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Lecture 2

ASTA01H3 Lecture Notes - Lecture 2: Barred Spiral Galaxy, Observable Universe

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Kristen Menou

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- Although clouds of gas are thinner than the best vacuum produced in laboratories on earth, it is
these clouds that give birth to new stars
- The Sun formed from gas clouds 4.56 billion years ago
- The milky way can be defined as a cloudy band of stars
- Our galaxy is called the milky way, it has a small spindle-like structure in the centre called the
galactic bar
- Our galaxy contains over 100 billion stars, (10^11 stars or 100 giga stars)
- The disk of our galaxy is roughly 80,000 ly in diameter
- Our galaxy is only one of a hundred billion (10^11 or 100 giga) galaxies scattered through the
observable universe
- The local group includes a few dozen galaxies such as Andromeda or the Milky Way which measure
roughly 17 million ly in diameter
- There are 3 main categories for the shape of a galaxy; it can either be a spiral, elliptical (smooth
without arms) or irregular
- Clusters of galaxies are grouped into superclusters- clusters of clusters and the superclusters are
linked to form long filaments and walls outlining voids that seem nearly empty of galaxies
- Each of the billion of galaxies contains billions of stars
- (10^11) x (10^11) = 10^22 stars = 10,000,000,000,000,000,000,000
- 10% or more of stars probably have families of planets like our solar system, and on some of those
thousand billions of billions of planets liquid-water oceans and protective atmospheres may have
sheltered the spark of life
- Our current understand of the formation of the universe leads us to believe that it has been 13.7
billion years (13.7 Gy) since the Big Bang, the instance the universe commenced itself, and perhaps
the beginning of time itself
- *Keep in mind that live you live on a planet, a moving platform*
- Earth rotates on its axis once a day
- Constellations originated in Mesopotamia, Babylon, Egypt and Greece beginning as 5000 years ago
- Today, only 48 of these ancient constellations are still in use
- In 1928, the international Astronomical Union (IAU) established 88 official constellations with
clearly defined permeant boundaries that together cover the entire sky
- A constellation now represents not a group of stars but a section of the sky
- The Big Dipper is an asterism which a less formally defined grouping of stars
- Another asterism is the Great Square of Pegasus that include three from Pegasus and Alteraz
- The brightest stars in a constellation are usually given Greek letters in order of decreasing
- Most individual star names derive from ancient Arabic
- Astronomers measure the brightness of stars using the magnitude scale, measured in Apparent
Magnitude (Mv)
- Ancient astronomers divied the stars into six brightness groups, the brightest were called first-
magnitude stars
- The larger the number, the fainter the star
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