ASTR 1010-Exam 2

Just as optical astronomers observe the visible light emitted by objects such as stars and galaxies, radio astronomers can also observe the radio waves emitted by these objects, as well as the radio waves emitted by gas and dust. However, radio telescopes are different from optical telescopes in important ways.

In general, radio telescopes are BIGGER than optical telescopes. BECAUSE radio wavelengths are much longer than optical wavelengths.

focus

(of telescope) point where the rays of light converged by a mirror or lens meet

What percentage of X-ray light at 10 nm will reach a detector on a telescope at sea level?

0% Atmospheric opacity plays a major role in the selection of locations that astronomers build telescopes. The opacity of the atmosphere measures what percentage of photons are absorbed by the atmosphere before reaching the detector. The percentage of light transmitted through the atmosphere (the percentage of photons that will be received) is given by transmissance= 100%- opacity

Space Weather

1) Coronal mass ejections are the most energetic eruptive events to occur on the sun. 2) Solar stroms can potentially expose passengers in high-flying aircrafts and astronaults to increased amounts of radiation.

Which of the following are benefits of using telescopes located on Earth compared to telescopes in space? (You may choose more than one answer.)

1) Earth-based telescopes are easier to repair if something breaks or malfunctions. 2) Telescopes on Earth cost less to build and operate.

An astronomer wants to observe at a particular wavelength, using one of the telescopes below. Rank these telescopes from WORST to BEST ANGULAR RESOLUTION, based on the diffraction limit. The sizes indicate the diameters of the telescopes.

1) personal 8-inch telescope 2) Nickel 1 meter telescope on Mt. Hamilton 3) 2.1 MeterOtto Struve Telescope at Mcdonalds Observatory 4) Mayall 4-Meter Telescope at Kitt Peak National Obervatory 5) 10.4 meter grand Telescope Canaris

List some of the advantages of space telescopes over ground-based telescopes

1) they can "see" the entire electromagnetic spectrum from space, whereas on Earth, the atmosphere absorbs some wavelengths of light 2) they produce clearer images since they are above the Earth's atmosphere which affects the "seeing" of the telescopes when starlight shifts as it encounters moving pockets of air, known as turbulence 3) they can operate 24/7 since they do not have to contend with weather or wait for the Sun to set

Hopefully, you were able to pick up on some of the more difficult aspects of spacecraft communication from the video, but let's go over these in some detail.

1. We use radio waves to send our uplink and downlink communication. We also use radio waves for our WiFi signals, GPS satellites, listening to radio stations, etc. so it is a very busy part of the electromagnetic spectrum. We must be careful to broadcast our spacecraft communications at the right frequency in order to hear the spacecraft's signal relative to that background noise. 2. Not only must the DSN be talking and listening to the correct frequency, but the communication signals coming from deep space are very weak. The intensity of light falls off according to an inverse square law which causes the signal to become weaker the farther it travels from its source. As an example, the Voyager spacecraft recently passed the edge of our solar system and is now considered to be in interstellar space. The Voyager spacecraft sends a signal to Earth at 19 Watts, but the signal's power has been reduced to 10E-19 Watts by the time it reaches the DSN telescopes. 3. The delay time between flicking a switch and having your lamp turn on is almost nothing on Earth. However, the radio waves used to send communication are traveling the speed of light, but they must cross immense distances. As an example, when NASA sent information to the New Horizons craft that was flying by Pluto in 2015, the information took nearly 5 hours to reach the spaceprobe. Then the information returned by the spaceprobe took 5 hours to travel back to Earth.

Use Google to find out how many orbits the astronauts onboard the International Space Station complete each day. Round your answer to the nearest whole number.

15.54

Roughly how many manmade objects are tracked in their orbits around Earth?

18,000

In 1989 at solar maximum, there were 157 sunspots visible during the year. What would be the next year you would expect to observe about the same number of sunspots?

