CL - Stars (remote)[40]-1 (1)

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California State University, Bakersfield *

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1609

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Astronomy

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Apr 30, 2024

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docx

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Computer Lab – Stars (Virtual Lab Remote Edition) Star Data Launch the Voyager 4 program from csub.apporto.com. Single-click on any bright star to bring up its Info Panel. The Info Panel information has the following meanings: Under the General tab: Name: The star's Arabic name and other common names. The star's Bayer name is a Greek letter followed by the constellation. The Greek letter Alpha usually indicates the brightest star in the constellation, Beta would be the second brightest, and so on. The Flamsteed Number of the star, for example "58 Ori" would mean this is the 58 th star (counting west to east) in the Orion constellation. The Harvard Revised (HR) number of the star in the Yale Catalog. The Smithsonian Astrophysical Observatory (SAO) Catalog number. The star's number in the Henry Draper (HD) Catalog of spectral types. Plus, additional catalog numbers for the star. Type: Such as Star, Double Star, Moon, Variable Star, Constellation, Galaxy, etc. Magnitude: The apparent magnitude of the star, lower values mean brighter looking stars. Right Ascension: The star's right ascension. The year following the R.A. value is what year this position was calculated for, the value changes year to year due to precession and proper motion. Declination: The star's declination. Distance: The distance to the star in parsecs and light-years. Size: Stars will have no angular size listed because they appear as just a dot in the sky, objects like planets and nebulae which have visible extent will have sizes. Azimuth: The azimuth of the star in the sky for the current viewing location and time. Altitude: The altitude of the star in the sky for the current viewing location and time. Under the Visibility tab: Name: [Same as above.] Rise: Time and azimuth that the star rises. Transit: Time and altitude when the star crosses the meridian. Set: Time and azimuth that the star sets. Midnight Transit: Time of year when the star is visible all night. Solar Conjunction: Time of year when the star is near the Sun – it is not visible at all at night. In Constellation: The constellation in which the star resides. Stars – 1
Under the Physical tab: Name: [Same as above.] Spectral Type: The spectral class of the star (following the OBAFGKM system) followed by the luminosity class of the star (a roman numeral between I and VII). Color Index: Exactly how this is calculated is not important. Hot, blue stars will have a color index that is negative, white or yellow stars near zero, and cooler red stars will have positive values. Proper Motion: The Proper Motion of the star, the rate at which it appears to move across the celestial sphere, because of its actual motion through space (or the combination of its movement and our Sun's movement). This gives the rate at which both the Right Ascension and Declination of the star change with time in arc seconds per year. Radial Velocity: The Radial Velocity of the star, how fast it is moving towards or away from the Earth and Sun. Positive values are stars moving away. Absolute Mag.: The absolute magnitude of the star which is the apparent magnitude the star would have if viewed from a distance of 10 parsecs. The absolute magnitude is a way of stating the star's luminosity, how much total light it emits. Temperature: The star's surface temperature in Kelvin. Questions: [Selected answers are given at the end of these instructions.] 1. For the star you chose, what is its: (a) common name? Sirius (b) type? Double Star (c) distance away in light years? 8.601 ly (d) transit time today? 5:40 PM (e) spectral type? A1Vm (f) direction of motion, towards us or away? [Hint: See the Radial Velocity entry above.] Away Stars – 2
