Astronomy basics
61 flashcards covering Astronomy basics for the ACT Science section.
Astronomy basics explore the fundamental aspects of the universe, including stars, planets, galaxies, and other celestial objects. It's the study of how these elements form, move, and interact, helping us understand the scale of space and phenomena like the solar system's structure or the life cycles of stars. This knowledge connects everyday observations, such as night sky patterns, to broader scientific principles.
On the ACT Science section, astronomy questions often appear in data representation passages, research summaries, or conflicting viewpoints, typically involving graphs, tables, or diagrams of astronomical data. You'll need to interpret trends, like planetary orbits or star brightness, and answer multiple-choice questions that test reasoning and evidence analysis. Common traps include misreading scales on charts or confusing similar concepts, such as comets and asteroids, so focus on applying logic to data rather than memorizing details. Practice identifying key information in passages quickly.
Terms (61)
- 01
Solar System
The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, including planets, moons, asteroids, comets, and dwarf planets.
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Planet
A planet is a celestial body that orbits a star, is massive enough to be rounded by its own gravity, and has cleared its orbit of other debris, as defined by the International Astronomical Union.
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Star
A star is a massive, luminous sphere of plasma held together by gravity, which produces energy through nuclear fusion in its core, like the Sun.
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Galaxy
A galaxy is a massive system of stars, stellar remnants, interstellar gas, dust, and dark matter, all bound together by gravity, such as the Milky Way.
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Universe
The universe is the entirety of space, time, matter, and energy, including all galaxies, stars, planets, and other forms of matter and radiation.
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Rotation
Rotation is the spinning of a celestial body on its axis, causing day and night on Earth, for example.
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Revolution
Revolution is the movement of one celestial body in an orbit around another, such as Earth's yearly orbit around the Sun, which defines a year.
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Orbit
An orbit is the gravitationally curved trajectory of an object around a point in space, like a planet around a star, following Kepler's laws.
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Eclipse
An eclipse occurs when one celestial body moves into the shadow of another, such as a solar eclipse when the Moon blocks the Sun's light from Earth.
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Phases of the Moon
The phases of the Moon are the different illuminated portions visible from Earth as it orbits the Earth, caused by the changing angles of sunlight.
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Seasons
Seasons are the periodic changes in weather and daylight hours caused by the tilt of Earth's axis as it orbits the Sun, leading to variations in solar radiation.
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Equinox
An equinox is one of two times a year when the Sun crosses the celestial equator, resulting in nearly equal day and night lengths worldwide.
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Solstice
A solstice is one of two times a year when the Sun reaches its highest or lowest point in the sky at noon, marking the longest or shortest day in the Northern Hemisphere.
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Constellation
A constellation is a group of stars that forms a recognizable pattern in the night sky, often named after mythological figures or animals.
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Light Year
A light year is the distance that light travels in one year in a vacuum, approximately 9.46 trillion kilometers, used to measure vast astronomical distances.
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Parallax
Parallax is the apparent shift in position of a nearby star against the background of distant stars when viewed from opposite sides of Earth's orbit.
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Red Shift
Red shift is the lengthening of light waves from distant galaxies moving away from us, indicating the expansion of the universe as per Hubble's law.
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Electromagnetic Spectrum
The electromagnetic spectrum is the range of all types of electromagnetic radiation, from radio waves to gamma rays, which astronomers use to study celestial objects.
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Wavelength
Wavelength is the distance between successive peaks of a wave, such as light, and determines its color in the visible spectrum.
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Frequency
Frequency is the number of waves that pass a point per second, measured in hertz, and is inversely related to wavelength in electromagnetic radiation.
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Telescope
A telescope is an instrument that collects and magnifies light from distant objects, allowing observation of stars, planets, and galaxies.
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Spectroscopy
Spectroscopy is the study of the interaction between matter and electromagnetic radiation, used in astronomy to determine the composition of stars and galaxies.
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Doppler Effect
The Doppler effect is the change in frequency or wavelength of a wave relative to an observer moving towards or away from the source, explaining red shift in astronomy.
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Black Hole
A black hole is a region in space where gravity is so strong that nothing, not even light, can escape, formed from the collapse of massive stars.
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Nebula
A nebula is a vast cloud of gas and dust in space, often serving as a stellar nursery where new stars are formed.
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Comet
A comet is a small, icy body that orbits the Sun and develops a visible coma and tail when it approaches the inner Solar System.
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Asteroid
An asteroid is a rocky body orbiting the Sun, mostly found in the asteroid belt between Mars and Jupiter, varying in size from small rocks to dwarf planets.
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Meteor
A meteor is the streak of light produced when a small particle from space enters Earth's atmosphere and burns up, commonly called a shooting star.
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Kepler's First Law
Kepler's first law states that planets orbit the Sun in elliptical paths, with the Sun at one focus of the ellipse.
