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Naked Eye Astronomy Notes

Naked Eye Astronomy

Orion in the UK Sky

  • Orion is a readily identifiable constellation in the night sky.

  • Visible in the UK during winter, from late autumn to early spring.

  • Best views are in December and January.

  • Appears as a large hourglass shape.

  • Orion's Belt is formed by three bright stars in the middle.

  • Prominent due to its bright stars and distinct pattern.

Betelgeuse

  • A bright red star marking Orion's left shoulder (or right, from Earth's perspective).

  • It is a red supergiant, near the end of its life.

  • Could explode as a supernova within the next 100,000 years.

  • Brightness is variable, with significant dimming observed.

  • Astronomers closely monitor it for signs of an imminent explosion.

The Dog Star - Sirius

  • Sirius is the brightest star in the night sky.

  • Located in the constellation Canis Major, following Orion.

  • Can be found by following Orion's Belt downward and slightly to the left.

Planet Overview

  • Mercury

    • Name Origin: Messenger god

    • Size: 0.38 \times (Earth=1)

    • Position from Sun: 1st

    • Visibility: Low on horizon at dawn/dusk; hard to spot

  • Venus

    • Name Origin: Goddess of love

    • Size: 0.95 \times

    • Position from Sun: 2nd

    • Visibility: Very bright; "Morning Star" or "Evening Star"

  • Earth

    • Name Origin: Not from mythology

    • Size: 1 \times

    • Position from Sun: 3rd

    • Notes: We live on it!

  • Mars

    • Name Origin: God of war

    • Size: 0.53 \times

    • Position from Sun: 4th

    • Visibility: Reddish; best every ~2 years when close

Differences Between Moons, Planets, and Galaxies

  • Moons

    • Natural satellites that orbit planets.

    • Vary in size and composition.

    • Some have atmospheres or underground oceans (e.g., Europa, Titan).

  • Planets

    • Large celestial bodies that orbit a star.

    • Have cleared their orbit of debris.

    • Spherical due to their gravity (e.g., Earth, Jupiter).

  • Galaxies

    • Vast collections of stars, planets, gas, and dark matter bound by gravity.

    • Contain billions of stars.

    • Can be spiral, elliptical, or irregular in shape.

Milky Way

  • Distance from Earth: Earth is about 26,000 light-years from the centre.

  • Size: About 100,000 light-years in diameter.

  • Contains 200-400 billion stars.

The Life Cycle of a Star: Small vs. Large Stars

  • Stars form from clouds of gas and dust.

  • Life cycle depends on their mass.

Small Stars (Like the Sun)

  1. Nebula: A cloud of gas and dust collapses under gravity.

  2. Protostar: The core heats up as it contracts, but fusion hasn't started yet.

  3. Main Sequence Star: Hydrogen fusion begins, and the star remains stable for billions of years.

  4. Red Giant: When hydrogen runs out, the core contracts while the outer layers expand and cool.

  5. White Dwarf: The outer layers are shed, and the hot, dense core is left behind.

  6. Black Dwarf (Theoretical): Over time, the white dwarf cools completely and no longer emits light.

Large Stars (Much Bigger than the Sun)

  1. Nebula: Like small stars, they form from collapsing gas and dust.

  2. Protostar: The core heats up as gravity pulls in more mass.

  3. Main Sequence Star: Hydrogen fusion occurs at a much faster rate due to the star's high mass.

  4. Supernova: Once fusion stops, the core collapses and triggers a massive explosion, scattering elements into space.

  5. Neutron Star or Black Hole:

    • Neutron Star: If the remaining core is 1.4-3 times the Sun's mass, it becomes a neutron star.

    • A dense, rapidly spinning remnant.

    • Black Hole: If the core is even more massive, gravity collapses it into a black hole, from which not even light can escape.

Key Differences:

  • Small stars end as white dwarfs and eventually black dwarfs (if given enough time).

  • Large stars explode as supernovae, leaving behind either a neutron star or a black hole.