Estimate the Number of Galaxies in the Universe

Estimate the Number of Galaxies in the Universe

In 1996 the Hubble Space Telescope ‘created’ a photograph of what was thought to have been a relatively empty area of the sky. To create the image astronomers made the telescope stare at the same place for 10 days. Below is what they found:

The image became known as The Hubble Deep Field(HDF) and covers an area of one thirty-millionth of the sky. This is equivalent to the area of sky that would be hidden behind the queen’s eye of a 5p piece when held at arms length. Astronomers began saying things such as “One peek into a small part of the sky, one giant leap back in time”. The HDF represents a narrow view of the universe and is essentially a deep “core sample” of space. The image is similar to a geologic core sample of the Earth’s crust. Just as a core sample represents history of the evolution of the Earth’s surface, the HDF contains information about the universe at many different stages in time. See the image on the next page, note the scale across the bottom of the image.

Astronomers have long tried to estimate the number of galaxies in the universe and this image became very significant in that quest. The HDF was considered to be a representative sample of the typical distribution of galaxies in space because the universe, statistically, looks largely the same in all directions. Given that it is a ‘core sample’ and representative of the whole sky it could therefore be used to estimate the total number of galaxies in the universe.

And this is what you will do today.

The Hubble Deep Field is a ‘core sample’ of the universe since just before the Big Bang

So, if we were to count all of the galaxies (that’s every single little dot!) in the HDF and multiply this number by 30,000,000 then we’d have an estimate of the number of galaxies in the universe. But this would take a considerably long time to count. Thankfully we can make use of another aspect of the method by which the image was made.

The HDF is made up of four separate pictures which are combined, like tiles, to create a mosaic. The image below shows the HDF split into these four pieces. You’ll notice that one (‘PC’) is much smaller than the others as it provides even more detail of a smaller ‘field’. The main threeimages are labelled A, B and C and are what we will use for our activity.

We can split these three images up even further and invert the colours for ease of counting. See the next few pages.

All that we need do now is to choose and count just one of these areas. Then multiply up to create an estimate.

You can check your estimate with the real answer using these links:

Hubble Deep Field Image A

Hubble Deep Field Image B

Hubble Deep Field Image C

Where in the Sky?

Internet Links

Hubble Space Telescope

NASA’s Amazing Space (Educational resources)

Hubble Deep Field Newscenter Website

Where in the sky is the Hubble Deep Field?

The image, full resolution download options

Check the answer