# Scale Models and a Cosmic Calendar

## Astronomy 10, Vista College - Spring, 2004

In class, we constructed some scale models, in order to give us a better understanding of the locations and sizes of objects in the universe. Think of a model car. Someone went out, measured a real car, shrunk every dimension by the same factor, and created a model of the car that looks exactly like the real thing, only smaller! Scale models can give us a very good idea of what something really looks like, even if the original object, is too big to fit into a room (think of a scale model of a building). For sample calculations that go into making such scale models, see the handout on Unit conversion and Scale Models.

## Scale Models

### Scale of the Solar System

In class, we constructed a scale model of the solar system. See the solutions to this in-class activity for details. At the end of this exercise, we found that if the Sun is represented by a volleyball in Berkeley, then the next star (represented by another volleyball) is in Caracas, Venezuela! Imagine how difficult it would be to see a Jupiter-sized (0.9 inch) or Earth-sized (0.08 inches) object near this star! Clearly, for this scale model, only a few of the nearest stars would fit on the surface of the Earth. To get a picture of things more distant, we need a new scale model!

### Scale of the Galaxy and Other Galaxies

Let's change the scale of our model, and make a new model. In our new scale, each cm in the model will represent 1 x 1019 cm in real life! On this scale, each light year is 1 mm. The separation between the Sun and the nearest stars (SF to Caracas in our previous model) is about 4.4 mm, or smaller than the width of your little finger!

On this scale, the stars are the size of individual atoms, but now we can visualize the scale of the Milky Way Galaxy (MWG). The MWG is 100,000 light years across, which makes it about 100 yards (= football field) across in our model!

How many stars are in our galaxy? 100 billion. If you counted one star each second, it would take you more than 3,000 years to count them all! (You can verify this by dividing 100 billion by the number of seconds in a year).

Where is the next galaxy? Let's shrink the scale again! If the MW Galaxy were represented by a grapefruit, then the next galaxy would be ~ 25 grapefruits away! Although the distances between stars are HUGE relative to the sizes of stars, the distances between galaxies are only tens of times the sizes of galaxies.

There are about 100 billion galaxies in the observable universe (this just means the part that we can see). This means that there are 1022 stars in the observable universe! This is more stars than there are grains of sand on all the beaches of Earth.

### Brief history of the Universe

We'll talk about all of this in much more detail later. This brief introduction is here simply to give you an idea of where we came from, so that you have an idea of the "big picture".
• Observations show that the universe is expanding. The distances between the galaxies are increasing with time.
• Expansion started with the Big Bang. The observed rate of this expansion allows us to trace the universe back to when the expansion began, the "Big Bang", and gives us an estimate of the age of the universe: 10-16 billion y
• Gravity has slowed the expansion over time, and local clumps of matter have collapsed to form galaxies, stars, planets.
• All matter was created in the Big Bang, but at first only hydrogen and helium.
• All other elements manufactured in stars through nuclear fusion (makes heavier elements from lighter ones). Carbon, gold, all of it! We are made of stuff that was once part of stars!

### Cosmic Calendar

In class, we imagined that the history of the universe was scaled to a single year! The Big Bang happened the first instant of Jan 1, and the present time is just before midnight on Dec 31. If the universe is assumed to be 12 billion years old, then each month on our calendar represents about a billion years!

• Milky Way Galaxy - formed sometime in Feb
• Solar system formed from cloud of gas/dust 4.6 billion yr ago = Aug 13
• earliest (unicellular) life on earth by about 3.4 billion yr ago = mid-September
• invertebrate life only 600 million y ago ~ Dec 13
• Dinosaurs began just after midnight on Dec 25 (~ 250 million y ago), and ended suddenly about 2am Dec 30 (65 million y ago)
• Earliest human ancestors walked upright ~ 3-5 million y ago, ~9pm Dec 31
• Homo Sapiens appeared 300-400 000 y ago = 13 min ago = 11:47pm Dec 31
• Agriculture rose 30 sec ago = 11:59:30pm
• Egyptians built pyramids 13 sec ago = 11:59:47pm
• Columbus sailed to America in 1492 = 1.3 sec ago = 11:59:58.7pm
• Average college student born less than 0.1 sec ago 11:59:59.9 pm Dec 31

We are truly latecomers to the universe!