Astrophysics Discoveries of the 20th Century | Astrophysics Discoveries of the 20th Century

Astrophysics in the 20th Century

Hubble's Discoveries:

Edwin Hubble (1889-1953) is a central figure in the development of observational cosmology. From 1922 to 1936 Hubble solved four of the central problems in cosmology.

From 1922 to 1925 Hubble devised a classification system for galaxies by grouping them according to their content, distance, shape, size, and brightness. The galaxy classification system had become the Hubble morphological (The study of form, structure and the like. ) sequence of galaxy types. Previous to this, no satisfactory system existed, thus Hubble's powerful, yet simple scheme was an incredibly important scientific achievement.

 By his discovery of Cepheids in 1924 and with parallel work in M33 and M31, Hubble proved decisively that these stars are outside of our galaxy. Thus bringing to a close the debate of whether there are other galaxies in the Universe beyond the Milky Way.

 From 1926 to 1936 Hubble spent studying the distribution of galaxies and thus determining them to be homogeneous in distance. Hubble's 1934 paper was so detailed and convincing that it brought the problem of the mean galaxy distribution, which was by then more than 100 years old, to a close. Attempts were then made to measure the radius of curvature of space.

 By 1936 Hubble, together with Milton Humason, showed beyond doubt the linear velocity-distance relationship (redshift-distance relationship). This led to the formulation of the Empirical Redshift Distance Law for galaxies, now days known as Hubble's Law.The law states that the greater the distance between any two galaxies, the greater their relative speed of separation ie Recessional Velocity = Hubble's Constant  distance (mpc). An integral part of this equation is "Hubble's Constant" which refers to how fast the velocities of the galaxies increase with their distance from the Earth. To this day "Hubble's constant has not been given a concrete value. Currently it is believed that it lies in the range from 50 km/sec/mpc to 100 km/sec/mpc. Hubble's Law helped astronomers to determine the age of the Universe and proved the concept of an expanding Universe. This discovery later resulted in the formulation of the Big Bang theory.

The above has briefly outlined Hubble's major contributions into the field of astrophysics, but he produced other works of influence as well. Among them:

 He discovered the asteroid 1373 Cinciannati on August 30, 1935

 He also obtained extensive evidence that the laws of physics outside our galaxy are the same as on Earth.

 He was also instrumental in the design of the Hale Telescope, which was set up on Mount Palomar Observatory.

 He solved the problem of the source of radiation and the nature of the spectra of diffuse nebulae, recognizing the difference between emission and reflection nebulae and proving the source of radiation.

 Measured for the first time the surface brightness profiles of E galaxies and thus proved the basic model of the profile laws.

 Hubble began the detailed study of the stellar content of the nearby galaxies.

 He discussed the sense of rotation of the spiral arms of the individual galaxies.

________

20. Century Discoveries:
1905. : Albert Einstein introduces his theory of special relativity and other key theories in physics on the photoelectric effect and Brownian motion .
1910. : Ejnar Hertzprung and Henry Norris Russel publish their Hertzsprung-Russel diagram. They also develop the theory of the evolution of stars.
1912. : Henrietta Leavitt showed that there is a relation between "absolute luminosity" and the "period of pulsation" through her studies of Cepheid variable stars .
1917. : Karl Schwarzschild suggests the existence of black holes that swallow matter.
1920. : Arthur Eddington worked out the basic relationships between the mass, radius and central temperature of stars. This showed that the interior of the stars were at temperatures above a million degrees Kelvin.
1922. : Niels Bohr's new quantum theory resulted in establishing that the make up of the atmospheres of stars are mainly hydrogen and helium.
1930. : Robert Trumpler discovered interstellar dust. This resulted in subsequent studies of the space between stars and how new stars constantly formed from the interstellar dusts and gases.
1930. : Hans Bethe and collaborators discovered the "proton-proton" and "carbon-nitrogen cycle"
1934. : Karl Jansky had detected radio emissions from the Milky Way
1940. ; John Hey detected radio emissions from the Sun as well as the first radio point sources.
1955. : Fred Hoyle and Martin Schwarzchild calculated a model for the cycle of life of a star after it has used up all of the hydrogen at its center.
1957. : Space Age dawns.
1965. : Arno Penzias and Robert Wilson made the discovery of microwave background radiation
1967. : Bob Wagoner, Willy Fowler and Fred Hoyle proved that helium, deuterium and lithium were originated form the nuclear reactions that occurred in the early stages of the Universe.
1970. : Arno Penzias and collaborators made the discovery of carbon monoxide (CO) in outer space.
1970. : Launch of the Uhuru satellite marked a great step for x-ray astronomy
1974. : Stephen Hawking says black holes can radiate energy slowly.
1980. : Space shuttles take off
1980. : Luis Alvarez and son Walter Alvarez say a large meteorite, asteroid or cote impact caused mass extinction of dinosaurs and other animals 65 million years ago. The controversial theory will become widely accepted and the impact crater later identified.
1990. : Fred Hoyle suggests a link between influenza pandemics and sunspot outbreaks.

