DAVID A. HARDY / SCIENCE PHOTO LIBRARY DAVID A. HARDY / SCIENCE PHOTO LIBRARY
Worlds within worlds. Artwork showing how everything in the Universe contains smaller objects. The sequence moves from top left to bottom right, starting with a galaxy (top left). The next image is an atom, the basic unit of matter. Atoms are made of tiny particles, the heaviest of which are in its core or nucleus (centre). The nucleus is made of a mixture of protons and neutrons (orange and green), one of which is shown at lower right. Neutrons and protons contain three smaller particles known as quarks (red, bottom right).
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