🔭 Stuff Every Astronomer Should Know: Training Guide

The electromagnetic spectrum

Light is a spectrum of electromagnetic waves. It runs from long radio waves to very short gamma rays. Gamma rays have wavelengths under 0.01 nanometer and carry the most energy per photon.

Visible light is only a narrow slice. Our eyes see wavelengths from about 400 nanometers in the violet to about 700 nanometers in the red. That band passes through air well, which is one reason eyes evolved to use it.

Herschel discovered infrared in 1800 by placing a thermometer just past the red end of a spread spectrum. The thermometer warmed, showing invisible light. Later Jansky found the first cosmic radio waves coming from the Milky Way.

Spectra and what they reveal

A normal star shows a continuous rainbow crossed by dark absorption lines. Fraunhofer counted about 600 dark lines in sunlight. Thin glowing gas shows bright emission lines instead.

Each element leaves its own pattern of lines. Astronomers found helium in the Sun's spectrum before it was found on Earth. This is spectroscopy: doing chemistry at a distance.

The Doppler effect measures motion along our line of sight, called radial velocity. A source moving toward us has a blueshift. A source moving sideways, neither toward nor away, shows no Doppler shift.

For speeds far below light speed, the fractional wavelength shift equals speed divided by light speed. A shift of 1/1000 of the wavelength means about 300 kilometers per second. Absorption lines are good targets because their rest wavelengths are known precisely.

Brightness, color, and distance

Light spreads over a sphere. If you move twice as far from a star, the same light covers four times the area, so you receive one quarter. This inverse square law is the key link between apparent brightness and distance.

Hotter objects peak at shorter wavelengths. A stove burner goes from dull red to orange as it heats. Among stars, blue-white stars are hotter than the Sun, and red stars are cooler.

Parallax is the apparent shift of a nearby star against distant background stars as Earth orbits the Sun. Distance in parsecs equals one divided by parallax in arcseconds. A star at 0.1 arcsecond is 10 parsecs away. The first successful stellar parallax measurement came in 1838.

Astronomers use light travel time for distances. One astronomical unit, the Earth to Sun average distance, is about 8.3 light-minutes. A light-year is about 9.46 trillion kilometers. Looking at the Moon means seeing it about 1.3 seconds in the past.

Magnitudes measure brightness. Five magnitudes are exactly a factor of 100 in brightness. The scale runs backward: smaller or negative numbers mean brighter objects. Absolute magnitude places every star at 10 parsecs so true brightness can be compared.

Optical telescopes and aperture

A telescope's main lens or mirror is its aperture. Light gathering depends on area, so a 4-meter mirror collects 16 times more light than a 1-meter mirror. Bigger aperture, not bigger magnification, is what shows fainter objects.

The main optic forms an image, and the eyepiece magnifies it. Changing eyepieces changes magnification. Too much magnification enlarges atmospheric shimmer as well as the target.

Glass lenses bend each color by a slightly different amount. This causes chromatic aberration, where colors focus at different spots and blur the image. Mirrors avoid this problem by reflecting every color the same way.

Isaac Newton built the first successful reflecting telescope in 1668. Reflectors use a curved mirror and can be supported from behind. Nearly every large research telescope today is a reflector.

The largest working refractor, at Yerkes Observatory, is 40 inches. Big lenses sag under their own weight because they can only be held at the edges. That limit is why giant telescopes use mirrors.

Adaptive optics uses a sensor to measure atmospheric distortion and a flexible mirror to cancel it as often as 500 times per second. The Keck telescopes combine 36 hexagonal segments into one 10-meter mirror. The best optical sites are high, dark, and dry.

Beyond visible light

Infrared astronomy is best from mountaintops, airplanes, or space because water vapor and carbon dioxide absorb infrared. High and dry locations put much of that gas below the telescope.

X-rays are absorbed by Earth's atmosphere. Chandra orbits above the atmosphere, up to 139,000 kilometers out, and focuses X-rays with nested mirror shells hit at a glancing angle.

