Introduction to Astronomy
Astronomy is the scientific study of celestial objects (such as stars, planets, comets, and galaxies), the physics, chemistry, and evolution of objects in the universe, and phenomena that originate outside Earth’s atmosphere. Astronomy provides a visual tour of our solar system and beyond, helping us understand our place in the vast cosmos.
Our Solar System
Formation and Age
- Formed about 4.6 billion years ago from a dense cloud of interstellar gas and dust
- The cloud collapsed, possibly due to the shockwave of a nearby exploding star (supernova)
- This formed a solar nebula – a spinning, swirling disk of material
- The Sun’s core temperature is about 27 million degrees Fahrenheit (15 million degrees Celsius) – hot enough to sustain nuclear fusion
Composition
- Includes the Sun, eight planets, five officially named dwarf planets, hundreds of moons, and thousands of asteroids and comets
- Eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
- Five officially recognized dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris
Key Solar System Facts
- The Sun contains more than 99% of the mass of the solar system
- Our solar system orbits the center of the Milky Way galaxy at about 515,000 mph (829,000 kph)
- It takes our solar system about 230 million years to complete one orbit around the galactic center
- Venus is the hottest planet in our solar system, even though Mercury is closer to the Sun
- Jupiter is the largest planet – if it was a hollow shell, 1,000 Earths could fit inside
- Earth is quite small within the solar system – it would take 1.3 million planets the size of Earth to fill up the Sun
Classification of Planets
Terrestrial Planets: Mercury, Venus, Earth, and Mars
- These inner planets have solid surfaces
- Closest to the Sun
Gas Giants: Jupiter and Saturn
- Don’t have hard surfaces but have swirling gases above a core
Ice Giants: Uranus and Neptune
- Uranus is the seventh planet and third largest
- Neptune is the eighth and most distant planet, fourth largest
- Uranus rotates on its side, appearing to roll around the Sun like a ball
The Sun
- A 4.5 billion-year-old yellow dwarf star
- About 93 million miles (150 million kilometers) from Earth
- Without the Sun’s energy, life as we know it could not exist on Earth
- About 100 times wider than Earth and 10 times wider than Jupiter
Moons and Other Objects
- Our solar system has hundreds of moons orbiting planets, dwarf planets, and asteroids
- Mercury and Venus are the only planets with no moons
- Jupiter and Saturn lead our solar system’s moon counts
- All four giant planets (Jupiter, Saturn, Uranus, and Neptune) have ring systems
- Some asteroids, like Chariklo, also have rings
Boundaries of the Solar System
- Beyond the Kuiper Belt is the Oort Cloud, a giant spherical shell surrounding our solar system
- The Oort Cloud extends from 5,000 to 100,000 astronomical units (AU) from the Sun
- One AU is about 93 million miles (150 million kilometers)
- The heliosphere is the bubble created by the solar wind – a stream of electrically charged gas blowing outward from the Sun
Stars and Stellar Objects
Star Formation and Life Cycle
- Stars form in large clouds of gas and dust called molecular clouds
- Molecular clouds range from 1,000 to 10 million times the mass of the Sun
- Stars are giant balls of hot gas – mostly hydrogen, with some helium and small amounts of other elements
- Each star has its own life cycle, ranging from a few million to trillions of years
- When stars like our Sun eventually die, all that’s left is the core, called a white dwarf, which gradually cools over billions of years
- High-mass stars follow a different path, eventually creating elements as heavy as iron before exploding as supernovae
Star Facts
- The universe could contain up to one septillion stars – a one followed by 24 zeros
- Our Milky Way galaxy alone contains more than 100 billion stars
- In 1967, pulsars were discovered, which were soon shown to be rapidly spinning neutron stars
- The first exoplanet (planet outside our solar system) was confirmed in 1992
- Astronomers have now found over 4,000 exoplanets
Galaxies and the Universe
Galaxy Types and Composition
- There are probably more than 170 billion galaxies in the observable universe
- Astronomers sort galaxies into three main types by their shapes: spirals, ellipticals, and irregulars
- Lenticular galaxies are a cross between spirals and ellipticals, with a central bulge and disk but no arms
- Irregular galaxies have unusual shapes like toothpicks, rings, or groupings of stars
- Galaxies are sprawling systems of dust, gas, dark matter, and anywhere from a million to a trillion stars held together by gravity
- Nearly all large galaxies are thought to contain supermassive black holes at their centers
Our Milky Way Galaxy
- The Milky Way is a spiral galaxy with a disk of stars spanning more than 100,000 light-years
- Earth is located along one of the galaxy’s spiral arms, about halfway from the center
- Our solar system takes about 240 million years to orbit the Milky Way just once
- The Milky Way contains 100 to 400 billion stars that spin around Sagittarius A*, a supermassive black hole with a mass of about 4 million suns
The Universe at Large
- A 2016 study estimated that the observable universe contains two trillion galaxies
- Only about 4.9% of the mass-energy of the Universe is atoms (the kind of stuff we, stars, and galaxies are made of)
- About 26.8% of cosmic mass-energy is invisible dark matter
- Dark energy accounts for 68.3% of the mass-energy of the Universe
- The universe is expanding, as demonstrated by Edwin Hubble
Nearby Galaxies
- The Andromeda Galaxy is the closest large galaxy to our Milky Way
- Light from Andromeda takes more than 2.5 million years to reach us
- Andromeda is visible to the naked eye in the Northern Hemisphere
- If the Milky Way were the size of a single tennis ball, the Andromeda Galaxy would lie 5.6 feet away
Interesting Astronomical Phenomena
Active Galaxies
- About 10% of known galaxies are active, with centers appearing more than 100 times brighter than the combined light of their stars
- Excess energy comes from areas near the galaxies’ central supermassive black holes
- The Milky Way is not currently an active galaxy, though it likely experienced a burst of activity in the past few million years
Cosmic Microwave Background
- The heat of the Big Bang fireball is still around us today, called the Cosmic Microwave Background
- Its temperature is 2.725°C above absolute zero (-270°C) and is essentially the same everywhere
Resources for Further Learning
Online Resources
- NASA’s Solar System Exploration: https://science.nasa.gov/solar-system/
- NASA’s Universe Exploration: https://science.nasa.gov/universe/
- The Schools’ Observatory: https://www.schoolsobservatory.org/
Books and Publications
- “An Introduction to Basic Astronomy Concepts” by Chris McMullen
- Astronomy Magazine: https://www.astronomy.com/
- Sky & Telescope Magazine
Observation Tools
- Eyes of the Solar System – NASA’s simulation tool that uses data and images from NASA missions to give you a simulated view of our solar system
- Stellarium (free planetarium software)
- Sky Safari (mobile app)