Doppler Effect in Astronomy

Doppler Effect

• The apparent change in the wavelength of radiation.
• This is caused by the motion of the source.

Doppler Effect on Earth

• Observed in sound waves from car horns or sirens.
• Perceived change in pitch (frequency) due to the vehicle's motion.
• High pitch implies movement towards the observer, conversely, low pitch suggests movement away.
• Depends on the relative motion between the source and observer.
• Shifts an object's entire spectrum towards red or blue.

Doppler Effect in Astronomy

• Notice a shift by comparing it to something.
• Motion towards results in a shift to shorter wavelengths.
• Motion away results in a shift to longer wavelengths.
• Faster motion leads to bigger shifts.
• Slower motion leads to smaller shifts.
• Redshift and blueshift indicate the Doppler Effect (or Doppler Shift).
• Changes are often small, looking at the shifting of spectral line patterns.
• The Doppler Shift does not alter the object's overall color.

Properties of Stars

• Common characteristics include formation from gas/dust clouds and similar chemical composition.
• Stars vary in size, age, brightness, mass, and temperature.
• Differences come down to luminosity, surface temperature, and mass.

Brightness of Stars

• Apparent brightness is the amount of radiative power reaching Earth.
• Measured by the rate of photons collected, also known as energy flux.
• Refers to how bright stars appear in the night sky from Earth.
• An object appears bright either due to its proximity or inherent brightness.
• The Sun appears bright because of its proximity and not its true brightness compared to other stars.
• Luminosity (absolute brightness) measures the total radiative power emitted by a star.
• It's an object's true brightness, describing the amount of power a star produces.
• Luminosity is found by multiplying the power produced per area by the surface area of the star.
• Apparent magnitude describes how bright a star appears.
• Absolute magnitude describes how bright it actually is.

The Inverse Square Law of Light

• Area of Sphere = 4π(radius)2
• Luminosity remains constant through each sphere.
• Brightness is calculated by dividing luminosity by area.
• There is a relationship between apparent brightness and luminosity depends on distance [apparent brightness = absolute brightness (luminosity) / 4πR^2].
• A star's luminosity can be determined when distance and apparent brightness are measured [absolute brightness luminosity = 4πR² x apparent brightness].

Stellar Luminosity

• Most luminous stars have a luminosity of 10^6 times that of the Sun.
• Least luminous stars have a luminosity of 10^-4 times that of the Sun.
• L sun refers to the luminosity of the sun.
• Photometry the process of measurements of the apparent brightness of stars.
• Astronomical photometry began in 150 B.C.E. by Hipparachus.
• Hipparchus sorted stars into six brightness categories referred to as magnitude.
• First-magnitude stars were the brightest, while sixth-magnitude stars were barely visible.
• Apparent magnitude can be used to measure apparent brightness; a smaller magnitude indicates higher brightness.

Star Color and Temperature

• Blue indicates that very hot stars emit much additional radiation in the ultraviolet
• Cool stars emit most of their light energy at red wavelengths.
• Hottest stars have temperatures exceeding 40,000 K.
• A chart of star color and temp - Blue(25,000 K) e.g. Spica, White(10,000 K) e.g. Vega, Yellow(6,000 K) e.g. Sun, Orange(4,000 K) e.g. Aldebaran, Red(3,000 K) e.g. Betelgeuse.

Brightness

• Light obeys the Inverse Square Law.
• The amount of light received follows the relationship 1/d^2.
• Brightness (apparent) refers to energy (light) is dependent on 1 m2 for each second
• Brightness (Flux) is equal to amount of energy (light) dependent on 1 m2 for each second
• Luminosity (L) = total amount of energy a star emits per second = absolute brightness.
• The formula for brightness is Luminosity/Distance^2
• Astronomers sometimes use a unit called apparent magnitude to measure apparent brightness
• Smaller magnitude = brighter star

Luminosity

• Luminosity depends on the temperature of the star and the size of the star
• The bigger the surface area (size), the higher the luminosity.
• The higher temperature means the higher the luminosity.
• Examples: L = R^2 T^4

Hertzsprung-Russell Diagram (H-R Diagram)

• Can plot several different related quantities.
• Used to plot luminosity versus temperature (or spectral type)
• It is useful because it wasn't a complete scatterplot
• Most stars fall somewhere on the main sequence of the H-R diagram.
• Main Sequence = Hydrogen “burning” in the core

Interstellar Medium (ISM)

• Space is not actually empty, contains clouds of gas and dust.
• Acts to obscure or alter our view of distant objects/place where dense clouds are where stars form
• Gas and dust clumps: 75% H, 25% He by mass (90% H,10% He by number)
• Extremely cold, ~ 10-50K (-250 C), Huge in size ~ 30 l.y. Diameter, Density 1-1000 atoms/cm3.
• Has Gas and dust clouds known as nebulae in the ISM.

