Astronomy 25 Astronomy 25 Before the 1920 s s telescopes would detect fuzzy light objects. Anything back then that could not be seen clearly was called a nebulae.
Immanuel Kant (1724-1804) 1804) Kant had the idea that these fuzzy objects are distant star systems.
This telescope came to southern This telescope came to southern California the 100 inch telescope at Mount Wilson. Edwin Hubble figured this out while looking at M 31 a spiral nebulae.
Hubble was the first to see Hubble was the first to see individual stars in it. He got the distance wrong it was over double that. But it was far enough away to know this was outside our galaxy and was in fact an individual galaxy of stars.
In the 1920 s s to photograph a galaxy the film would be exposed all night. The biggest and brightest galaxies are Spiral and Elliptical.
Spiral arms have bright emission Spiral arms have bright emission nebulae and hot young stars showing new stars being born.
2/3rds of all spirals show a boxy or 2/3rds of all spirals show a boxy or peanut shaped bar running thru the middle of it. middle of it. This is a barred spiral galaxy. This is a barred spiral galaxy.
There is a nuclear bulge in all There is a nuclear bulge in all spirals.
The lit part of spirals can be 20,000 The lit part of spirals can be 20,000 to 100,000ly across in diameter.
Spirals can have a billion to 1000 Spirals can have a billion to 1000 billion suns masses. Their luminosity ranges from 100 million to a 100 billion times the sun luminosity.
Elliptical galaxies are made of old Elliptical galaxies are made of old stars. Round or oval in shape with no arms. And no dust to block the view.
There are also rare giant ellipticals. Brightness 10 to the 11 th power solar luminosities. The mass is about 10 to the 13 power solar masses.
Anything that does not fit elliptical Anything that does not fit elliptical or Spiral galaxies is put in the category of Irregular galaxies. They have lower mass and brightness. They appear chaotic lots of star formation but have popi and popii stars in them.
Two examples are the Large and Two examples are the Large and Small Magellanic clouds.
The Small cloud may have run into The Small cloud may have run into the large cloud 200 million years ago.
Collisions and mergers of galaxies Collisions and mergers of galaxies can make large ellipticals. Individual spirals can stop star formation and arms become less evident over long periods time. To calculate a galaxies mass is not to hard to do now. All you do is measure how fast the outermost stars are moving around the center of the galaxy.
One way to describe a galaxy is to One way to describe a galaxy is to compare it mass to how bright it is. This is the Mass to light ratio. Our sun is rated at a one. Galaxies that still have stars being made so the ratio of one to 10. Ellipticals have older stars that may have finished there cycles no longer shine so the ratio can be 10 20.
If dark matter is present these If dark matter is present these ratios of mass to light could be as high as 100.
The first thing you have to learn The first thing you have to learn about a galaxy is how far away it is. This is hard due to most galaxies are so far away that you cannot see individual stars in them. Cepheid variables works for galaxies near us. Hubble telescope used this technique to a distance of 108 million ly.
Type I supernova gives us this Type I supernova gives us this standard brightness. They all have the same brightness 10 to the 10 th power solar luminosities.
Farther distances can be measured Farther distances can be measured by the total light given off by the galaxy itself. In isolated galaxy has no comparison so distance cannot be calculated. The newest way of measuring involves a relation ship noticed in galaxies. The faster a spiral galaxy rotates the brighter it is. The more massive a galaxy has the faster it rotates.
Rotational velocity can be Rotational velocity can be measured by looking at 21cm radiation.
Another way is looking at Elliptical Another way is looking at Elliptical galaxies. There is little gas and dust in ellipticals. We look at them we see individual stars. There is a lumpy bumpy appearance by the distribution of light from the stars in it. Distant elliptical galaxies the appearance is smooth. Now you can measure distance by the amount of bumpiness in the light distribution.
One of the velocities of a spiral was One of the velocities of a spiral was moving away from us at 1800Km/s. Slipher s work gained more weight when Hubble found ways to measure the distance to spiral nebulae. Hubble had help thou.
Milton Humason was that help.
The farther away the galaxy is the The farther away the galaxy is the faster it was moving away. Some are moving at 20,000km/s This is the formula for this relationship. V = H X D V= recession speed D = distance H = Hubble constant This equation is called Hubble s s Law.
This is expressed as Km/s/millionly The current value for Hubbles constant is 20km/s/millionly. A galaxy 100 million ly away is moving at 2000km/s
With Hubbles constant all we need is the spectrum of the galaxy. Measure the doppler shift then we know how far away it is.
Measuring the spectrum gives us Measuring the spectrum gives us the speed of the galaxy plug in equation with hubble constant you can solve for distance.
This means we live in an This means we live in an expanding universe. Clusters move away from other clusters of galaxies.