#### Transcript Used for stars w/in a few hundred LY

```Objectives:
Learn what units scientists measure
distances in space.
Define and use an astronomical unit to
measure distances in space.
Define and use a light year to measure
distances in space.
For example:
Earth is 93,000,000 miles from the sun.
Earth is 46,000,000 miles from Mars.
Pluto is 3,720,000,000 miles from the sun.
Our sun is 25,800,000,000,000 miles from the
nearest star, Proxima Centauri.
Our galaxy, The Milky Way, is
600,000,000,000,000,000 miles in diameter (across).
Scientists do not want to work with large
numbers, so they make up new units to measure
distances in space with.
The two units used in astronomy are:
Astronomical Units (A.U.s)
Light Years
•Used to measure distances within the solar system.
• 1 A.U. =
the average distance between the Sun and
Earth.
• 1 A.U. =
93 million miles / 149.6 million kilometers
Distances Within the Solar System
Planet
Mercury
Distance from
Sun in kms
Distance in A.U.s
57,900,000
0.387
108,200,000
149,600,,000
227,900,000
0.723
1.000
1.524
Jupiter
Saturn
Uranus
Neptune
778,400,000
1,427,000,000
2,896,600,000
4,520,000,000
5.203
9.539
19.182
30.214
Pluto
5,899,900,000
39.439
Venus
Earth
Mars
•A light year is the distance light can travel in
one year.
• Light travels at the speed of 300, 000 km/s
(186,000 mi/s)
• 1 LY = 6 trillion miles (9.5 trillion kilometers)
6 x 10 12 mi (9.5 x 10 12 km)
•Used to measure distances outside our solar
system
Example: Distance between the sun and
another star or galaxy
Proxima Centauri is the closest star to our sun.
Proxima Centauri is 4.6 light years away.
How many miles away is Proxima Centauri?
• 4.6 light years x 6 trillion mi= 27.6 trillion miles away
(27,600,000,000,000 miles or 2.76 x 10 13 )
• Since Proxima Centauri is 4.6 light years away, it
takes 4.6 years for the light from Proxima Centauri to
reach us.
How can we tell how far away a star is?
PARALLAX = APPARENT CHANGE IN POSITION OF
AN OBJECT WITH RESPECT TO A MORE DISTANT
BACKGROUND.
•As Earth moves in its orbit, stars are viewed from
different angles and appear to move against the
background of more distant stars. The degree of
“apparent” motion can be converted into LYs.
•ASTRONOMERS MEASURE THE PARALLAX OF
NEARBY STARS TO DETERMINE THEIR DISTANCE.
(Used for stars w/in a few hundred LY)
•Degrees parallax can be converted to light years.
http://instruct1.cit.cornell.edu/courses/astro101/java/parallax/parallax.html
M13 is about 145 light-years in diameter,
and it is composed of several hundred
thousand stars.
M13 is 25,100 light-years away from Earth.
Use parallax to determine which
star is closer to Earth?
February
August
```