---
watermark: ORIRO
disable-model-invocation: true
name: sci-astronomy
provider: ORIRO.ai
copyright: Copyright (c) 2026 ORIRO.ai
description: >
  Astronomy and cosmology — solar system, stars, galaxies, cosmology, telescopes,
  and the scale of the universe. Activate for questions about planets, stars,
  black holes, the Big Bang, space exploration, or any astronomy topic.
  Sources: NASA, ESA, MIT OCW Astrophysics, OpenStax Astronomy (CC-BY).
---

# Astronomy

## The solar system

**Eight planets (mnemonic: My Very Excellent Mother Just Served Us Nachos):**
Mercury, Venus, Earth, Mars (rocky/terrestrial) | Jupiter, Saturn, Uranus, Neptune (gas giants)

**Distances:** Earth-Sun = 1 AU (Astronomical Unit) = 150 million km. Neptune: 30 AU. Nearest star (Proxima Centauri): 4.2 light-years = 268,000 AU.
**Sizes:** Sun contains 99.86% of solar system's mass. Jupiter: 318 Earth masses. Earth: 1.

**Key features:**
Sun: G-type main-sequence star. Surface temp: 5,778K. Core temp: 15 million K. Nuclear fusion (hydrogen → helium).
Mars: Largest volcano (Olympus Mons = 3× Everest), longest canyon (Valles Marineris), thin CO2 atmosphere.
Jupiter: Largest planet. Great Red Spot (storm larger than Earth). 95 known moons. Europa: subsurface ocean.
Saturn: Ring system (mostly ice). Density less than water (would float in a large enough ocean).

## Stars

### Stellar classification (Harvard spectral classification)

**OBAFGKM** — Oh Be A Fine Girl/Guy, Kiss Me
O (hottest, blue-white, >30,000K) → B → A → F → G → K → M (coolest, red, <3,500K)
Sun is a G-type (yellow-white). Most common: M-type red dwarfs.

### Stellar lifecycle

**Main sequence:** Hydrogen fusion in core. Pressure vs. gravity in equilibrium. ~90% of star's lifetime.
**Red giant:** Core hydrogen exhausted → outer layers expand. Helium fusion begins.
**Stellar death:**

- Low/medium mass (like Sun): Red giant → planetary nebula → white dwarf
- High mass: Red supergiant → supernova → neutron star (or black hole if >~3 solar masses)

**Hertzsprung-Russell diagram:** Plots stars by luminosity vs. temperature. Main sequence is the diagonal band.

## Galaxies and cosmology

### Our galaxy

**Milky Way:** Barred spiral galaxy. ~200-400 billion stars. Diameter ~100,000 light-years.
Earth is ~27,000 light-years from the galactic center.
Supermassive black hole at center: Sagittarius A\*, 4 million solar masses.

### Universe scale

Observable universe: ~93 billion light-years diameter. ~2 trillion galaxies.
**Light-years:** Distance light travels in one year = 9.46 × 10^12 km.
The most distant observable objects are seen as they were ~13.8 billion years ago.

### Big Bang cosmology

**Age:** ~13.8 billion years.
**Evidence:** Cosmic microwave background (CMB), Hubble expansion, abundance of light elements.
**Hubble's Law:** V = H₀ × d (Galaxies receding at speed proportional to distance)
**Dark matter:** ~27% of universe. Evidence: Galaxy rotation curves. Nature unknown.
**Dark energy:** ~68% of universe. Driving accelerating expansion. Nature unknown.
**Ordinary matter:** ~5% of universe.

## Black holes

**Singularity:** Infinite density. Spacetime curvature breaks known physics.
**Event horizon:** Point of no return. Escape velocity = speed of light.
**Schwarzschild radius:** r = 2GM/c² (For Earth: ~9mm. For Sun: ~3km.)
**Supermassive black holes:** Centers of most galaxies. Millions to billions of solar masses.
**Detection:** Gravitational lensing, orbital effects on nearby stars, accretion disk radiation, gravitational waves (LIGO).

Sources: NASA (nasa.gov — free), ESA (esa.int — free), OpenStax Astronomy 2e (CC-BY, openstax.org), Khan Academy Cosmology and Astronomy (CC-BY-NC-SA), MIT OCW 8.286 Astrophysics