1. Astronomy 105 Laboratory
Lab 10

2. Lab 10
Hubble’s Law

3. Edwin Hubble ( )
Measured distances to nearby galaxies using Cepheid variables
Galaxies are islands of stars
Developed a classification scheme for galaxies.
Discovered the Expansion of the Universe
Space telescope named after him!

4. Hubble’s Law
Hubble Measured distances to nearby galaxies using Cepheid variables
Other astronomers measured galaxy velocities (redshifts)
Hubble Diagram
Redshifts

5. The Universe is Expanding!!
Recessional Velocity is Proportional to Distance
V = 0
2800
2100
1400
700
km/sec
10 Mpc
20 Mpc
30 Mpc
40 Mpc
Milky Way A B C
V = 0
2800
2100
1400
700
km/sec
10 Mpc
20 Mpc
30 Mpc
40 Mpc
Milky Way A B C
Alien’s Galaxy
The Universe is Expanding!!

6. Hubble’s Law v = Ho d y = mx + b Hubble’s Law
Hubble Diagram
Equation of a Straight Line
y = mx + b
y - recessional velocity(v) x - distance(d)
m = slope = Hubble’s Constant (Ho)
Hubble’s Law
v = Ho d

7. Finding a Galaxy’s Distance Hubble’s Law
Hubble Diagram
v = Ho d
d = v / Ho
To Find Distance: Measure recessional velocity (red shift)
132 Mpc

8. Important: Do not write or mark on the Hubble Handout

9. Sample Galaxies
Distance
images
Recessional Velocity
spectra

10. Measuring Distance
Elliptical galaxies - same physical size (0.032 Mpc)
Distances inversely proportional to angular size

11. Measuring Distance 0.032 Mpc d = θ S = r θ r = S / θ
Measure to nearest 0.5 mm and convert to radians
θ in radians
r = S / θ
1.0 mm

12. Measuring Recessional Velocity
redshifted H line nm nm
The H line has a rest wavelength of nm but is redshifted in a galaxy’s spectrum. Find the redshifted wavelength of the H line (λ)
Measure distance (mm) between the nm line and the H line
Convert the measured distance from mm to nm using the plate scale and then add nm.
Recessional Velocity
λo = nm (laboratory wavelength of the H line)
C = 300,000 km/sec
c (λ – λo) λo
v =

13. Hubble Diagram Procedure - plot data - draw best fit line
v = Ho x d
Hubble Diagram x
Procedure
- plot data x
- draw best fit line
rise
- find slope (Ho) x
slope = rise/run x
run x

14. Estimating the Age of the Universe
60 mph, 120 mi
Milky Way
M100
t = 0 
t = present
Hubble’s Law v = Ho d

15. Example: Ho = 60 km/s/Mpc
conversion
conversion
t = 16 billion years

16. End

17. Lab Quiz 10

18. Stars within a cluster have certain properties in common
Stars within a cluster have certain properties in common. Which one of the following is not a common property?
distance
age
color
chemical composition 23

19. The age of a star cluster is determined from the ______.
mass of the brightest giant star
main-sequence turnoff point
mass of the coolest main-sequence star in the cluster
reddest stars in the cluster 28

20. How does interstellar dust affect the distance measurements to stars?
it makes them appear cooler
it makes them appear hotter
it makes them appear farther away than they actually are
it make them appear closer than they actually are 28

21. Edwin Hubble discovered most galaxies show a redshift in their spectra
Edwin Hubble discovered most galaxies show a redshift in their spectra. This indicates the galaxies are
stationary in space relative to the Milky Way.
moving away from the Milky Way.
moving toward the Milky Way.
cooler than Milky Way.
warmer than the Milky Way. 30

22. The most distant galaxies have the ________ redshifts.
smallest
largest 16

23. Converting Angles from Arc Seconds to Radians
Plate Scale
𝟏𝟓𝟎 𝒂𝒓𝒄 𝒔𝒆𝒄=𝟏𝟖 𝒎𝒎
𝑷𝒍𝒂𝒕𝒆 𝑺𝒄𝒂𝒍𝒆= 𝟏𝟓𝟎 𝒂𝒓𝒄 𝒔𝒆𝒄 𝟏𝟖 𝒎𝒎 =𝟖.𝟑 𝒂𝒓𝒄 𝒔𝒆𝒄 𝒎𝒎
Type equation here.
Unit Conversion (mm to radians)
𝜽= 𝒊𝒎𝒂𝒈𝒆 𝒔𝒊𝒛𝒆 𝒎𝒎 ×𝑷𝒍𝒂𝒕𝒆 𝑺𝒄𝒂𝒍𝒆 ( 𝒂𝒓𝒄𝒔𝒆𝒄 𝒎𝒎 ) 𝟐𝟎𝟔,𝟐𝟔𝟓 𝒂𝒓𝒄 𝒔𝒆𝒄 𝒓𝒂𝒅
Example: Measured galaxy size is 1.0 mm
𝜽= 𝟏.𝟎 𝒎𝒎×𝟖.𝟑 𝒂𝒓𝒄𝒔𝒆𝒄 𝒎𝒎 𝟐𝟎𝟔,𝟐𝟔𝟓 𝒂𝒓𝒄 𝒔𝒆𝒄 𝒓𝒂𝒅 =𝟒.𝟎× 𝟏𝟎 −𝟓 𝒓𝒂𝒅