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The Color of the Stars

8-10-year-old students

In this experience, we will learn how to classify the stars according to the light that we receive from them, and in particular according to their color.


The material for the classroom before coming to ESAC can be divided into two categories:

  • For students:
    • Introduction to ESA (6:46 min)
    • Departure into Space (4:02 min)
    • Further than the Solar System (6:03 min)


Recent publications have brought up again the topic about the number of galaxies in our Universe. Apparently, there are 20 times more galaxies than it was considered in 2015, in total about 2 billion galaxies.  More developed cameras allowed scientists to detect fainter objects and therefore older galaxies. The number of galaxies can be obtained by counting them in an image or by derivation of the number of galaxies from what we call the mass function, which provides, by mass-range a tendency to galaxy generation.

As it can be seen in the image below from the Hubble satellite, there is a great variety of galaxies shape (spiral, lenticular, elliptical) and size (large, medium and dwarf). Our galaxy, the Milky Way, is a very normal spiral galaxy with a diameter of 100.000 light-years (1 light-year = 9,46 × 1012 km) and with 200.000 to 400.000 million of stars. The closest galaxy to ours is Andromeda galaxy, at 2.5 million light-years.

The Hubble Ultra Deep Field is seen in the ultraviolet, visible, and infrared light. Image Credit: NASA, ESA, H.Teplitz and M. Rafelski (IPAC/Caltech), A.Koekemoer (STScI), R.Windhorst (Arizona State University), and Z. Levay (STScI).


In the spiral galaxies, such as the Milky Way, the stars are mainly hosted in their spiral arms. We are located quite far from the center of our Galaxy within a cloud that is called "The Local Bubble", which is approximately 60 light-years across. If the following image this was our galaxy our Sun will be located were the arrow indicates.

The Sun, so important for us as the source of our life, is a very standard star compared to others in the Galaxy (the Milky Way) and other galaxies.  

Depending on the place and conditions where the stars form they will begin with a more or less sophisticated chemical components, will be more or less massive, and will be isolated or accompanied (see The Herztsprung-Russell diagram). 

From a visual point of view, we can know some of the properties of a star by their colors and by their chemical composition (spectra).

Eta Carina Nebulae, collected by a Cassegrain Telescope 16" f/3 (1200mm). Credits: Johannes Schneider

For example, cold stars are redder. This could be because they are older or because they do not have much mass. Massive and younger stars are bluer.

When we look at the stars from the Earth, we see them moving across the sky. If we took a picture of the sky at night, exposing during the whole night, we would be able to see a picture such as the one below. As you can see all the stars always move in the same way ( to the center of the circle). In the Earth's northern hemisphere, this reference point is the Polar star.

Teide Sky Trails. Credits: Daniel Lopez