This collection of activities will allow you to journey with your students to these distant explosions, by following exactly the same procedures used by today's astronomers. Your students will review some basic physics concepts about light, learn about redshifted spectra and Hubble's Law, and use them to find out how far away, and how powerful, gamma ray bursts truly are. Your students will "be the astronomer!"
Students will work with a real optical spectrum of a gamma ray burst afterglow taken at the Keck Observatory. They will measure the shifts of spectral lines, deduce the velocity of the astronomical object, and then use Hubble's Law to get its distance. Finally, if we know how far away an object is, and how much energy it deposited on our detector, we can calculate how much energy was emitted at the source, using the "one over r-squared" fall-off relation of light.
These activities reinforce basic physics concepts (light, the electromagnetic spectrum) and chemistry concepts (spectral lines, spectroscopy). Students experience how these concepts are cast in a mathematical form, and how the concepts combine to answer a fundamental question about a mysterious phenomenon in our universe (process of science). They are most appropriately used in a 11th or 12th grade physics or chemistry class, but can be modified for use in a mathematics class, or with bright, motivated younger students.
To use these activities, follow the Activity Flow Chart. Specifically:
Step 1: Introduce the students to the big idea: what we know from stars and galaxies is all based on what we measure using detectors either on Earth or in orbit. How do we go from this information to an estimate of how far away the objects are, and how much power they are putting out? Let students brainstorm possible paths to these answers.
Step 2: Measure how light falls off with the Fall-off of Light Activity. Your class might break up into several groups, whose results could be averaged together. How close did your class come to the actual behavior of one over r-squared?
Step 3: Present the information found in Introduction to Hubble's Law and the Doppler Shift. The central, most important concept to get across is that Hubble discovered that the further a galaxy is from Earth, the greater its redshift - i.e. the faster it is receding from us. We can turn this idea around: if we measure the redshift of an object we can calculate its distance from Earth.
Step 4: Now, have your class perform the Keck Spectrum Activity. Students will use state-of-the art, timely astronomical data to measure the redshift of the gamma ray burst, and then use this information to estimate its distance from Earth. The final step of this activity applies the 1/r2 fall-off of light, the distance to the object, and the power measured at our detector, to estimate the total power emitted by the gamma ray burst.
Think of light as a wave travelling away from the source. At a distance r1 from the source, all the energy from the light is found on a spherical surface with radius r1. The area of such a sphere would be 4(r1)2. As the wave continues to travel away from the source to a distance of r2, the light is then all found on a spherical surface whose area is 4(r2)2. Keep in mind that the total amount of light emitted by the source, let's call it L, is constant. What is changing as we move away from the source is the area over which it is spread. Let us then compare the amount of light per unit area at a distance of r1 to that at r2. At r1, the light per unit area, L1, is equal to L/4 (r1)2. And at r2, the light per unit area, L2, is equal to L/4(r2)2. Solving each equation for L gives us L= L1 x 4(r1)2 = L2 x 4(r2)2. So the amount of light per unit area at L1 relative to the amount of light per unit area at r2 is then
L1/L2 = (r2/r1)2
Think about what this means. If r1 is 5 cm and r2 is 10 cm, then there is 1/4 as much light per square cm at r2 as at r1. So the distance changed by a factor of 2, but the amount of light per square cm changed by a factor of 4. What if r1 was 5 and r2 was 50? How much less light per square cm do you have there?
Conclusion: We say that the intensity, or amount of light per square cm, changes as 1/distance squared (i.e., 1/r2) away from the source. How does this help us to achieve our Objective? If we measure X amount of energy per square cm in our detector, then we know that the source must have emitted energy equal to 4r2 times X!
Now let's confirm it!
Want to convince yourself that light really does fall off as 1/r2? Here is a quick, easy way!
v = Ho * d
where v represents the galaxy's recessional velocity, d is its distance away from Earth, and Ho is a constant of proportionality called Hubble's Constant. This equation is known as Hubble's Law. The exact value of the Hubble Constant is still somewhat uncertain. Hubble's first determination of it was a value of 550 kilometers per second for every megaparsec in distance (km/sec/Mpc). Today, it is generally believed to be around 65 km/sec/Mpc. (A megaparsec is given by 1 Mpc = 3 x 106 light-years). This means that a galaxy 1 Mpc away will be moving away from us at a speed of around 65 km/sec, while another galaxy 100 Mpc away will be receding at 100 times this speed. In essence, the Hubble Constant reflects the rate at which the Universe is expanding.