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Kepler's Second Law
Kepler's second law states that a line segment joining a planet and the Sun sweeps out equal areas in equal times, meaning planets move faster when closer to the Sun.
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Kepler's Third Law
Kepler's third law states that the square of a planet's orbital period is proportional to the cube of the semi-major axis of its orbit, expressed as T² ∝ a³.
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Newton's Law of Universal Gravitation
Newton's law of universal gravitation states that every particle attracts every other particle with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.
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Geocentric Model
The geocentric model is an ancient view that placed Earth at the center of the universe, with all celestial bodies orbiting around it.
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Heliocentric Model
The heliocentric model places the Sun at the center of the Solar System, with planets orbiting around it, as proposed by Copernicus.
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Big Bang Theory
The Big Bang theory is the leading explanation for the origin of the universe, positing that it began as a hot, dense point that expanded about 13.8 billion years ago.
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Cosmic Microwave Background
The cosmic microwave background is the thermal radiation left over from the Big Bang, providing evidence for the expansion of the universe.
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Hubble's Law
Hubble's law states that the velocity at which a galaxy is moving away from us is proportional to its distance, indicating the universe's expansion.
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Absolute Magnitude
Absolute magnitude is a measure of a star's intrinsic brightness as it would appear from a standard distance of 10 parsecs, allowing comparison of star luminosities.
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Apparent Magnitude
Apparent magnitude is how bright a star appears from Earth, depending on its actual brightness and distance from us.
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Hertzsprung-Russell Diagram
The Hertzsprung-Russell diagram is a scatter plot of stars showing the relationship between their absolute magnitudes or luminosities versus their stellar classifications or temperatures.
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Supernova
A supernova is a stellar explosion that occurs when a star reaches the end of its life cycle, briefly outshining an entire galaxy and dispersing elements into space.
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Milky Way
The Milky Way is the galaxy that contains our Solar System, a barred spiral galaxy with billions of stars, gas, and dust.
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Solar Eclipse
A solar eclipse happens when the Moon passes between the Earth and the Sun, blocking the Sun's light and casting a shadow on Earth.
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Lunar Eclipse
A lunar eclipse occurs when the Earth passes between the Sun and the Moon, casting a shadow on the Moon and making it appear red.
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Tides
Tides are the rise and fall of sea levels caused by the gravitational forces of the Moon and Sun on Earth's oceans.
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Gravitational Force
Gravitational force is the attraction between two masses, which keeps planets in orbit and governs the motion of celestial bodies.
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Escape Velocity
Escape velocity is the minimum speed needed for an object to break free from the gravitational attraction of a celestial body, such as 11.2 km/s from Earth.
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Celestial Sphere
The celestial sphere is an imaginary sphere of arbitrarily large radius centered on the Earth, used to map the positions of stars and other celestial objects.
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Right Ascension
Right ascension is the angular distance of a celestial object eastward along the celestial equator from the vernal equinox, measured in hours, minutes, and seconds.
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Declination
Declination is the angular distance of a celestial object north or south of the celestial equator, similar to latitude on Earth.
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Zodiac
The zodiac is a band of the celestial sphere that extends about 8 degrees north and south of the ecliptic, containing the apparent paths of the Sun, Moon, and planets.
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Precession
Precession is the slow wobble of Earth's axis over a 26,000-year cycle, which causes the position of the equinoxes to shift gradually.
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Andromeda Galaxy
The Andromeda Galaxy is the nearest spiral galaxy to the Milky Way, about 2.5 million light-years away, and is on a collision course with our galaxy.
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Exoplanet
An exoplanet is a planet that orbits a star outside our Solar System, often detected through methods like the transit method or radial velocity.
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Pulsar
A pulsar is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation, appearing to pulse as it rotates.
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Quasar
A quasar is an extremely luminous active galactic nucleus, powered by a supermassive black hole, often found at the centers of distant galaxies.
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Roche Limit
The Roche limit is the minimum distance at which a celestial body, held together only by its own gravity, will disintegrate due to tidal forces from a larger body.
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Difference between rotation and revolution
Rotation is the spinning of a body on its axis, causing day and night, while revolution is its orbital motion around another body, causing yearly cycles.
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Kepler's Third Law formula
Kepler's third law is mathematically expressed as T squared equals four pi squared times r cubed divided by G times M, where T is the orbital period, r is the semi-major axis, G is the gravitational constant, and M is the mass of the central body.
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Example of red shift calculation
In a red shift calculation, if a galaxy's spectral line shifts from 500 nm to 550 nm, the redshift z is 0.1, indicating it is moving away at about 30,000 km/s, assuming low speeds.
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Strategy for identifying constellations
To identify constellations, locate key stars or patterns relative to the North Star, use seasonal sky charts, and account for light pollution to enhance visibility.