high school Exoplanets & Galaxies

Exoplanets & Galaxies

A galaxy is a huge gravitationally bound collection of hundreds of billions of stars, gas, and dust, and our Sun is one star in the Milky Way. Hubble classification sorts galaxies into spiral, elliptical, and irregular types, and exoplanets are found mainly by radial-velocity and transit methods. The Milky Way is an SBb spiral about 100,000 light-years across. Change the galaxy type and compare the shapes of spiral, elliptical, and irregular galaxies.

Galaxies — cities of stars

🌌 What is a galaxy?

①A galaxy is an enormous gravitationally-bound collection of hundreds of billions of stars, gas, and dust

②Our Sun is one of ~200 billion stars in the Milky Way

③There are hundreds of billions of galaxies in the universe

Types of galaxies (Hubble classification)

Galaxy typeSpiral galaxy

💡 Key observations

①Spiral — spiral arms, young stars + abundant interstellar matter

②Elliptical — ball-shaped, mostly old stars, little gas

③Irregular — no distinct shape (e.g., Large Magellanic Cloud)

The Milky Way

Size of the Milky Way

diameter ≈ 100,000 light-years / Sun: ~26,000 ly from center

barred spiral (SBb), ~200 billion stars

Milky Way structure

Components of the Milky Way

Exoplanet detection methods

Exoplanet search

Main detection methods

Method	Principle	Features
Radial velocity	Doppler effect on the star (star wobbles)	works well for massive planets
Transit method	planet passing in front of star → dip in brightness	measures planet size
Direct imaging	block starlight to observe the planet	technically very difficult
Microlensing	gravity amplifies background starlight	one-time event observation

Radial velocity principle

spectral shift: toward planet → blueshift, away → redshift

star + planet orbit a common center of mass → detect tiny wobble of the star

Worked Examples and Exam Practice

Example 1

A galaxy with no spiral arms, ball-shaped, mostly old stars, and little gas is which Hubble type?

Clear spiral arms mean a spiral galaxy; round and dominated by old stars means an elliptical galaxy.

Little gas and dust, so almost no new stars form, is also typical of elliptical galaxies.

▸ Elliptical galaxy

Hubble classes: spiral (S), elliptical (E), irregular (Irr). The Milky Way is a barred spiral (SBb).

Example 2

Which exoplanet detection method uses the periodic slight dimming of a star as a planet passes in front of it, and what does it reveal?

When a planet passes in front of its star, part of the starlight is blocked and the brightness dips (a transit).

Using this dip is the transit method, and the size of the dip estimates the planet's size.

▸ Transit method; it reveals the planet size

The radial-velocity (Doppler) method estimates planet mass; the transit method estimates size.

Mock-assessment style

Which statement about galaxies and exoplanet detection is correct?

①The Milky Way is an elliptical galaxy

②The transit method uses the brightness dip as a planet passes in front of a star

③The radial-velocity method directly measures a planet's size

④The Sun lies at the center of the Milky Way

⑤Direct imaging is the easiest detection method

▸ ② The transit method uses the brightness dip as a planet passes in front of a star

As a planet crosses in front of its star, part of the light is blocked and brightness periodically dips; the size of the dip estimates the planet size.

The Milky Way is a barred spiral (①), radial velocity estimates mass (③), the Sun is about 26,000 light-years from the center (④), and direct imaging is technically very hard (⑤).

Summary

Galaxies & exoplanets core

Hubble: spiral (S) / elliptical (E) / irregular (Irr) / Milky Way: SBb, 100k ly

main exoplanet methods: radial velocity (Doppler), transit (brightness change)

🎯 Exam Points

①Hubble: spiral (Sa–Sc), elliptical (E0–E7), irregular (Irr)

②Milky Way: barred spiral (SBb), 100k ly diameter, Sun ~26k ly from center

③Radial velocity: Doppler shift of star → estimates planet mass

④Transit: brightness dip → estimates planet size

⑤Habitable zone (Goldilocks): distance where liquid water can exist

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Properties of Stars

Expanding Universe & Big Bang

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