The Milky Way Galaxy: galac1c geography

Transcription

1 The Milky Way Galaxy: galac1c geography From Ki8 Peak, looking south toward the constella1on Sagi8arius, summer 1me: the Milky Way! Next, Look at the en1re sky as shown by sofware Stellarium

2

3

4 The solar system is surrounded by the Milky Way. It is brightest towards the constella1on of Sagi8arius. We see the nearby stars in all direc1ons, but the more distant stars merge into the band we call the Milky Way Suppose we ask what it would look like if we could fly far away and view it. How can we figure out what we would see? if you were in the middle of New York, could you use informa1on about city landmarks to create a map?

5 We have discussed various astronomical objects. What do you think these are? (These are all taken from the NOAO image gallery)

6 Next, How far way do you think each one is? M17, young star cluster M13, globular cluster NGC1232, Spiral galaxy M33, spiral galaxy Planetary nebula (dying star) From these pictures, you can t tell how far away they are!

7 Lecture- tutorial: Milky Way Geography As you learn about different types of objects in the sky, we can ask if they are found in par1cular loca1ons. In par1cular, we are going to explore how different objects are distributed in the Milky Way. Our first step is just to see in what direc1on we find them. open cluster: group of about 20 to 200 stars, some1mes with glowing gas, heated by very hot stars, surrounding it. Examples: Pleiades globular cluster : star cluster of up to a million stars, so 1ghtly packed that the center is unresolved. No gam amongst the stars galaxy our Milky Way is our home galaxy. So other galaxies must be much farther away. How do we know their distance? How do we dis1nguish them from clusters? planetary nebula dying sun- like star, shedding gas The second step will be to ask how far away they are: this will require that we use some new methods.

8 What have we found? The open clusters are in the plane of the milky way The globular clusters are in the direc1on of tsagitarius The galaxies are all over, although not in the plane of the Milky

9 Review: how do we measure distance to stars? Parallax: geometric method for nearby stars Parallax allows us to measure distance for nearby stars. We can then calculate how bright the star would be at any distance we choose a standard distance of about 30 light years. What can we learn if we do this for all the stars in a cluster, like the Pleiades?

10 Take an observing break! The moon, Saturday: about azimuth 230, al1tude about 20 degrees. The moon last night, 5:40 pm. Azimuth (measured) 210 degrees, al1tude about 30 degrees Tonight?

11 The Pleiades (Subaru in Japanese, the Homeless Women in an O odham story), a cluster of stars visible to the naked eye The HR diagram for the Pleiades: we can measure the distance to these stars by parallax, and compute how bright they would be compared to the sun, above. How can we measure their temperature?

12 Review last class: spectra From the spectra of a hot gas, we can determine a. The type of gas (its composi1on) b. The temperature of the gas c. The velocity toward or away from us of the object d. All of the above

13 Let s explore how we can measure the temperature of a star from its spectrum, or even easier, from its color, using a fun website:

14 Lecture- tutorial, as a class

15 Challenge ques1on: measuring magnitudes

16 The Pleiades again, The HR diagram for the Pleiades, as astronomers would show it:

17 Consider a more distant open clusters too far for parallax! Chi Persei, one of the double cluster We can measure the apparent brightness of each star: since the stars are all in a cluster, at the same distance, we can plot along this with the color of each star.

18 When we compare the HR diagrams, we can compute how much fainter Chi Persei is compared to the Pleiades thus how much farther it is. For those taking algebra: our black box calculator for three quan11es m M = 5 log (distance) 5

19 Review We can measure the apparent brightness of stars in a cluster as well as their colors (which tells us temperature) Then, ploing an HR diagram allows us to compute how much fainter, thus how much farther away, that cluster is compared to, say, the Pleaides

20 What do we find when we measure distances to lots of open clusters? We confirm that they are all within the plane of the Milky Way. Let s go back to the idea of a map of New York when we are at Lexington and 29 th St. But since we look at the band of the Milky Way, we will measure around in a circle like azimuth. What do you suppose this is showing us? Hint: The direc1on to 0 degrees is toward the constella1on Sagi8arius

21 Globular clusters (green) are centered in Sagi8arius: seen from Arizona Whatwhat would we see from the southern hemisphere?

22 So south to Chile

23 So we can now draw a cartoon of our Milky Way galaxy Remember, this is only a cartoon: we cannot leave even oru solar system let along our galaxy. Distance from the center to our sun: a future class! Mul1ple spiral arms Bar in the center, and very massive object in center (Gautham s lecture)