IYA 2009 “You are Galileo” Project

Let’s Observe the Andromeda Nebula (M31)!
Observation & Sketch Worksheet
80 years ago, there was a great debate concerning the nature of M31.
It took Edwin Hubble's observation to reveal the true nature of M31.
This caused a breakthrough in our understanding of the universe.
What is M31? Let’s see for ourselves using a telescope!

NameAddress

Age■ Let’s observe and make a sketch of M31
1
Observation Date
Place of observation:MonthDiameter of Telescope

DatePowerxcm

Field of View°* The power of the telescope can be determined by dividing the focal length of the telescope with the focal length of the eyepiece.
* The field of view depends on the telescope and eyepiece, so check the telescope’s manual or the vendor’s web page.

Make a large sketch and make it as detailed as possible.

Draw the size of M31 relative
to the field of view of the telescope.

■ Write down what you noticed, and note anything you find in particular.

© IYA 2009 Japan Committee

IYA 2009 "You are Galileo!" ProjectLet’s Observe the Andromeda NebulaPost Observation StudyName■About 80 years ago, we did not know what M31 was. The primely problem was whether M31 resided

in our own Milky Way galaxy or whether it exsited outside of the Milkyway Galaxy independently.
In 1920, the Great Debate over this problem was held between two American astronomers:
Harlow Shapley and Heber Curtis.

Solar SystemShapley■ This problem was later solved by Hubble.
Hubble used the two following facts:
1. Cephide variable stars:

These are stars that cyclically turn bright and then dark.
Also, the cycle and brightness correlate, and longer thecycle, brighter the star. This means we can learn the realbrightness of the star from its cycle.

(bright)

M31 is simply a partof our Galaxy.
The Universe is notas large as Curtis
says!

As the figure below shows, light disburses as distance increases. If thedistance doubles, the brightness decreases to 1/4. If the distance triples,the brightness decreases to 1/9. This means that the apparent brightnessof a star is inversely proportional to the square of distance.
Thus, we can calculate the distance by comparing the actual
brightness of a star and its apparent brightness.

12(dark) Length of Cycle
Time
Hubble calculated the distance to M31 from these two properties.
First, he measured the cycle of a cephide variable star in M31.
(dark)

Curtis

2. Relationship between apparent brightness and distance.

The Real Brightness

Apparent Brightness(bright)

M31 is far and separate
from our Galaxy.
The Universe is muchlarger than our Galaxy!

3→from there, calculated its true brightness → from the difference between its real brightness and apparent brightness he calculated that:

The M31 is at 900,000 light years away.
1 light year = the distance light travels in a year = 9.5 trillion km = 5.9 trillion miles■This was three times larger than the width of our galaxy estimated at that time by Shapley: 300,000 light years.
Thus, () was right!

→The Universe is far larger than our galaxy!

(At our current calculation, the stellar disk of our galaxy is approximately 100,000 light-years in diameter,
and M31 is 2.3 million light-years way from us.)■ What is the nature of this M31? What could it be doing so far away?
Let’s use are sketches of M31 and consider this.

© IYA 2009 Japan CommitteePost Observation Study

IYA 2009 “You are Galileo” Project
Let’s Observe the Andromeda Nebula (M31) !
Name■The size of the M31 we saw in the telescope waslight years.

Then, how large is M31 compared to our galaxy?
In our observation, did we see the whole M31?
What might M31 be?

→ Let us compare M31 and our Milky way Galaxy.

↓ M31 taken with a telescope with a diameter of 7.6 cm
and long exposure time.
© National Astronomical Observatory of Japan↓ Our Galaxy (a model of the view from the side)

Bulge: about 20,000 light years

Disk: about 100,000 light years
The size of the disk of our Galaxy is about
100,000 light years. The bright part in thecenter is called the bulge. The diameterof the bulge is about 20,000 light years.

Unlike when you observe it with a 4cm telescope,
you can see that the center of M31 is very bright.
A 7.6cm telescope can gather more light, so you canobserve lights that you could not see with a 4cm telescope.
With the 4cm telescope, you were only
looking at the bright section in the center.

→ Write down your own thoughts.

■Write down what you’ve learned today and what you would like to learn more about.

© IYA 2009 Japan Committee

M31 is separate galaxy outside of our galaxy. We were looking at the bright bulge in the center of M31.

IYA 2009 “You are Galileo” Project

Post Observation StudyLet’s Observe the Andromeda Nebula (M31)!
Name■Since M31 is at so far away, it looks very small. How big is it actually?
Let’s calculate using our sketches!

■First, let’s find the apparent size of M31 in angler degrees.
This is the apparent angle of the width of a celestial object projected on the celestial sphere.

■Calculate the apparent angle from the size of M31 relative to the telescope’s field of view.
The apparent size of M31 (in angle units)
Size of M31=

Size of M31 on your sketch (in cm)

×Telescope’s field of view (degree)

Telescope’s field of view on your sketch (in cm)

The telescope’s field of view

=cm
8 cm

=

×

Telescope’s field of view (degree)degree■Let’s calculate the actual size.
We want the length of circular arc of a circle with a radius ofdegrees

2.3 million
light years

2.3 million light years and an angle ofdegrees, so...

Apparent size of part of M31 you sketched =degrees

2.3 million light years ×2 × 3.14 ×=

360

light years© IYA 2009 Japan Committee