SCALES: GALACTIC

DISTANCE TO STARS

Parallax is the shift in perspective viewing a nearby object relative to a more distant one.

The parallax angle is the angle between the sightline to a nearby object from each eye.

Parallax = a shift in apparent position of star over the course of half a year.

Remember: the parallax is too small to be seen by the unaided eye (< 1 arc minute).

Geshe ActivityLet’s see how far away the stars must be for the parallax to be too small to observe by eye.

Use your eyes, one at a time, to represent the view from Earth on opposite sides of its orbit. How far are stars before no shift is detected?

How to find distance by measuring parallax:

Parallax and distance are related to each other by a simple equation.

1 parsec = distance at which a star has a parallax of 1 arcsecond.

1 arcsecond = 1/60 arcminute = 1/3600 degree = a very small angle!

1 parsec = 206,000 astronomical units = 3.26 light-years = a very large distance!

Not to scale

SCALE MODEL II

A great island of stars in space, all held together by gravity and orbiting a common center of gravity

M31, Andromeda

Galaxy

Sun shrunk 1,400,000km to 0.2mm

That factor is 7,000,000,000, 000 or 7 trillion

Geshe ActivityWork in groups to set your expectations of the distances between stars and size of a galaxy.

If the Sun is now 0.2 mm in diameter how big and far away are the gas giant planets and the nearest stars, and how big is a whole galaxy?

Sun Star 110Earth Terrestrial 1Jupiter Gas Giant 11Neptune Gas Giant 4Comet Debris 0.008

Earth-Sun distance (AU) / Earth Diameter = 11,500

RELATIVE SIZE:

Sun Star 0Earth Terrestrial 1Jupiter Gas Giant 5Neptune Gas Giant 30Comet Debris 50,000Proxima Nearby Star 265,000

Earth-Sun distance = AU

RELATIVE DISTANCE:

This model shrinks the scale by another factor of 20,000. The Sun as a grain of sand: scale 1:7,000,000,000,000.

• The Earth is a small speck 22mm away• The Moon is a tiny speck 22mm away• Jupiter is a small grain 11cm away• Neptune is a tiny grain 70cm away• The nearest star is about 6 km away

SPACE SCALE MODEL II:

THE MILKY WAY

We live in a “city of stars” called the Milky Way. It rises high across the summer sky in the southern hemisphere.

The scale model sets the nearest star distance as 6 km so 1 light-year is about 1.5 km.

The Sun’s neighborhood in the Milky Way.

© 2004 Pearson Education Inc., publishing as Addison-Wesley

Diameter of disk = 100,000 l.y. (30,000 pc)

Radius of disk = 50,000 l.y. (15,000 pc)

Number of stars = 400 billion

Thickness of disk = 1,000 l.y. (300 pc)

Sun is in disk, 28,000 l.y. out from thecenter

Layout of the Milky Way Galaxy.

Science is Seeing

The Milky Way is Empty

The stars in the Milky Way are very widely spread out. The 400 billion grains of sand span 60x the Earth’s area, that is 1 m3 of star stuff, so only one part in 10,000 billion billion has anything.

Scientific Visualization