2000

For which of the following latitudes on the Sun is the rotation rate the fastest?

22 Degrees South The Sun has a differential rotation rate that varies with latitude. Different latitudes rotate at different speeds, something that is impossible for solid objects such as the Earth. Astronomers can observe these different rates by watching sunspots as they move with the rotation across the face of the Sun. Regions near the Sun\'s poles rotate once in about 36 days, while the Sun\'s equatorial regions rotate once about every 25 days. Therefore, the fastest rotation rate for a set of solar latitudes will be the one that is nearest the Equator.

The Hobby-Eberly Telescope has a diameter of 9.2 m. The WIYN Observatory Telescope has a 3.5-m diameter. How many times larger is the light gathering power of the Hobby-Eberly Telescope compared to the WIYN Observatory Telescope?

6.9

Of the satellites in low-Earth orbit, how much of it classifies as "space junk?"

95%

Choose the correct statement below.

A star uses fusion as an energy source by building larger atoms from smaller atoms.

The Hubble Space Telescope obtains higher resolution images than most ground based telescopes because it is

Above Earth's atmosphere

The steps in which the large, outer planets formed were

Accretion of planetesimals to form a core, followed by gravitational capture of hydrogen and helium gas.

The formation of terrestrial-type of planets around a star is most likely to have occurred by what process?

Accretion, or slow accumulation, of smaller particles by mutual gravitational attraction.

Satellites that study the Earth are commonly found in what type of orbit?

Low-Earth orbit

sunspot cycle (pt 1)

Between 1826 and 1850, Heinrich Schwabe, a German pharmacist and amateur astronomer, kept daily records of the number of sunspots. What he was really looking for was a planet inside the orbit of Mercury, which he hoped to find by observing its dark silhouette as it passed between the Sun and Earth. He failed to find the hoped-for planet, but his diligence paid off with an even-more important discovery: the sunspot cycle. He found that the number of sunspots varied systematically, in cycles about a decade long. What Schwabe observed was that, although individual spots are short lived, the total number visible on the Sun at any one time was likely to be very much greater at certain times—the periods of sunspot maximum—than at other times—the periods of sunspot minimum. We now know that sunspot maxima occur at an average interval of 11 years, but the intervals between successive maxima have ranged from as short as 9 years to as long as 14 years. During sunspot maxima, more than 100 spots can often be seen at once. Even then, less than one-half of one percent of the Sun's surface is covered by spots ([link]). During sunspot minima, sometimes no spots are visible. The Sun's activity reached its most recent maximum in 2014.

Match the following characteristics to the correct astronomical imager.

CCD-images are saved directedly in digital form, divided in a array of pixels, and more sensitive to light. Photographic Plate-must be developed with chemicals before images are analyzed, no longer used in professional astronomy.

What causes the Sun's magnetic field to become tangled?

Different latitudes of the Sun rotate at different rates, causing the magnetic field to get tangled.

What is the best place to put a telescope designed for radio observations?

On Earth's surface

You look at two stars that are separated in the sky by 0.1" using a telescope with an angular resolution of 0.9". What will you see?

One point of light that is the blurred image of both stars

Which of the following are possible effects of solar activity (flares and coronal mass ejections)?

POSSIBLE-damage of sensitive electrons on satelilltes, disruption of radio signals, and auroara borelais. NOT POSSIBLE-destruction of asteroids in the asteriod belt, and melting of the Earth's polar caps.

Match the following properties with the appropriate part of the Sun.

Photosphere-temperature of 5800k, location of sunspots, and normally visible surface of the sun. Cornoa-temperature of 1 to 2 million k, emits X-rays, visible during the eclipse, and irregular in shape.

What type of electromagnetic radiation does the DSN use to communicate with spacecraft?

Radio

For each property below, determine whether it applies to reflecting, refracting, or both telescope types.

