Chapter 12: Properties of Stars

Chapter 1
How Science Works

  • The Scientific Method
  • Evidence
  • Measurements
  • Units and the Metric System
  • Measurement Errors
  • Estimation
  • Dimensions
  • Mass, Length, and Time
  • Observations and Uncertainty
  • Precision and Significant Figures
  • Errors and Statistics
  • Scientific Notation
  • Ways of Representing Data
  • Logic
  • Mathematics
  • Geometry
  • Algebra
  • Logarithms
  • Testing a Hypothesis
  • Case Study of Life on Mars
  • Theories
  • Systems of Knowledge
  • The Culture of Science
  • Computer Simulations
  • Modern Scientific Research
  • The Scope of Astronomy
  • Astronomy as a Science
  • A Scale Model of Space
  • A Scale Model of Time
  • Questions

Chapter 2
Early Astronomy

  • The Night Sky
  • Motions in the Sky
  • Navigation
  • Constellations and Seasons
  • Cause of the Seasons
  • The Magnitude System
  • Angular Size and Linear Size
  • Phases of the Moon
  • Eclipses
  • Auroras
  • Dividing Time
  • Solar and Lunar Calendars
  • History of Astronomy
  • Stonehenge
  • Ancient Observatories
  • Counting and Measurement
  • Astrology
  • Greek Astronomy
  • Aristotle and Geocentric Cosmology
  • Aristarchus and Heliocentric Cosmology
  • The Dark Ages
  • Arab Astronomy
  • Indian Astronomy
  • Chinese Astronomy
  • Mayan Astronomy
  • Questions

Chapter 3
The Copernican Revolution

  • Ptolemy and the Geocentric Model
  • The Renaissance
  • Copernicus and the Heliocentric Model
  • Tycho Brahe
  • Johannes Kepler
  • Elliptical Orbits
  • Kepler's Laws
  • Galileo Galilei
  • The Trial of Galileo
  • Isaac Newton
  • Newton's Law of Gravity
  • The Plurality of Worlds
  • The Birth of Modern Science
  • Layout of the Solar System
  • Scale of the Solar System
  • The Idea of Space Exploration
  • Orbits
  • History of Space Exploration
  • Moon Landings
  • International Space Station
  • Manned versus Robotic Missions
  • Commercial Space Flight
  • Future of Space Exploration
  • Living in Space
  • Moon, Mars, and Beyond
  • Societies in Space
  • Questions

Chapter 4
Matter and Energy in the Universe

  • Matter and Energy
  • Rutherford and Atomic Structure
  • Early Greek Physics
  • Dalton and Atoms
  • The Periodic Table
  • Structure of the Atom
  • Energy
  • Heat and Temperature
  • Potential and Kinetic Energy
  • Conservation of Energy
  • Velocity of Gas Particles
  • States of Matter
  • Thermodynamics
  • Entropy
  • Laws of Thermodynamics
  • Heat Transfer
  • Thermal Radiation
  • Wien's Law
  • Radiation from Planets and Stars
  • Internal Heat in Planets and Stars
  • Periodic Processes
  • Random Processes
  • Questions

Chapter 5
The Earth-Moon System

  • Earth and Moon
  • Early Estimates of Earth's Age
  • How the Earth Cooled
  • Ages Using Radioactivity
  • Radioactive Half-Life
  • Ages of the Earth and Moon
  • Geological Activity
  • Internal Structure of the Earth and Moon
  • Basic Rock Types
  • Layers of the Earth and Moon
  • Origin of Water on Earth
  • The Evolving Earth
  • Plate Tectonics
  • Volcanoes
  • Geological Processes
  • Impact Craters
  • The Geological Timescale
  • Mass Extinctions
  • Evolution and the Cosmic Environment
  • Earth's Atmosphere and Oceans
  • Weather Circulation
  • Environmental Change on Earth
  • The Earth-Moon System
  • Geological History of the Moon
  • Tidal Forces
  • Effects of Tidal Forces
  • Historical Studies of the Moon
  • Lunar Surface
  • Ice on the Moon
  • Origin of the Moon
  • Humans on the Moon
  • Questions

