Hubble Deep Field

The Hubble Deep Field (HDF) is an image of a tiny region in the constellation Ursa Major, captured by the Hubble Space Telescope. This image covers an area about 2.6 arcminutes across—so small that it is like looking at a tennis ball from 100 metres away. It was created by combining 342 separate pictures taken over ten days in December 1995.

Because the area is so small, most of the objects in the picture are faraway galaxies, some of the youngest ever seen. The Hubble Deep Field has become very important for scientists who study the early universe.

Later, similar pictures were taken in other parts of the sky, like the Hubble Deep Field South and the even deeper Hubble Ultra-Deep Field. These images helped scientists understand that the universe looks mostly the same in all directions, supporting the idea that Earth is in a typical place.

Conception

One of the main goals of the Hubble Space Telescope was to study distant galaxies in more detail than ground-based telescopes. Because Hubble orbits above Earth’s atmosphere, it can capture clearer images of visible and ultraviolet light. Even with an early problem, the telescope could still see farther galaxies, showing us how they looked billions of years ago.

After repairs in 1993 improved Hubble’s vision, scientists used it to look at very distant and faint galaxies. They noticed big differences between galaxies today and those from billions of years ago. In 1995, the director of the Space Telescope Science Institute decided to use special observation time to study a typical patch of sky with the telescope’s camera, helping us learn more about the early universe.

Target selection

The Hubble Deep Field image was taken of a small area in the constellation Ursa Major. Scientists chose this area carefully because it had to meet several important conditions. The area needed to be far from bright stars and other objects that could interfere with the observations.

Scientists also wanted the area to be in a part of the sky that Hubble could observe continuously without being blocked by the Earth or the moon. They selected a spot that was about 2.6 arcminutes wide, which is very tiny compared to the whole sky. This careful selection allowed scientists to see very distant galaxies clearly.

Observations

The Hubble Deep Field used special tools called filters to capture images of a tiny part of the sky. Scientists picked four filters to record different colors of light, including blue, red, and near-ultraviolet light.

Between December 18 and 28, 1995, the Hubble Space Telescope took 342 pictures of this area. It orbited Earth about 150 times during this period. The telescope spent many hours capturing each color of light, creating a detailed view of distant stars and galaxies.

Data processing

The Hubble Deep Field image was created using a careful process to clean up the data. Bright spots caused by cosmic rays were removed by comparing images taken one after another. Scientists also removed streaks from space debris and satellites, as well as scattered light from the Earth.

The cleaned images were combined using a special method called "drizzling," where the telescope was pointed slightly differently between each exposure. This improved the final image's sharpness. The result was a colorful picture made from combining images taken at different wavelengths, though the colors shown are an approximation rather than the true colors seen by the human eye.

Contents

The final images from the Hubble Deep Field were released in January 1996 at a meeting of the American Astronomical Society. They showed about 3,000 distant, faint galaxies, including both irregular and spiral galaxies. Most objects in the image are faraway galaxies, with only a few nearby stars.

The image also includes about fifty blue points of light. Some of these are likely areas where new stars are forming, while others might be distant quasars. Early ideas suggested these points were not white dwarfs, but newer studies show that some white dwarfs can appear blue, so they might be present in the image too.

Scientific results

The Hubble Deep Field image helped scientists learn amazing things about space. It showed many distant galaxies that were much farther away than we knew before — some so far that their light had traveled about 12 billion years to reach us. These discoveries helped us understand how galaxies change over time.

Scientists also learned that in the early universe, galaxies bumped into each other more often, forming bigger galaxies. The image helped us see how the rate of new stars being born has changed over the universe’s history. It also showed there weren’t many faint stars in our galaxy’s outer parts, which helped scientists think about what dark matter might be.

Multifrequency followup

Scientists used many different telescopes to study the Hubble Deep Field in many kinds of light. They found that some galaxies that look faint in ordinary light shine brightly in infrared light because of lots of dust and new stars being born. They also used X-ray telescopes and found six X-ray sources, which came from different kinds of galaxies, including some very active ones.

Radio telescopes also found many radio sources in the same area, matching up with galaxies seen in ordinary light. These observations help us learn more about the distant universe and the many different kinds of objects within it.

Subsequent HST observations

The Hubble Deep Field South looks very similar to the original Hubble Deep Field, supporting the cosmological principle that the universe looks roughly the same in all directions.

Later, an even deeper view called the Hubble Ultra-Deep Field was created, showing galaxies that may have formed very early in the universe’s history. This was followed by the Hubble eXtreme Deep Field, completed in 2012, which revealed galaxies thought to have formed within the first 500 million years after the Big Bang.