Hubble launched in 1990 with a flawed mirror. Astronauts installed corrective optics in December 1993. The James Webb Space Telescope operates near the second Lagrange point, 1.5 million kilometers from Earth, where its sunshield blocks Sun, Earth, and Moon at once.

The Vera C. Rubin Observatory will re-image the whole southern sky every few nights for 10 years. Its 8.4-meter telescope and the largest digital camera ever built make a movie of the changing sky.

Radio and interferometry

Radio waves are very long, so a single giant radio dish sees less detail than a small optical telescope. Resolution depends on aperture and wavelength.

Radio astronomers link dishes into interferometers. At a fixed wavelength, the finest detail an interferometer can resolve is set by the distance between telescopes, not by the size of each dish.

The Very Long Baseline Array links 10 radio dishes stretching between the Virgin Islands and Hawaii. Each records waves with precise timing, and computers combine the data later for extremely fine resolution.

Practical observing

Dim red light preserves dark-adapted vision better than white light. A red screen setting makes a phone less disruptive at an observing site.

City light pollution brightens the sky background. It hides faint stars, nebulae, and the Milky Way because the background washes out their contrast.

Binoculars are often better than a telescope for a beginner. They are portable, easy to aim, and show a wide field of view, which makes finding bright targets easier.

Mapping the sky

The celestial sphere is an imaginary surface centered on the observer or Earth. Stars are not really fixed to one shell, but projecting them onto it makes positions easier to describe.

To map the sky, astronomers split the full circle into 24 equal hours of right ascension. One hour therefore covers 15 degrees of the circle. Declination tells you how far north or south an object is from the sky's equator, just as Earth latitude tells you how far from the equator.

The ecliptic traces the Sun's apparent yearly path against the background stars. It crosses the celestial equator at the two equinox points.

Seasons and solar angle

Earth's 23.5-degree axial tilt causes the seasons. Distance from the Sun is not the main cause, and Earth is actually closest to the Sun during Northern Hemisphere winter.

A higher summer Sun concentrates the same incoming sunlight over a smaller area. More daylight time also adds heating. A low winter Sun spreads the same sunlight over a larger patch of ground.

Moon phases and eclipses

The Sun always lights half of the Moon. As the Moon moves around Earth, we see different parts of that sunlit half. The complete phase cycle takes about 29.5 days.

A solar eclipse does not happen at every new Moon because the Moon usually passes above or below the Sun. The Moon's orbit is tilted by about 5 degrees, so alignment near a node is needed.

A lunar eclipse happens when Earth lies between Sun and Moon, so the full Moon enters Earth's shadow. The tilted orbit means this precise alignment does not occur every full Moon.

Spring tides occur near new and full Moon, when solar and lunar tidal effects reinforce each other. Neap tides come during first and third quarter Moon, when the Sun and Moon pull at right angles to Earth.

Time and precession

A solar day is longer than a sidereal day. After one rotation relative to distant stars, Earth has moved about one degree around the Sun. About four more minutes of rotation bring the Sun back to the same meridian.

Earth's spin axis wobbles over a cycle of about 26,000 years. The Sun and Moon torque Earth's equatorial bulge and drive much of this slow precession.

Polaris will not always be the North Star. Precession shifts the north celestial pole across the sky, so other stars take that role over millennia. Vega will be closest to the north celestial pole in about 14,000 years.

Kepler's laws

Planetary orbits are ellipses, and the Sun sits at one focus. A circle is the special ellipse whose eccentricity is zero and whose two foci coincide.

A planet moves fastest at perihelion, its closest point to the Sun. Kepler's second law says the Sun-planet line sweeps equal areas in equal times, so the planet covers more arc when it is nearer.

For solar orbits measured in years and astronomical units, the period squared equals the semimajor axis cubed. An object at 4 AU has a period of 8 years because 4 cubed is 64 and its square root is 8.

Newton's gravity

Newtonian gravity follows an inverse-square law. Doubling the separation between two bodies multiplies the force by one quarter. This is the same pattern as starlight dimming with distance.

Newton's law contains both masses multiplied together. Doubling either mass doubles the mutual force if the separation stays fixed. The two bodies feel equal-magnitude forces in opposite directions.

