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Home , Betelgeuse, Blue supergiant star

Lab Title: Observing Project – , , and the Moon

Your name:______Instructor's Initials:______

Lab partners:______

Your write-up for this lab shall contain answers to all the questions in this lab. You will be able to write most of your answers in the space provided in this write-up, but some discussion of your observations should be written up separately at the end. No Conclusions section is required for this exercise. If you write down notes on your observations, hand those in as well.

Purpose: In this lab you will get a chance to observe three objects in the Orion – the supergiant , the blue , and Orion’s . You will then apply principles of blackbody radiation to estimate some of the properties of these objects. Finally, we’ll re-visit Saturn and the Moon for some more detailed observations.

Required materials: 1. Warm clothes (even though the daytime temperatures are fairly warm, it still cools off quickly after dark, and you won't be moving around much). 2. If you have one, a flashlight covered with red cellophane or similar material (to preserve night vision). 3. Pencil, eraser, and blank sheets of paper for sketches. 4. For the write-up you will need a scientific calculator, access to an introductory astronomy textbook, and a computer with internet access.

Introduction:

Betelgeuse

The great star Betelgeuse, also called Alpha Orionis, is one of the two that dominate the constellation Orion. We will come to the other, Rigel, in a moment. The name Betelgeuse derives from the phrase "ibt al jauza," meaning the "armpit of al-jauza," al-jauza being an ancient Arab deity known as the "Central One." Betelgeuse lies near the upper left-hand corner of the constellation Orion, representing the right (since he is facing you) shoulder of the ancient Greek hunter. This is one of the larger stars visible without a ; indeed it is one of the larger stars yet discovered. At its most likely distance of 425 light , its measured angular size yields a radius of 4.4 x 108 km. Betelgeuse is a highly evolved star, one whose central hydrogen fuel supply has run out. As a result, the core has contracted into a hot dense state, and the outer portions have swelled outward. We do not really know the star's condition at the moment, but odds are that it is now in the process of fusing helium into carbon and oxygen in its core. Betelgeuse has a number of other interesting properties. Its brightness is known to vary over long periods of time. It is ejecting part of itself in the form of a strong wind and is surrounded by a huge shell of dust of its own making. The wind and variability are perhaps related to huge hot spots on the star's surface.

1 Depending on its actual mass, Betelgeuse will eventually meet one of two fates. If its mass is near the high end of current estimates (about 17 times the mass of the ), the core will fuse elements through neon, magnesium, sodium, and silicon all the way to iron. It will then run out of fuel, collapse, and explode as a "," most likely leaving behind a compact about the size of a small town. This would be a truly spectacular event. If Betelgeuse were to supernova today, it would become as bright as a crescent Moon, would cast strong shadows on the ground, and would be easily seen in full daylight, similar to a supernova witnessed across Europe and in China in 1054 A.D. If the star’s mass is near or below the lower end of estimates (about 12 times the mass of the sun), then Betelgeuse may eventually become a shrunken and dense about the size of . Even then it will be unusual. Most white dwarfs are made of carbon and oxygen, whereas Betelgeuse has enough mass to become a rare neon-oxygen white dwarf. The only way we will know for sure is to wait – a long proposition since the “death” of Betelgeuse may not happen for a few million years.

Rigel

Rigel, also known as Beta Orionis, is another supergiant in the constellation Orion. Its name comes from the same root as Betelgeuse's, originally "rijl al-jauza," meaning the "foot of al-jauza.” Rigel lies near the lower right-hand corner of Orion and represents his left foot. It is usually pictured as perched upon a fainter star, Cursa (Beta Eridani), which represents the hunter's foot stool. Though Rigel is Orion's Beta star, it actually appears somewhat brighter than the Alpha star, Betelgeuse. Since we know that Betelgeuse’s brightness is variable, this suggests that Betelgeuse was once brighter than Rigel. Today Rigel ranks 7th in apparent visual brightness, and Betelgeuse ranks 9th. In contrast to the red supergiant Betelgeuse, Rigel is a "blue supergiant." It is accompanied by a fairly bright, seventh companion nine arcseconds away. Normally such a star is easily found in a small telescope, but Rigel's brilliance nearly overwhelms it. The companion, at least 50 times farther from Rigel than Pluto is from the Sun, is itself a double , composed of much less massive stars that are fusing hydrogen into helium. Similar to Betelgeuse, the fate of Rigel is uncertain. With an original mass around 17 times that of the Sun, Rigel is in the process of dying, and is most likely fusing helium into carbon and oxygen in its core. The star seems fated to supernova, though it might just make it under the wire as a heavy oxygen-neon white dwarf. Rigel is part of a large association of related stars. These include the stars of Orion's Belt, the Orion Nebula and its illuminating stars, and many of the other hot blue-white stars in the constellation.

