Changes

The first person to discover the true distance and nature of Andromeda was [[Heber Curtis]], when in 1917 he examined several previous photographs of the galaxy, finding 11 more novae, noting that they were 10 magnitudes fainter then similar novae in our own galaxy.  Because of this, he became a proponent of the "island universes" hypothesis, estimating Andromeda to be 500,000 light years distant, and as a separate galaxy.  The debate to whether Andromeda was another galaxy (and indeed if there were other galaxies) was conclusively settled when in 1923, when [[Edwin Hubble]] discovered the first extragalactic [[Cepheid variable]]s, located within Andromeda, showing that it was not a nebulae within the [[Milky Way]], but another distant galaxy.<ref>http://adsabs.harvard.edu/abs/1929ApJ....69..103H</ref>   

The first person to discover the true distance and nature of Andromeda was [[Heber Curtis]], when in 1917 he examined several previous photographs of the galaxy, finding 11 more novae, noting that they were 10 magnitudes fainter then similar novae in our own galaxy.  Because of this, he became a proponent of the "island universes" hypothesis, estimating Andromeda to be 500,000 light years distant, and as a separate galaxy.  The debate to whether Andromeda was another galaxy (and indeed if there were other galaxies) was conclusively settled when in 1923, when [[Edwin Hubble]] discovered the first extragalactic [[Cepheid variable]]s, located within Andromeda, showing that it was not a nebulae within the [[Milky Way]], but another distant galaxy.<ref>http://adsabs.harvard.edu/abs/1929ApJ....69..103H</ref>   

The distance to the galaxy was again doubled when in 1953 when a second dimmer type of Cepheid variable was discovered.  This distance was further refined in the 1990s using the [[Hipparcos]] satellite to calibrate Cepheid distances using [[red giant]]s and red clump stars.<ref>http://arxiv.org/abs/astro-ph/?9802121</ref>

The distance to the galaxy was again doubled when in 1953 when a second dimmer type of Cepheid variable was discovered.  This distance was further refined in the 1990s using the [[Hipparcos]] satellite to calibrate Cepheid distances using [[red giant]]s and red clump stars.<ref>https://arxiv.org/abs/astro-ph/?9802121</ref>

==Environment and Structure==

==Environment and Structure==

The Andromeda galaxy is wider, and more luminous then our own Milky Way. Andromeda also appears to contain far larger amount of stars, and is some 25% more luminous then are own galaxy. However our galaxy has 3 to 5 times the rate of star formation at present, as well as twice the rate of new [[supernova]]e.  This suggests that Andromeda had a much greater period of star formation in the past but has since become far more quiet then our own galaxy in that arena.<ref>http://www.springerlink.com/content/1x7ebfnhu9mm6wqd/</ref>  

The Andromeda galaxy is wider, and more luminous then our own Milky Way. Andromeda also appears to contain far larger amount of stars, and is some 25% more luminous then are own galaxy. However our galaxy has 3 to 5 times the rate of star formation at present, as well as twice the rate of new [[supernova]]e.  This suggests that Andromeda had a much greater period of star formation in the past but has since become far more quiet then our own galaxy in that arena.<ref>http://www.springerlink.com/content/1x7ebfnhu9mm6wqd/</ref>  

Although larger, the Andromeda galaxy appears to have a lower mass (300-400 billion solar masses for visible material), then our own Milky Way (at least 600 million solar masses for visible material).  This would make the Andromeda galaxy significantly less dense then our own.  These results are from calculated estimates of the total halo masses of the two galaxies accounting for the gravitational effects of [[dark matter]].  This suggests Andromeda has a total mass of around 700 billion to 1.2 trillion solar masses compared to 0.93 to 1.9 trillion or more for the Milky Way.<ref>http://arxiv.org/abs/0801.1232</ref>

Although larger, the Andromeda galaxy appears to have a lower mass (300-400 billion solar masses for visible material), then our own Milky Way (at least 600 million solar masses for visible material).  This would make the Andromeda galaxy significantly less dense then our own.  These results are from calculated estimates of the total halo masses of the two galaxies accounting for the gravitational effects of [[dark matter]].  This suggests Andromeda has a total mass of around 700 billion to 1.2 trillion solar masses compared to 0.93 to 1.9 trillion or more for the Milky Way.<ref>https://arxiv.org/abs/0801.1232</ref>

One of the star clouds in Andromeda has its own NGC (New General Catalogue) number, known as NGC 206.  The region is one of the largest star-forming region known in the [[Local Group]], filled with massive, young blue stars.  The region is located in the southwestern outer edge of the spiral disk.  It was first noted by [[William Herschel]] and was recorded as H V.36 in his catalog in 1786.

One of the star clouds in Andromeda has its own NGC (New General Catalogue) number, known as NGC 206.  The region is one of the largest star-forming region known in the [[Local Group]], filled with massive, young blue stars.  The region is located in the southwestern outer edge of the spiral disk.  It was first noted by [[William Herschel]] and was recorded as H V.36 in his catalog in 1786.