Astronomy 101 Fall 2010

Astronomy 101 Fall 2010

Homework #4

Astronomy 101 – Fall 2010

Due November 4, 11 a.m.

  1. Page 126, #14

Why not Hubble? Of the more than 200 extrasolar planets discovered, only one likely planet has ever been imaged by the Hubble Space Telescope. What limit’s Hubble’s ability to image planets around other stars?

There are two main factors that limit Hubble’s ability to directly image planets orbiting other stars. First is the tremendous distance between any given star and our Sun relative to the tiny orbital distance of a potential planet. Any telescope, including Hubble, would need extremely good resolution in order to image such a planet, increasingly so for more and more distant stars. To make matters more difficult, stars are overwhelmingly bright compared to their potential companion planets. In order for the Hubble Space Telescope to see a planet directly, the light from the star needs to be blocked out - without blocking the orbit of the planet - before it reaches the telescope’s detector, otherwise the detector will be flooded with light from the star alone. Consequently, imaging extrasolar planets directly is quite a challenge, even for Hubble!

  1. Distinguish between the terms “fusion” and “fission”.

Fusion describes the process by which two light atomic nuclei are slammed together at high speeds to form a heavier nucleus. In this case, the total mass of the new, heavy nucleus is less than the mass of the two original nuclei put together. In order not to violate any principles of nature, this mass cannot simply vanish during the reaction; instead, it is converted into a great amount of energy according to Einstein’s famous equation E = mc2. Conversely, fission describes the process by which a single heavy nucleus is split apart into two lighter nuclei. Now, however, the combined mass of the two lighter nuclei is less than the mass of the original heavy nucleus; therefore, the process of fission also converts mass into energy. Fission is also described as radioactive decay and radioisotopic dating can be used to estimate the age of a rock.

  1. Review Figure 5.3 on page 78. Describe in your own words how an aurora is produced.

In general, the Earth’s magnetosphere shields us from the stream of charged particles that make up the solar wind. However, some of these solar wind particles are accelerated along the Earth’s magnetic field lines, creating bands of charged particles that come closest to the Earth’s surface near the North and South poles. These trapped particles excite particles in the Earth’s upper atmosphere, which in turn emit photons (light) as they return to their lowest energy states, thus creating the phenomenon we call the aurora.

  1. List three stellar properties that can be determined from the spectrum.

By looking at absorption lines in a stellar spectrum, we can determine the composition of the star’s cool, thin outer atmosphere, the surface temperature of the star, and how fast the star is moving either towards or away from us. Each element has a specific pattern of absorption lines that it is able produce under the right conditions. Most directly, these unique signatures tell us the chemical composition of the outer atmosphere. However, the complete pattern of lines for a certain element may not show up in the absorption spectrum; instead, we only see the portions of the spectrum that can be produced at that temperature. Therefore, we can glean information about the surface temperature of the star from the spectrum as well. (As a check, we can also verify that the wavelength at which the intensity of the star peaks in the continuous spectrum gets shorter as the surface temperature increases, and more light is produced overall for these hotter stars.) Finally, we can use the Doppler shift of the spectrum to measure how quickly the star is moving towards or away from us. If a star is moving towards us, we will see familiar patterns of spectral lines shifted slightly to shorter, bluer wavelengths. Similarly, if the star is moving away from us, its spectral lines will be shifted to longer, redder wavelengths. The faster the star is moving, the larger the shift will be.

Additional properties:

Magnetic fields—the presence and strength of magnetic fields can be deduced by studying the splitting of spectral lines.

Spectral type—the appearance of the spectrum is basically determined by the temperature of the star, so astronomers have set up a classification system, OBAFGKM.

Luminosity class or size—from subtle differences in the spectral lines you can determine whether the star is a supergiant, giant, dwarf, or regular main sequence star. Combined with the spectral type, the luminosity class can be used to assign an absolute magnitude for the star. When apparent magnitude is combined with the absolute magnitude (distance modulus), the distance can be determined using the spectroscopic parallax method as you did in Lab #8.

  1. a) The parallax of the star Epsilon Eridani is 0.31 arcseconds. What is the distance to Epsilon Eridani in parsecs? In light years? b) The distance to Aldebaran is 16 parsecs. What is Aldebaran’s parallax angle?