Astronomy

The Stars

Star cluster NGC 457, the Owl Cluster, in the

constellation of Cassiopeia

http://www.buytelescopes.com/gallery/view_photo.asp?pid=10298&sg=9&page=3

Distances to the Stars

How can we measure the distances to the nearest stars?

Parallax Method:Stars close to our sunMeasure star’s position once, then 6 months laterNearby stars appear to shift back and forth relative

to more distance starsAmount of shift can be used to calculate distances

to stars

Stellar Parallax

•A star close to our sun appears to shift back and forth compared to the more distant stars in the background

•This diagram is not to scale. The shifting of stars due to parallax is incredibly tiny and is measured in arc seconds Earth in

summerEarth in winter

Star close to our sun

Parallax Animation

http://www.astro.washington.edu/labs/parallax/solar.htmlCool parallax demo:

Arc Seconds

Seconds of Arc: Stellar parallaxes are measured in arc seconds (“)

Arc seconds are tiny divisions of a degree Remember that 360° makeup a circle1º = 60’ = 3600”1’ = 60”1” = 1/3600 degree1” = width of an aspirin tablet one mile away

ParsecDistance in light years to a star showing one arc

second (“) of parallaxParsec (pc): Distance measure = 3.26 light

yearsThe closest star is Alpha Centauri at 4.3 lyTo calculate a star’s distance from observed

parallax:Star’s Distance (in pc) = 1/parallax (“)

Newton’s Prism Experiment (1665)

•Isaac Newton discovered that light could be broken down into component colors by using a prism

•Newton isolated a single color and passed it through a second prism indicating that the prism wasn’t introducing false colors but that they were a true property of light

•The separated light is known as a spectrum

Visual Spectrum

Overhead Spectra Demo

Teacher demo

Spectrum of Light

Spectroscopy: Analysis of spectraSpectroscope: Device attached to a telescope,

splits light of a star into its spectrum of colorsSpectroscopy reveals what stars are made of

Spectra of Stars

3 Types of Spectra:1. Continuous: Complete array of all the

rainbow colors. (incandescent light bulb)2. Emission (Bright-Line): A pattern of bright-

colored lines emitted by hot gas (neon light, overhead fluorescent bulb)

3. Absorption (Dark-Line): A pattern of dark lines across a continuous spectrum. Created when light passes through a cool gas. (Stars, the sun)

Three Types of Spectra

Solid array of rainbow colors

Mostly dark with a few brightly colored lines

Mostly continuous but with a few missing dark lines

Spectral Tube Demo

Demo

Animation

Animation 4.1: Spectra

Spectroscope

Elements

Atoms are the smallest unit of matter that still retains that matter’s known properties

Atoms create the spectra types100 types of unique atoms are known,

each is an elementPeriodic table

Bohr Model

Bohr atom model: A nucleus made of positively charged protons surrounded by the same number of negatively charged electrons

Electrons are confined to a set of allowed orbits around the nucleus

Bohr Model of Nitrogen

•The Bohr model of an atom of the element Nitrogen contains 7 protons (+) in the nucleus and 7 electrons (-) arranged in two energy levels surrounding the nucleus

•Nitrogen is the 7th element in the periodic table due to its having 7 protons

Nucleus

Energy levels

http://education.jlab.org/qa/atom_model.html

Binding Energy

Binding Energy: energy that holds electrons in place around the nucleus

Each element has its own unique set of allowed electron orbits or energy levels

Jumping Electrons

Binding Energy: energy that holds electrons in place around the nucleus

Each element has its own unique set of allowed electron orbits or energy levels

Ground State: Undisturbed atom, electrons in allowed orbits, lowest energy

Excited State: Electrons will jump to higher energy levels, release light particle (photon) when falling back

Excite Me

•When an atom is excited, an electron jumps momentarily to a higher energy level or “orbit”

•When the electron jumps back down to its previous level, the atom emits a photon of light

