Universe Overview
Big Bang and Universe Age
- The universe began approximately 13.8 billion years ago with the Big Bang
- The universe is constantly expanding, and the expansion is accelerating
- The observable universe contains an estimated two trillion galaxies
- The universe’s first stars ignited approximately 180 million years after the Big Bang
- The first galaxies formed by around 400 million years after the Big Bang
Cosmic Microwave Background (CMB)
- The CMB is leftover radiation from the Big Bang, detected throughout the universe
- CMB radiation was released about 380,000 years after the Big Bang
- CMB was accidentally discovered in 1965 by Arno Penzias and Robert Wilson
- The CMB has a nearly uniform temperature of 2.725 Kelvin
- Small fluctuations in CMB temperature reveal the seeds of galaxy formation
- Regarded as one of the strongest pieces of evidence for the Big Bang theory
Composition of the Universe
- Ordinary visible matter makes up only about 4.9% of the universe’s mass-energy
- Dark matter accounts for approximately 26.8% of the universe
- Dark energy accounts for about 68.3% of the universe
- Dark energy has repulsive gravity that accelerates the universe’s expansion
Galaxies
Galaxy Types
- Spiral Galaxies: Contain spiral arms extending from a central bulge (like the Milky Way)
- Elliptical Galaxies: Ellipsoidal shape, little gas and dust, older stars
- Lenticular Galaxies: Cross between spiral and elliptical, with disk but no arms
- Irregular Galaxies: No definite shape, often result of galaxy interactions
Galaxy Facts
- Galaxies contain stars, planets, gas, dust, and dark matter
- Most large galaxies have supermassive black holes at their centers
- Galaxies can organize into groups (up to 100 galaxies) or clusters (thousands of galaxies)
- The largest galaxies can contain trillions of stars and span more than a million light-years
- Most galaxies formed between 10-13.6 billion years ago
- Galaxies grow through collisions and mergers with other galaxies
- About 10% of galaxies are considered “active,” with extremely bright centers
The Milky Way Galaxy
- The Milky Way is a spiral galaxy with a disk spanning over 100,000 light-years
- Contains 100-400 billion stars
- At its center is a supermassive black hole called Sagittarius A*, with mass of 4 million suns
- Earth is located on a spiral arm, about halfway from the center
- Our solar system takes about 240 million years to orbit the galactic center once
- The Milky Way is part of the Local Group of galaxies
- The Andromeda Galaxy (nearest large galaxy to us) is on a collision course with the Milky Way
Stars
Star Formation and Life Cycle
- Stars form in large clouds of gas and dust called molecular clouds
- Stars are primarily composed of hydrogen and helium
- Main sequence stars (like our Sun) fuse hydrogen into helium
- Low-mass stars live longer than high-mass stars
- A star’s life cycle depends on its mass:
- Low-mass stars eventually become white dwarfs
- High-mass stars can explode as supernovae, leaving neutron stars or black holes
Star Facts
- The universe may contain up to one septillion stars (1,000,000,000,000,000,000,000,000)
- The Milky Way alone contains more than 100 billion stars
- Stars that are stably fusing hydrogen into helium are called main sequence stars
- The Sun is a G-type main sequence star, midway through its life
- Stars don’t actually twinkle—the effect is caused by Earth’s atmosphere
- Most stars move through the universe in clusters, but our Sun travels alone
- Stars were some of the earliest objects to form in the early universe
Our Sun
- The Sun is about 4.6 billion years old (midway through its main sequence phase)
- The Sun contains 99.86% of the mass in our solar system
- The Sun’s core temperature is about 15 million degrees Celsius (27 million °F)
- It takes light from the Sun about 8 minutes to reach Earth
- The Sun is 400 times larger than the Moon but also 400 times farther away, making them appear the same size in our sky
Solar System
Formation and Age
- Our solar system formed about 4.6 billion years ago from a solar nebula
- A solar nebula is a spinning, swirling disk of gas and dust
- The Sun formed at the center, while planets formed from the remaining material