Refracting Telescopes-can be affected by chromatic aberration, used as primary lens Both-Bring Light to a Focal Point Reflecting-Modern Telescopes Used Today, Use Primary Mirror

GPS satellites are often parked in what type of orbit?

Semi-synchronous orbit

How are space probes powered?

Space probes can be expensive to plan, launch, and maintain so they are designed to be as small and as compact as possible. Spacecraft usually contain a power supply that generates electricity, an attitude control system for pointing the spacecraft, science instruments to gather information, and a communication system to send and receive information from Earth. Space probes are powered in one of two ways: Solar panels and Radioistope Thermoelectric Generators (RTGs). 1) Solar panels: Watch the following video to discover how solar panels harness the Sun's energy and turn it into electricity. The video refers to solar panels on Earth's surface, but they work the same in space. Of the many spacecraft with solar panels that have been launched, only Juno (to Jupiter) and Rosetta (to Comet 67-P) have ventured beyond the asteroid belt. 2) Radiostope Thermoelectric Generators: Watch the following video to discover how RTGs operate:

Benefits of space probes

Space probes come in a variety of flavors and their job is to collect reconnaisance or in-depth information about bodies in the solar system by observing, orbiting, landing, or crashing into the body of interest. Below are some of the ways that you have likely personally enjoyed the fruits of space probe labor. 1. Many of the space pictures that have captured your imagination over the years were taken by the Hubble Space Telescope which is one of NASA's Great Observatories. 2. Accurate weather forecasting is aided by space-based weather satellites which monitor storms and weather patterns on the Earth's surface. 3. Global Positioning Satellites (GPS) lead us to destinations. 4. Satellite radio, TV, and Internet keep you connected to the world.

Why are there three DSN sites on Earth?

The Deep Space Network (DSN) operates three radio telescope arrays that are situated 120 degrees, in longitude, apart from one another. As Earth's rotation causes one DSN site to set, the next DSN site picks up the radio communications so that we have 24 hour access to our space probes throughout the solar system.

The core

The core is approximately 20% of the size of the solar interior and is thought to have a temperature of approximately 15 million K, making it the hottest part of the Sun.

Describe two reasons why the orbit of a satellite might be altered once it is placed in orbit.

The two reasons why the orbit of a satellite might be altered once it is placed in orbit are: gravity and collision.

Which of these statements are true of refracting telescopes, and which are false?

True-heavy and difficult to support, suffer from chromatic aberration. False-less expensive than other types of telecopes. Refracting telescopes gather light using a lens. This is in contrast to reflecting telescopes which gather light using a primary mirror. The very first telescopes ever made and used were refracting telescopes, but they have several disadvantages which make them unsuited for use in large research telescopes today. Glass refracts different wavelengths of light differently, so point sources viewed through a refracting telescope can have circular rainbows around them. This optical effect is called chromatic aberration and it is difficult to correct in lenses of any size. Since the lens must transmit the light, lenses can only be supported around their edges. The large lenses needed for research astronomy are extremely heavy and tend to distort under their own weight. Also because the light is passing all the way through the lens, the entire lens must be free of defects that may arise during the casting process. Making such high-quality lenses is prohibitively expensive. Using a primary mirror instead of a primary lens deals with all of these issues.

Sort the following statements about sunspots as either true or false.

True-usually appears in groups, observed on the photoshere. False-hotter than the photosphere travels across the surface of the sun while the Sun itself remains rigid and stationary.

How do spacecraft communicate with Earth?

Uplink-Earth sends commands "up" to spacecraft. Orientation of spacecraft. When to turn an instrument on/off. Changes in trajectory. Where to "phone" again. Downlink-spacecraft sends demands "down" to Earth. Health of spacecraft systems. Relay of pictures and scientifc information recorded.

Which of the following solar phenomena vary with the solar cycle?

VARIES-sunspots, cornoal mass ejections, and magnetic fields. DOES NOT VARY-size of the sun's core, average temperature of the photosphere.

