11-16-2025, 07:17 PM
(This post was last modified: 11-16-2025, 07:29 PM by Leejohnston.)
⭐ How Stars Move — Galactic Orbits Explained Simply
Stellar & Galactic Dynamics — The Lumin Archive
? Overview
Every star in the galaxy — including the Sun — is constantly moving.
This thread explains:
• Why stars orbit the galactic centre
• Why their speed stays almost constant
• How dark matter shapes stellar motion
• The different types of stellar orbits
Clear diagrams included.
? 1. Why Stars Orbit the Galactic Centre
A galaxy is a massive rotating disk of:
• Stars
• Gas
• Dust
• Dark matter (MOST of the mass)
Gravity pulls everything toward the centre.
So stars orbit the same way planets orbit the Sun —
but on a much bigger, messier scale.
Simple diagram:
The Sun completes **one orbit every ~230 million years**.
? 2. Why Stars Move at Nearly the Same Speed
Here is the surprising part:
Stars far from the centre move just as fast as stars close to it.
This is impossible if only visible matter existed.
Rotation curve:
Solution: Dark matter.
A massive dark matter halo provides extra gravity.
? 3. Types of Stellar Orbits
Not all stars follow the same type of path.
Here are the main orbit families:
1️⃣ Circular Orbits (Disk Stars)
Stars like the Sun follow smooth, nearly circular paths in the galactic disk.
Diagram:
2️⃣ Eccentric Orbits (Halo Stars)
These stars plunge in and out of the galaxy on stretched, elliptical paths.
3️⃣ Box Orbits (Bulge Stars)
Stars near the centre bounce around in complex, chaotic patterns.
4️⃣ Resonant Orbits (Barred Spirals)
Some stars are trapped in gravitational “sweet spots” created by the bar.
? 4. The Sun’s Real Motion (More Than You Expect)
The Sun is not simply orbiting.
It is also:
• Wobbling up and down
• Oscillating around the galactic arm
• Drifting through the local stellar flow
Combined motion:
Total speed through space:
~220 km/s around the galaxy
+ ~20 km/s up/down
+ ~12 km/s relative to nearby stars
? 5. How Stellar Motions Reveal the Galaxy’s Mass
By measuring:
• Star velocities
• Orbital radii
• Motion patterns
Astronomers can calculate the **total mass** of the Milky Way — visible AND invisible.
Current best estimate:
~1 trillion solar masses
with 85–90% of it being dark matter.
? Summary
• Stars orbit the galactic centre due to gravity
• Speeds stay constant because of the dark matter halo
• Stars follow different orbit types depending on location
• The Sun’s motion is a complex layered combination
• Stellar orbits provide our best evidence for dark matter
Written for The Lumin Archive — Expanding the universe, one learner at a time.
Stellar & Galactic Dynamics — The Lumin Archive
? Overview
Every star in the galaxy — including the Sun — is constantly moving.
This thread explains:
• Why stars orbit the galactic centre
• Why their speed stays almost constant
• How dark matter shapes stellar motion
• The different types of stellar orbits
Clear diagrams included.
? 1. Why Stars Orbit the Galactic Centre
A galaxy is a massive rotating disk of:
• Stars
• Gas
• Dust
• Dark matter (MOST of the mass)
Gravity pulls everything toward the centre.
So stars orbit the same way planets orbit the Sun —
but on a much bigger, messier scale.
Simple diagram:
Code:
* ★ *
★ | ★
★ (•) ★ ← Stars orbiting
★ | ★
★ • ★
Galactic centreThe Sun completes **one orbit every ~230 million years**.
? 2. Why Stars Move at Nearly the Same Speed
Here is the surprising part:
Stars far from the centre move just as fast as stars close to it.
This is impossible if only visible matter existed.
Rotation curve:
Code:
Velocity ↑
Observed: ──────────────── (flat curve)
\
\_Predicted (visible mass only)
Distance →Solution: Dark matter.
A massive dark matter halo provides extra gravity.
? 3. Types of Stellar Orbits
Not all stars follow the same type of path.
Here are the main orbit families:
1️⃣ Circular Orbits (Disk Stars)
Stars like the Sun follow smooth, nearly circular paths in the galactic disk.
Diagram:
Code:
------(orbit)------
\ ★ /
\ /
\ /
[•] Galactic Centre2️⃣ Eccentric Orbits (Halo Stars)
These stars plunge in and out of the galaxy on stretched, elliptical paths.
3️⃣ Box Orbits (Bulge Stars)
Stars near the centre bounce around in complex, chaotic patterns.
4️⃣ Resonant Orbits (Barred Spirals)
Some stars are trapped in gravitational “sweet spots” created by the bar.
? 4. The Sun’s Real Motion (More Than You Expect)
The Sun is not simply orbiting.
It is also:
• Wobbling up and down
• Oscillating around the galactic arm
• Drifting through the local stellar flow
Combined motion:
Code:
~~~~ zig-zag up/down ~~~~
-------> ---> ---> ---> ------> (main orbit)Total speed through space:
~220 km/s around the galaxy
+ ~20 km/s up/down
+ ~12 km/s relative to nearby stars
? 5. How Stellar Motions Reveal the Galaxy’s Mass
By measuring:
• Star velocities
• Orbital radii
• Motion patterns
Astronomers can calculate the **total mass** of the Milky Way — visible AND invisible.
Current best estimate:
~1 trillion solar masses
with 85–90% of it being dark matter.
? Summary
• Stars orbit the galactic centre due to gravity
• Speeds stay constant because of the dark matter halo
• Stars follow different orbit types depending on location
• The Sun’s motion is a complex layered combination
• Stellar orbits provide our best evidence for dark matter
Written for The Lumin Archive — Expanding the universe, one learner at a time.