A few definitions Light year (ly): Distance light travels in one years (about 10 trillion km) Andromeda Galaxy – 2.4 million light years away.

A few definitionsLight year (ly): Distance light travels in one years (about 10 trillion

km)

Andromeda Galaxy – 2.4 million light years away.

A Few Definitions

Star: A self-luminous, gravitationally bound ball of gas that shines (or used to shine) because of nuclear reactions in its core. The Sun is a typical star (and by far the nearest star).

Planet: A body massive enough to be round due to gravity (≥ 600 km diameter, but not massive enough for nuclear reactions to begin), orbiting a star, and far more massive than all other objects in its vicinity.

A Few DefinitionsPlanetary system: A collection of planets and smaller bodies

orbiting a star (solar system).

Galaxy: Large (typically 5,000 to 200,000 ly across), gravitationally bound system of hundreds of millions (up to a trillion) stars

Universe: “All that there is.” (Actually, there could be many other, physically disjoint universes in a “super-Universe” or “multiverse”!)

Galaxy

The Universe

Distance (AU)Astronomical Unit (AU): the average distance between the Earth

and Sun 150,000,000 km (90,000,000 miles). Stars are much farther away.

AU and Light years are convenient ways to measure distance.

Distance (Light Years)

We use light years to measure distances outside of our solar system. The distances are far to big to measure using the Earth – Sun distance as a reference. So we use the distance light travels in one year (a constant value). 1 light year ≈ 6 trillion miles. Alpha Centauri is 4.2 ly away (easier than imagining 25,200,000,000,000 miles!).

Distance

If stars and galaxies are SO FAR, FAR away...

How do we know so much about them?

The Electromagnetic Spectrum

The supreme informant of our knowledge of the universe is from the electromagnetic spectrum

that the stars emit. (gamma, x-ray, UV, visible light, IR, radio)

Electromagnetic Field

Electric field of a stationary charge. Magnetic field of a stationary magnet.

James Clark Maxwell (1831-1879)

The Electromagnetic Wave

Visible light is one type of electromagnetic radiation. Different colors of light correspond to E&M waves having different wavelengths (λ). The waves consist of self-propagating, oscillating electric and magnetic fields that are perpendicular to each other and to the direction of motion.

The Electromagnetic Wave

Gamma RaysX RaysUltraviolet (UV)Visible (optical)Infrared (IR)Radio