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The Expanding Universe

Part 3 : T he S pectra f rom G alaxies A re

Redshifted.

In 1 912, A merican a stronomer

Hδ

Hγ

Hβ

3C 273

Redshift

Vesto S lipher b egan t o o bserve m ore d istant

objects u sing a s pectrometer, a nd h e n oticed

that m ost o f t hem h ad a d istinct s hift t oward

the r ed e nd o f t he s pectrum. H e r ecognized

Blue

Red

this a s a D oppler s hift c aused b y t he m otion

of t he o bjects. A r edshift m eans t hat t he

source o f l ight i s m oving a way f rom t he

observer.

Comparison spectrum

Figure 5

s hows a n e mission s pectrum f rom

Hδ

Hγ

Hβ

the q uasar c alled 3 C 2 73. W e s ee t hat t he

400 nm

500 nm

600 nm

emission l ines d ue t o h ydrogen a re s hifted

to t he r ed e nd o f t he s pectrum. T he l arger

the r edshift i s, t he g reater t he r elative

Figure 5

R edshifted s pectrum f rom q uasar 3 C 2 73

motion.

Edwin H ubble e xtended t his o bservation t o m ore o bjects, a nd f or s everal o bjects p lotted t he r edshift v ersus t he d istance t o t he

object. T ry i t f or y ourself. I n

T able 1

, u se t he r edshift o f t he s pectral l ines t o d etermine t he s peed o f e ach g alaxy. T o d o t his, u se a

ruler t o a lign t he l eftmost s pectral l ine p rovided w ith e ach g alaxy w ith t he s peed s cale a t t he b ottom o f

the t able.

Table 1

Activity 2

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