Detection Methods in Astronomy

• Much of the incoming solar radiation does not make it to the Earth’s surface – due to atmospheric absorption

• Electromagnetic radiation that will pass through the Earth’s atmosphere can be studied using ground-based detectors

• Other regions of the electromagnetic spectrum must be detected by space-based instruments• The Hubble Space Telescope is a good example of

an instrument outside Earth’s atmosphere

Intro

Earth Motions

• Daily Rotation on its axis (daily cycle)• Rotation – spin on an internal axis• Proofs: Foucault pendulum

• Annual revolution around the sun (annual cycle)• Revolution – movement of one mass around another• Proofs: stellar parallax and aberration of starlight

• Precession – the slow change of the earth’s rotational axis (now at 23.5o) – see more info in chapter 15

Section 16.3

Experimental Proof of the Earth’s Rotation: Foucault Pendulum

The pendulum does not rotate with reference to the fixed stars.

Section 16.3

Parallax

• Parallax – the apparent motion, or shift, that occurs between two fixed objects when the observer changes position

• Parallax can be seen with outstretched hand• The motion of Earth as it revolves around the

Sun leads to an apparent shift in the positions of the nearby stars with respect to more distant stars

Section 16.3

Stellar Parallax

• The observation of parallax is indisputable proof that the Earth revolves around the Sun.

• In addition, the measurement of the parallax angle is the best method we have of determining the distance to nearby stars

Section 16.3

Aberration of Starlight

• A 2nd proof of Earth’s orbital motion• Telescopic observations of a systematic change

in the position of all stars annually• Due to the motion of the Earth around the Sun

• Angular discrepancy between the apparent position of a star and its true position, arising from the motion of an observer relative to the path of the beam of light observed• This is similar to what you see when driving in the rain

Section 16.3

Aberration of Starlight

• This discrepancy is very small and is measured in a -- parsec

• Parsec parallax + second• Recall that a circle contains 360o. Each degree

is divided into 60 minutes, and each minute into 60 seconds

• Therefore 1 second = 1/3600 degree• Parsec = the distance to a star when the star

exhibits a parallax of 1 second.

Section 16.3

Terrestrial Planets

• The terrestrial planets include: Mercury, Venus, Earth, Mars

• Due to physical/chemical characteristics they resemble Earth

• All four terrestrial planets are • Relatively small in size and composed of rocky

material and metals• Relatively close together and close to Sun• Have no rings• Only Earth and Mars have moons• Only Earth has surface water and oxygen

Section 16.4

Mercury

• Mercury is the closest planet to the Sun• Mercury has the shortest period of revolution (88

days), and is the fastest moving• Temperatures on Mercury range from about

473oC on the side facing the Sun to about -173oC on the dark side

• Due to its small size and closeness to the Sun, Mercury has practically no atmosphere

• It has a density close to that of Earth.

Section 16.4

Mercury, a Terrestrial Planet

Rotates 3 times while circling the Sun twice

Section 16.4

The Messenger spacecraft mapped the surface of Mercury. It was the first spacecraft to orbit the planet.

Venus

• Venus is the closest planet to Earth

• Venus is the third brightest object in the sky

• The surface of Venus cannot be seen from Earth, due to dense, thick clouds that cover the planet

• Magellan radar images indicate that the surface of Venus is composed of black, hot rock• Most surface rocks appear

to be volcanic

Section 16.4

The Atmosphere of Venus

• Venus’ atmosphere is composed of 96% CO2

• It is so dense that the surface of Venus has a pressure of 90 atm

• Atmosphere rotates faster than solid planet.• Retrograde rotation of planet• The large percent of CO2 in the atmosphere results in

high surface temperatures (477o C) due the “greenhouse effect”

• Radar images have revealed relatively few impact craters• Most of these craters are fairly large, because the

smaller incoming objects are consumed by Venus’s thick atmosphere

Section 16.4

Mars

• Mars has a red color, as viewed from the Earth, and was named for the Roman god of war

• The surface of Mars has two outstanding features that have intrigued scientists for decades; polar ice caps and extinct volcanoes

• The ice caps are composed of frozen CO2 in the winter and CO2 vapor with frozen water in the summer

• The red color is thought to be due to fine grain iron oxide minerals.

