Astrophysics For Dummies
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The path to understanding astrophysics is both thought-provoking and brain-stretching. How did the universe come into existence, when will it end, and what role do our familiar planets and stars play in the grand scheme of the cosmos? There are many more questions in astrophysics than there are answers. The goal of this book is to put you in a position where you’re able to better formulate those questions, and know where to go for answers.

As you’re getting started on your journey, use this Cheat Sheet to answer some of the first questions that come to mind.

Top three websites for the latest astrophysics discoveries

Ready to take your interest in astrophysics a step further? Just about everything and everyone has a website, but that doesn’t mean they’re all equally useful (or even accurate). Here are some of the best places to start expanding your budding knowledge, and keep up on the latest discoveries.

  • NASA: Start here not only for information about current NASA programs and missions but also to uncover a treasure trove of information on all things space, including the latest astrophysics discoveries.
  • Hubble Space Telescope: If you’re looking for images taken by the venerable Hubble Space Telescope, look no further. The official site is home to official images and videos, as well as technical information about the telescope and its data. Most of the image releases also have an accompanying short article that describes the science behind the image, which is a great way to keep up on both theory and observation.
  • James Webb Space Telescope: This is the definitive site for current images from the new JWST, an extremely powerful space-based telescope capable of detecting infrared wavelengths. JWST observations will capture data from exoplanets and far-distant galaxies that were impossible to detect with previous generations of telescopes.

A timeline of key discoveries in astrophysics

Don’t let the trees cloud your view of the forest. While the fields of astronomy, physics and astrophysics have had numerous points of innovation throughout humanity’s relatively short period of existence, a few rise to the top. They are:

  • 13.8 billion years ago: The Big Bang
  • 4.6 billion years ago: Formation of the solar system
  • 30,000 BCE: Humans first moved from Asia to North America
  • 3000–1520 BCE: Stonehenge is built, one of the first constructions with astronomical significance
  • 2000 BCE: Solar and lunar calendars developed in ancient Mesopotamia and Egypt
  • 270 BCE: Ancient Greek Aristarchus first proposed the heliocentric theory of the universe
  • 1200-1300 CE: Early Chinese development of solid rocket propellant
  • 1543: Nicolaus Copernicus’s theory of a heliocentric universe published
  • 1608: Invention of the telescope
  • 1609–1618: Johannes Kepler formulated laws of planetary motion
  • 1610: Galileo discovered the moons of Jupiter, definitively proving wrong the geocentric theory.
  • 1687: Isaac Newton’s description of gravity published
  • 1781: William Herschel’s first discovery of a planet via telescope, Uranus
  • 1867: James Maxwell’s proposition that light waves longer than infrared existed
  • 1915: Einstein’s theory of general relativity published
  • 1926: Robert Goddard launched first rocket with gasoline and liquid oxygen fuel
  • 1927: Georges Lemaître proposed that the universe exploded into creation from a single point
  • 1958: NASA established by President Eisenhower’s National Aeronautics and Space Act
  • 1961: Yuri Gagarin’s first human spaceflight
  • 1969: Neil Armstrong’s first steps on the Moon
  • 1970: Stephen Hawking’s work connecting black hole singularities and gravity
  • 1975: Founding of the ESA, European Space Agency
  • 1990: Launch of the Hubble Space Telescope
  • 2006: SpaceX first Falcon 1 rocket launch
  • 2021: Launch of the James Webb Space Telescope

Top six misconceptions about astrophysics

You’ve probably got one of those friends who thinks they know everything. Be the first to prove them wrong if they try to convince you of any of these common misconceptions about how the world works.

  • Black holes aren’t really black. Einstein’s theory of general relativity paved the way for understanding the effects of gravity in an ever-expanding universe. As light and matter move toward a black hole’s singularity, light emissions shift from visible to non-visible (microwave and infrared, for example) wavelengths but don’t disappear completely. Though never visually observable, traces of that light still exist — and, as Stephen Hawking discovered, even black holes emit very small amounts of radiation. As long as there’s even the tiniest amount of light, a black hole can never be black.
  • A parsec is a unit of distance, not time. Despite what the world of science fiction would have you believe, one parsec is a measurement of length equal to 3.26 light-years. And since we’re on the subject, a light-year is also not a unit of time; one light-year is how far light travels in a year, or 5.88 × 1012 miles (9.46 × 1012 km).
  • The Big Bang didn’t sound like a cannon firing. Although the name might make you think that the birth of the universe came with dramatic sound effects, the Big Bang itself would have been silent. By definition, sound waves are created when something vibrates through a medium (water or air, for example). At the time of the Big Bang there was no space or air (or anything else) for sound waves to move through.
  • The Moon does not only come out at night. Because the Earth rotates about its axis every 24 hours, the Moon is only above the horizon for half of that time — 12 hours. When the Moon is below the horizon for you, you can’t see it though during those hours, but the Moon is visible to people living in the opposite hemisphere.
  • The Earth is not at the center of our solar system. The geocentric model of the universe was first posed by ancient Greek astronomers in 380 BCE and remained a popular theory until disproved by Galileo in the 17th century. Using some of the world’s first telescopes, Galileo used studies of the Moon and Venus to show that the concept of phases meant that the Sun had to be at the center of the universe, not Earth. He also discovered moons orbiting Jupiter, definitive proof that not everything orbited the Earth. While we’re at it, the Earth also is not flat. Again, give a shout-out to the ancient Greeks; observations of lunar cycles showed early astronomers that the Moon had to be spherical and, ergo, so was Earth.
  • Comets, meteors, shooting stars, and asteroids are not all the same. Shooting stars and meteors are, though! They’re chunks of rock that burn up as they pass through the Earth’s atmosphere. Asteroids are rocky celestial objects, usually larger than meteors, that orbit the Sun. Comets also orbit the Sun, but at a larger distance than asteroids, and consist of cosmic dust and ice instead of rock. Just to make matters more confusing, tiny pieces of asteroids or comets can fall to earth as meteors, and if any of this material makes it through the atmosphere to hit the ground, the resulting space rock is called a meteorite.

