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.
Here you can use the galaxy-type slider to 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)
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Shapes of spiral, elliptical, and irregular galaxies
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?
1
Clear spiral arms mean a spiral galaxy; round and dominated by old stars means an elliptical galaxy.
2
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?
1
When a planet passes in front of its star, part of the starlight is blocked and the brightness dips (a transit).
2
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
1
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.
2
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).