Download Your Star: _____________________ d = 1 / p

Astronomy C10, Section 123/114
Worksheet 6
Constructing a Temperature-Luminosity Diagram
Vega (example)
Your Star: _____________________
1. Distance
d=1/p
3. Temperature
If distance is in parsecs
and parallax is in arcsec
Write down your star's parallax:
3000 Å
λpeak = ______
Compare this with the Sun:
Now immediately calculate the distance:
λpeak
7
d = ________
parsecs
λpeakSun
exact: 7.69 pc
L
b×d
bSun
=
2
W/m
3
10 W/m
2
= 3 × 10-11
Compare its distance with the Sun's*:
d
=
dSun
7
pc
5 × 10-6 pc
=1.4 × 10
6
Finally, compare its luminosity with the Sun.
L
LSun
= ( 3 × 10-11 ) × ( 1.4 × 106 )2
=
=
3 × 10-11 ×
60
Finally, convert this to Kelvins.
10000 K
) × 6000 K = ______
Once you have the temperature and the luminosity, plot
your star on the board.
exact: 9630 K
Compare its brightness with the Sun*:
3×10-8
3000 Å =
0.6
5000 Å
T
TSun = 1 / 0.6 = 1.7
T = ( 1.7
-8
3×10
b = ________
W/m2
=
Compare the temperature with the Sun:
2
Write down your star's apparent brightness.
Round it to one significant digit.
b
1 / λpeak
Write down the wavelength at which the
star's spectrum peaks. Round to one
significant digit.
0.13 arcseconds
p = ________
2. Luminosity
T
2 × 1012
exact: 53
* Normally, we compare the brightness to another (distant)
star instead of the Sun, since the Sun is obviously much
brighter and closer than other stars. We use the Sun here
because we want to use solar units throughout.
4. Radius (optional)
R2
S
L / T4
Write down the luminosity and temperature
you calculated, as compared to the Sun.
L = 60
LSun
T = 1.7
TSun
Use these to calculate the surface area,
compared to the Sun.
S =
(
SSun
60
) / ( 1.7 )4
= 8
Now calculate the radius, compared to the
Sun.
R = (
8
)1/2 = 2.8
RSun
exact: 2.6
5. Classification (optional)
What is the spectral type of the star? (See CS-156) A
What is the evolutionary class of the star? (See CS-160)
main sequence
For comparison:
-2
REarth ≈ 10 RSun
-1
RJupiter ≈ 10 RSun
AU ≈ 200 RSun