IYA 2009 “You are Galileo Project”

Let’s Observe M31 and M45!
Observation and Sketch
An 18 century French Astronomer, Charles Messier made acatalogue of celestial objects that appeared blurry. Theseobjects were later called by their number with a “M” at the beginning. Later astronomers have found that his catalogues included
many different kinds of celestial objects. The true nature of M31
was especially ground-breaking. Let’ s observe and compare
M31 and M45 and try to discover their true natures.

NameAddressAge■M45 and M31 look completely different from each other when seen through a telescope. Let’s Observe, Sketch and Compare.

M31DateMonthDay:

M45

Site

Diameter of Telescope

cm Powerx

DateMonthDay:

Site

Diameter of Telescope

cm Powerx* The Power of the Telescope can be calculated by dividing the focal length of the telescope with the focal length of the eyepiece.
* M31 is darker then M45, so it’s hard to find and see.
If you couldn’t see it this time, try again when you are visiting places that are darker in the night.

■Write down what you noticed, and anything peculiar you found.

© IYA 2009 Japan Committee

IYA 2009 “You are Galileo” Project

Let’s Observe M31 and M45!
Post Observation Study
Name■The photoes below are of M31 and M45 taken using a telescope with a large diameter.

Photoes provided byM45
National Astronomical Observatory of Japan

Center of M31■Let’s list the characteristics of M31 and M45 from our sketches, observations and photoes above.
M31M45© IYA 2009 Japan Committee

IYA 2009 “You are Galileo” ProjectLetʼs Observe M31 and M45!
Post Observation Study
Name
■M45
M45 is also known as Pleades, or “Seven Sisters” (in English), and “Subaru” (in Japanese)
You can see that M45 is a cluster of stars.
It includes a couple hundred stars and it resides at around 400 light years* from Earth.
How many stars could you count with your Telescope?
( *1 light year = the distance light travels in a year = 9.5 trillion km = 5.9 trillion miles)■M31
M31 looks spread out and blurry. You cannot tell whether it is a cluster or not from the pictures or sketches,
as you could with M45.
Observations performed in the 19th century proved that M31 was also a cluster of stars.
The reason M31 was blurry and the stars could not be distinguished was because M31 was even further than M45.

So, how far is M31?
■There was a debate held between two American astronomers, Harlow Shapley and Herber Curtis,
over how far M31 was. Did M31 reside within our Milky-Way Galaxy, or did it exists outside of our galaxy
independently?
M31 is simply apart of our Galaxy.
The Universe isnot as large asCurtis says!

↑Solar System

M31 is far andseparate fromour Galaxy.
The Universe ismuch larger thanour Galaxy!
CurtisShapley■ This problem was later solved by Hubble. He used the two following facts.
1. Cephide variable stars
These are stars that turn bright and then dark
cyclically. The length of the cycle and brightness
correlate: brighter stars have a longer cycle. Thismeans we can calculate the true brightness of astar from its cycle.
(bright)

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

(bright)

The Real Brightness

Apparent Brightness(dark)

2. Relationship between apparent brightness and distance.

Time(dark)

1

2

Length of Cycle

3© IYA 2009 Japan Committee

IYA 2009 “You are Galileo” Project

Let’s Observe M31 and M45!
Post Observation Study

Name

Hubble calculated the distance to M31 using these two properties.
First, he measured the cycle of a cephide variable star in M31.
→From the cycle, he calculated that star’s true brightness.
→From the difference between the apparent and true brightness he calculated the distance.
20

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 Sharply:
300,000 light years.

This means (15) Was right!

→The Universe is far larger than our galaxy!
→The fact that M31 looks spread even though it is so far away
means M31 is a very large celestial object!
(Modern Astronomers estimate the size of our galaxy at around 100,000 light years
and M31 at around 120,000 light years. The distance between Earth and M31 isestimated at around 2,300,000 light years)

10■Let’s draw the relative positions of the Milky-Way Galaxy, M45 and M31 into
the figure space at the right.
Let’s use a reduced scale of 100,000 light years = 1cm(10,000 light years → 1mm)
● Milky-Way Galaxy: around 100,000 light years →● M31: around 120,000 light years →1 cm５cm● Distance between our galaxy and M31: 2,300,000 light years →● Distance between Earth and M45: 400 light years →cmmm(M45 is in the Milky-Way Galaxy)･･･Way too close to draw!

* Earth and M45 are too small to be for its shape to be drawn at this scale. Represent them with dots.
* You can see that M31 is very large compared to Earth and M45 and that it is very far.

© IYA 2009 Japan Committee0↑

Solar System(Earth)←Milky-WayGalaxy

IYA 2009 “You are Galileo” Project

Let’s Observe M31 and M45!
Post Observation Study
What is the true nature of M31 that is solarge and far away? Let’s consider this bycomparing M31 against our Milky-WayGalaxy.

Name↓ Our Milky-Way Galaxy
around 20,000 light years
buldge↓ An Overview of M31 (photographed using a telescopewith a diameter of 7.6cm)

Around 120,000 light years

Disk : around 100,000 light years

What is M31? Let’s write down our thoughts.

You can see that it’s center is very bright.

This is a imageof a part ofM31 photographed using
the Subaru
Telescope.

→You can see
individual stars
as white dots!

■Write down what you have learned, and what you want to learn in the future.

Image provided by National Astronomical Observatory of Japan.

Our Milky-Way Galaxy is shaped like a disk.
The bright area in the center is called the bulge.

Image provided by National Astronomical Observatory of Japan.

© IYA 2009 Japan Committee