Unit 4 - Exploring Our UniverseChapter 10 - Stars, Galaxies, and the Universe

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Below is a rendition of what our galaxy, the Milky Way, looks like:

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A galaxy is a collection of hundreds of billions of stars that are heldtogether by gravitational forces.

Each star in a galaxy can be classified according to the following:

Colour

Luminosity (brightness)

Surface Temperature

Size/Mass

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The Life Cycle of Stars

What cycle a star goes through is determined by what mass the star firstdevelops after its formation in a nebula. There are three main life cycles ofstars:

1 Low Mass - becomes a white dwarf → black dwarf.

2 Medium Mass - becomes a red giant → white dwarf → black dwarf.

3 High Mass - becomes a supergiant → supernova → a neutron staror a black hole.

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Definitions:

White Dwarf - A star, around the size of the earth, that hasundergone gravitational collapse. It begins hot and white and endscold and dark (black dwarf).

Supernova - The explosion of a high mass star, during which thestar’s luminosity increases by roughly 20 magnitudes. This explosionforms the heavier elements, such as iron and nitrogen, and spewsthem outward into space.

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Neutron stars - One of the ending phases of a high mass star. Thecompression and collapse of the core forms neutrons. These stars aremade of the densest material known. A star 10 to 40 times the Sun’smass will become a neutron star.

Black holes - One of the ending phases of a high mass star. A regionof space where gravitational forces are so strong that nothing, noteven light, can escape. After exploding as a supernova, the star’s coreis under so much gravitational force that nothing can stop itscollapse, not even the formation of neutrons. A star greater than 40times the mass of the Sun will become a black hole.

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The Universe and its Origins

What is the Universe? The Hubble Space Telescope, an orbiting telescope,has taken images of galaxies beyond our own Milky Way galaxy. There arearound 10,000 visible galaxies in just this one image.

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The Universe - all existing matter and space considered as a whole. Theuniverse is believed to be at least 10 billion light years in diameter,containing an estimate of 100 to 200 billion galaxies.

Hubble’s picture of Andromeda - our closest galaxy.

Watch: “Hubble’s sharpest and biggest picture taken of Andromeda”Unit 4 - Exploring Our Universe 8 / 1

The First Moment - The Big Bang

Edwin Hubble (early 20th century astronomer) made two discoveries thatchanged astronomy:

1 There are many galaxies exist beyond the Milky Way.

2 Almost all galaxies are moving away from each other.

He discovered the second fact through an understanding of the wavenature of light and the colours that we see.

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The conclusion: The universe is expanding. Therefore, if we rewind theclock of time, we must find that the universe was closer together. In fact,if we continue on rewinding, we will eventually arrive at the “firstmoment”, where all matter and energy was confined in one infinitely densepoint.

The Big Bang Theory: first proposed by the Belgian priest and physicistGeorges Lemaıtres, states that the universe formed when an infinitelydense point suddenly and rapidly expanded in a single moment.

Watch: “Stephen Hawking - The Big Bang”Unit 4 - Exploring Our Universe 10 / 1

Evidence for the Big Bang

Expansion of the universe - the “redshift” of galaxies.

Microwave Background - if the Big Bang occurred, the early universemust have been extremely hot and filled with short-wave gamma rayradiation. As the universe expanded, it cooled and the wavelengthmust have lengthened to X-rays, ultraviolet, visible light, infrared, andmicrowaves. This predicted Cosmic Microwave Background has beenaccurately measured by orbiting detectors.

Extra fact: The age of our universe (∼ 13.7 billion years old) is based onthe observed expansion of the universe. Knowing the constant rate ofexpansion allows scientists to determine how long ago the universekicked-started into existence. The observed microwave backgroundradiation supports this estimation.

View: “Timeline of the Universe”

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