Galaxies: Seeing The Universe Section 3

Galaxies Seeing the Universe Section 3

What Exactly are They? • Galaxies are groups of stars, gas, and dust held together by gravity. – May contain as many as one trillion stars

• They vary in size but our galaxy (the Milky Way) is 100,000 light years across • The closest galaxy to the Milky Way is Andromeda – 2 million light years away…

Types of Galaxies • Spiral – have spiral arms that wind outward from inner regions – The Milky Way and Andromeda are spiral (so is the picture in the background) – Some are “barred spirals” where the arms extend from a central “bar” of stars

• Elliptical – shaped like large 3-D ellipses (like a football). – Some are large enough that several Milky Ways could fit inside of them

• Irregular – just like it sounds, they don’t have a definite shape and don’t fit easily into other classifications

Elliptical Galaxy

Notice other spiral galaxies in the background

An irregular galaxy

The Milky Way • Our home galaxy is 100,000 light years across – Our solar system is about 30,000 Ly from the center and makes one full orbit in 240 mil years • This means that we’re travelling at 235 km/s

• If there’s low light pollution, you can see the Milky Way on a clear night – It looks like a band of misty light stretching through the night sky – All the stars in the night sky belong to

The Milky Way as seen from a road in Death Valley

Expansion of the Universe • What does an ambulance sound like as it passes you? – Have you noticed the drop in pitch as it travels away from you? – This is called the Doppler effect

• Light from other stars does the same thing!

The Doppler Shift • Just like sound waves, light that travels toward us is compressed and light that travels away is elongated – Think about the ambulance, the pitch lowers because the sound waves are being stretched out

• Light that is moving away from us (stretched) is shifted toward the red end of the color spectrum • Light that is moving toward us (compressed) is shifted toward the blue end

Normal waves

Elongat ed waves

Compressed waves

Shifted red

Shifted blue

Red Shift • Edwin Hubble discovered that all galaxies outside of our Local Group have a red shift – What direction are all of these galaxies moving then?

• If they all have red shifted light, they all must be moving away from the Earth • Scientists determined that if they’re all moving away, they all must have

The Big Bang Theory • States that the universe began 12-15 billion years ago in a massive explosion – The entire universe began to expand everywhere at the same time

• As matter cooled, it clumped together to form hydrogen and helium gasses – Eventually, gravity caused this matter to compact and form the first stars – The first stars were born after about one

Faith and the Big Bang • “Faith is being sure of what we hope for, and certain of what we cannot see” Heb. 11:1 – Take comfort in the complexity of life and the universe itself – The Bible says that the wisdom of men is like foolishness to God. 1 Cor. 1:25, 3:19 – The goal of science is to explain EVERYTHING and scientists are going to do just that.