Transcript How Far To That Star?

How Far
To That Star?
Parallax
and the
Standard Candle Method
Two Ways to find How Far
The Parallax Method uses the change in the direction
to the star as the Earth orbits
to find the distance to a relatively nearby star. (less
than 250 parsecs, or 815 LY)
It uses Triangulation to find the distance.
The Standard Candle Method compares Apparent
magnitude to Absolute Magnitude
to find the distance to more distant stars and even
other galaxies
It uses the Inverse Square Law.
A Third Method-Red Shift
Another way to find the distance is to measure
the amount of red-shift in the star’s or galaxy’s
light.
• It is based on the assumption that due to the
Big Bang the farther away an object is the
greater its red-shift must be.
• It is only usable for extremely distant objects
(millions of LY away!).
Astronomers use this method as a last resort due
to it being based on an assumption.
Using Parallax
To use parallax you first have to
find the measure of the angle
To do this you
Measure the difference in
direction to the star compared to
a reference object at the
maximum displacement in
degrees (P).
( Should be 6 months apart)
Next calculate the distance using basic
trigonometric function: Tangent.
The distance to the star will be the baseline, “B”
(in this case B = 2 AU or 300,000,000 km, the
average distance from the Earth to the Sun).
Divided by the tangent of “P.”
(see diagram & formula on board)
Parallax and the Parsec
To make using parallax easier Astronomers invented
the measure of distance called a parsec.
A parsec is defined as the distance at which a star will
have a parallax of one second of an arc as seen from
Earth.
That is a “parallax-arc second” or “parsec.”
The distance that this is true is about 3.26 LightYears, thus
1 parsec = 3.26 LY
If the parallax angle is given in arc-seconds it is…
(Formulae on Board)
Practice
Find the distance to the following imaginary stars.
(Most are too close, only the last is realistic.)
Example: mP = 20°
1. mP = 15°
2. mP = 8°
3. mP = 1.2°
4. mP = .08°
5. mP = .00016°