The Milky WayAstronomy

What is a galaxy?

A galaxy is a collection of stars, gas, and dust bound together by gravity.

Our own galaxy is called the Milky Way Galaxy

Herschel’s first map of Milky Way

Interstellar Dust

It’s hard to see our own galaxy, like being in the middle of a forest of trees

Our galaxy is filled with gas and interstellar dust, which makes it hard to see through

Types of Nebulae

Light hits the dust and can produce…

1. Dark nebula if dust blocks lots of light

2. Emission nebula if dust is close to stars—reddish

3. Reflection nebula if dust is far from stars—bluish

Example

Match up the following to the images…

1. Dark Nebula

2. Emission Nebula

3. Reflection Nebula

(a) (b)

(c)

(a)

(b)

(c)

What can we see through dust?

Longer waves like infrared andradio waves

Radio waves go through gas and dust clouds (called 21 cm radiation)

Previous experiments about size and shape of the Milky Way were off because they didn’t account for interstellar gas and dust.

Image of the Milky Way galaxy in infrared

Globular Clusters

about 150 Globular Clusters found around the halo of our Milky Way

globular clusters contain redder, older stars

Spiral arms contain younger, bluer stars

halo

Spiral Arm structure?

Based on redshift of the 21 cm radio waves halo

Milky Way Galaxy

Contains about 200 billion stars (our Sun is only one of them)

Spiral structure with arms

About 100,000 light years across, and we are 30,000 ly from center

Supermassive black hole of 2 1/2 million solar masses at center (called Sgr *A)

Milky Way Galaxy

Contains about 200 billion stars (our Sun is only one of them)

Spiral structure with arms

About 100,000 light years across, and we are 30,000 ly from center

Supermassive black hole of 2 1/2 million solar masses at center (called Sgr *A)

Milky Way Galaxy

Can we really be looking at it from a top-down view or a side view?

No. We can’t leave the galaxy…it’s too big. These are just diagrams.

Milky Way Galaxy

So do we have any real pictures of our galaxy?

Yes. This is what it looks like in the sky at night.

And here’s what it looks like in other wavelengths…

Milky Way Galaxy Visible light panorama over a year

A few warm up questions…

Why is it so hard to get an idea of the size and shape of our own Milky Way galaxy?

a) Because we are so far away from it

b) Because there’s a lot of dust that blocks our view

c) Because a black hole blocks our view

d) Because the Sun blocks our view

Example

Which type of electromagnetic wave is able to pass through interstellar gas and dust without getting absorbed?

a) Visible light

b) Gamma ray

c) X ray

d) Radio wave

Example

Where are we located in the Milky Way?

a) At the center

b) Inside the halo

c) In a spiral arm

d) At the very outside edge

Example

Our Sun is one star inside the Milky Way. How many stars are there inside the Milky Way?

a) About 2,000

b) About 2,000,000

c) About 2,000,000,000

d) About 200,000,000,000

e) About 2,000,000,000,000,000

Example

Globular clusters contain many red stars. This indicates that globular clusters typically contain

a) Older stars

b) Younger stars

c) Stars that are the hotter than the Sun

d) The hottest stars in the galaxy

Example

Match up the following with the diagram shown.

1. Location of the Sun

2. Galactic disc

3. Globular clusters

4. Location of supermassive black hole