of plasma off the corona - the source of auroras and of the storms that can knock out satellites.

The life cycle

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Every star starts the same way and ends in one of two ways, and mass is what decides.

1. Nebula - a cloud of gas and dust.
2. Protostar - gravity pulls the cloud together and it heats.
3. Main sequence star - fusion begins and the star is stable. The Sun is here, and stars spend most of their lives at this stage.

Then the fork:

A supernova is the explosion, not the remnant. What is left afterward is a neutron star, or a black hole if enough mass remains. Questions offer supernova as an end state, and it is an event.

A black hole is a region whose gravity is strong enough that light cannot escape, which is why it is black rather than because it is empty.

Scale and structure

A galaxy is a vast collection of stars, gas and dust bound by gravity. Ours is the Milky Way, a spiral galaxy about 100,000 light years across.

Galaxies come in three shapes: spiral (a flat disk with arms, still forming stars, like ours), elliptical (a smooth football or sphere of older stars) and irregular (no clear shape). The nearest large galaxy is Andromeda, also a spiral. The Milky Way, Andromeda and a few dozen smaller galaxies form a cluster called the Local Group.

A constellation is a pattern of stars as seen from Earth - the stars in it are usually nowhere near each other in space. Orion is the one with the belt of three stars. Polaris, the North Star, sits almost directly above the North Pole, in Ursa Minor, which is why it has guided navigators for centuries.

A light year is a distance, not a time - the distance light travels in a year, about 6 trillion miles. That is the most reliably asked definition in the topic, because the word ends in "year".

The Big Bang theory holds that the universe began in an extremely hot, dense state and has been expanding ever since, about 13.8 billion years ago. The evidence includes the observed expansion of galaxies away from each other and the cosmic microwave background.

Exploring space

A few firsts are asked: Apollo 11 put the first people on the Moon in 1969. The two Voyager probes, launched in 1977, are the farthest human-made objects, now beyond the planets. The Hubble Space Telescope orbits above the atmosphere, because ground telescopes see through air whose turbulence makes stars twinkle and blurs the image.

What you can skip

Astronomy's questions reward the big facts and a short list of names. Across all 2,104 General Science questions in our bank, each of these comes up once:

• The laws of orbital motion as equations. Kepler's third law appears once. Know that planets closer to the Sun orbit faster.
• Individual missions beyond the famous three. OSIRIS-REx and the Mars Science Laboratory come up once each; Apollo 11, Voyager and Hubble cover the rest.
• Stellar astrophysics. Metallicity, accretion disks, the Eddington limit and the rest of the specialist vocabulary were removed from the bank as beyond the test's scope.

Where people lose points

Saying moon phases are caused by Earth's shadow.

Saying seasons are caused by distance from the Sun. The axial tilt causes them.

Naming Mercury as the hottest planet. Venus is.

Swapping rotation and revolution.

Treating a light year as a unit of time.

Saying red stars are hottest. Blue are.

Giving supernova as a star's final state. It is the explosion.

Work one in under a minute

Why does the northern hemisphere have summer in June?

Not because Earth is closer to the Sun - it is actually farthest in early July.

Because Earth's axis is tilted about 23.5 degrees, and in June the northern hemisphere is tilted toward the Sun. That gives more direct sunlight and longer days.

The check that settles it: the southern hemisphere has winter at the same moment, and both hemispheres are the same distance from the Sun.

Where this leads

The gravity holding these systems together is the physics of the Newton's laws lesson, and the light carrying the information is the waves lesson's subject.