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Hubble's Law

Hubble's Law

    

Origin

    American astronomer Edwin Hubble created Hubble's Law. You may have heard the name Hubble from the Hubble Space Telescope telescope. During the 1920s, Hubble noticed (through the Hale telescope at Mt. Palomar) that the galaxies were moving away from us. Their recession velocity was greater the further away they were from us. He discovered that the recession velocity and distance to that galaxy gave a fairly linear slope. That is Hubble's Law.



Explaining Hubble's Law

    Hubble's Law is v = H0D (recessing velocity = Hubble constant × proper distance). To find the Hubble constant, astronomers need to find out how fast the astronomical object moves away from us and the distance to it. 


Vocabulary: 

  • Doppler Effect - change in wavelength.
  • Redshift - an increase in a wavelength. 
  • Absorption lines - the patterns of light emitted from an object. 
  • Recession Velocity - the rate at which an astronomical object moves away from us.

Finding Recession Velocity


   You use the Doppler effect to find the recession velocity. The light from stars and galaxies moves away from us, changing the wavelength- astronomers call this redshift. To measure redshift, astronomers need to find the patterns in absorption lines. When redshift changes the wavelength and light, astronomers measure how much it has shifted and find the recession velocity. 


Finding the Proper Distance

    If an object is outside our galaxy, scientists compare the object's actual brightness to the brightness of what we see from Earth to find the distance. Canadian astronomer Wendy Freedman used red giants to find the Hubble constant. Red giant stars are stars at the end of their life. Their core jumps to 100 million degrees Fahrenheit while their brightness drastically drops. Scientists find the maximum brightness of a dying star nearby and use it for faraway galaxies. Using the red giant method, Freedman got a Hubble constant of 69.8 km/s/Mpc. The Hubble constant has multiple answers, from 69.8 km/s/Mpc to 74 km/s/Mpc, and if there is that big of a gap, that means something significant is missing. 

The simple version of all this is four words. The universe is expanding.

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