Chapter 6
The Terrestrial Planets

  • Studying Other Planets
  • The Planets
  • The Terrestrial Planets
  • Mercury
  • Mercury's Orbit
  • Mercury's Surface
  • Venus
  • Volcanism on Venus
  • Venus and the Greenhouse Effect
  • Tectonics on Venus
  • Exploring Venus
  • Mars in Myth and Legend
  • Early Studies of Mars
  • Mars Close-Up
  • Modern Views of Mars
  • Missions to Mars
  • Geology of Mars
  • Water on Mars
  • Polar Caps of Mars
  • Climate Change on Mars
  • Terraforming Mars
  • Life on Mars
  • The Moons of Mars
  • Martian Meteorites
  • Comparative Planetology
  • Incidence of Craters
  • Counting Craters
  • Counting Statistics
  • Internal Heat and Geological Activity
  • Magnetic Fields of the Terrestrial Planets
  • Mountains and Rifts
  • Radar Studies of Planetary Surfaces
  • Laser Ranging and Altimetry
  • Gravity and Atmospheres
  • Normal Atmospheric Composition
  • The Significance of Oxygen
  • Questions

Chapter 7
The Giant Planets and Their Moons

  • The Gas Giant Planets
  • Atmospheres of the Gas Giant Planets
  • Clouds and Weather on Gas Giant Planets
  • Internal Structure of the Gas Giant Planets
  • Thermal Radiation from Gas Giant Planets
  • Life on Gas Giant Planets?
  • Why Giant Planets are Giant
  • Gas Laws
  • Ring Systems of the Giant Planets
  • Structure Within Ring Systems
  • The Origin of Ring Particles
  • The Roche Limit
  • Resonance and Harmonics
  • Tidal Forces in the Solar System
  • Moons of Gas Giant Planets
  • Geology of Large Moons
  • The Voyager Missions
  • Jupiter
  • Jupiter's Galilean Moons
  • Jupiter's Ganymede
  • Jupiter's Europa
  • Jupiter's Callisto
  • Jupiter's Io
  • Volcanoes on Io
  • Saturn
  • Cassini Mission to Saturn
  • Saturn's Titan
  • Saturn's Enceladus
  • Discovery of Uranus and Neptune
  • Uranus
  • Uranus' Miranda
  • Neptune
  • Neptune's Triton
  • Pluto
  • The Discovery of Pluto
  • Pluto as a Dwarf Planet
  • Dwarf Planets
  • Questions

Chapter 8
Interplanetary Bodies

  • Interplanetary Bodies
  • Comets
  • Early Observations of Comets
  • Structure of the Comet Nucleus
  • Comet Chemistry
  • Oort Cloud and Kuiper Belt
  • Kuiper Belt
  • Comet Orbits
  • Life Story of Comets
  • The Largest Kuiper Belt Objects
  • Meteors and Meteor Showers
  • Gravitational Perturbations
  • Asteroids
  • Surveys for Earth Crossing Asteroids
  • Asteroid Shapes
  • Composition of Asteroids
  • Introduction to Meteorites
  • Origin of Meteorites
  • Types of Meteorites
  • The Tunguska Event
  • The Threat from Space
  • Probability and Impacts
  • Impact on Jupiter
  • Interplanetary Opportunity
  • Questions

Chapter 9
Planet Formation and Exoplanets

  • Formation of the Solar System
  • Early History of the Solar System
  • Conservation of Angular Momentum
  • Angular Momentum in a Collapsing Cloud
  • Helmholtz Contraction
  • Safronov and Planet Formation
  • Collapse of the Solar Nebula
  • Why the Solar System Collapsed
  • From Planetesimals to Planets
  • Accretion and Solar System Bodies
  • Differentiation
  • Planetary Magnetic Fields
  • The Origin of Satellites
  • Solar System Debris and Formation
  • Gradual Evolution and a Few Catastrophies
  • Chaos and Determinism
  • Extrasolar Planets
  • Discoveries of Exoplanets
  • Doppler Detection of Exoplanets
  • Transit Detection of Exoplanets
  • The Kepler Mission
  • Direct Detection of Exoplanets
  • Properties of Exoplanets
  • Implications of Exoplanet Surveys
  • Future Detection of Exoplanets
  • Questions

Chapter 10
Detecting Radiation from Space

  • Observing the Universe
  • Radiation and the Universe
  • The Nature of Light
  • The Electromagnetic Spectrum
  • Properties of Waves
  • Waves and Particles
  • How Radiation Travels
  • Properties of Electromagnetic Radiation
  • The Doppler Effect
  • Invisible Radiation
  • Thermal Spectra
  • The Quantum Theory
  • The Uncertainty Principle
  • Spectral Lines
  • Emission Lines and Bands
  • Absorption and Emission Spectra
  • Kirchoff's Laws
  • Astronomical Detection of Radiation
  • The Telescope
  • Optical Telescopes
  • Optical Detectors
  • Adaptive Optics
  • Image Processing
  • Digital Information
  • Radio Telescopes
  • Telescopes in Space
  • Hubble Space Telescope
  • Interferometry
  • Collecting Area and Resolution
  • Frontier Observatories
  • Questions

