History and Discovery of Milky Way Galaxy.
                     DISCOVERY AND HISTORY 
OF MILKY WAY GALAXY
INTRODUCTION
• Milky Way Galaxy, large spiral system consisting of several hundred billion stars, one of 
which is the Sun. It takes its name from the Milky Way, the irregular luminous band of 
stars and gas clouds that stretches across the sky as seen from Earth.
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GALAXIES
• Until about 100 years ago, the Milky Way galaxy was thought to be only a few thousand 
light years across, and most thought it was the entire universe. Other galaxies had been 
discovered, but they were thought to be smaller objects within our galaxy.
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FIRST GALAXY 
• The first galaxies were identified in the 17th Century by the French astronomer Charles 
Messier, although at the time he did not know what they were. 
• Messier, who was a keen observer of comets, spotted several other fuzzy objects in the 
sky which he knew were not comets.
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NEBULA
• In 1923 Hubble was studying the Andromeda "Nebula" (now called the Andromeda 
Galaxy), when he realized that one of the objects, he was observing was in fact a 
Cepheid variable star.
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CEPHEIDS
• Cepheids are stars whose brightness changes periodically over time, and they had been 
discovered by the American astronomer, Henrietta Leavitt, in the early 1900ʼs. 
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PERIOD-LUMINOSITY
• Leavitt found what is now known as the Period-Luminosity (P-L) relationship, a link 
between the luminosity (brightness) of a Cepheid and its period. 
• By measuring the period of a Cepheid (by observing its brightness changes over several 
days or weeks), the P-L relationship can be used to determine its actual brightness. 
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HUBBLEʼs LAW
• In his paper, Hubble plotted a graph of the velocity of galaxies against their distances. 
This plot showed that most galaxies are moving away from us, but also that the speed 
at which they are moving away is proportional to their distance distant. This became 
known as Hubbleʼs Law.
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RECENT ADVANCES
• Recent advances in astronomy have now narrowed down the value of the slope of the 
graph called “Hubbleʼs constant”, and results are converging to an accepted value of 
~65 km/s/Mpc.
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MOTIONS OF ROUGHLY 1 BILLION STARS
• On April 25, 2018, a European spacecraft by the name of Gaia released a staggering 
quantity of information about the sky. Critically, Gaia’s years-long data set described the 
detailed motions of roughly 1 billion stars. 
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A VIOLENT YOUTH
•  With Gaia, astronomers have found direct evidence of cataclysmic collisions. 
Astronomers assumed that the Milky Way had a hectic youth, but Helmer Koppelman, 
an astronomer, used the Gaia data to help pinpoint specific debris from one of the 
largest mergers.
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