Escape and Lagrange points

Escape velocity from Earth's surface is about 11 kilometers per second, ignoring air resistance. It depends on the body's mass and launch radius, not on the escaping object's mass.

A Lagrange point is a spot that stays fixed relative to two bodies like the Sun and Earth. Gravity is not absent there; it is balanced with the orbit.

In the ideal system, L4 and L5 can be stable. L1, L2, and L3 are unstable and require stationkeeping. Asteroids sharing a planet's orbit around L4 or L5 are called Trojans.

Planets in brief

Venus is hotter at the surface than Mercury because its massive carbon dioxide atmosphere traps outgoing heat. Its average surface temperature is about 467 degrees Celsius.

Mars hosts Olympus Mons, the largest volcano in the solar system. From base to summit it stands more than 40 kilometers tall, far higher than Mount Everest above sea level.

Jupiter's Great Red Spot is a giant atmospheric storm. Jupiter lacks a solid surface, and its atmospheric vortices can persist for many years.

Saturn's bright main rings are made of separate chunks of ice and rock, not a solid sheet. Particles range from dust-sized icy grains to chunks as large as houses or mountains.

Uranus's rotation axis is tilted about 98 degrees. This extreme geometry gives the planet unusually long, intense seasons.

Neptune has the fastest measured winds on any planet. Voyager 2 measured winds near the Great Dark Spot reaching about 2,000 kilometers per hour.

Dwarf planets and small bodies

Under the 2006 IAU definition, a planet must clear its orbital neighborhood. A dwarf planet orbits the Sun and is nearly round but has not cleared its orbit. Pluto was reclassified as a dwarf planet in 2006.

Most asteroids orbit in the main belt between Mars and Jupiter. The belt is not a tightly packed obstacle course; enormous gaps separate its objects. Ceres is its largest object and is a dwarf planet.

The Kuiper Belt is a disk-like region of icy bodies beyond Neptune. Its main region begins near 30 AU from the Sun. Pluto, Haumea, and Makemake are Kuiper Belt dwarf planets.

The Oort Cloud is thought to be a giant spherical shell of distant icy bodies. It has never been directly observed. Its outer edge may lie around 100,000 AU from the Sun.

A comet's tails generally point away from the Sun. Solar radiation pressure and the solar wind push dust and ionized gas outward. A comet usually develops a curved dust tail and a straighter ion tail.

In space a space rock fragment is a meteoroid. Its glowing atmospheric passage is a meteor. Any fragment that survives to the ground is a meteorite. Most meteors burn up completely.

Remarkable moons

Europa likely has a salty liquid-water ocean beneath its icy shell. Its ocean may contain about twice as much water as Earth's global ocean despite Europa's smaller size.

Enceladus hides a global liquid-water ocean beneath its crust. Its south-polar jets spray icy grains, water vapor, and simple organic chemicals into space, and some material supplies Saturn's E ring.

Titan is the only moon with a thick atmosphere. It is mostly nitrogen with about 5 percent methane, and methane drives clouds, rain, and complex organic chemistry. Its surface pressure is about 60 percent greater than Earth's.

Formation and exoplanets

The solar system began forming about 4.6 billion years ago from a dense gas-and-dust cloud. Gravity collapsed part of an interstellar cloud into a rotating solar nebula, and most material collected at the center to form the Sun.

Rocky planets formed mainly near the young Sun because heat vaporized many icy materials there. Refractory rock and metal condensed near the Sun, while volatile ices survived farther out and helped build giant-planet cores.

The transit method finds an exoplanet when it crosses in front of its star and dims the star slightly. If we know the star's size, the size of the dip gives the planet's size.

The first exoplanet around a regular Sun-like star was found in 1995 at 51 Pegasi by Michel Mayor and Didier Queloz. They measured the star's tiny Doppler wobble. The Doppler method alone gives only a planet's minimum mass.

A star's habitable zone is the distance range where a planet's surface temperature could allow liquid water. It is not a guarantee of life. Fainter stars have smaller, closer-in habitable zones.

