Sci - Lab

PHYS 105 Unit 6 Lab

Application - Dating Star Clusters and Red Shift

Stellar Clusters are very useful laboratories for studying stellar evolution nearby. This is because:

• All cluster stars are at the same distance • All cluster stars have the same physical age • As the cluster ages, different phases of stellar evolution are represented • A schematic representation of this is shown below.

HR diagram Young Cluster In this case we see the Main Sequence fully represented. The blue dots represent massive stars with short main sequence lifetimes. These stars would have masses greater than 2 solar masses. The yellow dots represent stars with masses between 1 and 2 solar masses and the red dots represent stars with masses less than 1 solar mass. Since the cluster is very young, no Red Giants have appeared.

HR diagram 10 Million Years Old Now we are seeing the disappearance of the most massive main sequence stars, as their main sequence lifetimes are less than 10 million years. These stars have evolved to become the first generation of Red Giants.

HR diagram 1 Billion years Old Now we see that all the blue stars have evolved off

the main sequence and that some of the Red Giants have evolved to become White Dwarfs. As the main sequence lifetime of a 2 solar mass star is about one billion years, then there are no stars still on the main sequence with masses higher than this. Thus, identifying the most massive star, which is still on the main sequence, is an effective means of determining the age of a stellar cluster.

HR diagram 10 Billion years Old All of the yellow stars have no disappeared. The Giant Branch is now well populated, Overall, the cluster colors would be very red since all the stars are either red giants or low mass main sequence stars. The number of white dwarfs has increased quite bit. In the next few billion years the cluster will continue to evolve and slowly fade as all its former members end up as white dwarfs, neutron stars or black holes.

Part 1: Exploring a Stellar Nursery.

Although most of the stars in the universe were born several billions of years ago, star formation continues today. This new Hubble image shows a very compact star-forming region -- the N83B nebula -- in a small part of a galaxy called the Large Magellanic Cloud. This galaxy is relatively nearby -- 165,000 light-years from our own Milky Way -- and can easily be seen from the Southern Hemisphere with the naked eye.

Researchers say the stars are young, massive and ultra-bright. They are seen in the new image just as they are born and emerge from the shelter of their prenatal molecular cloud.

Their high mass means that the young stars evolve rapidly. Because of this, it is difficult for astronomers to catch such events. Furthermore, the stars spend a good fraction of their youth hidden

from view, shrouded by large quantities of dust in the cloud from which they form. The only chance is to observe them just as they start to emerge from their cocoon.

Astronomers from France, the United States and Germany who studied the nebula said several individual stars are responsible for lighting it up. The star at the center of the nebula, just below the brightest region, is some 30 times more massive and almost 200,000 times brighter than our Sun, the researchers said.

The hottest star in N83B is 45 times more massive than the Sun and is embedded in the brightest region in the nebula. This bright region, just above the center, is only about 2 light-years across. The region's small size and its intense glow are telltale signs of a very young, massive star.

Measurements of the age of this star and neighboring stars in the nebula show that they are younger than the nebula's central star. Their formation may have been triggered by the violent wind from the central star. This possible chain-reaction of stellar births seems to be common in the universe, researchers said.

To the right of the glowing N83B is a much larger diffuse nebula, known as DEM22d, which is partly obscured by an extended lane of dust and gas. The colors in the image have meaning. Red indicates ionized hydrogen in one electric state; green, ionized oxygen; blue, ionized hydrogen in another electric state. The blue corresponds to the warmest regions, the red to the coldest. The full image represents an area that is 55 by 108 light-years.

Questions:

1. How does the nebula keep pushing gases out? 2. How is the process of nebula expansion related to how planetary nebula form? 3. Did the Sun have a similar birth? Why or why not? 4. Other than age, what can astronomers learn from studying these nurseries?

Part 2: Dating the Pleiades and 47 Tucanae

47 Tucanae

M45, the Pleiades

In class we have touched on many characteristics of stars: their distance, intrinsic luminosity, surface temperature, composition, mass and radius. However, this tells us very little about their actual histories. In order to study the life cycle of stars, we would like to know the age of the stars we observe. Star clusters give us an opportunity to determine the age of their member stars, because we can assume that all the stars were born at roughly the same time.

Normally, a Hertzsprung-Russell (H-R) diagram plots the spectral type of a star (or its temperature, which is equivalent) against the star's intrinsic luminosity (or absolute magnitude). However, in this lab we are going to construct a slightly different kind of H-R diagram, one which features the color of stars.