(g) absolute magnitude? 1.45 Magnitudes The Voyager 4 program doesn't list the star's luminosity in watts nor does it list the brightness we see in W/m 2 . Voyager uses an older system, the magnitude system. The star's brightness is indicated by the "Magnitude:" value under the General tab. The magnitude system dates back to Hipparchus who called the brightest stars magnitude 1, next brightest magnitude 2, down to the dimmest stars visible with the naked-eye, magnitude 6. The magnitude system is still in use today but in a slightly modified form. Using telescopes, we can see stars dimmer than were visible to Hipparchus, these have magnitudes like 7, 8, 9, etc. Note that dimmer objects have higher magnitude values. Because we can measure the apparent brightness very accurately, we now allow magnitudes in-between whole numbers, there are stars with magnitudes like 4.76 and 0.79. Very bright objects will have magnitudes with negative numbers, the magnitude of the Sun is -26.7. Use the Voyager 4 program to look up the magnitude of each of the following objects. You can use menu choices under the Center menu to find all of these objects. Also fill in the distances for the Moon (in km) and planets (in AU). Object Magnitude Distance Object Magnitude Distance Moon -11.8 4000214 km Sirius -1.44 8.6 LY Sun -26.7 1 AU Betelgeuse 0.56 428 LY Venus -3.9 1.68210 AU Polaris 2.00 431 LY Mars 1.1 2.00992 AU Orio n Nebula 4.00 1761 LY Pluto 14.5 34.97701 AU Andromed a Galaxy 3.50 3.13 MLY Questions: 2. Which is brighter today, Venus or Sirius? Sirius is brighter 3. Would you expect the Moon's magnitude to vary from day to day? Why? A lot or a little? Yes, the Moon's magnitude does vary from day to day, but the variation is relatively small. 4. Can Pluto be seen with the naked eye? No These values are all apparent magnitudes, how bright they look, not how bright they really are (their luminosity). The Andromeda Galaxy has the highest luminosity of all the objects in this list, it puts out enough light that it can be seen with the naked-eye on Earth Stars – 3
despite being over two million light-years away. No surprise it's so luminous, the Andromeda Galaxy is the combined light of a trillion stars! Of the objects in the above list, Pluto has the lowest luminosity. That’s because (1) it emits no light on its own, (2) it is very far from the Sun, and (3) its small size means it reflects little of the feeble sunlight reaching its vicinity. Inverse-Square Law The magnitude system indicates how bright objects appear to us. A lower magnitude means brighter. In particular, every 5 steps of magnitude represent a factor of 100 in brightness. So a star with magnitude -1 appears 100 times brighter to us than a star of magnitude +4. How bright the star appears to us is determined by how much total light it emits and how far away from us it is. The effect of distance on how bright the object looks is described by the inverse-square law. It says that if we move n times further away from an object that object should appear n 2 times dimmer. Let’s check this using the Sun. Select the Window/Location Panel menu and click on the Solar System tab. Drag the Distance slider to 1 AU, you can just leave the Longitude and Latitude values as they are. Select the Center/Planets/Sun menu and record the magnitude value of the Sun under the General tab) in the top line of the table below. Sun: Distance Magnitude 1 AU -26.72 10 AU -21.72 100 AU -16.72 You should've got -26.72 for the Sun's magnitude, this is the brightness of the Sun as seen from Earth since the Earth is usually about 1 AU from the Sun. change the Distance slider in the Location Panel to 10 AU. Record the new magnitude and repeat for a distance of 100 AU. 10 AU is 10x further from the Sun than 1 AU, so by the inverse-square law the Sun should appear 10 2 = 100 times dimmer. But 100 times dimmer on the magnitude scale means 5 steps higher, so the Sun’s magnitude at 10 AU should be -26.72 + 5 = -21.72 . Was it? It’s another factor of 10 in distance from 10 AU to 100 AU, so that should be five more steps of magnitude to -16.7. Question: 5. What would the magnitude of the Sun be if viewed from a distance of 1,000,000 AU? [You can’t set this on Voyager, so you need to work out the answer mathematically. That’s six factors of 10 (10 6 ) further than the 1 AU distance so six factors of 100 in decreased brightness which is six steps of 5 higher on the magnitude scale: -26.72 + 5 + 5 + … + 5 = ?] 3.28 magnitude Stars – 4
