A Sense of Time and Scale
in the Universe

## Changing Perceptions of the Universe

Our understanding of the Universe with respect to it size, temporal duration, the kinds of events that take place in it, and the kinds of objects that it contains has undergone serious revision in the last few centuries:
• Only in the last 400 years or so have we realized that the earth is not the "center", and that the Universe is immense.

1. The Sun would hold 1.3 million Earths.
2. There are 200 billion "Suns"in a galaxy like our own Milky Way Galaxy.
3. Astronomers can see billions of galaxies.
4. We don't know whether the Universe has an "end" or not; we are not completely sure even of the full meaning of the question.

• Only in the last 50 years have we realized that the Universe is not static.

• The entire Universe is expanding.
• Events of incredible violence take place in the Universe.

• In the last half-century we have come to believe that the Universe contains objects that are truly bizarre as measured in human terms.

1. Neutron Stars and Pulsars
2. Black Holes
3. Quasars
4. Exploding and Colliding Galaxies

• Only in the last 200 years have we begun to appreciate the age of the Universe.

1. The Universe is probably 10-20 billion years old.
2. Our Solar System is probably 4-5 billion years old.

Thus, the Universe of the modern astronomer would be largely unrecognizable to her counterpart from a few centuries ago. We can only speculate whether our present understanding of the Universe will appear as quaint 400 years from now as the views from 400 years ago appear to us.

## Scientific Notation

Scientists studying the very small and the very large try to use systems of units that are most convenient for the particular scales of the phenomena they are interested in. For example, we often use kilometers rather than millimeters to describe the distance we travel by car. However, there are often still a wide range of scales relevant to any particular phenomena and sometimes the very large mixes with the very small. Scientific notation is a convenient way to deal with this: We write e.g.

3000 = 3 x 10^3

600000000 = 6 x 10^8

52345 = 5.2345 x 10^4

10^n x 10^m = 10^(n+m)

e.g. 10^3 x 10^4 = 10^7

(10^n)^m = 10^(n x m)

e.g. (10^3)^8= 10^24

## A Sense of Scale

If the solar system were the size of a table, the Andromeda Galaxy would lie at 10 times the distance to the moon and the most distant galaxies would lie at 60 times the distance to the Sun.

If the sun were the size of a golf ball in new york the next nearest star would be a golf ball in chicago.

If you look at a picture of the earth which is about 8cm in diameter, all of the atmosphere relevant to our life is the thickness of a thread.

Earth Radius = 6.3 x 10^8 cm

Sun Radius: 7 x 10^10 cm

Earth-Moon Mean Distance: 4 x 10^10 cm

Earth-Sun Mean Distance: 1.5 x 10^13 cm (=astronomical unit)

Nearest non-solar star to Earth (proxima centauri) : 4 x 10^18 cm (= 4 light years)

NOTE: LIGHT YEAR IS DISTANCE LIGHT TRAVELS IN 1 YEAR

Radius of Galaxy: 4.5 x 10^22 cm (= 15 kiloparsecs)

Distance to Nearest Galaxy: 2 x 10^24 cm

Typical distance to the most distant objects we can see (quasars): 10^29 cm (= 10^11 light years).

## A Sense of Time

There is great evidence that the universe is expanding. We know this from observations of distant galaxies that we can tell are receding from us. The expansion can be traced back to a time at which the universe was much hotter and smaller in scale than it is now. The beginning of this expanding phase is often called the Big Bang, where the expansion was most rapid.

If we compress the time since the Big Bang into one year, and make the time of the Big Bang January 1, then

• The Earth was formed in mid-September.
• The mammals appeared on December 26.
• All human prehistory (from the first known stone tools) and history have occurred in the last 1/2 hour of New Year's Eve.
Thus, all of human history is but a fleeting instant on the cosmic timescale.

## A Sense of Energy and Power

Car consumes 20,000 Watts of power (energy per unit time, joules per second)

Sun emits the power of 10^22 cars.

Hydrogen Bomb releases 10^17 joules in a subsecond blast which equals energy usage in the entire US for a few days.

Supernova releases (exploding star) energy of 10^22 hydrogen bombs in a second, the equivalent to the luminosity from 10 billion suns, or an entire galaxy

The must lumunous sources of all: "gamma-ray bursts" release the equivalent of 10^30 hydrogen bombs in the same time scale explosion.

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