Star Properties studied so far…
•
•
•
•
•
•
•
Period
Mass
Eccentricity
Red shift
Velocity
Temperature (Wiens Law)
Composition
– which elements and how much of each
• Color
• Age
A star with a continuous spectrum shines through
a cool interstellar cloud composed primarily of
hydrogen. The cloud is falling inward toward the
star (and away from Earth). Which best describes
the spectrum seen by an Earthbound observer?
1) blue-shifted hydrogen emission lines
2) a red-shifted hydrogen continuum
3) red-shifted hydrogen emission lines
4) red-shifted hydrogen absorption lines
Velocity = distance / time
How do we measure the velocity of a star?
How do we measure the velocity of a star?
l0
l
z = red shift =
𝜆 − 𝜆0
𝜆0
How do we
measure the
velocity of a
star?
z=
𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦
𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 (𝑐)
or
Velocity = c × z
How do we measure the velocity of a star?
What do you get if your l0 Ca line at 393.3 nm in
the lab appears at 401.8 nm in the sky?
red shift =
401.8 − 393.3
393.3
= 0.0216
Velocity = cz
and the speed of light is 3 x 105 km/s
So the star is receding at a speed of
V = (3 x 105 km/s) x 0.0216 = 6,480 km/s!
How do we measure the velocity of a star?
What do you get if your l0 Ca line in the lab at
393.3 nm appears at 786.6 nm in the sky?
red shift = ____
Velocity = c x z,
and the speed of light is 3 x 105 km/s
So the star is receding at a speed of:
V = (3 x 105 km/s) x __ = ______ km/s
How do we measure the velocity of a star?
What do you get if your l0 Ca line at 393.3 nm in
the lab appears at 786.6 nm in the sky?
red shift =
786.6 −393.3
393.3
=1
Velocity = cz
and the speed of light is 3 x 105 km/s
So the star is receding at a speed of
V = (3 x 105 km/s) x 1 = 300,000 km/s
How do we measure the age of the universe?
We can measure velocity
We can measure distance
red shift =
786.6 −393.3
393.3
=1
Velocity = cz
and the speed of light is 3 x 105 km/s
So the star is receding at a speed of
V = (3 x 105 km/s) x 1 = 300,000 km/s
Star Properties studied so far…
•
•
•
•
•
•
•
•
Period
Mass
Eccentricity
Red shift
Velocity
Distance
Temperature (Wiens Law)
Composition
– which elements and how much of each
• Color
How do you measure the distance to a star?
Use 2 measurements of a star’s exact position in the sky,
taken exactly 6 months apart.
Hubble found the relationship between a galaxy's velocity (the radial
component, in a straight line) away from us (v) and its distance from us (d)
approaches a fairly linear one, which is known as Hubble's Law:
v = H0 x d
So H0 is the slope of that line, which he found to be 73.3 km/s/Mpc
Or about 3.26 million light years
dis tan ce
time 
velocity
So time = 1/H.
Use this graph to tell us the age of the universe?
Because we know that the slope of the line in the plot is
73 km/s/Mpc, we can simply write:
1Mpc s
1 year
3.09  10 km
T 


73km 3.156  10 s
1Mpc
 1.34  10 years  13.4 billion years
19
0
7
10
Life Cyclye of a Star
http://aspire.cosmic-ray.org/labs/star_life/hr_interactive.html
We can detect the velocity of a star through the Doppler
effect by
1) measuring the shift in distance of the star.
2) taking photographs six months apart.
3) applying the inverse square law of brightness.
4) measuring the shift in wavelength of a spectral line.
The light from the east limb (edge) of the
Sun is blue-shifted and the light from the
west limb is red-shifted. This is because
1) different kinds of atoms emit light at
the opposite edges.
2) the Sun is rotating.
3) the distance from the Sun to the
Earth changes.
4) the two sides of the Sun are at
different temperatures.
Most of the brightest stars in the sky are
1) relatively hot main-sequence stars that
are relatively close to the Sun.
2) relatively cool main-sequence stars that
are relatively far from the Sun.
3) relatively cool main-sequence stars
that are relatively close to the Sun.
4) giant stars and relatively hot main
sequence stars.