Research Skills Discovering the Universe for Yourselfrfinn/research/asteroids/Summer08-ObservingIntro.pdfDiscovering the Universe for Yourself M arkTwin( 1835–90) American author,

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Research Skills• KEEP GOOD NOTES

– someone else should be able to reproduce yourresults

• Read, read, read, read, read• Web page - should be daily log

– summary of what you read– description of what you did– useful formula, procedures– results– unanswered questions

• Email web page to Rose at the end of each day• Uncomfortable is OK

Discovering the Universe for Yourself

Mark Twain (1835 – 1910)American author, from Huckleberry Finn

We had the sky, up there, all speckled withstars, and we used to lay on our backs andlook up at them, and discuss about whetherthey was made, or only just happened.

Constellations

In ancient times,constellations only

referred to the brighteststars that appeared to

form groups, representingmythological figures.

Today, constellations are well-defined regions of thesky, irrespective of the presence or absence of bright

stars in those regions.

88 constellations

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The stars of aconstellation onlyappear to be close

to one another.

Usually, this isonly a projection

effect.

The stars of aconstellation maybe located at verydifferent distances

from us.

Stars are named by a Greek letter (α, β, γ,) according totheir relative brightness within a given constellation + the

possessive form of the name of the constellation:

Betelgeuse = α Orionis,

Rigel = β Orionis

Betelgeuse

Rigel

Orion

How do we locate objects in thesky?

(1) Know your reference points.(2) Locate an object by its altitude (above horizon) and direction (along horizon)

We measure the sky in angles...

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Angle measurements:

• Full circle = 360º

• 1º = 60′ (arcminutes)

• 1′ = 60″ (arcseconds)

Review: Coordinates on the Earth• Latitude: position north or south of equator

– projection of latitude on to sky is Declination

• Longitude: position east or west of prime meridian (runsthrough Greenwich, England)– projection of longitude on to sky is Right Ascension

The Celestial Sphere• Zenith = Point on

the celestial spheredirectly overhead

• Nadir = Point on thec.s. directlyunderneath (notvisible!)

• Celestial equator =projection of Earth’sequator onto thec.s.

• North celestial pole= projection ofEarth’s north poleonto the c.s.

Coordinates of the Celestial Sphere• Right ascension

– RA or α– angle along CE from

vernal equinox– measured in hours

• 1 hr = 15 0

• 60 arcmin = 1 hr• 60“ = 1’

– “longitude”

• Declination– Dec or δ– angle above CE– measured in degrees

• 60 min = 10

• 60 sec = 1 min

– “latitude”

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The Celestial Sphere The Celestial Sphere

• Positions of starsindicate location onearth - latitude

• Elevation of Northstar = geographiclatitude ℓ (northernhemisphere)

ℓ

90o - ℓ

Apparent Motion of theCelestial Sphere

Apparent Motion of the Celestial Sphere II

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The North Starlies above North Celestial Pole, soit appears stationary as Earth spins

Why do stars rise andset?

Earth rotates counter-clockwisewhen looking down at the NorthPole, so stars appear to rise inthe east and set in the west.

The Annual Motion of the sun

Due to Earth’s revolution around the sun, the sun appearsto move through the zodiacal constellations.

The sun’s apparent path on the sky is called the ecliptic.

Equivalent: The ecliptic is the projection of Earth’s orbitonto the celestial sphere.

The Seasons (I)Earth’s

equator isinclined

against theecliptic by

23.5º.

The differentincidence

angle of thesun’s rays iscausing theseasons on

Earth:

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The Seasons (II) The Seasons (III)

Northern summer =southern winter

Northern winter =southern summer

The Seasons (IV)

sun

Earth in JulyEarth inJanuary

Earth’s distance from the sun has only a very minorinfluence on seasonal temperature variations.

Earth’s orbit (eccentricitygreatly exaggerated)

The Motion of the Planets (I)

The planets are orbiting the sun almost exactly in theplane of the ecliptic.

The moon is orbiting Earth inalmost the same plane (ecliptic).

Jupiter

MarsEarth

Venus

Mercury

Saturn

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Mercury appears at most~28º from the sun.

It can occasionally be seenshortly after sunset in the

west or before sunrise in theeast.

Venus appears at most ~46º from the sun.

It can occasionally be seenfor at most a few hours aftersunset in the west or before

sunrise in the east.

Apparent Motion of the Inner Planets Precession (I)

Gravity is pulling on a slanted top. => Wobbling around the vertical.

The sun’s gravity is doing the same to Earth.

The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t.

the ecliptic takes about 26,000 years and is called precession.

Precession (II)As a result of precession,

the north celestial polefollows a circular patternon the sky, once every

26,000 years.

It will be closest to Polaris~ A.D. 2100.

~ 12,000 years from now, itwill be close to Vega in the

constellation Lyra.

There is nothing peculiar about Polaris at all(neither particularly bright nor nearby etc.)

SkyGazer

• We will learn more about the motions ofthe stars using SkyGazer

• Complete Motions of Stars lab

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The Magnitude Scale

First introduced by Hipparchus

(160 - 127 B.C.):

• Brightest stars: ~1st magnitude

• Faintest stars (unaided eye): 6th magnitude

More quantitative:

• 1 mag. difference gives a factor of 2.512 in apparent brightness(larger magnitude = fainter object!)

• 1st mag. stars appear 100 times brighter than 6th mag. stars

The magnitude scale system can be extendedtowards negative numbers (very bright) and

numbers > 6 (faint objects):

Sirius (brightest star in the sky): mv = -1.42

Full moon: mv = -12.5

Sun: mv = -26.5

Magnitudes

• Magnitudes are a logarithmic measure ofthe flux from an object.

• Flux (F) is related to an objects luminosity(L) and distance (r) as:

Think-Pair-Share

• What flux is measured from the sun at thesurface of Mars, at a distance of 1.5 AUfrom the sun?

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Magnitudes

• Magnitude

• Magnitude difference

Absolute Magnitude

• magnitude an object would have at adistance of 10 pc.

• This is known as distance modulus.

Definition of a parsec

• Distance from which radius of Earth’sorbit, 1 AU, subtends 1 second of arc.

Think-Pair-Share

• Use trigonometry to determine what aparsec is in terms of AU.

• What is a parsec in units of cm?

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Parallax

• In general, we can relate the parallax of anobject in arcseconds to its distance inparsecs using the following:

Think-Pair-Share

• The apparent magnitude of the sun is-26.81. What is its absolute magnitude?

Magnitude Zeropoint

• The star Vega is by definition m = 0.

• All magnitudes are measured relative toVega.

Photometric Bands

• Magnitudes are usually measured at aparticular wavelength

Traditionalfilter set

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Photometric Bands

• The Sloan Digital Sky Survey (SDSS)uses different filters

SDSSfilter set

Charge Coupled Device (CCD)

• highly sensitive photon detector

• divided into a large # of small areas calledpixels