Brief Astronomy Notes

The age of the universe is 13.77 billion years +/- 59 million years.
The current observable size of the universe is about 92 billion light years -- 45.7 ly in any direction.

One light year is the distance light travels in one year. Light travels 286,000 miles per second (300,000 km/s), 670,616,629 mph - 6 trillion miles per year (5.8786E+12).

There are an estimated hundreds of billions to 2 trillion galaxies in the (known - visable) universe with a few million stars (dwarf galaxies) to one hundred trillion stars (giant galaxies) each. The Milky Way contains an estimated 100 - 400 billion stars.

Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3000 - 300,000 light years) and separated by distances on the order of millions of parsecs (or megaparsecs).

The majority of galaxies are gravitationally organized into groups, clusters and superclusters. The Milky Way is part of the Local Group of more than 50 galaxies, which is dominated by it and the Andronmeda Galaxy, and is part of the Virgo Supercluster.

The Milky Way has a diameter of at least 30,000 parsecs (97,500 ly) and is separtated from the Androneda Galaxy, its nearest neighbor, by 780,000 parsecs (2.5 million ly).

Astronomers classify stars by color and luminosity plotted to a chart called the Hertzsprung Russell Diagram. The majority of stars on the diagram lie along the Main Sequence. Stars evolve onto and then off the relatively narrow band of the main sequence during their lifetime.

Classifications of the main sequence stars include O, B, A, F, G, K, M, L and T. The Sun is classified as a 'G2' star.

Medium, Sun size, main sequence stars Hydrogen => Helium -- four Hydrogen atoms --> one Helium. The lifespan of stars the size of the Sun is thought to be 10 billion years.

Red Giant Phase: The outer layer of mid-sized stars collapse until temperatures become hot enough to fuse Helium into Carbon (6C). The pressure of fussion provides an outward thrust that expands the star to many times its original size, forming a red giant. Eventually it will eject its nebula and fade away from a white dwarf to a black dwarf star.

Massive Stars (8 - 10 Sols) continue to fuse heavier elements from Oxygen (8O) to Iron (26Fe). Iron is the most stable element in the Universe. Within hundres of millions years they collapse to form super-dense white dwarfs. Only the most massive stars explode in supernova, spewing the heavier elements in their cores across the galaxy. The heavier elements are made only in the last two seconds of a massive star's exisitance.

The remaining core may form a neutron star.

Smaller stars (1/4 of Sol) collapse directly into white dwarfs and eventually cool to become black dwarfs. The life cycles of smaller stars are thought to last tens of billions of years (Class K stars 17 - 70 billion) to over a trillion years (Class M amd smaller stars).

Red and brown dwarfs are substellar objects not massive enough to sustain hydrogen fusion.

History of Astronomy

Eratosthenes c.276-c.195 BCE -- Librarian at the Library of Alexandria. Accurately calculated the circumference of the Earth as well as the tilt of the Earth's axis.

Claudius Ptolemy c.100 - 170 CE -- developed arithmetical techniques for calculating astronomical phenomena utilizing deferents and epicycles:

1 Moon
2 Mercury
3 Venus
4 Sun
5 Mars
6 Jupiter
7 Saturn
8 Fixed Stars
9 Primum mobile

Nicolas Copernicus 1473 - 1543 CE -- Formulated a model of the universe that placed the Sun rather than Earth at the center of the universe.

Tycho Brahe 1546 - 1601 CE -- Astronomer, and writer known for his accurate and comprehensive astronomical observations.

Johannes Kepler 1571 - 1630 CE -- Best known for his laws of planetary motion.

Galileo Galilei 1564 - 1642 CE -- The "father" of observational astronomy.

Edwin Hubble 1889 - 1953 CE -- Hubble provided evidence that the recessional velocity of a galaxy increases with its distance from the Earth, a property now known as "Hubble's law". The Hubble–Lemaître law implies that the universe is expanding.

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