6. The star Altair is about 1,000,000 AU (=15.8 LY) from the Sun. Would you be able to see the Sun with the naked eye from Altair? It's unlikely that the Sun would be visible with the naked eye from Altair. Again, the further away from an object you are, the dimmer it will appear; but the variation follows a well-understood law, the inverse-square law. If you know how much total light a star emits (its luminosity) and how much light actually reaches us here on Earth (the magnitude), you can calculate how far away the star is. The mathematics is complex, but the concept is simple. Well, there is one complicating factor; interstellar (between stars) dust can absorb some starlight causing dimming (“extinction”) separate from the inverse-square law. Planetary Magnitudes Select the Earth tab in the Location Panel, this should put you back on Earth viewing from Bakersfield. Select the Tools/Planet Report… menu and then select the "Apparent Magnitudes" option from the pop-up menu (probably after moving or hiding other windows). Click the check boxes so that all the planets are selected (you can't select the Earth because we are viewing from Earth and it doesn't appear in the sky), then click "Update". The chart shows the magnitudes of these planets month-by-month for a year, you can click on “Next Interval” and “Last Interval” to change years (that may be necessary to see some of the features described below). The white curve for Venus is almost always the highest, this means Venus is almost always the brightest planet in our sky. Why Venus? For a few reasons; Venus is close to the Earth, Venus is close to the Sun, and Venus is surrounded by clouds which reflect most of the sunlight hitting them Questions: 7. Mars is always closer to us than Jupiter, why is Jupiter usually brighter in our sky? Jupiter's larger size and its relatively consistent distance from Earth result in its generally brighter appearance compared to Mars in our sky. 8. Mercury's brightness (the brown line) rises and drops more than any other planet. What is happening when it drops so low? When Mercury's brightness drops significantly, it is typically due to its position near inferior conjunction The main factors determining what magnitude a planet will appear to have are • how close it is to the Sun (inverse-square law) • how close it is to the Earth (inverse-square law) Stars – 5
• the size of the planet (bigger reflects more light) • phase as seen from Earth (how much of reflected light heads our way) • albedo of planet (‘albedo’ measures how reflective the body is) Magnitudes from Jupiter Close the Planet Report window. Click on the Solar System tab in the Location Panel, select Jupiter from the pop-up window, and drag the distance slider down to zero. We are now viewing from Jupiter's location. Select the Tools/Planet Report… menu again. These lines now represent the varying brightness of planets as seen from Jupiter. Sadly, Voyager won't let us turn on the plot for Earth (a bug in the program); from Jupiter, Earth's brightness plot would be similar to that of Venus. 9. Which planet generally appears brightest in Jupiter's sky? Click the "Next Interval" button a few times to make sure of your answer. Venus Star Colors Select the File/Open Settings… menu, find the "110 Settings" folder, and open the Settings File called "Enhanced Colors". The program is now displaying the stars with larger dots and with brighter colors. Click on a bright star that appears blue in color to get its Info Panel. Check the "Spectral Type" under the Physical tab for your star, it should be either type O or B (if not, try clicking on another blue star). The spectral class follows the OBAFGKM system, for example B8 or K3. The luminosity class (listed immediately following the spectral class) is a Roman numeral with the following meanings: I (Ia and Ib) Supergiant stars II, III, IV Giant stars (mostly Red Giants and Yellow Giants) V, VI Main Sequence Stars VII White Dwarf Example: If the star’s “Spectral Type” says “G2V”, that means the star has spectral class G2 and luminosity class V. In the following table, record the spectral class (like M1 or G5), luminosity class (I, II, … , or VII), distance (in light years), magnitude, absolute magnitude ("Absolute Mag."), and temperature for 12 stars. Remember, 'magnitude' measures the star's brightness in our sky while 'absolute magnitude ' measures its luminosity (and that lower values mean more light). Stars 1 and 2 on the list must be O or B types. Types 3 and 4 must be type A (they will appear white or blue-white on screen). Stars 5 and 6 type F (white), 7 and 8 type G (yellow), 9 and 10 type K (orange), and 11 and 12 type M (red). Stars – 6
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