Chapter 11
Our Sun: The Nearest Star

  • The Sun
  • The Nearest Star
  • Properties of the Sun
  • Kelvin and the Sun's Age
  • The Sun's Composition
  • Energy From Atomic Nuclei
  • Mass-Energy Conversion
  • Examples of Mass-Energy Conversion
  • Energy From Nuclear Fission
  • Energy From Nuclear Fusion
  • Nuclear Reactions in the Sun
  • The Sun's Interior
  • Energy Flow in the Sun
  • Collisions and Opacity
  • Solar Neutrinos
  • Solar Oscillations
  • The Sun's Atmosphere
  • Solar Chromosphere and Corona
  • Sunspots
  • The Solar Cycle
  • The Solar Wind
  • Effects of the Sun on the Earth
  • Cosmic Energy Sources
  • Questions

Chapter 12
Properties of Stars

  • Stars
  • Star Names
  • Star Properties
  • The Distance to Stars
  • Apparent Brightness
  • Absolute Brightness
  • Measuring Star Distances
  • Stellar Parallax
  • Spectra of Stars
  • Spectral Classification
  • Temperature and Spectral Class
  • Stellar Composition
  • Stellar Motion
  • Stellar Luminosity
  • The Size of Stars
  • Stefan-Boltzmann Law
  • Stellar Mass
  • Hydrostatic Equilibrium
  • Stellar Classification
  • The Hertzsprung-Russell Diagram
  • Volume and Brightness Selected Samples
  • Stars of Different Sizes
  • Understanding the Main Sequence
  • Stellar Structure
  • Stellar Evolution
  • Questions

Chapter 13
Star Birth and Death

  • Star Birth and Death
  • Understanding Star Birth and Death
  • Cosmic Abundance of Elements
  • Star Formation
  • Molecular Clouds
  • Young Stars
  • T Tauri Stars
  • Mass Limits for Stars
  • Brown Dwarfs
  • Young Star Clusters
  • Cauldron of the Elements
  • Main Sequence Stars
  • Nuclear Reactions in Main Sequence Stars
  • Main Sequence Lifetimes
  • Evolved Stars
  • Cycles of Star Life and Death
  • The Creation of Heavy Elements
  • Red Giants
  • Horizontal Branch and Asymptotic Giant Branch Stars
  • Variable Stars
  • Magnetic Stars
  • Stellar Mass Loss
  • White Dwarfs
  • Supernovae
  • Seeing the Death of a Star
  • Supernova 1987A
  • Neutron Stars and Pulsars
  • Special Theory of Relativity
  • General Theory of Relativity
  • Black Holes
  • Properties of Black Holes
  • Questions

Chapter 14
The Milky Way

  • The Distribution of Stars in Space
  • Stellar Companions
  • Binary Star Systems
  • Binary and Multiple Stars
  • Mass Transfer in Binaries
  • Binaries and Stellar Mass
  • Nova and Supernova
  • Exotic Binary Systems
  • Gamma Ray Bursts
  • How Multiple Stars Form
  • Environments of Stars
  • The Interstellar Medium
  • Effects of Interstellar Material on Starlight
  • Structure of the Interstellar Medium
  • Dust Extinction and Reddening
  • Groups of Stars
  • Open Star Clusters
  • Globular Star Clusters
  • Distances to Groups of Stars
  • Ages of Groups of Stars
  • Layout of the Milky Way
  • William Herschel
  • Isotropy and Anisotropy
  • Mapping the Milky Way
  • Questions

Chapter 15
Galaxies

  • The Milky Way Galaxy
  • Mapping the Galaxy Disk
  • Spiral Structure in Galaxies
  • Mass of the Milky Way
  • Dark Matter in the Milky Way
  • Galaxy Mass
  • The Galactic Center
  • Black Hole in the Galactic Center
  • Stellar Populations
  • Formation of the Milky Way
  • Galaxies
  • The Shapley-Curtis Debate
  • Edwin Hubble
  • Distances to Galaxies
  • Classifying Galaxies
  • Spiral Galaxies
  • Elliptical Galaxies
  • Lenticular Galaxies
  • Dwarf and Irregular Galaxies
  • Overview of Galaxy Structures
  • The Local Group
  • Light Travel Time
  • Galaxy Size and Luminosity
  • Mass to Light Ratios
  • Dark Matter in Galaxies
  • Gravity of Many Bodies
  • Galaxy Evolution
  • Galaxy Interactions
  • Galaxy Formation
  • Questions

