Astronomy 105 Laboratory Lab 10

Lab 10 Hubble’s Law

Edwin Hubble (1889-1953) 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!

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

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!!

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

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

Important: Do not write or mark on the Hubble Handout

Sample Galaxies Distance images Recessional Velocity spectra

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

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

Measuring Recessional Velocity redshifted H line 388.87 nm 501.57 nm The H line has a rest wavelength of 396.85 nm but is redshifted in a galaxy’s spectrum. Find the redshifted wavelength of the H line (λ) Measure distance (mm) between the 388.87 nm line and the H line Convert the measured distance from mm to nm using the plate scale and then add 388.87 nm. Recessional Velocity λo = 396.85 nm (laboratory wavelength of the H line) C = 300,000 km/sec c (λ – λo) λo v =

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

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

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

End

Lab Quiz 10

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

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

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

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

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

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