Animation

Animation 4.2: Absorption and Emission of a Photon

Emission Lines

Excited atoms create bright colored emission lines due to their jumping electrons

Each chemical element has its own unique set of bright emission lines

Emission spectra of various elements

Emission Spectrum of an Element

By analyzing the light of a burning element in the laboratory, its unique bright spectral lines can be observed and recorded

Absorption Lines

Correspond to the bright emission linesUnique dark absorption lines produced

when an atom absorbs light, causes electrons to jump

Absorption lines represent light subtracted from the continuous spectrum

Observation of emission or absorption lines in spectra allows identification of the chemical element that produced them

ABSORPTION SPECTRUM• A pattern of dark lines across a

continuous spectrum• Light passing through a cold gas• If plotted as a graph, absorption

lines appear as dips

EMISSION SPECTRUM• A pattern of bright-colored lines with

black gaps.• Hot, glowing gas• If plotted as a graph, emission lines

appear as peaks

Spectra of Hydrogen

Stellar Spectra

Star light can be broken down with a spectrometer

Dark absorption lines can be observedAbsorption lines can be matched to specific

chemical elementsStellar absorption spectra are created when light

created inside a star passes through relatively cooler layers of gas in the outer atmosphere of the star before traveling into space

Absorption Spectra

O

B

A

F

G

K

M Coolest stars

Hottest starsThe unique chemical elements in stars create dark, absorption lines, the “fingerprints of the stars”. Various stellar spectra are shown in the image, each band is a different star, and they range from cool stars at the bottom to hot stars at the topOBAFGKM is the spectral sequence of stars

Stellar Spectra

Spectral Classes

Cooler stars display more absorption lines

Chemical Composition of StarsSun, first star to be analyzed (1814)Fraunhofer (1814) recorded the strongest absorption

lines, named Fraunhofer lines in his honorAstronomers have since recorded thousands of dark

lines in the sun’s spectrumComparison with spectral lines produced in laboratories

on earth have enabled the identification of 70 different elements in the sun

Stars are primarily hydrogen and heliumThe coolest stars allow actual molecules, compounds of

more than one element, to survive

Fraunhofer Lines

Some of Fraunhofer’s original drawings of the sun’s spectrum

Stellar Elements

Comparison of the sun’s absorption spectrum with the emission spectrum of iron allows the identification of iron in the sun’s outer atmosphere

Betelgeuse Spectra

•Betelgeuse is so cool that the star allows complete molecules such as TiO to survive in its atmosphere

Spectral Classes

Absorption lines used to classify stars into 7 spectral classes

Originally in alphabetical order, Annie Cannon (1863-1941) rearranged them into the present form of O B A F G K M (“Oh Be A Fine Girl/Guy Kiss Me”)

Politically Incorrect

Temperature of Stars

The OBAFGKM sequence of spectral classes is also a temperature sequence

O stars are hottest (> 30,000 K), M stars coolest (< 3,500 K)

Vega & Sirius, O stars (10,000 K)The sun is a G star (5-6,000 K)Antares & Betelgeuse, M stars (3-3,500 K)

Spectral Class and Star Color

•The diagram shows the star color that corresponds to each spectral class

•Many stars have colors that are visible with the naked eye and in telescopes

Click: Stellar Spectra Mini Exercise

Star Colors: Big & Little Dippers

Where is Polaris?

Orion

Betelgeuse

Rigel

•Look for these star colors when you see Orion

•Red areas represent glowing gas in space (nebulas), most is too faint to see with the unaided eye

Belt

Sword

Orion Nebula in Sword of Orion

Andromedawww.scienceandart.com

Galaxy (M31)

The Andromeda Galaxy is visible to the naked eye—at 2.3 million light years it is the most distant object visible to the naked eye

Planck Curves and Blackbodies

Blackbody is a theoretical object that absorbs all of the light that strikes it

Absorbed light heats the blackbodyThe blackbody then reemits the light at different

wavelengthsPlanck curves are graphs of the types of light reemitted

by blackbodies of different temperaturesThe shape of a blackbody curve is a function of the

blackbody’s temperature Ideal blackbody curves (Planck curves) were first

discovered by Max Planck in 1900 (photo)

3 Planck Curves

Three blackbodies at three temps

•Shown is a plot of intensity versus wavelength for blackbodies at different temperatures.