Chapter 16
The Expanding Universe

  • Galaxy Redshifts
  • The Expanding Universe
  • Cosmological Redshifts
  • The Hubble Relation
  • Relating Redshift and Distance
  • Galaxy Distance Indicators
  • Size and Age of the Universe
  • The Hubble Constant
  • Large Scale Structure
  • Galaxy Clustering
  • Clusters of Galaxies
  • Overview of Large Scale Structure
  • Dark Matter on the Largest Scales
  • The Most Distant Galaxies
  • Black Holes in Nearby Galaxies
  • Active Galaxies
  • Radio Galaxies
  • The Discovery of Quasars
  • Quasars
  • Types of Gravitational Lensing
  • Properties of Quasars
  • The Quasar Power Source
  • Quasars as Probes of the Universe
  • Star Formation History of the Universe
  • Expansion History of the Universe
  • Questions

Chapter 17
Cosmology

  • Cosmology
  • Early Cosmologies
  • Relativity and Cosmology
  • The Big Bang Model
  • The Cosmological Principle
  • Universal Expansion
  • Cosmic Nucleosynthesis
  • Cosmic Microwave Background Radiation
  • Discovery of the Microwave Background Radiation
  • Measuring Space Curvature
  • Cosmic Evolution
  • Evolution of Structure
  • Mean Cosmic Density
  • Critical Density
  • Dark Matter and Dark Energy
  • Age of the Universe
  • Precision Cosmology
  • The Future of the Contents of the Universe
  • Fate of the Universe
  • Alternatives to the Big Bang Model
  • Space-Time
  • Particles and Radiation
  • The Very Early Universe
  • Mass and Energy in the Early Universe
  • Matter and Antimatter
  • The Forces of Nature
  • Fine-Tuning in Cosmology
  • The Anthropic Principle in Cosmology
  • String Theory and Cosmology
  • The Multiverse
  • The Limits of Knowledge
  • Questions

Chapter 18
Life On Earth

  • Nature of Life
  • Chemistry of Life
  • Molecules of Life
  • The Origin of Life on Earth
  • Origin of Complex Molecules
  • Miller-Urey Experiment
  • Pre-RNA World
  • RNA World
  • From Molecules to Cells
  • Metabolism
  • Anaerobes
  • Extremophiles
  • Thermophiles
  • Psychrophiles
  • Xerophiles
  • Halophiles
  • Barophiles
  • Acidophiles
  • Alkaliphiles
  • Radiation Resistant Biology
  • Importance of Water for Life
  • Hydrothermal Systems
  • Silicon Versus Carbon
  • DNA and Heredity
  • Life as Digital Information
  • Synthetic Biology
  • Life in a Computer
  • Natural Selection
  • Tree Of Life
  • Evolution and Intelligence
  • Culture and Technology
  • The Gaia Hypothesis
  • Life and the Cosmic Environment

Chapter 19
Life in the Universe

  • Life in the Universe
  • Astrobiology
  • Life Beyond Earth
  • Sites for Life
  • Complex Molecules in Space
  • Life in the Solar System
  • Lowell and Canals on Mars
  • Implications of Life on Mars
  • Extreme Environments in the Solar System
  • Rare Earth Hypothesis
  • Are We Alone?
  • Unidentified Flying Objects or UFOs
  • The Search for Extraterrestrial Intelligence
  • The Drake Equation
  • The History of SETI
  • Recent SETI Projects
  • Recognizing a Message
  • The Best Way to Communicate
  • The Fermi Question
  • The Anthropic Principle
  • Where Are They?

Apparent Brightness


A nearby flashlight may appear to be brighter than a distant streetlight, but in absolute terms (if you compared them side by side) the flashlight is much dimmer. This statement contains the essence of the problem of determining stellar brightness. A casual glance at a star does not reveal whether it is a nearby glowing ember or a distant great beacon. Astronomers must distinguish between how bright a star looks and how bright the star really is. Apparent brightness is the brightness perceived by an observer on Earth and absolute brightness is the brightness that would be perceived if all stars were magically placed at the same standard distance. There can be a great difference between the total amount of radiation a star emits and the amount of radiation measured at the Earth's surface.


