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Astronomy Cheatsheet

A handy reference guide covering essential astronomical concepts, formulas, celestial objects, and observational tips.

Basic Concepts & Units

Fundamental Units

Astronomical Unit (AU)

Average distance between Earth and the Sun.
1 AU ≈ 149.6 million kilometers

Light-Year (ly)

Distance light travels in one year.
1 ly ≈ 9.461 × 10^12 kilometers

Parsec (pc)

Distance at which 1 AU subtends an angle of 1 arcsecond.
1 pc ≈ 3.26 light-years

Coordinate Systems

Right Ascension (RA)

Analogous to longitude on Earth, measured in hours, minutes, and seconds.

Declination (Dec)

Analogous to latitude on Earth, measured in degrees, arcminutes, and arcseconds.

Altitude (Alt)

The angle between the object and the observer’s local horizon.

Azimuth (Az)

The angle along the horizon, measured from North towards East.

Important Constants

Speed of Light (c)

c ≈ 299,792,458 m/s

Gravitational Constant (G)

G ≈ 6.674 × 10^-11 N(m/kg)^2

Stefan-Boltzmann Constant (σ)

σ ≈ 5.670 × 10^-8 W/(m^2 K^4)

Celestial Objects

Planets

Terrestrial Planets

Mercury, Venus, Earth, Mars. Rocky surfaces, higher densities.

Gas Giants

Jupiter, Saturn. Primarily composed of hydrogen and helium.

Ice Giants

Uranus, Neptune. Contain heavier elements like oxygen, carbon, nitrogen, and sulfur.

Stars

Main Sequence Stars

Stars fusing hydrogen into helium in their cores (e.g., our Sun).

Red Giants

Stars that have exhausted hydrogen in their cores and have expanded.

White Dwarfs

Remnants of low- to medium-mass stars after they have shed their outer layers.

Neutron Stars

Extremely dense remnants of massive stars after supernova explosions.

Black Holes

Regions of spacetime with gravity so strong that nothing, not even light, can escape.

Galaxies

Spiral Galaxies

Disk-shaped galaxies with spiral arms (e.g., Milky Way, Andromeda).

Elliptical Galaxies

Smooth, oval-shaped galaxies with little or no spiral structure.

Irregular Galaxies

Galaxies with no defined shape.

Key Formulas & Laws

Kepler's Laws of Planetary Motion

1st Law: The orbit of each planet is an ellipse with the Sun at one of the two foci.

2nd Law: A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

3rd Law: The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. P^2 ∝ a^3

Newton's Law of Universal Gravitation

The gravitational force between two objects:
F = G * (m1 * m2) / r^2
where:

  • F is the gravitational force
  • G is the gravitational constant
  • m1 and m2 are the masses of the objects
  • r is the distance between the centers of the objects

Doppler Effect

Change in frequency of a wave (light or sound) due to the relative motion between the source and the observer.
λ = λ₀(1 + v/c)
where:

  • λ is the observed wavelength
  • λ₀ is the emitted wavelength
  • v is the relative velocity
  • c is the speed of light

Stefan-Boltzmann Law

The total energy radiated per unit surface area of a black body per unit time is directly proportional to the fourth power of the black body’s thermodynamic temperature.
j* = εσT^4
where:

  • j* is the total energy radiated
  • ε is the emissivity of the object (1 for a black body)
  • σ is the Stefan-Boltzmann constant
  • T is the absolute temperature in Kelvin

Observational Astronomy

Telescopes

Refracting Telescopes

Use lenses to focus light.

Reflecting Telescopes

Use mirrors to focus light.

Aperture

The diameter of the main lens or mirror; larger aperture gathers more light.

Filters

Light Pollution Filters

Reduce the effects of artificial light.

Narrowband Filters

Isolate specific wavelengths of light (e.g., H-alpha for nebulae).

Color Filters

Enhance contrast and details of planets.

Observational Tips

  • Allow your eyes to adjust to the darkness (dark adaptation) for at least 20-30 minutes.
  • Use averted vision to see faint objects by looking slightly to the side of them.
  • Plan your observing session ahead of time using star charts or planetarium software.
  • Keep a logbook of your observations.
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