Layers of the Sun

The Sun's core is its hottest part at about 15 million K. The core spans only about 20 percent of the solar interior, yet it makes all of the Sun's energy.

Energy from fusion takes about 100,000 to 1,000,000 years to work its way from the center to the surface. In the dense interior, a photon travels only a tiny distance before it is absorbed or scattered. The 8 minutes is the travel time from surface to Earth.

Granulation on the photosphere is a rice-grain pattern caused by rising hot gas in convection cells. Hot plasma bubbles up, cools, and sinks again, much like boiling oatmeal. The bright granules are rising columns.

The photosphere is the visible surface where the Sun becomes opaque. It is a layer only about 400 kilometers thick where gas changes from nearly transparent to nearly opaque. Its temperature is about 5800 K.

The Sun's corona is its outer atmosphere and is hotter than the visible surface, at a million degrees or more. It extends millions of kilometers out and gives off about half as much light as the full Moon.

Fusion and energy

In the Sun's proton-proton chain, four hydrogen nuclei fuse to make one helium nucleus. Protons fuse into deuterium, then helium-3, and two helium-3 nuclei make helium-4.

When hydrogen fuses into helium, about 0.7 percent of the mass turns into energy. The missing mass is 0.71 percent of the initial hydrogen. Even that tiny fraction releases a lot because the speed of light squared is enormous.

Neutrinos made in the core travel at nearly light speed and pass through matter almost untouched. They leave the Sun in about two seconds, carrying news of fusion happening right now, unlike photons which take far longer.

Solar cycle and wind

Sunspot numbers rise and fall in a cycle averaging about 11 years. Even at sunspot maximum, less than half of one percent of the Sun's surface is covered by spots.

The full magnetic cycle takes about 22 years to return to the same polarity. In each new sunspot cycle, the magnetic polarity of the leading spots in each hemisphere flips.

The solar wind is a stream of charged particles, mainly protons and electrons, flowing outward at about 400 kilometers per second. Corona gas is so hot and fast that solar gravity cannot hold it back.

Most of the solar wind pours out through coronal holes, where magnetic field lines open into space instead of looping back. Coronal holes are large dark regions of the corona that are relatively cool and quiet.

Space weather

The Carrington Event of 1859 was a huge solar storm after a bright solar flare. Auroras appeared as far south as Hawaii and the Caribbean, and telegraph systems were disrupted. It began the study of space weather.

Star properties and classification

A stable star balances inward gravity with outward gas pressure. This balance, called hydrostatic equilibrium, keeps the star from collapsing or flying apart.

A star's color mainly tells its surface temperature. Blue-white stars are hotter than the Sun, and red stars are cooler. Red stars give off much of their energy in the infrared, and blue stars in the ultraviolet.

The main stellar spectral classes from hottest to coolest are O, B, A, F, G, K, M. The Sun is a G star. Cooler objects beyond class M use the added classes L, T, and Y.

Different star spectra look different mainly because the stars have different temperatures. Most stars have nearly the same makeup as the Sun. Temperature decides which atoms are ionized or excited enough to make lines.

Objects below about 0.075 solar masses never get hot enough to fuse ordinary hydrogen. They glow faintly as brown dwarfs. The white dwarf limit of about 1.4 solar masses is a separate, much higher limit.

The Hertzsprung-Russell diagram plots luminosity against surface temperature. Ejnar Hertzsprung and Henry Norris Russell independently found that temperature and luminosity are related. Most stars fall on a band called the main sequence.

Main sequence and stellar masses

About 90 percent of stars are found on the main sequence because that is where stars spend about 90 percent of their lives fusing hydrogen. Near the Sun, about 10 percent of true stars are white dwarfs and fewer than 1 percent are giants or supergiants.

Mass mostly sets where a star sits along the main sequence. More massive stars sit higher, hotter, and brighter. Mass and composition alone determine the structure of a stable fusion-powered star.

Low-mass stars stay on the main sequence longer. A massive star has more fuel but burns it much faster. A 40 solar mass O star lasts about 1 million years, the Sun about 10 billion years, and a 0.4 solar mass M star about 200 billion years.

