Determining distances of objects in space from Earth
Three
primary means for determining distances:
1. Spectroscopy and the Doppler Effect.
a.
for
objects that are moving away or toward us at high speeds (objects outside our
galaxy)
b.
for
objects that are emitting some form of ER.
c.
If
an object that is emitting light (any form) is moving away from us at high
speeds, the light waves it emits will appear to be elongated. When compared to a normal spectrum, all
known spectral lines will be shifted toward the “red” end of the spectrum. Hence, red shifted. The degree to which the waves are red
shifted tells us the speed at which the object is traveling. Through mathematics, we can extrapolate its
distance. The opposite (moving towards
us) is blue shift.
2. Absolute vs. Apparent Magnitude
a.
Apparent magnitude = a measure of how bright a star appears to be to an
observer on Earth.
b.
Absolute magnitude = the actual luminosity of an object. It is determined through spectroscopy by
finding temperature and size of a star.
The bigger, hotter stars are brighter.
c.
Magnitudes are measured in numbers, the smaller the number, the greater
the brightness. For example; our sun’s
apparent magnitude is –26.7. It’s
absolute magnitude is 4.8; an average star.
Sirius has an apparent magnitude of –1.45; the brightest star in the
sky.
d.
Absolute magnitude is the
apparent magnitude a star would have if it were placed 32.6 light years
away. In other words, if a star’s
apparent magnitude and absolute magnitude were identical, how far away would
that star be? If its apparent magnitude
is greater than (smaller number) its absolute magnitude would the star be
further or closer than 32.6 light years?
e.
Sirius has an apparent magnitude 10 times greater than Antares. Yet Antares has a luminosity 250 times
greater than Sirius. What does that
tell us about their distances from Earth?
f.
The brightest stars (apparent magnitude) are considered first-magnitude
stars. The faintest stars, ones that
can barely be seen with the unaided eye are sixth-magnitude stars. The difference in brightness between
magnitudes is an order of 2.5. In other
words, a first-magnitude star is 2.5 times brighter than a second-magnitude
star. How many times brighter is a first-magnitude than a sixth-magnitude star?
g.
Cepheid variables – astronomers use these pulsating stars and the
principle of magnitudes to determine distances more accurately. For extra credit homework, explain this
concept.
3. Parallax
As the Earth orbits the sun it moves
large distances in space. In six
months, the Earth is about 300,000,000 kilometers from its starting point. When observing a close star, that star would
have appeared to move, when in actuality we moved. The apparent shift in distance is inversely proportional to its
distance from the Earth. In other
words, if the star appeared to move a greater distance over a period of time
than another star, it is closer than that other star.
Limitation:
can only be used for close stars, usually under 300 light years away.
Three
main units used for stellar distances:
a.
Light year = the distance light travels in one year
b.
AU (Astronomical Unit) = the average distance the sun is from the earth
(150,000,000 km)