Match each space telescope with a region of the electromagnetic spectrum that it primarily observes in. Some wavelengths may have more than one telescope.

X-Ray-Chandra Observeratory Visible-Hubble Space Telescope Infared-Splizter Space Telescope

What property of a reflecting telescope determines its light-gathering (or light-collecting) power?

area of the mirror telescopes are sometimes called \'light buckets\' because they collect light like buckets collect rain. The larger the telecope\'s mirror the more light it can collect, and the larger the bucket the more rain it can collect.

charge-coupled device (CCD)

array of high-sensitivity electronic detectors of electromagnetic radiation, used at the focus of a telescope (or camera lens) to record an image or spectrum

Astronomers use adaptive optics to

correct for blurring from Earth\'s atmosphere in real time.

Several radio telescopes are combined into an interferometer in order to

decrease the smallest angle which can be observed by the telescope. When two more more radio telescopes are combined into an array, they can be operated as an interferometer (also called an interferometric array). All of the dishes in the array observe the same object at the same wavelength at the same time. Because each dish is located at a different distance from the object, the signals reach each dish at slightly different times. When these signals are combined by a computer, an interference pattern is produced as the signals constructively (add together) and destructively (cancel) interfere. Using the resulting interference pattern, an image of the astronomical object can be created. The angular resolution of a telescope is determined by dividing the wavelength being observed by diameter of the telescope (angular resolution = wavelength / telescope diameter). Observing at shorter wavelengths and/or using a larger collecting area improves the angular resolution. Radio wavelengths are very long compared to those of visible light and so radio telescopes typically have poor angular resolutions. An important property of an interferometric array is that the angular resolution can be calculated using the size of the entire array, not just the diameter of a single dish in the array. This results in an interferometer having a very small angular resolution.

resolution

detail in an image; specifically, the smallest angular (or linear) features that can be distinguished

detector

device sensitive to electromagnetic radiation that makes a record of astronomical observations

aperture

diameter of the primary lens or mirror of a telescope

chromatic aberration

distortion that causes an image to appear fuzzy when each wavelength coming into a transparent material focuses at a different spot

sunspot cycle (pt 2)

however, the polarity of the leading spots is reversed in each hemisphere. For example, if during one cycle, the leading spots in the Northern Hemisphere all had the polarity of a north-seeking pole, then the leading spots in the Southern Hemisphere would have the polarity of a south-seeking pole. During the next cycle, the leading spots in the Northern Hemisphere would have south-seeking polarity, whereas those in the Southern Hemisphere would have north-seeking polarity. Therefore, strictly speaking, the sunspot cycle does not repeat itself in regard to magnetic polarity until two 11-year cycles have passed. A visual representation of the Sun's magnetic fields, called a magnetogram, can be used to see the relationship between sunspots and the Sun's magnetic field

telescope

instrument for collecting visible-light or other electromagnetic radiation

the convective zone

is the outermost layer of the solar interior. It is a thick layer approximately 200,000 kilometers deep that transports energy from the edge of the radiative zone to the surface through giant convection cells, similar to a pot of boiling oatmeal. The plasma at the bottom of the convective zone is extremely hot, and it bubbles to the surface where it loses its heat to space. Once the plasma cools, it sinks back to the bottom of the convective zone.

How important are these features for an astronomical telescope, in order from least important to most important?

magnification, resolving power, light gathering power

eyepiece

magnifying lens used to view the image produced by the objective lens or primary mirror of a telescope

the radiative zone

named for the primary mode of transporting energy across it. This region starts at about 25% of the distance to the solar surface and extends up to about 70% of the way to the surface. The light generated in the core is transported through the radiative zone very slowly, since the high density of matter in this region means a photon cannot travel too far without encountering a particle, causing it to change direction and lose some energy.

