First look
The idea, in plain words
Light takes time to cross space, so a telescope that looks far away looks back in time. This real photograph looks further back than almost any other. It holds nearly 10,000 galaxies, and the faintest are seen as they were when the universe was 800 million years old. They are not grand spirals. They are small and ragged, a zoo of odd shapes. Some look like toothpicks and some like links on a bracelet.
The Webb telescope sees further still. The small red smudge picked out here is a galaxy called JADES-GS-z14-0. Its light set out less than 300 million years after the beginning, and when this picture was published in May 2024 it was the most distant galaxy known. Galaxies had started to form almost as soon as there were stars to make them from.
They did not form whole. Gravity first gathered matter into small clumps, and gas falling into each clump made a small galaxy. Then the clumps fell together. Small galaxies joined into middling ones, and middling ones into big ones. Every large galaxy today is built from many smaller ones that no longer exist.
Stars were made fastest long ago. Adding up the light of galaxies at every distance shows that the making of stars reached its peak about 3.5 billion years after the beginning, and has been falling away ever since. Half of all the stars there are today had formed by the time the universe was about 5 billion years old. The universe is past its busiest days.
The shapes settled early too. This picture sets real photographs of galaxies side by side at three ages: today, 4 billion years ago, and 11 billion years ago. Even in the oldest set there are two families: blue, untidy galaxies still making stars, and heavy red ones that have stopped. The further back you look, the smaller and less finished they all are.
Our own galaxy went through all this. The Gaia spacecraft has measured how millions of stars near the Sun are moving, and about 30,000 of them move together in a way the rest do not. They are what is left of another galaxy, which the young Milky Way swallowed about 10 billion years ago. Its stars have been circling among ours ever since.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Galaxy
- A huge family of stars, gas and dust held together by gravity.
- Look-back time
- How long ago the light you are seeing set out.
- Dark matter
- Matter that gives out no light at all and is known only by the pull of its gravity.
- Galaxy merger
- Two galaxies falling together under gravity and settling into one.
- Spiral galaxy
- A galaxy shaped like a flat, turning disc with bright arms winding out from the middle.
- Elliptical galaxy
- A smooth, rounded galaxy with no disc or arms, made mostly of old stars.
- Redshift
- A stretching of light to longer, redder wavelengths.
- Milky Way
- The galaxy we live in. From inside it shows as a pale band of light across a dark sky.
Where this comes from
- ESA/Hubble, Hubble Ultra Deep Field (heic0406a)
- ESA/Webb, JADES-GS-z14-0 (30 May 2024)
- Madau and Dickinson, Cosmic star formation history (2014)
- ESA/Hubble, Hubble explores the origins of modern galaxies (heic1315)
- ESA, Galactic ghosts: Gaia uncovers major event in the formation of the Milky Way (31 October 2018)
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.