Nature of the Universe

Chapter 1

Units in Astronomy

We introduce the basic measuring units used by astronomers. Length, measured in meter (m), and time, measured in year (yr) or second (s), are familiar to us. The angular size, measured in degrees (°) or arc seconds, which is very important in astronomy, is not commonly used in everyday life.

Length Scales

Size does matter. The sizes of various astronomical objects are usually so big that we don't have any feeling about them. Here we list the length scales of some typical objects. You can see that, for example, the ratio of the size of our Sun to the size of the Earth is about the ratio of the size of a human to the size of a bee.

Length (m)Object
> 1026 Size of the universe
1026 Distance to quasars ( Here is one of the most famous quasars, 3C273. Courtesy NOAO/NSF.)
1024 Size of a typical cluster of galaxies ( One example of galaxy cluster, near 3C324. Courtesy STScI.)
1021 Galaxies ( A typical galaxy, M31. Courtesy NASA.)
1018 Globular clusters ( A typical globular cluster, M80. Courtesy STScI.)
1016 Nebula (clumps of gas and dust) ( The Horsehead Nebula, (C) Anglo-Australian Observatory and Photograph by David Malin.), light year
1013 The solar system
109 The Sun
107 The Earth
106 Great Wall
102 Buildings
100 Basic unit of distance, humans
10-2 Coins, bees, bugs
10-4 Diameters of hairs
10-5 Blood cells
10-6 Bacteria
10-7 Viruses
10-8 Macromolecules
10-9 Micromolecules
10-10 Atoms
10-14 Nuclei, protons, neutrons

Time Scales

What is the age of our universe and how does it compare to the average life span of a human being?

Long time agoThere was
1.5x1010 yrs Creation of the universe
1010 yrs Formation of galaxies
4.6x109 yrs Formation of the solar system
3x109 yrs Appearance of unicellar life
6x108 yrs Cambrian era (fossils of complex, hard-bodied animals)
0.65-2.5x108 yrs Dinosaurs
3x106 yrs Early hominids (mammals fossils)
3x105 yrs Appearance of homosapiens (the first real `human')
5x103 yrs Beginning of written human history
102 yrs Life span of a typical human
1 yr Earth orbiting once around the Sun
1 day Life span of some insects
1 hour Time span of this lecture
10 seconds Time to read this sentence
10-17 seconds Time for light to travel across an atom

Angular Measures

Objects look bigger when they are closer to us. If the distance between an object and the observer is halved, the apparent size of the object will be doubled. The apparent size or the angular size of the object is the angle sustained by it.

A full circle has 360 degrees (°). One degree equals 60 arc minutes (') and one arc minute equals 60 arc seconds (''). The angular size of the Moon viewed from the Earth is about 0.5°.

1°= 60',    1'= 60''

Usually, if an adult has longer arms, he or she will have bigger hands. Thus, the angle sustained by, for example, the fists when we stretch out our arms is about the same for every person. Thus, we can use our hands to measure roughly the angular sizes of various objects.

At arm's length, the angular size of our fists is about 10°, fingers is about 1° and the tip of the thumb to the tip of the little finger of an extended hand is about 20°. Note that the Moon is ``smaller'' than the finger.

Next chapter.

Title page.