The Hubble Deep Field: A Far Out View

Richard Strauss: Sprach Zarathustra (Theme song from 2001: A Space Odyssey)

The picture described on the previous page actually exists. The image was created by combining 342 separate exposures taken with the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope (HST) for ten consecutive days between December 18 and 28, 1995. Called the Hubble Deep Field, this image, shown below was the deepest picture of the visible Universe ever taken by humankind. The visible Universe describes the extent of the Universe that is observable by humans, which means there is even more out there past the limit to which we can see, about 14 billion light years away. The light from anything farther away than that has not had time to reach us because the Universe is only 14 billion years old. The HST took this picture by zooming in on a tiny area of the sky near the constellation Ursa Major, the Big Dipper. Below are two animations depicting how the picture was taken. Below those is the actual Hubble Deep Field.

(NASA)                                                                                                                   (NASA)

Click on the above image to see a larger version.

Every object you can see in the above image is an entire galaxy, except for the points of light that have crosses; those are stars within the Milky Way Galaxy in the foreground of the image. There are over 1,500 galaxies in this image and some of them are more than 12 billion light years away. That means they are close to the other side of the visible Universe. It also means that the light leaving those galaxies took more than 12 billion years to reach Earth. Think about that for a moment...It means that you are seeing those galaxies as they were 12 billion years ago. You are literally looking into the past. Those galaxies have probably drastically changed since the light captured on this picture left them.

Maybe you're asking yourself, "Well, did the astronomers just get lucky a find a piece of the sky where there were many galaxies? Are there just as many galaxies in other areas of the sky?"

To answer that question, astronomers used the HST to take another picture similar to this one in a part of the sky visible in the Southern Hemisphere and a second in the Northern Hemisphere near the constellation, Fornax. The picture below shows the HST pointing towards the location of the constellation Fornax. The constellation Tucana, the area of the sky where the Hubble Deep Field South picture was taken, is also shown on the bottom right. To the right of that picture is an image of the actual Hubble Space Telescope in orbit around Earth. What do you think those other two deep field pictures look like?

(NASA)                                                                                                                                               (NASA)

Another extremely deep picture taken by the HST in the Northern Hemisphere is called the Hubble Ultra Deep Field because it produced an even deeper view of the universe than the original Hubble Deep Field (In 2009, an even deeper picture was taken). The Hubble Ultra Deep Field was created by combining 800 exposures, each averaging 21 minutes, from two different cameras including the Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-object Spectrometer (NICMOS). The image only covers a tiny piece of the sky. It shows a piece of sky the equivalent of only 1/50 the area of the Full Moon, yet there are about 10,000 galaxies shown. If a similar number of galaxies could be found in all parts of the sky, there would be over 127 billion galaxies in the visible Universe! However, some estimates put the total number of galaxies in the visible Universe as high as 200 billion. The average number of stars in each galaxy is estimated to also be about 200 billion. However, some galaxies only have millions of stars and the biggest can have 1 trillion stars. If we use the estimated values of 200 billion galaxies in the visible Universe and 200 billion stars on average in each galaxy we find that there are over 40 sextillion--a 4 with 22 zeroes after it--stars in the Universe.


Click on the above image to see a larger version.

Shortly after the HUDF image was taken, another exposure taken by the HST, of a cluster of galaxies called Abell 2218, revealed a galaxy that is believed to be a bit farther than the farthest ones in the HUDF. That galaxy in the exposure of the Abell cluster is seen as it was over 13 billion years ago! That distant galaxy was discovered by way of a gravitational lens. A gravitational lens is a massive object, or objects in this case, that magnify and bend the light from distant objects behind them in a way similar to how a magnifying glass bends and magnifies light. It is the gravitational field of the massive object or objects that actually focuses and bends the light from the most distant object lined up behind it.

The farthest galaxies in these HST images appear as small red dots. Most appear red because their light waves have been stretched out to the far end of the visible light spectrum. This is known as Redshift and it follows the same principles as the Doppler Effect. We have all experienced the Doppler Effect with sound waves. When you hear a motorcycle or the sirens of a fire engine coming towards you the sound you hear is a higher pitch compared to when they are going away from you because the sound waves are squished together as they approach you. When the motorcycle or fine engine passes you the sound you hear immediately goes to a lower pitch because the sound waves are being stretched out as they move away from you. The same thing happens with light waves, except it is not the sound that changes, but color. Shortened or squished-up light waves moving towards you appear bluer and stretched out longer light waves moving away from you appear redder. The diagram below illustrates that effect.

The image above on the left is of a source of waves moving to the left. The frequency is higher on the left, and lower on the right. The image above on the right shows the Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to that of the Sun (left). (Wikimedia Commons)

Many of the galaxies in the Hubble Ultra Deep Field appear red due to the vast distance between us and them is increasing because of the expansion of the Universe. Edwin Hubble discovered the Universe was expanding and realized that the farther away an observer is from the object, the faster the object is receding from the observer because of the expansion. That rule is known as Hubble's Law.

Just how fast the Universe is expanding and if that expansion is changing is currently being hotly debated by astronomers. Some astronomers have produced evidence that the Universe is not only expanding, but also accelerating in its expansion, which means that the Universe would be getting even bigger at a faster rate than it has in the past! Other astronomers have produced evidence that the Universe is decelerating, which means the expansion might stop someday resulting in the Universe re-collapsing. Still other astronomers argue that the expansion rate is constant, which means that the Universe would expand forever. The reason there is so much indecision about the expansion rate of the Universe is that it appears to exist today right on the dividing line between those three possibilities.

The factors that govern which of those possibilities is correct include: the total amount of mass in the Universe and the magnitude of something called dark energy, which may or may not even exist! The question left unanswered is: Does gravity win and cause the Universe to slow down in its expansion or possibly re-collapse onto itself in a Big Crunch or does dark energy drive the expansion of the Universe into infinity?

Now that we have traveled the entire distance from Earth and have reached one edge of the visible Universe, as far as we can observe, it is time to see if there is anything that connects those galaxies in the Hubble Ultra Deep Field with life as we know it on Earth. Amazingly enough, there seems to be a very profound connection indeed. It all starts with the birth and death of stars.

Travel to: Dying Stars
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Website created March 2007 by Sam Singer, last updated December 2009