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Over the years, the Hubble Space Telescope amazed us again and again. He showed us planets, moons, star clusters, nebulae, galaxies … as many spectacular, beautiful and scientifically rich images that astronomers will study for many decades.
Of all the types of observations taken, one of the deepest is also one of the most subtle. It is based on a simple idea: what happens if we run this powerful observatory to a small place in space, a place that seems relatively empty, and let it simply accumulate light for hours, days , weeks? What will we see?
The image created from this idea was the Hubble Deep Field, an iconic cliché containing thousands of galaxies. It was followed by the Hubble Ultra Deep Field, which required longer exposure and further exploration of the Universe. Hubble eXtreme Deep Field followed. This observation is more than a million seconds.
So, how do you follow that? Well, all these images represent a relatively small area of the sky; you can easily block them with the tip of a pencil held at arm's length. The next obvious step is to widen the view and map more of the sky. A lot, a lot wider section.
And that's why, my friends, we have now this: The incredible Hubble Legacy Field, which is simply breathtaking.
You. GADS.
His filled with galaxies! The vast majority of the objects you see here are galaxies; only a handful are stars in our own galaxy*. To prepare the ground: We are in the galaxy of the Milky Way, which is full of stars. We can see beyond these in an intergalactic space, like being inside a house and looking out the window to homes beyond. In this case, the stars in the image look like dust spots on the window; you can see things well beyond.
The galaxies in this picture are really very far away. I guess most are at least a billion light-years away, and some are up to 13.3 billion light-years far away: So far we see them because they were not long after the first stars were formed after the Big Bang!
Literally, their light traveled for more than 13 billion years, or 96% of the age of the universe, before ending their journey in front of the Hubble mirror.
Note that some of these smaller, point-shaped galaxies may be closer to us and physically smaller, or be monsters seen from around the universe. It is difficult to distinguish this simply by looking at the image. There are ways to discern both; for example, color examination of galaxies can reveal whether they are small and close or large and far apart. Taking spectra (dividing their light into thousands of individual colors) can also directly reveal their distance. Astronomers will do just that for the galaxies seen here, so that they can be better studied.
The image displayed here is only 2,000 pixels wide, it took ten times reduce the massive original image of 20,791 x 19,201 pixels to fit that image. Plus, downloading an image of 118 megabytes would dispel the wrath of my editors! But you can download it directly if you wish.
Moreover, even that is not all. It was a little cropped to make it square. So, how much sky does he cover? This much:
Yes, the whole picture is about half a degree of side, about the size of the full moon in the sky. You can still block it with the tip of an extended finger, but it's still a huge step forward compared to the old deep fields.
I was going to write everything about the facts of this picture, but I think it's easier to give you the following points:
- The image represents 31 different projects on Hubble over a period of 16 years.
- More than 7,500 separate observations have been collected to achieve it. Only two colors (red and near infrared) are shown here; the actual observations cover several filters, from ultraviolet to infrared.
- The total duration of exposure is greater than 250 days: more than 21 million seconds. It's 2/3 of the year!
- There are more than 265,000 galaxies in the complete picture; about 200,000 are in the cropped version.
- That's 20 times more galaxies in the deeper deeper but smaller field.
- A similar set of observations was made on another part of the sky; this one is in progress and will consist of about 5,200 images.
The Legacy field contains both the Ultra Deep and Extreme fields, as well as a much larger survey called the Deep Depth Survey of Observatories (GOODS), an area observed by several space telescopes: Spitzer, Hubble, Chandra , Herschel and XMM-Newton. . The use of several observatories tells us much more about what happens in these galaxies because different processes generate different types of light.
I remember the 1990s, when Hubble (and myself) were young, thinking about what the observatory could do. I had not thought of using it to take thousands of images and mosaic them together, for the simple reason that so many people wanted to do so much specific scientific research with Hubble as the only one. The idea of using days or weeks of his time to see how deep it could go was actually laughable. Astronomers would be riots! But a few weeks were actually reserved for Deep Field, and so skeptics like me became lawyers.
And here we are, all those years later, seeing incredible images like this, not just once, but many times. We learned a lot about the Universe through these images and other observations that support them.
I have read a lot of articles on this science, and it is fascinating. But still … I have to wonder, as I always do, if the larger impact of these images is much more general.
Look in the sky and you will see thousands of stars, a black space between them. But it's an illusion, an effect of our little eyes taking a short exposure.
There are no white dots in the sky. At least not on the scale that your eyes can see; every point of the sky is covered by a galaxy, near or far. There is two trillion dollars galaxies in the observable Universe, and even as deep as Hubble's images, much remains to be seen.
The Universe is vast and deep, but the most amazing is perhaps that it is knowable. These images are still among the first steps to find out.
*Stars are usually distinguished from galaxies because of diffraction points, the lines that seem to cross the stars. These are caused by the metal fins holding the Hubble secondary mirror; the light diffracts very slightly passing these palettes, creating the spikes. There are usually four of them, like a line of sight, but this image is composed of many observations, some of which are rotated relative to others. When they are added together, you get several sets of diffraction spikes. It should be noted that some galaxies are so far apart that they appear as dots, or that they have a center as bright as they also generate diffraction points. This is not a sure way to distinguish stars from galaxies.
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