1. CHAPTER 28 STARS AND GALAXIES

2. 28.1 A CLOSER LOOK AT LIGHT
Light is a form of electromagnetic radiation, which is energy that travels in waves.
Waves of energy travel at 300,000 km/sec (speed of light
Ex: radio waves and x-rays
Electromagnetic waves do not need a medium to travel, they travel through space
Electromagnetic waves emitted by an object indicated what elements are present and its motion

3. Electromagnetic radiation waves are arranged into a continuum call the electromagnetic spectrum.
Wide range of wavelengths
Long wavelengths with low frequencies at one end, short wavelengths with high frequencies at the other end
Wavelength measured crest to crest/trough to trough
Frequency the number of that crests of the same wavelength that pass a point in one second.

4. Scientists study the visible light portion of the electromagnetic spectrum.
Spectra of a stars allow for astronomers to learn about the star’s elements and motion.
Spectra studied using a spectroscope

5. Three types of visible spectra
Continuous spectrum: unbroken band of colors, emitting all colors of visible light
glowing solids, such as a light bulb filament
glowing liquids, molten iron
hot, compressed gases inside a star
Emission spectrum: unevenly
spaced lines of different colors,
emitting light of only some
wavelengths
glowing thin gases
every element has its own unique
emission spectrum, element’s finger print
Absorption spectrum: dark lines that
cross a continuous spectrum.
forms when light from a glowing object pass through a cooler gas which absorbs some wavelengths
can determine what is present in the cooler gas by comparing emission and absorption spectrum

6. Doppler Effect is the apparent change in the wavelength of radiation or sound due to relative motion between the object and the receiver.

7. Doppler effect applies to light as well as sound.
Shift of the emission spectra can indicate if the object is moving towards or away from Earth
Shift towards the red end of the visible light spectrum, object moving away Earth – Redshift
Shift towards the blue end of the visible light spectrum, object moving towards Earth - Blueshift
Doppler effect determined that the universe is expanding

8. TELESCOPES
Optical Telescopes – gather far more light than an unaided eye and magnify imagers
Reflecting
Uses one lens at back to gather and focus light
Image reflected on to a small mirror and then the eye piece
Refracting
Uses two lenses
Lens at the front gathers light
Eyepiece magnifies image

9. Radio Telescopes Other Technical term for really big satellite dishes
Use to detect energy waves at frequencies lower than visible light
Other
Detects energy waves at frequencies higher than visible light
Usually satellites in space
Gamma ray
Background radiation
X-ray
Hubble (infrared)

10. 28.2 Stars and Their Characteristics
Observation of stars has been going on for over 5000 years
The grouping of stars are called constellations
Constellations
human grouping of stars
only appear together as viewed from Earth from a different angle they do not look like the constellation
Grouped together due to looking like they all have the same brightness (apparent magnitude)

11. Constellations (continued)
Constellations will change shape over thousands of years due to the universe expanding
Some constellations have been around for thousands of years other were recently conceived
Move across the sky from east to west (though Earth rotates west to east)
Big Dipper and Little Dipper two of the best know constellations point to the North Star

12. North Star Current north star is called Polaris
Sits directly over the North pole
Does not move to the naked eye
Very powerful tool for navigation
Due to Precession, Polaris will not always be the “North Star”

13. Circumpolar Constellations – constellations around the north star
Create star trails evidence for Earth’s rotation
North star appears fixed in the sky as Earth rotates
star trails from circumpolar constellations rotate counterclockwise around the north star
Circumpolar constellations seem to never set from some northern hemisphere latitude

14. The Constellations that dominate the night sky change from month to month. This is the result of the Earth’s change in position as it orbits the Sun.

15. Plain of the Ecliptic (path of Sun, Moon and Planets across the sky)
Curve in Plain of the Ecliptic is due to change in tilt of the Earth over the course of a year.

16. Distances to stars and other objects in space
Unit of measurements on Earth are not sufficient for space measurements
Astronomical Unit (AU) is used for the distance from Earth to the Sun (150 million kilometers)
Light year is the distance light travels in one year (9.5 trillion kilometers)
It is a distance measurement
Example light-years means that the light we see has been traveling for 4.2 years before we can see it (4.2 X 9.5 trillion km)
Parallax an apparent change in an object’s position due to a change in the observer’s position
Parsec short for “parallax second” equal to light-years.

17. Parallax
The further the object from the viewer, the less the parallax shift.

18. Stars Elements Hydrogen ~69% Helium ~29% Heavier elements ~2%
No two stars have the same proportions of elements
light that radiates is dependent on composition and temperature, this differs in every star
Star spectrum is its fingerprint

19. Mass, Size and Temperature
Stars vary greatly in masses, size and temperature
Cannot observe directly so we are estimating what the mass might be
Gravitational effect on bodies around the star help with estimating its mass
Star mass is expressed as multiples of the mass of our Sun (which has a stellar mass of 1)
Size varies more than mass
Smallest stars are smaller than Earth
Largest have diameters more than 2000 times that of our Sun
Stars differ even more in density
Betelguese is about one ten-millionth of our Sun
One star is so dense that one teaspoon would weight more than a ton on Earth

20. Star size comparison

21. Temperature of stars vary
Stars have Different colors which indicate different temperatures
Cool stars are redder in color
Ex. Betelguese with a surface temperature of 3000oC
Mid-temperatures are yellower in color
Ex. The Sun with a surface temperature of 5500oC
Hot stars are blue in color
Ex. Sirus
Astronomers group stars by temperature and color into spectral classes.
System is called the Harvard Spectral Classification Scheme