Orion’s Nebula

The Great Nebula in Orion, or Orion’s Nebula, is one of the most interesting objects in the sky. To the , it looks like a star in the sword of the constellation Orion, but with binoculars or a telescope, you can see that it is actually a large glowing cloud of material. This is believed to be a huge region about 1630 light years away. The bright emission from the nebula is the glow of many luminous, newborn stars shining on the surrounding gas and dust from which the region formed. Perhaps the most important part of the Orion Nebula is the

2 part we can't see: the opaque Orion . This is a huge volume of very cold gas that has a total mass of about 2000 times the mass of the Sun. The gas from this cloud is slowly collapsing to form stars. Whenever a bright, new star is formed, its light evaporates the opaque gaseous cocoon surrounding it, allowing us to see it. The stars that are being born in the Orion Nebula are part of what astronomers call an "." When the star formation is complete, what will remain is a clump of a few hundred to a thousand stars that are all roughly the same age (give or take a few tens of millions of years). Currently this stellar nursery is dominated by a few very massive, bright stars collectively referred to as the Trapezium. The stars in the Trapezium outshine all the rest of the stars in Orion’s Nebula combined. Astronomers speculate that some of the stars in this cluster may even have like Earth forming around them. Also known as M42, the Orion Nebula spans about 40 light years and is located in the same spiral arm of the as our Sun.

Analysis: Part 1: Orion

You will be given a chance to observe each of these objects through binoculars and a telescope. Keep a record of what you see. For Betelgeuse and Rigel, record the apparent size and color of each star for each of your observations (with your eyes, with the binoculars, and with the telescope). For Orion’s nebula, make a sketch of the nebula as you see it through the telescope. Be sure to include any stars you notice inside the nebula. Make notes about the color and appearance of the nebula and the stars. After your observations, proceed with the following analysis.

1. Compare the size of Betelgeuse with the size of our . The radius of Betelgeuse is estimated to be 4.4 x 108 km. Consult your astronomy book or the internet to find out how many kilometers are in 1 AU:

1 AU = ______km

Now convert the radius of Betelgeuse to AU. Do this by dividing the radius of Betelgeuse in km by the number of km per AU:

Radius of Betelgeuse = ______AU

If Betelgeuse were place in the center of our Solar System, approximately where would its surface lie relative to the orbits of the planets? (Compare the radius of Betelgeuse in AU to the semi-major axis of the planets also in AU, which you can find either in a text or on the web.)

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2. Let’s assume that Betelgeuse and Rigel can be adequately modeled as blackbodies. From your observations and the above discussion, you know that Betelgeuse appears red and Rigel appears blue. Just from this piece of information alone, which star would you expect to have the higher surface temperature? Why?

3. Now use Wien’s Law 2.9 #106 " = max T to estimate the actual surface temperature on each star. For this equation, λmax is measured in nanometers and T in . First, assume that the peak emission for Betelgeuse is λmax=1000.0 nm (). What surface temperature would this correspond to (solve Wien’s Law for T and plug in λmax=1000.0 nm)?

Surface temperature of Betelgeuse = ______Kelvins

Next, assume that the peak emission wavelength for Rigel is λmax=100.0 nm (). What surface temperature would this correspond to?

Surface temperature of Rigel = ______Kelvins

4. Next, use the Stefan-Boltzmann Law 4 F ="T to estimate the energy flux given off by each star. For this equation, F is the energy flux (in units erg/cm2/s), T is the temperature (again measured in Kelvins), and σ is the Stefan-Boltzmann constant (σ = 5.67x10-5 erg/cm2/s/K4). For the temperature of each star, use the values from Question #3.