As soon as the Sun sets over the Chilean Atacama Desert, ESO’s Very Large Telescope (VLT) begins catching light from the far reaches of the Universe. The VLT has four 8.2-metre Unit Telescopes such as the one shown in the photograph. Many of the photons — particles of light — that are collected have travelled through space for billions of years before reaching the telescope’s primary mirror. The giant mirror acts like a high-tech “light bucket”, gathering as many photons as possible and sending them to sensitive detectors. Careful analysis of the data from these instruments allows astronomers to unravel the mysteries of the cosmos. The telescopes have a variety of instruments, which allow them to observe in a range of wavelengths from near-ultraviolet to mid-infrared. The VLT also boasts advanced adaptive optics systems, which counteract the blurring effects of the Earth's atmosphere, producing images so sharp that they could almost have been taken in space.

Apparent brightness can be defined as the number of photons per second collected on the Earth from an astronomical source. Apparent brightness depends on the light-collecting aperture of the viewing device and on the distance to the source. A star appears much brighter as seen through a telescope than it does when viewed by the eye — the larger aperture of the telescope can collect many more photons per second. Similarly, the closer a star is, the brighter it appears — the number of photons intercepted by our light-gathering device decreases by the inverse square of the distance. Now you can understand why astronomers build larger and larger telescopes. A larger collecting area compensates for the diminishing amount of light that reaches us from more and more distant sources. Larger telescopes can detect more distant sources and they can capture more photons from nearby sources.

The most direct way to talk about apparent brightness is in units of photons per second. However, the most convenient way to measure apparent brightness is to express it as a ratio to the apparent brightness of the Sun or some other prominent star. This ratio allows us to compare how much brighter or dimmer a star is than our Sun or another familiar star. Apparent brightness defined in this way requires no units since the ratio of two numbers that have the same units is a pure number. Astronomers traditionally relate the apparent brightness of astronomical objects to the bright star Vega.


NASA's Spitzer Space Telescope recently captured this image of the star Vega, located 25 light years away in the constellation Lyra. Spitzer was able to detect the heat radiation from the cloud of dust around the star and found that the debris disc is much larger than previously thought. This side by side comparison, taken by Spitzer's multiband imaging photometer, shows the warm infrared glows from dust particles orbiting the star. The left image was taken at λ=24µm; the right at λ=70µm.

The list below shows the ratio of the apparent brightness of different objects to the bright star Vega:

• Sun: 4 × 1010
• Full Moon: 100,000
• 100-Watt light bulb at 100 meters: 27,700
Venus (at brightest): 58
Mars (at brightest): 12
Jupiter (at brightest): 3.6
• Sirius (the brightest star): 3.6
• Canopus (second brightest star): 1.9
• Vega: 1.0
• Spica: 0.4
• Naked-eye limit in urban areas: 0.025
Uranus: 0.0063
• Bright asteroid: 0.0040
• Naked-eye limit in rural areas: 0.0025
Neptune: 0.0008
• Limit for typical binoculars: 0.0001
• 3C 273 (brightest quasar): 8 × 10-6
• Limit for 15-cm (6-inch) telescope: 6 × 10-6
Pluto: 1 × 10-6
• Limit by eye with largest telescopes: 2 × 10-8
• Limit for CCDs with largest telescopes: 6 × 10-12
• Limit for the Hubble Space Telescope: 3 × 10-12

The Sun is by far the brightest object in the sky. For example, the Sun is 11 billion times brighter than Sirius, the brightest star. The actual ratio is 4 × 1010 / 3.6 = 11 × 109. The best telescopes in space can detect objects about 800 million times fainter than the eye can see! Again, the actual ratio is 0.0025 / 3 × 10-12 = 8 × 108. Three of the planets appear brighter than any star when they are closest to the Earth. However, each of them only intercepts a tiny fraction of the Sun's rays and imperfectly reflects them back to the Earth. Jupiter, for example, is about nine-billionth (3.6 / 4 × 1010) as bright as the Sun. You can also see the effect of the inverse square law in the relative brightness of the different planets. Planets further from the Sun appear fainter (although the different size of the planets plays a role, too). Uranus is only visible to the naked eye if you are far from city lights and Neptune and Pluto can only be seen with binoculars or a telescope. Pluto is over 50 million times fainter (10-6 / 58) than Venus at its brightest.