22. Actual brightness of a star is Luminosity
Dependent on size and temperature of the star
Distance from Earth not a factor
If two stars are the same size the hotter star would be more luminous
Apparent magnitude is how bright a star appears to an observer on Earth
Does not factor in distance
Absolute magnitude is the measure of how bright the star would be if all stars were the same distance from Earth (10 parsec)

23. Variable stars These stars show a variation in brightness over cycles
Can last days to years
Some change brightness as they expand and contract
One important class is called Cepheid variables
Yellow supergiant stars with a cycle of brightness ranging from days.
Most have cycle of 5 day.
Nonpulsating star may change in brightness due to the fact that is more than one star.

24. 28.3 LIFE CYCLE OF STARS
Stage 1
Stage 2
Stage 3

25. Stage 1 Stage 2 Stage 3 Main Sequence Star (stars like our Sun)
Main sequence stage, last
the longest
Star stays until all H2 fuel
is used up (converted to He)
He core shrinks and
Contracts to produce
additional heat.
H2 fusion starts in the outer
Layers causing the star to
swells– moves to next stage.
Main Sequence Star (stars like our Sun)
less than 8 solar masses
Nebula spins & flattens
To form a protostar
Protostar shrinks due to
gravitional attraction.
As temperature increases and pressure increases, light and heat are emitted
Energy is produced from
Nuclear fusion of H2 into
He begins – moves to next stage
Core temperature increases
to a point that He can fuse into
heavier elements (O2 and C)
H2 into He continues in the
outer layers.
When Hydrogen is exhausted
the outer layers blow away
leaving only the carbon-oxygen
core – White Dwarf.
Expelled layer absorbs the
white dwarfs ultraviolet emission
causing a halo affect called a
Planetary Nebula

26. Stage 1 Stage 2 Stage 3 Massive Star 8 solar masses or more
Same process however due
to the more massive size it
swells to an even larger size
than the main sequence star
does
Same process however
much more dust and gas
present
Fusion in core continues until iron
forms
Iron does not release energy so it
absorbs it, thus leading to a quick
collapse resulting in an explosion –
Supernova.
Massive star that is 8-15 solar
masses, creates a neutron star
following the supernova
Massive star that is more than
15 Solar masses, creates a
black hole following the supernova.

27. HR Diagram HR diagram gives you a picture of a star’s life.
There are thousands of variations of the HR diagram
All plot luminosity against temperature
Some include spectral class and absolute magnitude.
Spectral class is based on the temperature of the star
Luminosity is the actual brightness of a star, where as absolute magnitude is the brightness if all stars were the same distance from Earth.

29. HR Diagram Groups
Stars fall into one of several distinct groups on the HR diagram.
90% of stars fall in a band called the main sequence, which runs from the upper left corner to the bottom right corner. Stars in this band are called main sequence stars.
Giant stars grouping is found in the upper right corner just above the main sequence.
Larger stars are more luminous and have diameters from 10 to 100 times greater than the sun

30. HR Diagram Groups
Supergiants grouping is found in the upper right corner just above the giant stars.
Diameters that are more than 100 times that of the sun
Relatively cool stars but their size gives them their luminosity.
White Dwarfs grouping is found in the lower left corner, these stars are near the end of their lives.
Were once red giants
The red giants outer lost their outer atmosphere, leaving only a glowing stellar core.
Black holes and Neutron stars do not appear on the HR diagram because their luminosity is not great enough.

31. 28. 4 Galaxies and the Universe
The universe is everything that exists.
The observable universe is everything we can observe.
Astronomers are not sure how old the universe is since the light from the beginning has not yet reach us. The estimate is between billion years.

32. Galaxies and the Universe
Galaxies are hard to separate from stars without telescopes.
Hazy patches of light which when viewed through a telescope reveals thousands to billions of stars.
Astronomers estimate there are billion galaxies in the observable universe.
No two galaxies are the same, however, they are classified based on shape.

33. Galaxies and the Universe
There are three classifications
Spiral (ex. Milky Way)
Elliptical are near spherical to lens-shape
Irregular (ex. The two Magellanic Clouds)
Spiral
Elliptical
Irregular

34. Origin of the Universe
Big Bang Model – explains history of the universe from a tiny fraction of a second after it came into being up to the present
Best explanation for how the universe came to being
Developed due to observations of stars, galaxies and other objects with telescopes and experimenting with matter on Earth
Approximately billion years ago all matter in the universe existed in an incredibly hot and dense state, from which it expanded and cooled slowly condensing into stars and galaxies.
It is expanding at a very slow rate and is still expanding today

35. Origin of the Universe Evidence for the Big Bang Model:
The universe’s apparent expansion, distance between galaxies and groups of galaxies seems to be increasing with time.
Edwin Hubble’s discovery of the redshift in the spectra of galaxies supports the expansion of the universe
Discovery of radiation called cosmic background radiation apparently left over from the universe’s beginning.
Continues to be tested and examined to seek further evidence to support it
Some astronomers are considering alternative ways the universe has reached its present state.

36. Origin of the Universe
It is impossible to know for certain how the universe began
as long as the Big Bang Theory (model) survives crucial tests, it remains the best explanation for the origin of the universe
If it were to ever fail a test, then astronomers will have to look for a new theory/model