Galaxies
Galaxies A galaxy is a huge collection of billions of stars, gas, and dust held together by gravity There are three main types of galaxies. Spiral, Elliptical, and Irregular. There are 80-220 billion galaxies in the universe
The “Discovery” of Galaxies At the beginning of the 20th century, what we now call spiral galaxies were referred to as “spiral nebulae” and most astronomers believed them to be clouds of gas and stars associated with our own Milky Way. The breakthrough came in 1924 when Edwin Hubble was able to measure the distance to the “Great Nebula in Andromeda” (M 31, at right) and found its distance to be much larger than the diameter of the Milky Way. This meant that M 31, and by extension other spiral nebulae, were galaxies in their own right, comparable to or even larger than the Milky Way. Edwin P. Hubble (1889-1953)
Spiral Galaxies Pinwheel shape Huge, about 32,000 to 160,000 light years across Large amounts of gas and dust Young and old stars present New stars forming Revolve quickly around a central bulge, looks like a thin disk from the side with a halo present
The Nuclear Bulge of M31 Young stars have formed along the foreground spiral arm. M31’s two satellite galaxies M32 and NGC 205, both dwarf elliptical galaxies, are in the bottom center and upper right. (NOAO/AURA Photos)
Examples of Spirals
Elliptical Galaxies Round to oval shape Small amounts of cool gas and dust Massive, 6.5 million light years across Almost no new stars Rotate as a whole with each star having a unique orbit Mainly old stars present Bulge but no disk, halo present
Irregular Galaxies No regular shape Very large amounts of gas and dust Many new stars present with mostly young blue stars Huge, 3,200 to 32,500 light years across Halo is present, may show signs of a bulge Unsure of movement
Clusters of Galaxies Rather than occurring individually in space, galaxies are grouped in clusters ranging in size from a few dozens to thousands of galaxies. The Coma Cluster, shown at right, is 300 million light years from the Milky Way and contains more than 1,000 (and possibly as many as 10,000) galaxies. The Milky Way is a member of a small cluster called the Local Group which contains about 40 galaxies. The largest member of the Local Group is M 31, with the Milky Way coming in second in size. (NOAO/AURA Photo)
Milky Way Galaxy Our galaxy Spiral in shape 100,000 light years across
Units of distance – Light Year Light year is a unit of distance Light moves at a velocity of 300,000 kilometers per second or 186,000 thousand miles per second. So, in one year it can travel about 10 trillion km. The Andromeda galaxy is 2.3 million light years away and the nearest star Proxima Centauri is 4.24 light years away.
Units of distance- Astronomical Unit A little closer to home to home we use an AU or astronomical unit which is the average distance between the Earth and the sun to measure distances in our solar system. 1 AU=93 million miles Mercury =.39 Earth=1.0 Jupiter=5.2