_______

Fate of the universe:

The "Big Bang" theory is the currently widely accepted theory that describes the origin and evolution of the Universe. However, the future is not so clear-cut and at the moment there are three contradictory views on the eventual fate of the Universe.

One is that the expansion that was started by the Big Bang could go on forever.

The other is what is commonly known as "The Big Crunch."

Essentially, this theory suggests that the average density of the universe is enough to stop its expansion and begin a cosm-wide contraction. It is yet undecided what the results of the Big Crunch could be, with some astrophysicists theorising that all the matter and space-time in the universe will collapse into a "dimensionless singularity" i.e. mass will have no volume and space and time stop. Others argue that we live in an "oscillatory universe" that, after the Big Crunch, will start to expand once more and so any endpoint may progress to a new universe.

The third is what is known as " The Heat Death" (i.e. heat dies): the universe, with its continued expansion, flies more and more apart, so that all matter and energy will dissipate and all will become the ultimate cold void.

However, the evolution and ultimate fate of the Universe depends on our ability to determine the relative and total densities (i.e. the amount of matter) of radiation, baryonic matter, dark matter and dark energy and that is one of the main challenges facing cosmologists today.

____

Bibliography:

 "Astrophysics and Cosmology: the Golden Age." Internet WWW page at URL http://physicsweb.org/articles/world/12/12/16

 "Big Bang." Internet WWW page at URL www.crystainks.com/bigbang.html

 "Distant exploding stars foretell the fate of the Universe: it will expand forever." Internet WWW page at URL www.lbl.gov/Science-Articles/Archive/expand-forever.html

 "Edwin Hubble." Internet WWW page at URL www.pbs.org/wgbh/aso/databank/entries/bahubb.html

 "Edwin Powell Hubble." Internet WWW page at URL http://en.wikipedia.org/wiki/Edwin_Hubble

 "Edwin Powell Hubble." Internet WWW page at URL http://www.edwinhubble.com/hubble_bio_002.htm

 "Foundations of Big Bang cosmology." Internet WWW page at URL http://map.gsfc.nasa.gov/m_uni_101bb2.html

 "The Final fate of the Universe." Internet WWW page at URL www.pd.astro.it/E-MOSTRA/NEW/A9003DES.HTM

 "The pocket Macquarie Dictionary." General Editor: JRL Bernard. NSW, Australia: The Jacaranda Press, 1997.

 "The ultimate fate of the Universe." Internet WWW page at URL http://en.wikipedia.org/Ultimate_fate_of_the_Universe

 "Will this Universe ever end"." Internet WWW page at URL www.closertotruth.com/topics/universemeaning/213/213transcrips.html

 Internet WWW page at URL http://oposite.stsci.edu/pubinfo/spacecraft/Primer/Hubble_Expand.htm

 Internet WWW page at URL www.howstuffworks.com

 Internet WWW page at URL www.spacetoday.org/History/MillenniumTimeline/20thCenturyLate.html