How far? How big ? How many ?
Light - our measuring stick in space
How far does light travel in one second ? 300 000
kilometers, which equals seven times around the Earth.
Let's call this distance one light second. We can
use this as a ruler to measure distances in space. Beyond the Sun and its
planets we need a longer ruler, which is the distance light travels in one
year. This is called a light year - it is 32 million times
longer than a light second. Multiplying a light second by the number of
seconds in a year, we can answer the question:
How far is a light year? 10
trillion kilometers (or more exactly, 9.5 x 1012
The Earth, which is about 12000 kilometers in diameter (NASA).
The Sun and its planets
- How far is the nearest star (after the Sun) ? Proxima Centauri is 4.3 light years away. It is a dim red star
orbiting the bright double star, Alpha Centauri. These nearest stars are
270 000 times further from the solar system than the Sun is from the Earth.
- What are the smallest stars ? Neutron stars
collapse to about 15 kilometers in diameter. We observe them as pulsars.
- What are the biggest stars ? Red Giant stars
swell up to 500 times the diameter of the Sun. Betelguese, Antares and
Aldebaran are well known red giant stars.
Our Galaxy, the Milky Way
- How far is the Sun from the middle of our galaxy, the Milky Way ?
30 000 light years.
- How big is the Milky Way ? 100 000 light years
across. This is 100 million times the diameter of Pluto's orbit.
- How many stars are there in the Milky Way ? About 100 billion (= 1011).
Stars and dust clouds towards the center of the Milky Way. Stars form
half the detectable mass of the Milky Way. The other half is in the form of
gas and dust.
- How far away is the nearest galaxy ? The
newly discovered dwarf galaxy in Sagittarius, only 60 000 light years away, is
spiralling into the Milky Way.
The nearest large spiral galaxy like the Milky Way is the
Andromeda galaxy, about 2.7 million light years away.
- How many galaxies are there in the visible universe ? Over 2 million have been counted, but there could be about 100
- How big is the visible universe ? About 15 000
million light years.
Distant galaxies seen by the Hubble Space Telescope (STSCI/NASA).
More about Space and
Time, Stars and Planets