The Milky Way

Question 1

Describe the appearance of the Milky Way by the naked eye.

Answer 1

Faint
Fuzzy
Band of light
Stretching across the sky
With some darker patches

Question 2

What key constellations does the Milky Way pass through?

Answer 2

Auriga
[Asterism the Summer Triangle]
Cassiopeia

Question 3

What can you see of the Milky Way with binoculars or a telescope?

Answer 3

It is possible to resolve some individual stars.

Question 4

Where is the Milky Way visible in the Northern hemisphere?

Answer 4

In a line from Cassiopeia to Cygnus.

Galactic centre lies in Sagittarius.

Question 5

What do we look at when we view the Milky Way?

Answer 5

The plane of our galaxy, where most stars are located.

Question 6

What does the Milky Way look like when viewed from above?

Answer 6

A disc-shape/Catherine Wheel, rotating around a central bulge.

Question 7

How big is the disc?

Answer 7

30kpc wide

1-2kpc thick

Question 8

How many spiral arms are there?

Answer 8

4

Question 9

Where is the bulge?

Answer 9

It surrounds the nucleus.

Question 10

Where is the halo?

Answer 10

It lies outside the spiral arms.

Question 11

Where is the sun / solar system located?

Answer 11

In one of the 4 spiral arms, roughly 10kpc or 2/3 from the nucleus.

Question 12

How many stars are in the galaxy?

Answer 12

200-400 billion

Question 13

What are the two companion galaxies of the Milky Way?

Answer 13

Large and Small Magellanic Clouds.

Question 14

How far away is the Andromeda Galaxy?

Answer 14

The Andromeda galaxy is one of our closest neighbours, at 700kpc away.

Question 15

What are globular clusters?

Answer 15

A spherical collection of relatively old stars all orbiting a galactic core.

Question 16

Where is the supermassive black hole?

Answer 16

In the nucleus

Question 17

Where is gas and dust found?

Answer 17

In the nucleus, but mostly in the spiral arms.

Question 18

Where are the sites of star formation?

Answer 18

In the spiral arms - NOT in the nucleus!

Question 19

Where are the globular clusters?

Answer 19

In plane view: close to the bulge.

In cross-sectional view: close to the halo or central bulge.

Question 20

Why do astronomers use 21cm radio waves instead of visible light to observe the galaxy?

Answer 20

1. Visible light cannot penetrate the dust in the spiral arms of the galaxy, or the atmosphere (fully).
2. 1420MHz radio waves (or 21cm radio waves) can.
3. 1420MHz radio waves are absorbed and emitted by hydrogen in the spiral arms of the galaxy when its electron reverses its spin.
4. Radio telescopes tuned to this frequency detect this.
5. The wavelength of the radio waves in the lab can be measured and compared with the radio waves emitted from hydrogen.
6. This allows their wavelengths to be determined.
7. This allows Dopppler shift to be determined.
8. This means that by using 21cm radio waves, the galaxy’s speed of rotation can be determined.