Section 16.4

Factoids on Mars

Mount Olympus, the largest known volcano in the solar system, at 24 km in height, it is about three times that of Mauna Loa

Mars at closest approach

Mars – Valles Marineris

This canyon on Mars is 4000 km in length and 6 km deep

Geologists think that it is a crustal fracture caused by internal forces

Section 16.4

In 2006 the Opportunity rover reached the edge of Victoria crater

Section 16.4

The Jovian Planets

• Jupiter, Saturn, Uranus, Neptune • Much larger than the terrestrial planets• Composed mainly of hydrogen and helium

• The four Jovian planets have a very low average density (approximately 1.2 g/cm3)

• All four are thought to have a rocky core composed of iron and silicates

• Thick layers of frozen methane, ammonia, and water are found above the core

Section 16.5

Formation of the Terrestrial Planets

• The two least massive elements – H & He – were the most abundant when the planets started to coalesce about 5 billion years ago

• Due to the heat from the Sun most of these less massive elements escaped the gravitational pull of the inner planets• Leaving behind more of the massive elements and

resulting in thick rocky cores and higher densities for the inner planets

Section 16.5

Formation of the Jovian Planets

• The four large outer planets were much farther from the Sun and therefore much colder

• The Jovian planets retained most of their H and He which now surround their ice layers and innermost rocky cores

• As a consequence the Jovian planets have a much lower average density

Section 16.5

Jupiter

• Largest planet of the solar system, in both volume and total mass

• Named after the supreme Roman god of heaven because of its brightness and giant size

• Diameter is 11 times Earth’s -- 318 times more mass than Earth

• The average density of Jupiter approximately 1.3 g/cm3 • Jupiter is covered with a thin layer of clouds composed

of hydrogen, helium, methane, ammonia, and several other substances

• It is a fast rotator – taking about 10 hours to rotate.

Section 16.5

Jupiter

Section 16.5

Jupiter’s Great Red Spot (“eye”)

• The Great Red Spot has erratic movement, shape, color, and size – sometimes even disappearing

• Likely a huge counterclockwise “hurricane-like” storm, lasting hundreds of years

Section 16.5

Saturn

• Distinctive system of three prominent rings• Rings are inclined 27o to orbital plane

• The rings are thought to be composed of ice and ice-coated rocks (micrometers 10 m)

• Most spectacular sight that can be viewed from Earth with a small telescope

• Diameter is 9 times Earth’s -- 95 times more mass than Earth

• Average density of only 0.7 g/cm3

Section 16.5

Saturn and its rings

Section 16.5

Uranus

• Discovered in 1781 by William Herschel (1738-1822), an English Astronomer

• Thin ring system composed of boulder-size particles (>1m), with very little dust-size

• Average density of only 1.3 g/cm3

• Unlike the other planets Uranus revolves around the Sun on its side and rotates in a retrograde fashion

Section 16.5

Neptune

• Discovered in 1846 by Johann Galle, a German astronomer

• Englishman John Couch Adams and Frenchman U.J.J. Leverrier were mathematicians using Newton’s law of gravitation

• They noted that Uranus’ motion was disturbed and predicted the location of another planet – this is how Galle eventually discovered Neptune

• Neptune also has a large dark spot similar to Jupiter’s and thought to be the result of large wind systems

• Neptune and Uranus are similar in size and in the composition of their atmospheres

• In many respects these two planets can be considered twins

Section 16.5

Neptune

Section 16.5

Designations of Celestial BodiesInternational Astronomical Union (IAU)

• In 2006 the IAU adopted the following criteria for a solar system body to be a planet:• (1) It must be in orbit about the Sun.• (2) It must have sufficient mass for self-gravity to

form a nearly round shape.• (3) It must be the dominant body within its orbit.