•Blackbodies at different temperatures will appear as different colors or wavelengths.

•At higher temperatures the most intense wavelengths are shorter.

•The sun is very similar to the 6000 K curve. It’s peak wavelength is in the blue-green portion of the visible spectrum.

•Very hot stars have peak emissions in the ultraviolet and beyond, very cool stars can peak in the infrared.

Wien’s Law governs the peak wavelength, Stephan-

Boltzmann governs the intensity

Stars and Blackbodies (Planck Curves)

Star color is related to its temperatureHot stars are bluish white, cool stars are

reddishLight emitted by stars follows a Planck

curveA star’s Planck curve can be used to

estimate a star’s temperature

•A heated iron poker will begin to glow emitting photons. The intensity and wavelength of the radiation changes with temperature.

•As the object heats up, it gets brighter, emitting more photons of all colors (wavelengths), and the color of its greatest light output changes from orange to yellow to blue.

WHEN FIRST HEATED THE WHEN FIRST HEATED THE POKER GLOWS DIMMLY POKER GLOWS DIMMLY AND IS REDAND IS RED

AS THE TEMPERATURE AS THE TEMPERATURE RISES, THE POKER RISES, THE POKER BECOMES BRIGHTER BECOMES BRIGHTER AND GLOWS ORANGEAND GLOWS ORANGE

AT HIGHER TEMPERATURES AT HIGHER TEMPERATURES THE POKER BECOMES EVEN THE POKER BECOMES EVEN BRIGHTER AND GLOWS BRIGHTER AND GLOWS YELLOWYELLOW

Blackbody Radiation Example

Stars emit light that is close to an ideal blackbody. We can estimate the surface temperature of a star by examining the intensity of emitted light across a wide range of wavelengths.

Summary: Properties of Stars

Method of parallax—distances to starsSpectroscopy—Composition of starsPlanck curves—Star temperatures

Apparent Motion of Stars

Earth’s rotation: Stars rise and set Earth’s revolution: Stars change with the

seasonsEarth’s precession: Positions of stars

change in a 26,000 year cycle

Stars Move Through Space

Stars in our galaxy revolve around the galaxy’s center

220 million years for the sunStars have high velocitiesStars are so distant that they appear still

for thousands of yearsStar motions are revealed by measuring

and comparing positions over periods of time, and by analyzing spectra

Sun’s Revolution in Milky Way

http://www.envirotruth.org/images/graphics/suns_path.jpg

•The sun revolves around the center of the Milky Way galaxy every 220 my

•The sun is just one of around 100 billion other stars in the Milky Way

•The Milky Way is a flat disk of stars organized into spiral arms; the spiral rotates clockwise in this view

Space Motions of Stars

Space Velocity: True motion of a star in space, motion of a star with respect to our sun and earth

Space velocity exhibits two components :1. Radial Velocity: Motion towards or away from us

2. Proper Motion: Motion at right angle to us

Star Motions

V = Space Velocity

Vt = Proper Motion

Vr = Radial Velocity

Yellow circle=1st observation

Blue Arrowhead=2nd observation

star

The star moved along the blue arrow in the observation period

Doppler Shift

Radial velocity revealed by Doppler shiftA star’s spectrum exhibits a Doppler shift

if it is moving towards or away from usDoppler shift: When a source of waves

are approaching or receding, the observed wavelengths are changed.

Ex: A train rushing by, the pitch of the train’s whistle drops abruptly as it passes

Doppler Shift of Sound S = A moving source of

sound such as a train whistle

Observer 2 hears a higher pitch whistle, and the pitch drops suddenly when the

source passes

Observer 1 hears a lower pitch whistle than observe 2

For observer 2, the waves are pushed together

Doppler Shift cont.

