The Hubble Ultra Deep Field: A Hubble Image Tour

This long exposure of a tiny patch of the sky allowed astronomers to understand the scale, structure, and development of the universe much more completely and precisely than ever before. The image includes galaxies across cosmic history, including some that existed when the universe was only around 5 percent of its current age.

Each tiny dot in the image is a galaxy, a collection of millions to hundreds of billions of stars.

Some galaxies are large, well developed, and nearby, while others are small, shapeless, and distant.

Only a few of the Milky Way's stars can be seen in this image. They have a distinctive cross-like appearance, caused by light reflecting off struts inside the Hubble Space Telescope.

Spiral galaxies like this large one are relatively old. They take several billion years to achieve their characteristic shape.

Similarly, elliptical galaxies — like these two large, orange ones — are relatively old and have less gas and dust, so fewer new stars are born.

Younger galaxies can be full of gas and dust and may have the basic structure of modern galaxies, but they are still refining their shapes.

Gravity is causing some galaxies to pull at and distort each other, such as the bluish galaxies near the center of this zoomed view. Galaxies evolve by interacting and merging.

The small reddish galaxies are some of the first to form in the universe.