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Grade 10-11 (age 15-17)

high school Exoplanets & Galaxies

A galaxy is a vast system of hundreds of billions of stars, gas, and dust held together by gravity, and the Sun is one of the stars of our own galaxy, the Milky Way. Hubble sorted galaxies by shape into ellipticals, normal spirals, barred spirals, and irregulars, and the amount of gas and dust and the ages of the stars change along with the shape. Planets around other stars are too faint to see directly, so they are found by measuring how a planet makes its star wobble or briefly dims its light. In the diagram, switching the galaxy type shows each of the four shapes along with where the old stars, the young stars, and the star-forming regions are.

Galaxies, cities of stars

🌌 What is a galaxy?
①A galaxy is a system of hundreds of billions of stars and interstellar matter (gas and dust) bound by gravity and moving together
②The Sun is one of about 200 billion stars in the Milky Way, and the band of the Milky Way seen on a clear night is our galaxy's disk viewed from inside
③The observable universe holds more than hundreds of billions of galaxies
④The space between galaxies is mostly empty; even the nearest large galaxy, Andromeda, is about 2.5 million light-years away

Types of galaxies (Hubble classification)

Barred spiral galaxy (SB)
💡 How the types differ
①Elliptical (E): shaped like a ball or a rugby ball, with almost no interstellar matter, so few new stars form and most stars are old and red
②Normal spiral (S): spiral arms wrap around a central bulge of old stars, and the arms are rich in gas, dust, and young blue stars
③Barred spiral (SB): the spiral arms start at the ends of a bar running through the center, and the Milky Way is one of these
④Irregular (Irr): no regular shape and plenty of interstellar matter, so stars form actively; the Large Magellanic Cloud is a well-known example
⑤Hubble's classification is based on shape and does not mean ellipticals evolve into spirals

The Milky Way

Size of the Milky Way and the Sun's position
disk about 100,000 light-years across · Sun about 26,000 light-years from the center
A barred spiral with about 200 billion stars, with a supermassive black hole of about 4 million solar masses at its center.

Milky Way structure

Parts of the Milky Way

How exoplanets are found

Searching for exoplanets

Main detection methods
MethodPrincipleFeatures
Radial velocityas a planet tugs its star into a small wobble, the star's absorption lines shift back and forth between blue and redeasiest for massive planets close to their stars; gives the planet's mass
Transitthe star's brightness dips slightly and regularly whenever the planet passes in front of itthe depth of the dip gives the planet's size, but the orbit must be seen nearly edge-on
Direct imagingblock the bright starlight and photograph the light from the planetso hard that only large planets far from their stars can be imaged
Microlensingwhen a foreground star's gravity focuses and brightens a background star, a planet adds a small extra spike to the light curvecan find small, distant planets, but the event never repeats
How radial velocity works
star moving toward us → blueshift → moving away → redshift → the pattern repeats with the orbit
A star and its planet orbit their common center of mass. Even when the planet cannot be seen, the period and size of the star's wobble give the planet's orbital period and mass.

Worked Examples and Exam Practice

Example 1
A galaxy has no spiral arms, is round like a ball, is made mostly of old stars, and has little gas. What is it in the Hubble classification?
1
Clear spiral arms mean a spiral galaxy; a round, smooth shape made mostly of old stars means an elliptical galaxy.
2
Having little gas and dust, so that hardly any new stars form, is also a mark of an elliptical galaxy.
▸ an elliptical galaxy
In the diagram, the elliptical has an even reddish glow and no pink star-forming regions, while the spiral and irregular galaxies have them.
Example 2
Which exoplanet detection method uses the slight, regular dimming of a star when a planet passes in front of it, and what does it reveal?
1
When a planet passes in front of a star, it covers part of the star's surface and the star's brightness drops (a transit).
2
The method that uses this dimming is the transit method, and the deeper the dip, the larger the planet.
▸ the transit method; it reveals the planet's size
Radial velocity gives a planet's mass, and transits give its size. Using both together even gives the planet's density.
Mock-assessment style
Which statement about galaxies and exoplanet detection is correct?
①The Milky Way is an elliptical galaxy
②The transit method uses the dimming of a star when a planet passes in front of it
③The radial velocity method measures a planet's size directly
④The Sun is at the center of the Milky Way
⑤Direct imaging is the easiest way to find exoplanets
▸ ② The transit method uses the dimming of a star when a planet passes in front of it
1
When a planet passes in front of a star, it blocks some of the starlight, so the brightness dips regularly, and the size of the dip gives the planet's size.
2
The Milky Way is a barred spiral (①), the radial velocity method gives a planet's mass (③), the Sun is about 26,000 light-years from the center (④), and direct imaging is technically very difficult (⑤).

Summary

Galaxies and exoplanets in brief
Hubble classes: elliptical (E) · normal spiral (S) · barred spiral (SB) · irregular (Irr) / Milky Way: barred spiral, about 100,000 light-years across
Exoplanets are found from a star's radial velocity changes, transits, microlensing, and direct imaging.
🎯 Exam Points
①The Hubble classes are elliptical (E), normal spiral (S), barred spiral (SB), and irregular (Irr), and ellipticals have little interstellar matter and few young stars
②The Milky Way is a barred spiral about 100,000 light-years across, and the Sun lies in the disk about 26,000 light-years from the center
③The radial velocity method uses the Doppler shift of starlight to find a planet's mass
④The transit method uses how much the star dims to find a planet's size
⑤The habitable zone is the range of distances where a planet receives enough energy from its star for water to stay liquid
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Properties of Stars
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Expanding Universe & Big Bang

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