Astronomy 105G Lecture Notes, 16 January 2004

Announcements

• Reading: Ch. 1 pp. 25-32
• Homework 1 due Wed. Jan. 21, 10:30 am (** you will receive an email confirmation from me indicating that I received your homework; if you do not receive an email from me, that means I did not receive your assignment **)
• SIGN UP FOR A LAB SECTION IF YOU HAVE NOT ALREADY DONE SO

Scientific Notation

In astronomy, most of the time we deal with very large numbers. This is because the distances and sizes of most objects we discuss are very large. In order to abbreviate very large (or very small) numbers, we use scientific notation. This allows you to write a big number as N x 10^m, or "something times ten to the something." The way to rewrite a large number in scientific notation is as follows:

• put a decimal point after the first non-zero digit of the number
• count how many places you have to move to the right after the decimal point to get to the end of the number, and put that result in the exponent

Thus, we can rewrite the number 250,000,000 as 2.5 x 10⁸ because we put the decimal point after the first non-zero number (2), and then move over 8 spaces to the right to get to the end of the number.

The distance to Pluto is 4,538,940,000 km. What would this be in scientific notation?

To abbreviate a very small number, we follow similar rules:

• put a decimal point after the first non-zero digit of the number
• count how many places you have to move to the left from the decimal point to get to the start of the number, and insert that result as a negative exponent

Thus, we can rewrite the number 0.00000002137 as 2.137 x 10^-8. We put the decimal point immediately after the first non-zero number (2), and then moved over 8 spaces to the left to get from the decimal point to the start of the number.

The size of a dust grain in a planetary atmosphere is approximately 0.000001 m. What would this be in scientific notation?

Introduction to the Solar System

We can get a good overview of the contents of our solar system at this web site: http://www.nineplanets.org/. Our solar system contains one star (the Sun), nine planets, and many "small bodies" (asteroids and comets). The planets can be divided into two main categories: terrestrial, or Earth-like, planets, and Jovian, or Jupiter-like, planets. The Jovian planets are also referred to as the giant planets, or the gas giants.

Everything in our solar system revolves around the Sun. [We will talk more about this next week.] For now, let's distinguish between what is in our solar system and what is not.

All the stars that we see in our sky EXCEPT THE SUN are not in our solar system. There are 10¹¹ stars in our galaxy, which is a spiral galaxy known as the Milky Way. Our Sun is just one of these stars! Below is a picture of another spiral galaxy, taken with the Hubble Space Telescope.

Galaxies are sometimes found in groups; these are known as clusters or superclusters. Finally, the Universe is the sum of everything: all galaxies, stars, planets, etc. Your "address" in the Universe is really more complicated than what we write on an envelope!

Powers of Ten video

We will watch a short video that illustrates the size and scales of objects in the Universe by taking steps (in distance) of powers of ten. This movie was made in the 1970's so it does not reflect the most cutting edge research results of astronomy or particle physics, but it does an excellent job of giving you a feeling for the vastness of space.

Distance Units in Astronomy

There are two new units of distance that you will hear about in astronomy. The first is the light year. A light year is defined as the distance light travels in one year. This distance is 9.5 x 10¹² km. The speed of light is extremely fast, but it is finite. Specifically, it is 300,000 km/sec (3 x 10⁵ km/sec). It can travel 7 times around the circumference of the Earth in 1 second!!

Because space is so vast, it takes light a finite amount of time to reach us from its source. Here are some examples of light travel time:

• light travel time from Moon to Earth: 1.3 sec
• light travel time from Sun to Earth: 8 min.
• light travel time from Mars to Earth: 12 min.
• light travel time from Pluto to Earth: 14 hours
• light travel timem from nearest star: 4.3 years

A more common distance that we will come back to often in our discussion of planetary science is the Astronomical Unit (AU). The AU is defined as the average Earth-Sun distance. It corresponds to 150,000,000 km (1.5 x 10⁸ km). We compare everything else in the solar system to the Earth and its distance from the Sun:

Planet	Distance from Sun (AU)
Mercury	0.4
Venus	0.7
Earth	1.0
Mars	1.5
Jupiter	5.2
Saturn	9.5
Uranus	19.2
Neptune	30.1
Pluto	39.5

We will come back to these distances in Lab 2. You don't need to memorize them right now, just know the definition of an Astronomical Unit.