University of California, San Diego
Center for Astrophysics & Space Sciences

Gene Smith's Astronomy Tutorial

The Scale of the Universe

The Atom -- The basic unit of each chemical element is the atom. Atoms have a large nucleus, composed of protons and neutrons held together by the Strong Force. The electrons "orbit" the nucleus, attracted by the Electrical Force.

Atomic Particles
Particle Symbol Charge Mass
(g )
(amu )
proton p+ +1 1.673 x 10-24 1.00727 baryon
neutron n0 0 1.675 x 10-24 1.00866 baryon
electron e- -1 9.109 x 10-28 5.485 x 10-4 lepton

Atomic Nuclues - ~10-13cm.

Size of Atom - the radius of the first electron "orbit" in the Hydrogen atom is 1/2Å = 0.5 x 10-8cm.
The atom is about 100,000 times bigger than its nucleus. If the atomic nucleus were a pingpong ball (r ~ 1cm), its electrons would be in orbits about a kilometer away.

Molecules - Atoms build themselves into molecules (e.g. H2O) with sizes ranging from a few Å up to 10 cm for the largest molecules, such as DNA.

Cells - 10-2 cm.

Humans - The largest humans who don't play basketball are about 2 meters ( = 200 cm) in size; Professor Smith is 186 cm in height.

The Earth - The Earth is about 25,000 miles around, which makes its radius about 4000 miles. More precisely, the radius at the equator, RE = 6378 km = 6.378 x 108 cm. In other words, it would take about 7 million people to span the distance from San Diego to China through the center of the Earth.

The Sun - The Sun, our nearest star is 93 million miles = 150 million km = 1.5 x 1013cm away. Its radius , RS = 7 x 1010cm.

If the sun were a large grapefruit (r~ 7 cm), the Earth would be a pinhead 15 meters away.

The Solar System; Sizes are to scale, but distances are not.

The Stars - The nearest star, Alpha Centauri, is a triple star system in the constellation Centaurus. It is only visible from the Southern Hemisphere. The distance to Alpha Centauri is 4.34 light- years = 4 x 1018cm.

A light-year is the distance that light travels in one year:

If the Sun were a grapefruit in San Diego, the nearest grapefruit (star) would be 2500 miles away.

The Galaxy - The Sun is one of the 500,000,000,000 stars in the Milky Way Galaxy. The Milky Way is about 100,000 light-years = 1023cm across.

Other Galaxies - The Andromeda Galaxy, our nearest sizeable neighbor, is about 2 million light-years = 2 x 1024cm away.

The Universe - 1027cm: There may not be a "size" to the Universe; it may be infinite. But, a crude estimate of the scale of the Universe can be made from looking back to the "beginning". Astronomers believe that the Universe began with the Big Bang,some 10 - 20 billion years ago. So, the most distant objects that we can see are about 10 - 20 billion light-years (= 1027 cm) away.

From the size of the atomic nucleus,important for the fusion energy of the stars, at 10-13cm, to the gigantic extent of the Universe as a whole ~ 1027cm, the distance scales of interest in Astronomy span a factor of about 1040 -- 40 orders of magnitude!

  • Powers of ten a visual depiction of the scale of the Universe starting at 10 million l.y. from the Milky Way and zooming into the heart of the atom.

    Through the Universe at the Speed of Light  Education & Outreach  CASS Home  

    Conducted by Gene Smith, CASS/UCSD.
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    Prof. H. E. (Gene) Smith
    CASS   0424   UCSD
    9500 Gilman Drive
    La Jolla, CA    92093-0424

    Last updated: 16 April 1999