How big are you compared to objects in space?

Are black holes smaller than Earth? Are comets bigger than a car? Look no further for a quick guide to guesstimating size in the universe (and use these numbers to practice your metric system skills!)

  • Average person height: 0.00175 km
  • Height of the Empire State Building: 0.4 km
  • Typical comet diameter: 10 km
  • Typical asteroid diameter: 250 km
  • Our Moon’s diameter: 3,475 km
  • Mercury (the smallest planet in our solar system) diameter: 4880 km
  • Earth’s diameter: 12,742 km
  • Jupiter (the largest planet in our solar system) diameter: 139,822 km
  • Supermassive black hole diameter: 6,000,000 km
  • The Milky Way Galaxy diameter: 9.5×1017 km
  • Local Supercluster diameter: 9.5×1020 km

Astrophysics world record holders

If you’ve ever tried to break a world record, you know just how extreme humans like to be. Turns out, the universe is no different. Here are a few more-than-noteworthy achievements:

  • How fast can neutron stars spin? 700 times per second
  • Largest planet in our solar system? Jupiter, twice as big as all other planets in our solar system put together
  • Tiniest fundamental particle? Quark (infinitely small)
  • Most powerful celestial object? Quasar (trillions of times brighter than the Sun, 100–1000× brighter than our galaxy)
  • Densest object in the universe? Neutron star (billions of tons per cubic inch)
  • Heaviest object in the universe? Black hole (up to 100 billion solar masses)
  • Galaxy furthest away from Earth? Candidate HD1, located 13.5 billion light-years from us
  • Coldest celestial object? Boomerang Nebula, 1 degree K
  • Hottest celestial object? Supernova, up to 1 million degrees C

The units of astrophysics

Without units, it’d be difficult to figure out how far away a store or restaurant is. Now try to figure out the distance from the Earth to the Moon without units! Below is a listing of some of the most common units that astrophysicists frequently refer to.

Astrophysical Units

Unit name Abbreviation Description Value
Astronomical Unit AU 1 AU = distance from Earth’s orbit to Sun 93 million miles (150 million km)
Light year ly 1 light-year = distance traveled in a year at the speed of light 6 trillion miles (9.7 trillion km)
Cosmic year, or galactic year The amount of time it takes the Sun (and our solar system) to orbit the center of the Milky Way About 225 million years
Parsec pc The distance at which Earth’s orbit is visible as one arcsecond 3.26 light-years or 1.9×1013 miles (3×1013 km)
Megaparsec Mpc 1 million parsecs 1.9×1019 miles (3×1019 km)
Solar mass M☉ The mass of the Sun 4.4×1030 pounds (2.0×1030 kg)
Earth mass M⊕ The mass of the Earth 1.3×1025 pounds (5.97×1024 kg)
Electron Volt eV The energy gained by an electron traveling through a 1-volt potential; used for measuring speed of high-energy cosmic rays 1.6×10−19 J
Joule J A unit of energy or work, defined as the work of 1 Newton acting over 1 meter 107 ergs, or 0.737 foot-pounds
Kelvin K Measure of thermodynamic temperature Absolute zero: 0 K = –459°F = –273°C
Newton N Force required to accelerate 1 kg over 1 meter per second per second 1 kg⋅m/s2
Jansky Jy Unit of radio-wave emission strength 10−26 W/m2 per Hz

About This Article

This article is from the book:

About the book authors:

Cynthia Phillips, PhD, is a scientist at the NASA Jet Propulsion Laboratory. Previously, she worked at the SETI Institute for 15 years. Shana Priwer is a technical writer who has co-authored many projects with Cynthia Phillips, including the Frameworks series on architecture.

Cynthia Phillips, PhD, is a scientist at the NASA Jet Propulsion Laboratory. Previously, she worked at the SETI Institute for 15 years. Shana Priwer is a technical writer who has co-authored many projects with Cynthia Phillips, including the Frameworks series on architecture.

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