While there is certainly some scatter of measured observations off the line of best fit for Hubble's Law, it is nevertheless a good way to get a rough idea of how far away a very distant object is from Earth. All we need to do is to be able to determine the object's velocity.
The velocity is relatively easy for us to measure using the Doppler effect, or Doppler shift. An object in motion will have its radiation (i.e. light) shifted in wavelength according to the following formula:
where = measured wavelength, is the original (rest) wavelength, v is the speed of the object and c is the speed of light. The quantity on the left side of the equation above is usually called the redshift, and is denoted by the letter "Z".
Wavelengths can be measured in any units - Angstroms (10-10 meters), nm, mm, or km. As long as and are in the same units, they will cancel. The speed of light has a constant value of 300,000 km/sec. So, to determine the velocity of a galaxy (in km/sec) from its spectrum, we simply measure the wavelength shift of a known absorption line and solve for v.
A certain absorption line that is found at 5000 Angstroms in the lab is found at 5050 Angstroms when analyzing the spectrum of a particular galaxy. We then conclude that this galaxy is moving with a velocity v = (50/5000) * c = 3000 km/sec away from us.
Putting it all together now, if the object is moving away from us at 3000 km/sec, its distance from us (according to the Hubble Law) is
d = v/Ho = 3000/65 = 46 Mpc or 1.4 x 108 light-years
Another reason gamma ray bursts (GRBs) have remained so mysterious is that it is difficult to determine where they come from on the sky. Gamma rays are hard to focus, so for many years the uncertainty in the position of GRBs meant that looking for optical counterparts was like looking for a needle in a haystack. But recently the situation has improved.
On December 14, 1997, the BeppoSAX satellite measured the first 'afterglow' of a GRB, ushering in a new era of discovery of these powerful events. On July 3, 1998, astronomers at the Keck Telescope in Hawaii measured the optical afterglow of a GRB that contained many sharp spectral lines. We will use this spectrum and measure several spectral lines to determine the redshift of this object. From its redshift, and our understanding of the expansion of the universe we will measure the distance to this GRB, and ultimately the power of this explosion.
In the plot, the flux in micro-Janskys (µJy) is shown as a function of wavelength measured in Angstroms. We see a smooth energy distribution with several lines on top. These lines come from atomic transitions of elements in astronomical sources. Their relative spacing is determined only by the atom giving rise to the line. The lines marked Hd and Hg are due to neutral hydrogen. O II is from ionized oxygen. The 'rest' wavelengths at which these transitions occur in the lab are marked on the plot. The motion of the galaxy away from us, Doppler shifts the lines from their rest wavelengths.
Using any straight-edge, measure the wavelengths of the sharp Hd, Hg and O II lines in the spectrum, and record these in the table. Calculate the difference = ( - ), and the redshift (/) . In the steps below, use the average redshift of the three lines to estimate the distance to this gamma ray burst and the power it emitted.
A larger version of this plot is available.
Average Redshift R : _________ Average Velocity = R x 3 x 105 km/sec : ___________ Average distance: D=V / 65 km/sec/Mpc : ________________ Distance in cm: 1 Mpc = 3.26 x 106 ly, 1 ly = 9.5 x 1012 km, 1 km = 105 cm Power Emitted: P = 4D2 x measured flux (5.27 x 10-6 ergs/cm2):___________________
For More Information on Gamma Ray Bursts:
The Gamma-Ray Universe. D. Kniffen in American Scientist, Vol. 81, No. 4, pages 342-350; July 1993.
The Compton Gamma Ray Observatory. Neil Gehrels, Carl E. Fichtel, Gerald J. Fishman, James D. Kurfess and Volker Schonfelder in SCIENTIFIC AMERICAN, Vol. 269, No. 6, pages 68-77; December 1993.
The Gamma-Ray Burst Mystery. D. H. Hartmann in The Lives of Neutron Stars. Edited by A. Alpar, U. Kiziloglu and J. van Paradijs. NATO Advanced Studies Institute, Kluwer Academic Publishers, 1994.
Gamma Ray Bursts. G. J. Fishman and C. A. Meegan in Annual Review of Astronomy and Astrophysics, Vol. 33, pages 415-458; 1995.
BeppoSAX Mission home page is available on the World Wide Web at http://www.sdc.asi.it/