Instructor : Pete Kozich


Distance
• Measuring a star's distance is technical
• Stellar parallax
• Used for measuring distance to a star
•
Apparent shift in a star's position due to the orbital motion of Earth
• Measured as an angle
• Near stars have the largest parallax
• Largest parallax is less than one second of arc
(arcsecond)


Distance
• Distances to the stars are mind boggling
• Units of measurement
• Kilometers or astronomical units are too cumbersome to use
• Light-year is used most often
• Distance that light travels in 1 year
• One light-year is 9.5 trillion km (5.8 trillion miles)


Stellar brightness
• Controlled by three factors
• Size
• Temperature
• Distance
• Magnitude
• Measure of a star's brightness


Stellar brightness
• Magnitude
• Two types of measurement
• Apparent magnitude
• Brightness when a star is viewed from Earth
• Decreases with distance
• Numbers are used to designate magnitudes - dim stars have larger numbers; brightest are negative; log scale where 5 magnitudes = 100 times
• Absolute magnitude
• "True" or intrinsic brightness of a star
• Brightness at a standard distance of 32.6 light-years (10 parsecs)
• Vast majority of stars' absolute magnitudes are between -5 and
+15


Color and temperature
OBAFGKM (Oh Be A Fine Girl, Kiss Me)
• Hot star
• Temperature above 20,000 K
• Emits short-wavelength light
• Appears blue (O-, B class stars )
• Cool star
• Temperature less than 3000 K
• Emits longer-wavelength light
• Appears red (M class stars )
• Between 5000 and 6000 K
• Stars appear yellow
• e.g., Sun (G class stars)


Binary stars and stellar mass
• Binary stars
• Two stars orbiting one another
• Stars are held together by mutual gravitation
• Both orbit around a common center of mass
• Visual binaries are resolved telescopically
• Most stars in the universe are binary stars
• Used to determine stellar mass
• Stellar mass
•
Determined using binary stars – the center of mass is closest to the most massive star
• Mass of most stars is between one-tenth and ten times the mass of the Sun; most are less than the Sun, and lower estimate is debatable


Shows the relation between stellar
• Brightness (absolute magnitude) and
• Temperature

Diagram is made by plotting (graphing) each star's
• Luminosity (brightness) and
• Temperature


Parts of an H-R diagram
• Main-sequence stars
• ~ 90% of all stars
• Band through the center of the H-R diagram
• Sun is in the main-sequence
• Giants (or red giants)
• Very luminous
• Large
• Top the H-R diagram
• Very large giants are called supergiants
• Less than 1% of all stars


Parts of an H-R diagram
-Red dwarfs ~70% of all stars, orange dwarfs
~15%
-- White dwarfs
• Fainter than main-sequence stars
• Small (approximately the size of Earth)
• Lower-central area on the H-R diagram
• Not all are white in color
• Perhaps 10% of all stars
• Remains of older stars

Figure 24.5


Stars that fluctuate in brightness

Types of variable stars
• Pulsating variables
• Fluctuate regularly in brightness
• Expand and contract in size
• Eruptive variables
• Explosive event
• Sudden brightening
• Called a nova (outer layer of the star is ejected at high speeds)

 Between the stars is “the vacuum of space”, not a perfect vacuum, but very few molecules

Nebula (name of interstellar matter)
• Cloud of dust and gases
• Two major types of nebulae
• I. Bright nebula
• Glows if it is close to a hot star
• Two types of bright nebulae
• A) Emission nebula (absorbs UV radiation and emits visible light)
• B) Reflection nebula (reflects light from nearby stars)

Figure 24.8

Figure 24.9


Nebula
• Two major types of nebulae
• II. Dark nebula
• Not close to any bright star
• Appear dark
• Contains the material that forms stars and planets


Stars exist because of gravity

Two opposing forces in a star are
•
Gravity – contracts
•
Thermal nuclear energy – expands

Stages
• I. Birth
•
Occurs in dark, cool, interstellar clouds
• Gravity contracts the cloud
• Temperature rises
• Radiates long-wavelength (red) light
• Becomes a protostar


Stages
• II. Protostar
• Gravitational contraction of gaseous cloud continues
• Core reaches 10 million K
• Hydrogen nuclei fuse
•
Become helium nuclei
•
Process is called hydrogen burning
• Energy is released
• Outward pressure increases
• Outward pressure balanced by gravity pulling in
• Star becomes a stable main-sequence star


Stages
• III. Main-sequence stage
• Stars age at different rates
• Relatively steady, stable middle age for much of a star’s life
• Massive stars use fuel faster and exist for only a few million years
• Small stars use fuel slowly and exist for perhaps hundreds of billions of years… they can shine on for much longer than the universe is old!