Masses of stars are measured most directly from binary star orbits. Gravity links orbit size and period to mass. About half of stars are in binaries. A spectroscopic binary looks like one star, but its spectral lines shift back and forth as the pair orbits.

Life from main sequence to giant

When core hydrogen runs out, a star swells into a red giant. Hydrogen fuses in a shell around the hot helium core. The extra energy pushes the outer layers outward, so the star grows larger and brighter.

Helium fusion needs about 100 million K because helium nuclei carry twice the charge of hydrogen. At that temperature, three helium nuclei, also called alpha particles, fuse into one carbon nucleus.

A dying Sun-like star sheds its outer layers. Ultraviolet light and wind from the hot leftover core ionize the shells and make them glow as a planetary nebula. Planetary nebulae have nothing to do with planets.

End states and explosions

The most mass a white dwarf can have is the Chandrasekhar limit, about 1.4 solar masses. Heavier white dwarfs would have a radius of zero, so no white dwarf can be heavier than that.

White dwarfs have no fusion. Electron degeneracy pressure holds them up against gravity. Their electrons are packed so tightly they cannot be squeezed further, and this pressure needs no heat.

Fusion stops once a massive star builds an iron core because fusing iron absorbs energy instead of releasing it. Without outward pressure from fusion, gravity makes the core collapse.

When a massive star's iron core collapses into a neutron star, the result is smaller than about 20 kilometers across. Electrons are forced into nuclei, turning protons into neutrons, and the core becomes a ball of neutrons of extreme density.

During some supernovae, heavy nuclei beyond iron form when nuclei capture neutrons that later become protons. Neutron-star mergers are another confirmed site of this rapid neutron capture process, and they can create gold and platinum.

Jocelyn Bell discovered pulsars in 1967 as a graduate student. She found rapid, sharp, regular radio pulses from the constellation Vulpecula. Pulsars turned out to be spinning neutron stars, and the lighthouse model explains the pulses as sweeping beams.

A Type Ia supernova is the explosion of a white dwarf in a binary system. The trigger may be material from a companion star or a merger of two white dwarfs. Its calibrated brightness makes it useful as a standard candle.

Distance ladder and pulsating stars

Henrietta Leavitt discovered the cepheid period-luminosity relation in 1908. She studied variable stars in the Magellanic Clouds, all at about the same distance, and found that longer period meant greater luminosity.

For cepheid variable stars, a longer pulsation period means greater true luminosity. Timing a cepheid's brightness cycle gives its intrinsic brightness, and comparing that with how bright it looks gives distance. Cepheids are standard candles.

Parallax is the geometric method that anchors the nearby cosmic distance ladder. Earth's changing viewpoint during its orbit makes nearby stars appear to shift against distant backgrounds, and geometry converts that angular shift into distance.

Cepheids connect geometric parallax distances to nearby galaxies. Their pulsation period reveals their intrinsic luminosity, and calibrating cepheids with parallax extends the ladder.

Type Ia supernovae are standard candles that let astronomers measure distances far beyond individual cepheids. Their great luminosity makes them useful for very distant galaxies.

Elements heavier than primordial lithium mainly formed in stars and stellar explosions. Iron is the heaviest element a star's core fusion can build while still releasing energy.

The Milky Way and galaxies

The Milky Way is a barred spiral galaxy with a flattened stellar disk, spiral arms, and a central bar. Dust blocks our visible-light view across the disk, so infrared and radio surveys help map its structure. Its bright stellar disk is about 100,000 light-years across.

The Sun is in the Orion Spur, a minor spiral feature also called the Orion-Cygnus Arm, between larger spiral arms. It sits about 26,000 light-years from the Milky Way's center.

Hubble's tuning fork places ellipticals along the handle and then separates unbarred and barred spirals. The lenticular type S0 sits near the point where the diagram divides.

In Hubble's scheme, elliptical classes describe apparent flattening. E0 appears nearly round, while E7 is strongly elongated. The class can depend partly on the angle from which the galaxy is viewed.

In a barred spiral galaxy, spiral arms usually begin at the ends of the central bar. The Milky Way itself is classified as a barred spiral galaxy.