List some of the disadvantages of space-based telescopes over ground-based telescopes.

pace-based telescopes are more expensive because they must be placed into orbit and once something is broken, it most likely cannot be repaired.

A geo-stationary satellite orbits around the Earth once

per day.

prime focus

point in a telescope where the objective lens or primary mirror focuses the light

Which phenomenon would cause monochromatic light to enter the prism and follow along the path as shown in the figure?

refraction The phenomenon shown is refraction, where the higher index of refraction of the prism relative to air causes the path of the light beam to be deflected. The beam bends toward the normal when moving into a region of higher refractive index, and away from the normal when moving into a material of lower refractive index.

How the Solar System Was Formed?

step 1-the birth of our solar system began as dust and gases (nebula) started to gravitationally collapse step 2-the nebula contracted into a rotating disk that was heated by the conversion of gravitational energy into thermal energy step 3-cooling of the nebular cloud caused rocky and metallic material to condense into tiny solid particles step 4-repeated collisions caused the dust-sized particles to gradually coalesce into astroid-sized bodies step 5-

adaptive optics

systems used with telescopes that can compensate for distortions in an image introduced by the atmosphere, thus resulting in sharper images

reflecting telescope

telescope in which the principal light collector is a concave mirror

refracting telescope

telescope in which the principal light collector is a lens or system of lenses

The Sun's corona is much hotter than the photosphere, but it is not as bright because

the density of the corona is much lower, so the corona does not emit as much light.

the photosphere

the layer where the Sun becomes opaque and marks the boundary past which we cannot see

​granulation

the rice-grain-like structure of the solar photosphere; granulation is produced by upwelling currents of gas that are slightly hotter, and therefore brighter, than the surrounding regions, which are flowing downward into the Sun

seeing

unsteadiness of Earth's atmosphere, which blurs telescopic images; good seeing means the atmosphere is steady

In comparison to radio waves, visible light has

wavelengths that are shorter. Radio waves and visible light are two types of electromagnetic radiation and therefore both travel at the speed of light. Where they differ is in their wavelengths and frequencies. Visible light is of higher frequency and and shorter wavelength than radio waves.

corona

​(of the Sun) the outer (hot) atmosphere of the Sun

plage

​a bright region of the solar surface observed in the light of some spectral line

​solar wind

​a flow of hot, charged particles leaving the Sun

​plasma

​a hot ionized gas

​prominence

​a large, bright, gaseous feature that appears above the surface of the Sun and extends into the corona

Maunder Minimum

​a period during the eighteenth century when the number of sunspots seen throughout the solar cycle was unusually low

​coronal hole

​a region in the Sun's outer atmosphere that appears darker because there is less hot gas there

​coronal mass ejection (CME)

​a solar flare in which immense quantities of coronal material—mainly protons and electrons—is ejected at high speeds (500-1000 kilometers per second) into interplanetary space

​solar flare

​a sudden and temporary outburst of electromagnetic radiation from an extended region of the Sun's surface

​active region

​an area on the Sun where magnetic fields are concentrated; sunspots, prominences, flares, and CMEs all tend to occur in active regions

sunspot

​large, dark features seen on the surface of the Sun caused by increased magnetic activity

aurora

​light radiated by atoms and ions in the ionosphere excited by charged particles from the Sun, mostly seen in the magnetic polar regions

​chromosphere

​the part of the solar atmosphere that lies immediately above the photospheric layers

​differential rotation

​the phenomenon that occurs when different parts of a rotating object rotate at different rates at different latitudes

​transition region

​the region in the Sun's atmosphere where the temperature rises very rapidly from the relatively low temperatures that characterize the chromosphere to the high temperatures of the corona

​photosphere

​the region of the solar (or stellar) atmosphere from which continuous radiation escapes into space

​sunspot cycle

​the semiregular 11-year period with which the frequency of sunspots fluctuates

How much greater is the light collecting area of a 9-meter telescope than that of a 3-meter telescope?

Nine times