The numbers
- What a galaxy is
- Stars, gas and dust held by gravity
- The families
- Spirals, ellipticals, irregularsSorted by shape
- Ours
- A barred spiral
How it happens, step by step
Look at the shape
Galaxies are sorted into families by how they look, not by how big or how far away they are.
Spirals
They have a bright center and arms wound around it, full of gas and of young blue stars.
Barred spirals
They are spirals with a bar of stars across the center, and the arms start from the ends of the bar.
Ellipticals
No arms and no disk: just an oval of old stars, with little gas and few new births.
Irregulars
They have no set shape. They are often small, or they have been twisted by a close encounter.
To get an idea
In spirals new blue stars are born; in ellipticals, almost none anymore.
The shape tells the story: a big elliptical is usually the result of past crashes.
Fun facts
These are not in the book: there was no room on paper.
In elliptical galaxies there is very little gas, so almost no new stars are born.
The stars of elliptical galaxies are mostly old and tend toward red.
Irregular galaxies have no shape you can recognize, and they are often quite small.
A galaxy can be bent out of shape by a close pass from a bigger neighbor.
Big ellipticals are usually the result of crashes and mergers between galaxies.
Even two spirals that meet end up, over time, becoming an elliptical.
Test yourself
The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.
Question 1 of 5★ is the one from your book
How are galaxies sorted into families?
Question 2 of 5
What extra thing do barred spirals have?
Question 3 of 5
What is an elliptical galaxy like?
Question 4 of 5
Why are few stars born in ellipticals?
Question 5 of 5
Where do big ellipticals come from?
Riddles
I have no arms and no disk: I am an oval of old, reddish stars, with very little gas. Almost certainly I was born from a crash. What am I?
I have a bright center and arms that wind around me, and inside them young blue stars are born. I am the shape everyone draws. What am I?
Try it yourself
With even a modest telescope, galaxies look like fuzzy ovals: you cannot see their exact shape, and telling them apart is already an achievement.
Draw a spiral, an oval and a shapeless blob yourself: you have just sorted every galaxy in the universe.
The words
- spiral arm
- The curved band of stars and gas that winds around the center.
Sources: Wikidata (Q318), Wikipedia (Galassia).