Irregular galaxies lack a regular spiral or elliptical shape. Some were distorted by gravitational interactions with neighbors.

Andromeda, also called M31, is the nearest giant spiral galaxy to the Milky Way at about 2.5 million light-years. It is visible to the naked eye as a faint oval smudge from a dark site.

The Local Group includes the Milky Way, Andromeda, Triangulum, and over 50 other galaxies. Galaxy groups generally hold 100 or fewer galaxies, while clusters can contain hundreds to thousands. Superclusters are usually not gravitationally bound as single systems.

Black holes and active galaxies

The first image of a black hole, released 2019, showed the black hole in Messier 87. The Event Horizon Telescope linked radio dishes worldwide into an Earth-sized virtual telescope.

If the Sun could become a black hole, its Schwarzschild radius would be about 3 kilometers. The event horizon is the boundary from which nothing can escape.

The black hole at the center of our Milky Way has about 4 million solar masses. The first black hole found in a binary star system is Cygnus X-1, with about 21 solar masses.

An active galactic nucleus is powered by an active supermassive black hole. A quasar is a brilliant active galactic nucleus whose compact central region can outshine its host galaxy. Jets escape along the rotation axis.

Most visible AGN power originates in hot orbiting gas outside the event horizon. Gas in the accretion disk rubs, heats, and radiates before crossing the horizon. The black hole itself has no glowing material surface.

Gravitational lensing and waves

A background object can appear as several lensed images because light reaches us along different paths around a foreground mass. Gravitational lensing can also magnify sources too faint to observe directly.

With a source directly behind the lens, lensed images can merge into an Einstein ring. Imperfect alignment commonly produces arcs rather than a complete ring.

When a foreground star microlenses a background star, the background star's light is temporarily magnified. A planet can add a smaller feature, so microlensing can reveal planets far from their host stars.

LIGO's first direct gravitational-wave detection in 2015 came from two merging black holes. Each Advanced LIGO detector has two perpendicular 4-kilometer arms. The event shifted the arm length by about 4 times 10 to the minus 18 meters, a tiny relative length change called strain.

Gravitational-wave strain measures relative length change. A passing wave stretches and squeezes spacetime, and interferometers report the length change divided by the original length. Multiple detectors help distinguish a real signal from local noise.

Cosmology and the dark universe

Hubble's law states that a galaxy's recession speed is proportional to its distance. The compact form is velocity equals H0 times distance. The Hubble constant is usually expressed in kilometers per second per megaparsec.

Unexpectedly flat galaxy rotation curves imply dark matter beyond visible stars. Outer gas and stars orbit faster than visible matter alone permits, so an extended dark matter halo is needed.

Gravitational lensing can reveal dark matter in a cluster by tracing mass through light deflection. Comparing lensing mass with luminous matter exposes a large invisible component. X-ray observations of hot cluster gas provide another independent mass clue.

Dark matter accounts for about 27 percent of cosmic critical density. Ordinary matter contributes only about 5 percent, and dark energy dominates the remainder at about 68 percent. Dark matter has not been directly identified in an Earth laboratory.

Dark energy is the name for the unknown cause of accelerating cosmic expansion. Distant supernova measurements showed that the expansion accelerated, leading astronomers to infer dark energy. The acceleration began billions of years after the Big Bang.

The Big Bang was not an explosion at one location inside preexisting space. The scale of space itself changed throughout the universe. Every observer sees distant galaxies receding because expansion has no unique central galaxy.

The universe is about 13.8 billion years old. The cosmic microwave background was released about 380,000 years later, when electrons joined nuclei to form neutral atoms and photons stopped scattering constantly.

The present average temperature of the CMB is 2.73 K. It arrives from every direction with tiny temperature variations. The observable universe's present radius is about 46.5 billion light-years, larger than the light-travel time because space expanded while light was in transit.

Big Bang nucleosynthesis formed hydrogen, helium, and a small amount of lithium. The early universe stayed hot enough for fusion for only a few minutes, so heavier elements formed later in stars and stellar explosions.