Energy flux of Betelgeuse = ______erg/cm2/s

4

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Energy flux of Rigel = ______erg/cm2/s

5. Now use the energy flux to estimate the L of Betelgeuse: 2 L = 4"R F For the radius R use the value given in Question #1. For the flux F use your answer from Question #4.

Luminosity of Betelgeuse = ______erg/s

33 The luminosity of our Sun is L=3.83x10 erg/s. How much brighter than our Sun is Betelgeuse? (Just divide the luminosity of Betelgeuse by the luminosity of the Sun.)

6. While observing Betelgeuse and Rigel, you may have been disappointed that even through the telescope these stars only appear as very small points of light - they do not appear to have any size. This is because the angular resolution of our was not sufficient to resolve their surfaces (all of the light appears to come from a single point). Even though these stars are very large, they are also very far away. To see what kind of angular resolution we would need, let’s calculate the angular size of Betelgeuse as seen from Earth: D Angular size (in arcsec) = 206,265 d The number 206,265 is equal to the number of arcseconds in a complete circle divided by 2π. In this formula, D is the diameter of the object (in this case the diameter of Betelgeuse, D=2R) and d is the distance to the object. We said above that Betelgeuse is 425 light years (ly) from Earth. Since we have the radius of Betelgeuse in km, let’s convert 425 ly to km.

1 ly = 9.46x1012 km

d = 425 ly = ______km

Now calculate the angular size of Betelgeuse as seen from Earth using the formula above (remember to use the diameter of Betelgeuse and not the radius):

!

5

! Angular size of Betelgeuse = ______arcsec

The best angular resolution possible for a telescope is limited to: 5 # Angular resolution (in arcsec)= 2.5 "10 DT where λ is the wavelength of light being observed and DT is the diameter of the telescope. To be optimistic, let’s take the shortest wavelength of visible light λ = 400 nm (violet). The diameter of our telescope was about 20 cm. Before we can calculate the angular resolution, we have to convert the wavelength and the diameter to the same units (let’s use meters).

λ = 400 nm = ______m

DT = 20 cm = ______m

Now calculate the angular resolution of our telescope:

Angular resolution of class telescope = ______arcsec

Based upon these calculations, can we expect to resolve the surface of Betelgeuse?

Now let’s consider the Hubble (HST). Again, use the same wavelength λ, but this time the telescope diameter DT = 2.3 m. Now calculate the angular resolution of HST:

Angular resolution of HST = ______arcsec

Based upon this calculation, can we expect HST to be able to resolve the surface of Betelgeuse?

HST has in fact done this. You can see the first resolved picture of the surface of a star other than the sun at http://antwrp.gsfc.nasa.gov/apod/ap990605.html . What interesting feature was discovered in this observation?

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7. The Orion Nebula appears as a glowing cloud of gas. Again, our small telescope did not illustrate the full beauty of this celestial object. A nice picture of the Orion Nebula, the Horsehead Nebula, and the Orion Molecular Cloud is available at http://antwrp.gsfc.nasa.gov/apod/ap020530.html. As a preview to our emission spectrum lab, look up and discuss how astronomers might determine the chemical composition of this cloud.

Look in an astronomy text, or online, and say identify the type of spectrum you see from the gas in Orion’s Nebula? Blackbody? Emission-line? Absorption line? Some combination? Why?

Part 2: Saturn

One of the telescopes shall be aimed at Saturn. As you did in our first lab, look at Saturn and sketch it to the best of your ability. Look around Saturn for any points of light that might be moons, and be sure to include those in your sketch.

After lab, go to http://www.wwu.edu/depts/skywise/saturn.html This will show you a picture of Saturn, with its largest moons identified. Be aware that the image that you saw through the telescope may be reversed from the image shown on the web page. If the moons that you think you saw were on the other side of Saturn from the picture, click on “inverted view” below the image. Which moons do you think you saw through the telescope?

7 Part 3: The Moon

Back to the Moon. This time, we’ll focus on small portions of it. Below are shown, in cross section, two different types of crater, simple and complex. Find one of each on the Moon, and sketch it. On your sketch, identify the following features, if present: crater rim, crater floor, and central peak.

Discussion and Comments

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