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
|| 1.673 x 10-24
|| 1.675 x 10-24
|| 9.109 x 10-28
|| 5.485 x 10-4
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
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
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
Conducted by Gene Smith, CASS/UCSD.
You may send email to firstname.lastname@example.org
Prof. H. E. (Gene) Smith
CASS 0424 UCSD
9500 Gilman Drive
La Jolla, CA 92093-0424
Last updated: 16 April 1999