# 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 )
Mass
(amu )
Family
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.

Conducted by Gene Smith, CASS/UCSD.
Comments? You may send email to hsmith@ucsd.edu

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

Last updated: 16 April 1999