Notice in the list that the limit for naked eye observing is ten times better (or deeper or fainter) in a rural area than in an urban area. The same factor of ten reduction applies between a night sky with no Moon and with a full Moon. That's because there's another issue that affects the depth of viewing apart from the apparent brightness of an astronomical source: the brightness of the night sky. The night sky is never truly black, even with no Moon and when you are far from city lights. There's always some light from cities and industrial activity diffusing over large distances, and the air itself shines as molecules are excited by cosmic rays from space, a phenomenon called airglow. So the same little patch of sky that has starlight also has some light from the sky itself; the star is "competing" against this background and that affects the depth of viewing. For this reason, astronomers go to great lengths (and expense) to locate their major telescopes far from city lights, such as in Chile's Atacama Desert. Notice also in the list that the apparent brightness limit for the Hubble Space Telescope is two times better than for much larger ground-based telescopes. How can that be? It's the same reason — sky brightness. In space, there's no airglow and the sky approaches true black. This lets the Hubble Space Telescope make images ten billion times deeper than most people can see with their eyes!

Can the relative brightness of objects be used to estimate the distance to the nearest stars? Yes, if we make the bold assumption that the brightest stars are just like the Sun (this would be like assuming that the flashlight and the streetlight are intrinsically the same brightness). This is equivalent to assuming that stars emit the same number of photons per second as the Sun, and the difference in apparent brightness is a measure of how the photons thin out with increasing distance. This assumption also implies that stars with the highest apparent brightness and the nearest. By the inverse square law, the brightest few stars must be about √ (1010) = 100,000 times farther than the Sun. This is a distance of 1.5 x 108 x 105 ≈ 1013 kilometers, or about 1/3 parsec.

The 6000 stars visible with the naked eye span a range of 3.6 / 0.0025, or a factor of 1440. If we assume that they are also like the Sun, then we infer a distance range spanning a factor of √ (1440) ≈ 40 for the stars you can see in the night sky. We can also relate star brightness to a more familiar terrestrial object — a light bulb. From the list of ratios, we can calculate that the Sun is like a 100 Watt light bulb seen at a distance of √ (27,700 / 4 x 1010) x 100 = 0.08 meters or about 3 inches. (Don't try this; it will hurt your eyes just as staring at the Sun would!) On the other hand, the brightest star beside the Sun is like a 100 Watt light bulb seen at a distance of √ (27,700 / 3.6) x 100 = 8770 meters. This is like looking at a reading light in a house over five miles away. This gives a sense of the enormous range in brightness between our star and all the others.

It is easy to understand relative brightness using a linear scale. However, astronomers use a relative brightness scale based on logarithms. Why do astronomers use a different system? They are victims of history. When Hipparchus cataloged 1200 stars in about 130 B.C., he ranked their apparent brightness on a magnitude scale of 1 to 6, with 1st-magnitude stars the brightest and 6th-magnitude stars the faintest visible to the naked eye. Viewed with the naked eye, stars could only be classified with six gradations of brightness. A difference of one magnitude corresponds to a factor of roughly 2.5 in apparent brightness, five magnitudes represent a factor of 100 in brightness. The scale is reversed; higher numbers represent fainter objects. Vega defines the zero-point of the magnitude scale. Hipparchus invented this nonlinear scale to match the response of the eye. The eye is a logarithmic detector, which means it can accommodate a huge range of brightness. Evolution equipped us with sensors that let us see in bright sunlight but also let us hunt at night. (A more familiar example is the loudness of a sound, which is quoted in units of decibels, also a logarithmic scale. The decibel unit must be tied to a known sound intensity measured at a fixed distance, since loudness depends on how far you are from a sound.) The 2100-year-old magnitude system is so ingrained that astronomers continue to use it.

To be meaningful, apparent brightness must be specified at a particular wavelength. Stars have different colors, which means that the apparent brightness depends on the wavelength. Also, light detectors (the eye, photographic films, and electronic CCDs) have different sensitivities to particular colors or wavelengths. For this reason, astronomers specify exactly what wavelength any set of measurements refers to. Standards have been derived to express apparent brightness measured in blue light, red light, infrared light, radio waves, X-rays, and so on. Here we will usually be referring to a system having the same color sensitivity as the human eye, which is sometimes called visual apparent brightness (corresponding to a range of wavelengths centered on the green part of the visible spectrum).


Author: Chris Impey
Editor/Contributor: Audra Baleisis