Stages
• IV. Red giant stage
• Hydrogen burning migrates outward
• Star's outer envelope expands
• Surface cools
•
Surface becomes red
• Core is collapsing as helium is converted to carbon
• Eventually all nuclear fuel is used
• Gravity squeezes the star


Stages
• V. Burnout and death
• Final stage depends on mass
• Possibilities
• A. Low-mass star
• ~0.5 solar mass or less
• Red giant collapses
• Becomes a white dwarf

Figure 24.12 A


Stages
• Possibilities
• B. Medium-mass star
• Between ~0.5 and 2 solar masses
• Red giant collapses
• Planetary nebula forms
• Becomes a white dwarf
• Vast majority of all stars are low and medium masses

Figure 24.12 B

Figure 24.11


Stages
• Possibilities
• C. Massive star
• Over 2 solar masses? (uncertain exactly)
• Short life span
• From visible wavelengths, the star terminates in a brilliant explosion called a supernova
• Interior condenses
• May produce a hot, dense object that is either a neutron star or a black hole; black holes are from the most massive stars (more than 5 solar masses)

• Supernovae can shine brighter than their galaxies.
Amount of emitted energy tough to fathom.
• They are responsible for the production of heavy metals. Without them, it would have been impossible for humans to exist (no iron, for example). As such, life with the capabilities such as humans may not be able to exist on planets orbiting first generation stars. Also, supernovae good for generating titanium, zinc, gold, silver – many essential or luxurious resources we use today…

Figure 24.12 C


White dwarf
• Small (some no larger than Earth)
• Dense
• Can be more massive than the Sun
• Spoonful weighs several tons
• Atoms take up less space
• Electrons displaced inward
• Called degenerate matter
• Hot surface
• Cools to become a black dwarf


Neutron star
• Forms from a more massive star’s supernova remnants
• Gravitational force collapses atoms
• Electrons combine with protons to produce neutrons
• Small size– size of a city & rotate rapidly (tenths of a second)
•
Pea size sample
• Weighs 100 million tons
•
Same density as an atomic nucleus
• Strong magnetic field
• First one discovered in early 1970s
• Pulsar (pulsating radio source)
• Found in the Crab nebula (remnant of an A.D. 1054 supernova)

Figure 24.14


Black hole
•
More dense than a neutron star
• Tiny
• Intense surface gravity lets no light escape
• As matter is pulled into it
• Becomes very hot
• Emits x-rays
• Event horizon is small (area that it effects)
• Also have a visible effect on neighboring stars
• Gases are pulled from other stars


Milky Way galaxy
• Structure
• Determined by using radio telescopes
• Large spiral galaxy
• About 100,000 – 200,000 light-years wide
• Between 200 billion and 3 trillion stars
• Thickness at the galactic nucleus is only about
1,000 light-years
• Apparently four spiral arms of stars (two major)
• Sun in the middle of a spiral arm, the Orion Arm

Figure 24.18 A

Figure 24.18 B


Milky Way galaxy
• Rotation
• Around the galactic nucleus
• Sun rotates around the galactic nucleus once about every 225 - 250 million years
• Halo surrounds the galactic disk
• Oblong sphere, about 180,000 light years across
• Very tenuous gas and dark matter
• Numerous globular clusters


Other galaxies
• Four basic types of galaxies
• I. Spiral galaxy
• Arms extending from nucleus
• About 30% of all galaxies
• Large diameter of 20,000 to 200,000 light years
•
Contains both young and old stars
• e.g., Milky Way

Figure 24.20


Other galaxies
• Four basic types of galaxies
• II. Barred spiral galaxy
• Stars arranged in the shape of a bar
• Generally quite large
• About 10% of all galaxies, the Sombrero Galaxy in
Virgo
• III. Elliptical galaxy
• Ellipsoidal shape
• Mainly older stars
•
About 60% of all galaxies
• Most are smaller than spiral galaxies; however, they are also the largest known galaxies, many in the Virgo
Cluster

Figure 24.22


Other galaxies
• Four basic types of galaxies
• IV. Irregular galaxy
• Lacks symmetry
• About 10% of all galaxies
•
Contains mostly young stars
• e.g., Magellanic Clouds, which are companions of the Milky Way and have been distorted by its gravity


Galactic cluster
• Group of galaxies
• Some contain thousands of galaxies
•
Local Group
• Our own group of galaxies
• Contains more than 30 galaxies
• Milky Way and Andromeda galaxies are largest
• Supercluster
• Huge swarm of galaxies
• May be the largest entity in the universe


Doppler effect
• Change in the wavelength of light emitted by an object due to its motion
•
Movement away stretches the wavelength
•
Longer wavelength
• Light appears redder
• Movement toward “squeezes” the wavelength
• Shorter wavelength
• Light shifted toward the blue


Doppler effect
• Amount of the Doppler shift indicates the rate of movement
• Large Doppler shift indicates a high velocity
• Small Doppler shift indicates a lower velocity

Expanding universe
• Most galaxies exhibit a red Doppler shift
• Moving away

Figure 24.24


Expanding universe
• Most galaxies exhibit a red Doppler shift
• Far galaxies
• Exhibit the greatest shift
• Greater velocity
• Discovered in 1929 by Edwin Hubble
•
Hubble's Law – the recessional speed of galaxies is proportional to their distance
• Accounts for red shifts