Stars: Doppler shift is revealed by the positions of dark absorption lines

Spacing between individual absorption lines of an element remains constant, yet the entire set of lines can be shifted right or left compared to the background spectrum

Blueshift and Redshift

Blueshift: The star is approachingRedshift: The star is moving away

Redshift

•Compared to the reference at top, the twin absorption lines are progressively moved towards the red end of the spectrum as a star’s radial velocity increases

•The degree of shift is indicative of the speed of the star’s motion away from us

Reference

Twin spectral lines

Proper MotionProper motion, star motion perpendicular

to our line of sight to a starAverage proper motion for all visible stars

is less than 0.1” per yearProper motions are very tiny

Slow Motion

Big Dipper stars will appear much different in 50,000 years due to a high proper motion

Barnard’s Star has the highest observed proper motion. Comparison of telescope sketches or photos over a lifetime would reveal its motion

61 Cygni is also a high-proper motion star

Big Dipper Proper Motion

Barnard’s Star

•Highest proper motion star, lies at a distance of 6 light years from earth

•Photo a montage displaying 4 years of the star’s proper motion

61 Cygni Proper Motion

http://www.almanak.hi.is/61cygni.html

Hipparcos Web Site

Proper Motion Demohttp://www.rssd.esa.int/Hipparcos/TOUR/

tour.html

Luminosity

Luminosity, measure of a star’s total light outputLuminosity is the amount of light a star shines

into space each secondSun’s luminosity (L), L = 3.85x1026 watts,

equivalent to 3850 billion trillion 100-watt light bulbs

Rigel in Orion is about 60,000 times more luminous than the sun

Why does our sun appear much more luminous than Rigel?

Propagation of Light

Propagation, how light travels through space

Light moving away from a star becomes dimmer

Amount of starlight drops off as the square of the distance away from the star (inverse square relationship)

Propagation•The light from a star is spread further and further apart as it travels into space

•At 1 AU, the star’s light is spread into a 1x1=1 area

•At 2 AU, the star’s light is spread into a 2x2=4 area

•At 3 AU, the star’s light is spread into a 3x3=9 area

•This behavior is an inverse square relationship

Each sphere represents the same amount of light from the star

Star

Apparent Magnitude

Apparent magnitude, a measure of a star’s brightness from earth

Greek astronomer HipparchusTraditional magnitude scale is 1—6,

represents all stars visible to the unaided eye

“1” were the brightest, “6” were the faintest

Modern Magnitude Scale

Modern magnitude scale: A 1st magnitude star is exactly 100 times brighter than a 6th magnitude star

Astronomers found that some stars were brighter than 1, requiring zero and negative magnitudes

The smaller the magnitude number, the brighter the star

Magnitude scale from a star map

Magnitude Scale

•Magnitude 6 and less equal naked eye objects

•Binoculars can see to magnitude 11; An 8-inch aperture telescope can see down to magnitude 14

•Interestingly, the Hubble Space Telescope can see nearly as faint as our sun is bright

> Mag = < Bright

The magnitude scale can be considered a type of number line

8-inBN

Little Dipper Magnitudes

Little Dipper star map, identify some magnitudes

LD is circumpolar

Polaris is a 2nd magnitude star

Absolute Magnitude

Absolute magnitude measures a star’s true brightness

Absolute magnitude is the magnitude that a star would have at a distance of 10 parsecs (32.6ly)

Star

Polaris

Sirius

Apparent

Magnitude

+2.3

-1.5

Absolute

Magnitude

-4.6

+1.4

Distance

240 Parsecs

2.7 Parsecs

Sirius is the brightest star in our sky, which star, Sirius or Polaris, is truly the brightest?

