L13: Hazard Galaxy

What is the Galactic Habitable Zone (GHZ)?

The region of the galaxy where conditions are suitable for the formation of life.

What are the three main structural components of the Milky Way?

The bulge (old stars, gas, and dust at the centre), the disk (mostly young stars, gas, and dust concentrated in spiral arms, ~100,000 ly in diameter), and the halo (old individual stars and globular clusters, possibly over 130,000 ly across, also containing dark matter).

Where is the Sun located in the Milky Way?

Near the plane of the disk, approximately 25,000 light-years from the Galactic centre, in the Local/Orion Arm.

What is the Andromeda galaxy?

The nearest spiral galaxy to the Milky Way, approximately 2.5 million light-years away.

Why are spiral arms bright?

They contain many young, hot, luminous blue stars due to high mass density and star formation.

What is Special Place 1 — eccentricity?

The Sun's orbit is less elliptical than other stars, preventing it from entering the inner galaxy.

What is Special Place 2 — position in the plane?

The solar system lies about 20 light-years above the galactic plane, reducing gravitational disturbances.

What is Special Place 3 — co-rotation?

The Sun orbits near the co-rotation radius, reducing exposure to supernovae and comet encounters.

What is metallicity?

The fraction of heavy elements in a star, varying across the galaxy.

What is the minimum metallicity needed to form rocky planets?

Approximately 20-30% of the Sun's metallicity.

Can metallicity be too high?

Yes, it can lead to ocean planets and disrupt orbital stability.

What is the trade-off in distance from the Galactic Centre?

Too far reduces metallicity; too close increases radiation exposure.

What are the counterarguments to galactic constraints on life?

Very low metallicity might still allow for Earth-like planets; supernova radiation may not be fatal.

What is Nasty Threat 1 — the galactic centre?

The central black hole can produce bursts of high-energy radiation.

What is Nasty Threat 2 — supernovae?

Most dangerous in the inner galaxy; they occur within 33 light-years of Earth every 240 million years.

What is Nasty Threat 3 — gamma-ray bursts (GRBs)?

Powerful explosions that could cause mass extinction if directed at Earth.

What are the places to avoid in the galaxy for life?

Globular clusters, nebulae, massive young O-type stars, the galactic centre, and supernova remnants.

What is the overall conclusion about the Galactic Habitable Zone?

Only about 10% of star systems in the Milky Way may be suitable for life.

What makes Earth a special place — summary of factors?

Distance from the Sun, atmospheric composition, strong magnetic field, plate tectonics, and location in the GHZ.