- Located in the Orion Arm (a minor spiral arm) of the Milky Way galaxy
- Takes about 230 million years to complete one orbit around the galactic center
General Solar System Facts
- Includes the Sun, 8 planets, 5 officially recognized dwarf planets, hundreds of moons, and thousands of asteroids and comets
- Over 99.86% of the solar system’s mass is in the Sun
- Nearly 90% of the remaining mass is in Jupiter and Saturn
- Mercury and Venus are the only planets with no moons
- Jupiter and Saturn have the most moons (95 and 83 confirmed moons respectively)
- All four giant planets have ring systems, though Saturn’s are the most prominent
- The Oort Cloud, at the edge of our solar system, extends from 5,000 to 100,000 AU from the Sun (1 AU = Earth’s distance from the Sun)
Exoplanets
- Planets that orbit stars other than our Sun
- First confirmed exoplanet discovery was in 1992
- Over 5,000 exoplanets have been confirmed to date
- Exoplanets come in a wide variety of sizes and compositions
- Many have been found in the “habitable zone” of their stars
Black Holes
Black Hole Types
- Stellar Black Holes: Formed from collapsed massive stars, typically 3-20 times the Sun’s mass
- Intermediate Black Holes: Between 100-100,000 solar masses
- Supermassive Black Holes: Found at the centers of most galaxies, millions to billions of solar masses
Black Hole Facts
- Black holes are regions where gravity is so strong that nothing, not even light, can escape
- The boundary of a black hole is called the event horizon
- Time appears to slow down near a black hole due to gravitational time dilation
- Black holes don’t “suck” matter in—they attract matter like any massive object
- Black holes can merge, creating gravitational waves
- The first image of a black hole (M87*) was captured in 2019
Astronomical Measurements
Distance Scales
- Astronomical Unit (AU): Average distance from Earth to the Sun (about 93 million miles or 150 million km)
- Light-Year: Distance light travels in one year (about 5.88 trillion miles or 9.46 trillion km)
- Parsec: About 3.26 light-years
- The nearest star to our Sun, Proxima Centauri, is 4.24 light-years away
- The Andromeda Galaxy is approximately 2.5 million light-years from Earth
Time Scales
- Earth day: 24 hours (rotation period)
- Earth year: 365.25 days (orbital period)
- Jupiter day: About 10 hours
- Saturn day: About 10.7 hours
- Venus day: 243 Earth days (longer than its year)
- Mercury year: 88 Earth days
- Jupiter year: 11.9 Earth years
- Neptune year: 165 Earth years
Notable Space Observations
Telescopes and Missions
- Hubble Space Telescope: Launched 1990, revolutionized astronomy with deep space images
- James Webb Space Telescope: Launched 2021, infrared observatory studying early universe
- Voyager 1 & 2: Launched 1977, now in interstellar space
- Cassini-Huygens: Studied Saturn and its moons (1997-2017)
- Perseverance: Mars rover studying the planet’s geology and past habitability
Significant Discoveries
- 1964: Discovery of Cosmic Microwave Background radiation
- 1992: First confirmed detection of exoplanets
- 1995: Confirmation of first Jupiter-mass planet orbiting a Sun-like star
- 2012: Higgs Boson particle discovered, confirming the Standard Model of particle physics
- 2015: First detection of gravitational waves from merging black holes
- 2019: First direct image of a black hole (M87*)
Stargazing Tips
Best Objects for Beginners
- Moon: Best observed during partial phases when shadows highlight terrain
- Planets: Jupiter, Saturn, Mars, and Venus are easily visible with naked eye
- Star Clusters: Pleiades (M45) and Hyades in Taurus are visible to naked eye
- Nebulae: Orion Nebula (M42) visible with binoculars
- Galaxies: Andromeda Galaxy (M31) visible to naked eye in dark skies
Observing Equipment
- Naked Eye: Can see about 2,500-3,000 stars in dark skies
- Binoculars: Good for wide-field observing, moon craters, star clusters
- Small Telescope (3-6 inch): Good for lunar details, planets, bright deep sky objects
- Medium Telescope (8-10 inch): Good for galaxies, nebulae, and fainter objects
Dark Sky Preservation
- Light pollution affects 80% of the world’s population
- Finding a dark sky location dramatically improves astronomical viewing
- International Dark-Sky Association (IDA) designates dark sky preserves worldwide