Apparent & Absolute Magnitudes

Canis Major how it appears in our sky—note the brightness of Sirius, the brightest star in apparent magnitude

Canis Major in absolute magnitude, as if all of its stars were brought within 10pc of our sun. The true brightness of the stars is shown

http://media.skytonight.com/images/Sirius_Mags_m.gif

ComparisonsStar App Mag Abs Mag Spectral

ClassParallax

Alpha Centauri

-0.3 4.1 G 0.750”

Thuban 4.7 5.9 K 0.176”

Barnard’s Star

9.5 13.2 M 0.545”

Altair 0.8 2.3 A 0.202”

Comparisons Cont.

Which star is:(a) hottest? __________ (b) coolest? __________(c) brightest looking? __________ (d) faintest looking? ___________ (e) actually most luminous? _________(f) actually least luminous? __________ (g) closest? __________ (h) most distant? __________

Comparisons Cont.

Which star is:(a) hottest? Altair(b) coolest? Barnard’s Star(c) brightest looking? Alpha Centauri(d) faintest looking? Barnard’s Star (e) actually most luminous? Altair(f) actually least luminous? Barnard’s Star (g) closest? Alpha Centauri (h) most distant? Thuban

H-R Diagram

Hertzprung-Russell Diagram, a plot of luminosity (absolute magnitude) versus temperature (spectral class)

When plotted, stars fall into definite regions, not random

Relationship between luminosity and temperature

The diagram was independently created in 1910 by Ejnar Hertzsprung and Henry Norris Russell

Main Sequence: About 90% of stars, runs from upper left to lower right

Sun is a main sequence starUpper left—blue giants Lower right—red dwarfs (most common

star)Upper right—cool giants and supergiantsLower left—white dwarfs

H-R Diagram cont.

H-R Diagram

Sun

H-R Diagram again

Clickhttp://en.wikipedia.org/wiki/Hertzsprung-Russell_diagram

HR Diagram Regions

http://zebu.uoregon.edu/~soper/Stars/hrdiagram.html

Main Sequence

Star’s position on H-R Diagram determined by its mass

Main sequence, stars decrease in mass from upper left to lower right

Mass-luminosity relation: More massive a star, the more luminous it is

After a star forms, it quickly joins the main sequence where it spends most of its life

Mass-Luminosity Relationship

Msun=Mass of our sun

•Masses of stars decrease from upper left to lower right of HR diagram

http://zebu.uoregon.edu/~soper/Stars/hrdiagram.html

Sizes of Stars

Star size: From luminosity and temperature (stellar spectra)

Sun = 864,000 miles, the same as 109 earths placed end to end

Blue-white giants are 25 times the sun’s radius, supergiant stars such as Betelgeuse are 400 times the sun’s radius!

If our sun were replaced by Betelgeuse, its radius would extend beyond the orbit of Mars

White dwarfs are about the size of the earth

Main Sequence Star Sizes

Giant Star Example

The star V838 Monocerotis would extend beyond the orbit of Mars in our solar system

Double Stars

Binary Star: Pair of stars revolve around a common center of gravity. Twins

Binary stars are useful in calculating the masses of stars

Many visual binaries visible in telescopes, display color and brightness differences

Famous Double Stars: Mizar in Ursa Major, Albireo in Cygnus

Optical Double: Apparent double star, one member is actually much more distant, lined up by coincidence

Albireo•Albireo is a famous double star located at the foot of the Northern Cross (Cygnus)

•Albireo consists of two stars in orbit about each other, the brighter star displays an orange tint, and the fainter companion is blue

•The separation between the two stars, and their colors, are easily seen in a small telescope

Northern Cross

•Cygnus represents a swan in flight

•Many refer to the central portion of the constellation as the Northern Cross

•Albireo lies at the foot of the Cross

Dog Star and Pup

•Sirius, the night sky’s brightest star, is also a double

•Seeing the companion in a small telescope is extremely challenging

Orbital diagram of the Sirius system. Closest separation occurred in

1997

Sirius

Sirius B (“Pup”)

Canis Major

http://www.winshop.com.au/annew/CanisMajor.html

Sirius

150 Binary Stars

The orbits of 150 visual binaries