Emission Nebulae

• Three types, each different in look:
• Clouds of hydrogen can be heated when near a very hot star causes atoms to be ionized and electrons to be lost
• heated cloud of gas glows.
• Orion's Nebula is a famous example and easily visible with telescopes
• Reddish color

Reflection Nebulae

• Reflection nebulae do not make their own light, they scatter light from nearby stars.
• Clouds of dust scatter red and blue photons in different ways.
• Interstellar reddening happens when looking through dust, background stars appear redder.

Dark Nebulae

• Dense clouds of solid grains of material.
• It appears dark in visible light, unlike emission or reflection nebulae.
• It presents itself as dust clouds obscuring light appearing as a black void.

Giant Molecular Clouds

• The Orion Nebula is a giant molecular cloud (GMC)
• Containing many new stars.
• Stars tend to form in star clusters.

Protostars

• Protostars will eventually become stars.
• Giant cloud contracts and releases low energy red color
• Increased Temp increased Pressure = Start Nuclear reactions!
• Stars normally form in big bunches.
• A shock wave through a GMC can collapse smaller, denser regions.
• This forms a protostar and protostellar disk.
• Protostar contracts, increasing surface and core temperatures.
• A star is born when hydrogen fusion begins because the core gets hot enough.
• More massive protostars form FASTER
• Mass determines ability to make a star.

Mass

• Stars need at least 0.08 Msun to avoid becoming a brown dwarf and there is an upper limit on mass.
• If above 120 Msun, makes a binary system instead. Tells us how large or small in size it is (hydrostatic equilibrium) and how quickly a star goes through its fuel (mass-luminosity relation)

Star Formation

• Hydrogen to Helium Fusion all main stars on sequence.
• Use fusion to convert four Hydrogen atoms into one Helium atom
• Sun, and the other low mass stars, use the proton-proton chain (4^1 H → ^4 He + energy)
• Stars with slightly higher masses than the Sun (2 solar masses and above) use a more efficient process, called the CNO Cycle
• The inward pull of gravity is balanced by the outward push of pressure (hydrostatic equilibrium).
• Hydrogen fusion provides the outward pressure, so while on the main sequence, the mass of a star determines its temperature, luminosity, and size

Types of Main Sequence Stars

• Stars are usually of Low Mass (< 0.4 MSun), Medium Mass (0.4-8.0 Msun), High Mass (> 8.0 MSun).
• They all change (live) differently

Medium Mass (Sun) Stellar Changes

• Medium Mass = 0.4 – 8.0 Msun
• Core composition shift = H fuel runs out (core becomes He enriched)
• Step 1: Core collapses and hydrogen shell burns outward
• Step 2: Envelope expands (outward pressure)
• Step 3: Star becomes a Red GIANT
• The Red Giant will 10-100 DiameterSun 10% of the total lifetime of the star, have a cool surface temperature and a L because of size, also a T because of expansion
• The Core becomes super-dense (degenerate) leading to increased Temperature of Helium and Helium fusion
• Energy Helium Flash! - when He ignites in the core of the star and Helium shell burns star expands and cools

Helium Flash

• Compress the core to create high enough temp, 100 million K, to start fusing helium, or temperatures of billions of Kelvin to fuse heavier elements if it is repeating this stage if have a massive enough star.
• Lower mass stars (like the Sun), the onset of helium fusion can be very rapid, producing a burst of energy called a helium flash.
• The reaction rate settles down - Fusion in the core releases more energy/second than the core fusion of the main sequence stage, so the star is bigger, but stable! Hydrostatic equilibrium is restored until the core fuel runs out

Deaths of Low Mass Stars -- Planetary Nebulae

• The Sun will leave the main sequence when it can't fuse hydrogen into helium in its core as a reminder
• Then goes through a short phase of helium to carbon fusion
• Outer layers keep expanding, the core keeps contracting
• For a low-mass star like the Sun, the outer layers continue to expand until they drift away from the star form a planetary nebula + exposed core

White Dwarfs

• The outer layers of a low mass star form a planetary nebula, exposed core forms and becomes a white dwarf
• Stellar remnant balances gravity with electron degeneracy pressure
• There is an inverse relationship - the more mass a white dwarf has, the smaller its size
• The white dwarf is extremely dense. About half the Sun's total mass will be condensed down to the approximate size of Earth.
• One teaspoon of WD weighs ten tons!

