What is Astrophysics?
Physics applied to the universe. How do stars work? What are black holes? Why is the universe expanding? Astrophysics answers these questions using the same physics that works on Earth.
Stars: Birth, Life, Death
Birth: Gas clouds collapse under gravity, creating protostars. Temperature rises. At 10 million Kelvin, hydrogen fuses into helium—a star is born.
Life: Gravity pulls inward. Fusion pushes outward. This balance lasts billions of years.
Death: When hydrogen runs out, the star dies. Low-mass stars become white dwarfs. Massive stars explode as supernovae, leaving neutron stars or black holes. Elements created in stars become planets and you.
Gravity: Not a Force, But Curved Space
Einstein's insight: Gravity isn't a force pulling you down. Mass curves spacetime itself. Imagine spacetime as a rubber sheet. Heavy objects create dips. You're simply rolling down the slope of curved space.
Extreme version: Black holes are regions where spacetime curves so violently that nothing escapes—not even light.
The Big Bang: The Universe's Beginning
Discovery: Distant galaxies are moving away from us. The further, the faster. Rewind to the beginning: everything converges to a single point 13.8 billion years ago.
What it was: Not an explosion in space, but the beginning of space itself. Time, matter, and energy all emerged from an infinitely hot, infinitely dense state.
First moments: Too hot for atoms. Then it cooled. After 380,000 years, electrons and nuclei combined into atoms. Light could travel. Gravity amplified tiny ripples. Galaxies and stars formed.
Dark Matter & Dark Energy: 95% of Everything
Dark Matter (27%): Invisible. Detected only by gravity. Galaxies move too fast—they'd fly apart without it. Dark matter forms the web that holds galaxies together.
Dark Energy (68%): Even more mysterious. Makes the universe expand faster and faster. We have no idea what it is.
Bottom line: Ordinary matter—you, stars, planets—is only 5% of the universe. We mostly don't know what exists.
Life in the Universe
Requirements: Water, heavy chemical elements, time. We have all three on Earth. Likely elsewhere too.
The problem: We see no signals from aliens. Why? Possible answers: life is rare, civilizations destroy themselves, space is too vast, or we're not looking right.
Current approach: Hunt for exoplanets and look for biosignatures—chemical evidence of life.
How We Know This Stuff
We can't experiment on stars, so we observe. Telescopes collect light. Spectrometers analyze it. Mathematical models test predictions against data.
If predictions match observations: Theory is correct. If not, revise or discard.
This is how we know the universe's age, star composition, and that black holes exist—through observation and rigorous theory, not guesswork.