Weighing the Universe at Home

Part of the Hubble Deep Field
Part of the Hubble Deep Field
Credit: NASA

It is impossible to count all the stars in all the galaxies in the universe, but we can make a reasonable estimate. To do this, we try to count everything in a small patch of the sky and scale up to the whole sky. We can use a special observation taken by the Hubble Space Telescope: the Hubble Deep Field or HDF.

This is a very long-exposure observation of a single, small piece of sky that is otherwise not special at all - no big nearby galaxies, or bright stars and so on. Since we know how much of the sky the image covers, if you can count all the galaxies you can find in the observation, you can scale up and work out roughly how many galaxies there are in total, and from that how many stars they must have.

What to do

  1. First you need to get the HDF image which is here.
  2. Almost every dot you see is a galaxy, but if you try to count them all on one go, you will probably get lost, so it is easier to split the image up into strips or squares - 10 is a good amount. If you have a printer, you could print it out and cut it up, but it is OK to just work on your screen. If you have any image editing software like Microsoft Paint or Photoshop, you can use that to cut the picture up into about 10 strips or squares. If you are working with anyone else, you can share the pieces out between you.
  3. Then you should carefully count all of the galaxies on each piece. Write down how many you find.
  4. When you have counted all the galaxies on all the pieces, add all the numbers together.
  5. Although different galaxies are different sizes and shapes, on average there are about 100,000,000,000 stars in a typical galaxy, so now multiple your total number of galaxies by 100,000,000,000 (or 1x1011). If will be a very big number, so your calculator will give you the answer in standard notation. This is an estimate of the total number of stars in the HDF image.
  6. A typical star has a mass about 1030 kilograms (10 to the power 30 or 1E30 on a calculator), so multiply your number of galaxies by 1030 to estimate the total mass of all the stars in the HDF image.
  7. The HDF image covers just 1/95,000,000 of the whole sky, so finally multiply your total mass of stars by 95,000,000. That is an estimate of the total amount of mass of all the stars in all the galaxies in the observable universe!
In the boxes below, put how many galaxies you counted in each of the 10 strips Add up all of the galaxies in all of the 10 strips to give you the total number of galaxies
1 2 3 4 5 6 7 8 9 10 Subtotal
Equation   Answer
Number of galaxies = subtotal = (put subtotal here)
Number of stars = number of galaxies x 1x1011  =  
Mass of stars = number of stars x 1x1030 =  
Total mass in universe ​ = mass of stars x 95,000,000 =  


Some things to think about

  • Although almost all the objects in the HDF are galaxies, there are two individual stars which are in our own galaxy the Milky Way. Can you find them? (Hint: Observations of stars by telescopes usually have sharp "diffraction spikes" coming out of them).
  • This is only an estimate of the mass of the Universe, not an accurate measurement. This means that you are making some assumptions. How important do you think each of these potential problems is? In other words, which might make the biggest difference to the final answer?
    1. You might have miscounted the number of galaxies.
    2. You might have mistaken stars for galaxies.
    3. There might be lots of mass that is not made into stars, so you couldn't see it.
    4. This part of the sky might have more or less than the typical number of galaxies.

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