Supernovae

• Before a high-mass star dies, its core has fused elements all the way up to iron, which is the last energy source
• The inert iron core is now so dense that it collapses (it cannot stop and make a white dwarf), squeezing all of the electrons into the nuclei of atoms, converting all of the protons into neutrons instantaneously, releasing a huge number of neutrinos
• The “bounce back” from this collapse creates an enormous shock wave explodes the star (Type II Supernova)

Question 1

• Unknown stars' composition are Sodium, Hydrogen, Helium Oxygen Neon. (b)

Question 2

• Unknown star moving towards us, away from us, or not moving. (a&b)

Question 3

• The types of electromagnetic radiation from space that reach the surface of Earth are Radio waves and microwaves, X-rays and ultraviolet light, , infrared and gamma rays, and d. visible light and radio waves
• Earth's atmosphere allows radio waves and visible light to reach the ground

Question 4

• Spectral lines are shorter in wavelength if a light source is approaching you

Question 5

• It takes the Sun 27 days to complete one full rotation at the equator

Question 6

• The correct statement is that apparent magnitudes, mA > mB

Question 7

• Correct statement about absolute magnitudes mA > mB

Question 8

• Sun can be identified as a star on the basis of - Comparison with planets - Comparison with yellow stars - Comparison with other stars - Comparison with other galaxies (c)

Question 9

• Key physical feature of stars includes temperature, mass, and size; distance is not a significant one (d).

Question 10

• First step to find pile rock investigation is done by physical parameters (c.)

Question11

• The hottest star on diagram A

Question 12

• The most luminous star is classified by diagram B

Question 13

• Main-sequence star diagram: (D)

Question 14

• Star has largest radius diagram: (C)

Question 15

• Locations of the main sequence stars diagram: (W,X,Y,Z)

Question 16

• Sun - where is it shown on the diagram?: (Y)

Question 17

• Giant stars on the diagram: (S)

Question 18

• Coolest stars on the diagram (Z)

Question 19

• Where are giant stars (diag)- (S)

Question 20

• Diagram location of largest-sized stars: (T)

Question 21

• Brightest star on diagram location: (T)

Question 22

• Region for the placement of WD Os diagram: (U,V)

Question 23

• Color of the most blue diagramed - (W)

Question 24

• Least-massive MS Star on diagram: (Z)

Question 25

• Category Nebula (Orion): (d) all

Question 26

• Trifid Nebula Sagittarius Category: (a)

Question 27

• Main sequence stars on survey, 90 % are on, which of the main sequence. Their long lifespan? (a)

Question 28

• Fate: What lies for the remainder of the SL Stars?: (d)

Question 29

• Fate: What event might we be for the core to change MS Stars?: (e)

Question 30

• Time MS Star: (d)

Question 31

• Component of What Dwarfs: (c)

Question 32

• Which element is formed to create the new star: (d)

Question 33

• Supernova definition: (d)

Question 34

• Temp's for Nuclei react for elements? (d)

Question 35

• The eventual fate for massive stars: (e)

Question 36

• Approximation Star: (b)

Question 37

• Approximation of Star size: (d)

Question 1 Homework 10

• Doubles if distance, the four points decrease 14th. (d)

Question 2 Homework 10

• A Star with O magnitude: (a)

Question 3 Homework 10

• Intrinsity - Lumn depends on distance (d.)

Question 4

• What is luminously: (b)

Question 5

• HIgh Lum Result: (b)

Question 6

• If luminosities doubled in Star Radius: (a.)

Question 7

• Temput doubled? Lumos increase: (c.)

Question 8

• HR Plot Diagram = Temput Vs Star Strength (a)

Question 9

• HR Diagram with Location DW : (d.)

Question 10

• Kind stars is high? Lumm and Temp with Sequels: (c)

Question 11

• Position Star with MS = mass: (b)

Question 12

• Star forming in Equib Hydro = the meaning (b.)

Question 13

• Hi Star, low Lumin = D (b.)

Question 14

• Spectral for Star Cooling: What kind spectrum? (B)

Question 15

• Sun woul appear on HR diagram

Question 1

• Phase before the sun? (a).

Question 2

• Nebula that's orion? (a.)

Question 3

• Density Object?? (d)

Question 4

• Star is MS Star - Nuclar what now? (D).

Question 5

• Star are Main Seque - why?: (c.) =

Question 6

• The vent with Star in Gaint almost Hydrogen: (a)

Question 7

• Move Giant H -R way? ( c.)

Question 8

• Star Like Son Fuse? - The high enough (a)

Question 9

• Star goes glant? - Temperut: C.

Question 10

• Stars diff? Star: a

Question 11

• nebula shell lets it = a

Question 12

• Saller diameter

Question 13

• nucleus in Atomic= iron

Question 14

• Wd

Homework 13

Question 1

• Intersteller is comets, asteroid..

Question 2

• Stars- Gas in inter - stars .

Question 3

• Star finds ultra: plant

Question 4

• illum

Question 5

• Contracto

Question 6

• Reations

Question7

• charatictis = mass

Question 8

• reflection

Question 9

• .8

Question 10

=-wd