Sunday, June 8, 2014

My God! It's full of galaxies!

How many stars we can see with the naked eye on a clear, moonless night sky? Perhaps 5000 stars? May be 6000? Or may be 3000? Whatever is the number, it is in "thousands". It is also possible to see some foggy star clusters that are surrounding our 'Milkyway' Galaxy. Come to think of it, we can see our own Milkyway galaxy if you have a very clear night! (I highly recommend once in your lifetime to go out, and watch the Milkyway. It is awe inspiring!)  And you can also see our nearest neighboring galaxy (Andromeda).

What if we want to see galaxies much further away? And how far 'away' can we see? How *many* galaxies can we see? Fortunately, we have NASA's Hubble Space Telescope (HST) that can answer some of these questions. Astronomers asked HST to peer at one location in the sky, for a period of time, and created a picture of that sliver of the Universe. Here is that picture:
Fig. 1: An image of the Universe, from ultraviolet to infra-red part of the light, in one tiny part of the night sky, created by Hubble Space Telescope. There are about 10,000 galaxies in this picture. Some are so far away that the Universe was just a few hundred million years old when they were formed. Image courtesy: Hubble site
To say this picture is astonishing doesn't do justice to what we are seeing.  Almost every single point of light you see in this picture is a galaxy! Some are bigger, some are smaller than our own Milkyway galaxy. Since all of these galaxies are millions of light-years  away from us, that means we are seeing them how they were millions of years ago (because it took light that much time to reach us).  So there is also a "depth" (of time) included in this picture. Basically, we are looking back in time in the history of the Universe. It's like looking at your pictures, from childhood to the present, keeping the oldest photos further from us.

How big is this picture? It is 2.4 arc minutes wide. Well, that doesn't help. So what does it mean? Here is something that I thought would help. If you extend your arm and place your little finger up in the sky (see figure below), then the width of the little finger is about 1 degree.
Fig.2 : If I extend my arm out, pointing little finger to sky, the width of my finger makes 1 degree width in the sky.

Now, let me make a square box of 1 degree on each side.
Fig.3: A square box of 1 degree in the sky, compared to my little finger.  

There! Now, the size of the Hubble image shown in Fig. 1 would appear like this:
Fig. 4: The size of the Hubble image on the sky, compared to my little finger.
Can't see it? Here is a zoomed in picture:

Fig. 5: Zoomed in view of Fig. 4 above. The dot (or the tiny yellow square) on the left is the size of the Hubble image. The dot (or the black square) on the right is the actual Hubble image shown in Fig. 1 above.
 All those 10,000 galaxies that you see in Fig. 1 are in that tiny black dot above the tip of my little finger! And that is just a tiny speck of the sky.  Now, Imagine if you can do this on the whole sky (there would be nearly 25 million of those tiny black squares to fill up the sky! Note: this is a bit of an overestimate as I assumed the box is a 2.4 arc minute square). THAT is the observable Universe!

Next time when you look up in the sky, extend your little finger up and recall the above picture.  And realize that the Universe is always there, reminding us the insignificance of our momentary existence on a third planet going around a yellow star in one corner of a galaxy among billion others.

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