• The last statement disqualifies Pluto• The IAU established two new categories for objects that

orbit the Sun.• Dwarf planets is one of the categories• Pluto, Ceres, Eris, Haumea and Makemake are now

designated dwarf planets• So far, there are known more than 200 TNOs (Trans

Neptunian Objects) inside the solar system

Section 16.6

Outermost reaches of the Solar System

• The Kuiper Belt extends just beyond the orbit of Neptune and into the space of Eris.• Consists of comet and cometary material and other

small objects – Trans Neptunian Objects• Many astronomers put the edge of the solar system to

be at about 100 AU.• Voyager 1, launched in 1977, and in 2004 reached 100

AU. In 2010 it crosses the boundary of zero solar wind velocity.

• Sedna- the coldest, most distant object in the solar system: Dr. Mike Brown- CalTech scientist- used data from the Palomar Observatory to identify it.

Section 16.6

Origin of the Solar System

• Any theory that purports to explain the origin and development of the solar system must account for its present form

• According to our best measurements, our solar system has been in its present state for about 4.5 billion years

• A valid theory for solar system formation – must be able to explain a number of major properties of our solar system

Section 16.7

Major Questions Concerning Solar System

• Origin of material?• Forces that formed the solar system?• Isolated planets, circular orbits, in same plane?• Revolution (orbit) in the same direction?• Most Rotate in same direction (except two)?• Terrestrial versus Jovian planets?• Origin of the asteroids?• Origin of comets and meteoroids?

Section 16.7

The Formation of the Solar SystemCondensation Theory

Section 16.7

•Began with a large, swirling volume of cold gases and dust – a rotating solar nebula•Contracted under the influence of its own gravity – into a flattened, rotating disk•Further contraction produced the protosun and eventually accreted the planets•As particles moved inward, the rotation of the mass had to increase to conserve angular momentum (refer to the ice skater analogy)

Other Planetary Systems

• Are there other planetary systems in the universe?

• If so, we would expect to find some of these systems in different stages of formation• In other words, we should be able to find clouds of

gas and dust, primordial nebula, and protosuns, etc.

• We should also be able to use gravitational effects to detect small wobbles due to rotational objects in space

• These are called exoplanets or extra-solar planets

Section 16.8

Gravitational Effects

• A star with a large planet orbiting about it will have a small wobble superimposed on its motion as a result of gravitational effects

• This change in motion (the wobble) is likely to be very slight, but in some cases may be detected as a Doppler shift of the star’s spectrum• As the star approaches the observer, the

wavelengths are compressed (‘blue shift’)• As the star move away from the observer, the

wavelengths are lengthened (‘redshift’)

Section 16.8

Gravitational Effects

• The amount of wobble can be used to determine the planet’s mass (related to gravitational pull)

• The wobble’s cycle time can be used to determine the orbital period

• Once the orbital period is known, Kepler’s third law (T2= kR3) can be used to determine the planet’s average distance from the star

Section 16.8

Star WobbleDue to the gravitational pull of an orbiting planet.

Section 16.6

The wobbling in this illustration is greatly exaggerated!Section 16.8

Transit method

• A planet passing in front of its star as seen from Earth

• The star’s light will temporarily dim• The Kepler mission uses the transit method.

Section 16.8

First Planets Discovered Beyond our Solar System

• In 1992, using the Arecibo Observatory in Puerto Rico, astronomers reported the discovery of two objects revolving about a pulsar• Pulsars are very dense, rapidly rotating stars• Pulsars have a very precise rotation period• If the rotation period is disrupted, this would indicate

the presence of an object rotating about the pulsar

• These two objects are the first planets detected beyond our solar system

Section 16.8