Astro 122: Astronomy of Stars and Galaxies

Introduction: Methodology

Observations and measurements at different wavelenghts of light. Main bands RIVUXG - Radio, Infrared, Visual, Ultraviolet, X-rays, Gamma-rays
Establishing observed relations
Developing theories based on Physics that explain observations. Astronomy becomes Astrophysics
- At the heart - "Copernicus Principle" - we assume that the laws of physics are the same everywhere. Best support - atomic spectra from distant galaxies.
- Physical theories are formulated using math. Math is the most concise language to express relations between things. It also provides tool to deduce new results.
Understanding what new things theory predicts, designing tests and going back to observe

Lecture 1: Distances in Astronomy

Sizes of things

Powers of 10

When dealing with quantities (distances, sizes, masses ...) that can take a vast range of values, we use 'powers of 10' notation.

For example, distances in meters:

100 m = 10*10 m = 10² m
1000 m = 10*10*10 m = 10³ m = 1 km (kilo)
1000000 m = 10⁶ m = 1 Mm (Mega)
1000000000 m = 10⁹ m = 1 Gm (Giga)

Small quantities are divided by powers of ten, giving negative powers

0.01 m = 1/(10*10) m = 1/(10²)m = 10^-2 m = 1 cm (centi)
0.001 m = 1/1000 m = 1/(10³)m = 10^-3 m = 1 mm (milli)
0.000001 m = 1/1000000 m = 1/(10⁶)m = 10^-6 m = 1 micron
0.000000001 m = 1/1000000000 m = 1/(10⁹)m = 10^-9 m = 1 nm (nano)

Manipulating Powers of 10

how to write uneven numbers in powers of 10 ?

as the product of two terms: 13586 = 1.3568*10⁴

Powers of 10 are easy to multiply

(1.357*10⁴) * (0.962*10³⁵) = (1.357*0.962)* (10⁴*10³⁵) = 1.305*10⁴⁺³⁵ = 1.305*10³⁹

but not to add !

1.3568*10⁴ + 0.962*10³⁵ = ?? (1.3568*10^-31 + 0.962)*10³⁵

We say 'of order' when we speak aproximately, only as accurately as the power of 10.

Distance 1.3568*10⁴ m is 'of order' of 10⁴ m. What about 9.6*10¹² ?

Light units for distances

Light travels at the speed:
speed of light = c = 2.9979 x 10⁸ m/s

Define new units of length based on speed of light:

light-second = distance travelled by light in one second = 3 x 10⁸ m = 3 x 10⁵ km = 300,000 km

light-year = ly = distance travelled by light in one year = 9.46 x 10¹⁵ m = 9.46 x 10¹² km

Photo of Earth
from the Apollo 17 mission

The Earth

Radius = 6,378 km
Diameter = 12,756 km
Circumference = 40,000 km

Light could travel around the Earth's equator about 8 times in one second.

The Earth seen from the Moon

Distance between the Earth and the Moon is
3.84 x 10⁵ km.

Light takes 1.3 seconds to travel from the Earth to the Moon.

Distance between the Earth and the Moon is about
30 x larger than the Earth's diameter.

Photo showing Earth rising taken from the Moon

The Sun

The Sun's radius = 6.96 x 10⁸ m
= 6.96 x 10⁵ km

The Sun's diameter = 1.39 x 10⁶ km
= 4.6 light-seconds

The Sun is about 100 x larger in radius than the Earth.

The Sun's radius is about 1.8 x larger than the distance between the Earth and the Moon.

The Earth's Orbit Around the Sun

Distance between the Sun and the Earth
= 1.5 x 10¹¹ m
= 1.5 x 10⁸ km
= 500 light-seconds
= 8.3 light-minutes

An Astronomical Unit (AU) is defined to be the average distance between the Sun and the Earth.

1 AU is about 100 x larger than the Sun's diameter.

Other "Nearby" Distances:

Average distance between the Sun and Pluto = 40 AU = 5.5 light-hours

Closest star system to the Sun is Alpha Centauri.
Distance between the Sun and Alpha Centauri = 300,000 AU = 4.1 light-years

The Pleiades Star Cluster

The Pleiades is a cluster of over 3000 stars.

Cluster is about 13 ly across

Cluster is about 400 ly away from us

The Milky Way Galaxy

Our Sun is not at the centre of the Milky Way!

Distance from Sun to Galactic Centre = 2.8 x 10⁴ ly

Diameter of Milky Way's disk
= 10⁵ ly

The Andromeda Galaxy

Andromeda is the nearest big spiral galaxy similar to the Milky Way.

Distance from the Milky Way to Andromeda = 2 x 10⁶ ly

Andromeda and the Milky Way are the two largest galaxies in the Local Group of about 30 galaxies.

The Virgo Cluster of Galaxies

The Virgo Cluster is the closest cluster of galaxies, with about 2000 member galaxies.

Distance from the Local Group to the Virgo Cluster = 6 x 10⁷ ly

Size of the Virgo Cluster = 10⁷ ly

Superclusters

This map shows that galaxy clusters are arranged in arclike superclusters.

Size of largest superclusters = 10⁸ ly

The Hubble Deep Field

These galaxies are the most distant objects which have been observed in the Universe.

Typical distance = 10¹⁰ ly

Next Lecture: Angles
Read Chapter 1: 8th ed pages 8 - 15 or abridged 3rd Ed. pages 6 - 15