This isn't as easy as it sounds. Think, for example, of how hard it is to get two people to agree on the exact color of a sweater. Now imagine if we had to get all astronomers to agree on the color of every object in the sky! To avoid this situation, astronomers measure the brightness of a star through filters. A filter only lets light of specific wavelengths through. For example, if you were to look through a red filter, everything would appear different shades of red, because only red wavelengths of light can pass through the filter and into your eye.

Astronomers like to compare the brightness of a star (or anything, really) in one filter to its brightness in a different one. Since stars are blackbodies they will appear brighter in one filter than in another, and the difference between these brightnesses is a number, which we use to describe the stars' color. Using this technique allows us to give color a universal meaning.

Since color and temperature and spectral type are all equivalent, we can plot the color of a star against its brightness (measured in magnitudes) as a way of building an H-R diagram without taking the star's spectrum. This type of H-R diagram is called a "color-magnitude" diagram. This method is particularly useful with star clusters where taking the spectrum of thousands of closely spaced stars would be impossible.

Today we will be plotting actual data for two star clusters: an open cluster called M45 and a globular cluster called 47 Tucanae. Each cluster contains thousands of stars, but we will only plot the data for a representative few. The table below provides the data. The filter combination we will use is B-V: the difference between the star's brightness in a blue filter and in a yellow filter. The

important thing to know is that the bigger B-V is the redder the star is -- and the smaller it is, the bluer the star.

Questions

1. Graph the two clusters. How are their profiles different?

Plot B-V versus magnitude for each star on a piece of graph paper. Use a different plot or a different color for each cluster. Your x-axis is the color (B-V), and runs from -0.4 to +1.6 in both plots. Your divisions should be about 0.2. The y-axis is the apparent magnitude, and is different in each case. Check the numbers in the table to see what your maximum and minimum values should be. Don't forget that magnitudes are backwards, so that smaller numbers mean brighter stars!!!!

2. We usually plot absolute magnitude or luminosity on the y-axis of an H-R diagram. Why can we plot the apparent magnitude for cluster stars? (Hint: what's the difference between apparent and absolute magnitude?)

3. The lifetimes of different spectral types are given in Table 2. Use these lifetimes to estimate the age of 47 Tuc and M45. Explain your reasoning!

4. Based on your age estimates, what can you say about the lifetimes of open star clusters versus globular star clusters?

5. Why don't we see O and B type stars on these diagrams (B-V < -0.2)? 6. What factors determine how stars that evolve in clusters like these evolve?

Table 2: Main Sequence Lifetimes Spectral

Type Color B-V

Lifetime (years)

O -0.4 < 106 B -0.2 3 X 107 A 0.2 4 X 108 F 0.5 4 X 109 G 0.7 1 X 1010 K 1.0 6 X 1010 M 1.6 >1011

Table 1: Data for 47 Tuc and M45

47 Tuc M45 Star

Number Magnitude Color (B-V)

Star Number Magnitude

Color (B-V)

10012 19.6 0.76 133 14.4 1.28 10170 20.6 0.98 165 7.6 0.12 10200 21.0 1.05 345 11.6 0.84 10206 21.0 0.96 522 11.9 0.90 10278 21.6 1.23 697 8.6 0.35 10335 22.0 1.31 804 7.9 0.20 10359 22.2 1.23 950 4.2 -0.10 10489 22.6 1.33 1040 15.8 1.44 10610 23.0 1.45 1103 14.8 1.47 20028 17.6 0.53 1234 6.8 0.02 20034 17.7 0.58 1266 8.3 0.36 20049 18.0 0.57 1305 13.5 1.18 20070 18.4 0.60 1309 9.5 0.47 20104 18.8 0.65 1355 14.0 1.23 20130 19.1 0.69 1432 2.9 -0.09 20185 19.8 0.83 1454 12.8 1.16 20210 20.1 0.88 1516 14.0 1.31 20239 20.4 0.93 1766 9.1 0.47 20335 21.4 1.10 1797 10.1 0.56 20364 21.6 1.20 1924 10.3 0.62 30014 13.5 1.10 2168 3.6 -0.08 30103 15.5 0.82 2181 5.1 -0.08 40002 12.0 1.45 2209 14.4 1.47 40022 12.6 1.25 2406 11.1 0.76 40043 12.9 1.14 2425 6.2 -0.05 40130 14.0 0.99 2588 13.1 1.22 40135 14.0 0.69 2601 15.0 1.55 40144 14.0 0.79 2655 15.5 1.36 40164 14.0 0.59 2870 12.5 1.07 40351 14.9 0.85 2881 11.8 0.86 40628 16.2 0.73 40821 16.6 0.73 41051 16.9 0.70